WO1989003980A1 - Infrared emission detection - Google Patents

Abstract

The invention pertains to the means and method for observing the infrared spectrum obtained from the infrared emission of excited molecules in samples of interest (21). Over the wavelength interval from 1 to 5 microns, two strong emission bands were observed with a PbSe detector (24) when organic compounds were introduced into a hydrogen/air flame (21). The band at 4.3 microns (2326 cm-1) was due to the asymmetric stretch of carbon dioxide while the band at 2.7 microns was due to both water and carbon dioxide emission. The carbon dioxide emission at 4.3 microns was found to be most intense at the tip of the flame (21), and was found to increase with the amount of organic compound introduced into the flame (21). For chromatographic application, an optical filter was used to isolate the 4.3 micron emission band.

Description

INFRARED EMISSION DETECTION

DESCRIPTION Technical Field

This invention relates to infrared emission detection means and method for detecting selected molecules of interest in a gaseous sample. The invention is particularly applicable to the fields of gas chromatography, liquid chromatography, CO₂ detection, total organic carbon analysis and total inorganic carbon analysis. Prior Art

Combustion flames have long been employed analytically as spectroscopic sources. Although, the analytical application of combustion flames as spectroscopic sources has been studied in great depth, the work, to date, has been confined almost entirely to studies of the radiant emissions falling within the UV-visible region of the electromagnetic spectrum.

U.S. Patent No. 3,836,255 describes a spectro¬metric substance analyzer which monitors both emission and absorption. In this analyzer a fluid is cyclically heated and cooled wherein the radiation variation is characteristic of the substance of interest in the fluid.

U.S. Patent No. 3,516,745 describes a method for observation of gas spectral emissions. The gas is contained in a chamber where it is cyclically compressed and allowed to expand. The variation in infrared emission can be correlated to the concentration of gas within the piston. The oscillation excites or energizes the gas contained in the chamber to give off spectral emissions. U.S. Patent No. 3,749,495 like U.S. Patent No.

3,516,745 describes an IR emission analyzer where the sample is periodically compressed and expanded. The compressed gas becomes heated due to increased molecular collision and thereby produces infrared emissions. Comparison of the emissions of the compressed and expanded gas produces a differential emission dependent upon gas concentration.

Despite the fact that a sizable fraction of the total radiation emitted from combustion flames lies in the infrared region of the spectrum, the use of this emission for analytical purposes does not appear to have been studied.

In terms of optical radiation, the energy radiated from a combustion flame extends from the ultraviolet region of the spectrum to the far infrared region. Of the total energy radiated by the flame, emission from the ultraviolet and visible regions of the spectrum accounts for only about 0.4% (Gaydon, A. G.; The Spectroscopy of Flames; Chapman and Hall: London, 1974; pp 221-243). By contrast, it is estimated that infrared emission from a combustion flame may account for as much as 20% of the total energy radiated (Gaydon, A. G. ; Wolfhard, H. G.; Flames, Their Structure, Radiation and Temperature, 4th ed.; Chapman and Hall; London, 1979; pp. 238-259.) For transparent flames such as the hydrogen/air flame, the visible emission, is negligible and most of the radiated energy falls in the infrared region of the spectrum. In spite of these facts, the analytical applications of infrared emissions from combustion flames have not been studied.

Despite the lack of analytical studies, a great deal of work has been done on the infrared emissions from hot gases, primarily for the purpose of tracking jet aircraft and rockets for military applications. In much of this work, ordinary combustion flames have been employed as models for exhaust gases from airborne vehicles. Plyler (Plyler, E. K.; J. Res. Nat. Bur. Stand. 1948, 40, 113.), in particular, has studied the infrared emission from a Bunsen flame over the wavelength range from 1 to 22 jam. In the wavelength interval from 1 to 5 μm, Plyler found that two bands predominate as a result of infrared emission from molecules of CO₂ and H₂O

(Plyler, E.K.; J. Res. Nat. Bur. Stand. 1948, 40, 113.). One band is located at 2.7 μm (3704 cm^-1) while the other is located at 4.3μm (2326 cm^-1).

Studies of the infrared emission of carbon dioxide have shown that CO₂ emits strongly at 2.8 and 4.4μm (Gaydon,

A. G. , The Spectroscopy of Flames, Chapman and Hall: London,

1974; pp 221-243.) The longer wavelength band corresponds to the asymmetric stretch of the carbon dioxide molecule

(Nakamoto, K; Infrared Spectra of Inorganic and Coordination

Compounds; John Wiley: New York, 1963, p. 77.). Water, on the other hand, emits at 2.5 and 2.8 jam. The infrared spectrum observed from a typical combustion flame is a result of the superposition of these two emissions as modified by atmospheric absorption. Since the position of the CO₂ absorption band is shifted slightly with respect to the emission band, only a portion of the emission band undergoes atmospheric absorption (Curcio, J. A.; Buttrey, D. V. E.;

Appl. Opt. 1966, 5, 231.). The observed band at 4.4 um is due exclusively to carbon dioxide emission and appears shifted from the true 4.3μm CO₂ emission due to an alteration in the true band shape by atmospheric absorption by CO₂. The band observed from the flame at 2.8 μm is a result of the overlap of the water bands at 2.5 and 2.7 jam with the carbon dioxide band at 2.7 μm. Although other bands have been observed over the wavelength range from 1 to 22 μm, the bands at 2.7 and 4.3 μm are the two most intense emissions. The amount of infrared emission observed from flames is also dependent on a number of other parameters. Studies of flames have shown that most of the energy is lost by conduction and convection occurring upon mixing with the cooler atmospheric air (Gaydon, A. G.; The Spectroscopy of Flames; Chapman and Hall: London, 1974; pp. 221-243.) In addition, turbulent flow has been observed to decrease the amount of infrared radiation emitted (Gaydon, A. G.; Wolfhard, H. G. ; Flames, Their Structure, Radiation and Temperature, 4th ed.; Chapman and Hall: London, 1979; pp. 238-259.). Self absorption is another factor which can reduce emission. Thus, the size, shape, and nature of the flame are important parameters when considering the amount of radiation from the flame.

In addition, the region of observation within the flame is an important consideration. Spatially, the emission of infrared radiation is observed to be a maximum in the outer cone and the surrounding gases with little or no emission from the inner conal area (Gaydon, A. G. ; The Spectroscopy of Flames; Chapman and Hall: London, 1974; pp. 221-243.). Previous studies have shown that approximately one-seventh of the infrared emission comes from the inner conal area with the remaining six-sevenths originating in the outer cone/hot gas layer (Gaydon, A. G.; The Spectroscopy of Flames; Chapman and Hall: London, 1974; pp. 221-243.). The hot gaseous combustion products formed in the flame continue to emit above the visible portion of the outer cone until they are cooled by the entrainment of atmospheric air.

Finally, and most importantly from an analytical standpoint, the amount of infrared radiation emitted from the flame is a function of the number of CO₂ and H₂O molecules present in the hot gases. Thus both quantitative and qualitative analyses are possible.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically illustrates the apparatus used in Experiment 1 for the non-dispersive studies.

Figure 2 schematically illustrates the apparatus used in the wavelength-selective studies of Experiment 1.

Figure 3 schematically illustrates the optical system for use in Fourier Transform Interferometer studies.

Figure 4 schematically illustrates the preamplifier circuit for PbSe detector.

Figure 5 graphically illustrates the signal profile as a function of time for a 50 uL injection of toluene.

Figure 6 graphically illustrates the peak height singal as a function of injection volume in microliters for toluene.

Figure 7 graphically illustrates the effect of observation height above the burner on the signal when pure ethanol was aspirated into the flame.

Figure 8 graphically illustrates the signal obtained per mole of carbon as a function of the number of carbon atoms in the molecule.

Figure 9 graphically illustrates the effect of detector bias voltage on the signal observed at 4.3 um when

10% methanol/water mixture was pumped into the burner from the liquid chromatograph at 2 mL/minute. Figure 10 graphically illustrates the effect of chopping frequency on the signal observed when pure ethanol was aspirated into the flame at a steady rate.

Figure 11.1 - 11.2 are the infrared spectra obtained with the system for a hydrogen/air flame and an acetylene/air flame using the PbSe detector.

Figure 12.1 - 12.2 are the infrared spectra obtained with the system when CO₂ and ethanol were introduced into the flame.

Figure 13 graphically illustrates the signal observed at 4.3 μm as a function of CO₂ flow rate.

Figure 14 graphically illustrates the signal obtained at 4.3 μm as a function of volume of ethanol injected into liquid chromatograph.

Figure 15 is the chromatogram obtained when 50 uL of an equivolume mixture of methanol, ethanol and propanol were eluted from the liquid chromatograph. The order of elution was methanol, ethanol, propanol.

Figure 16 schematically illustrates the apparatus used in Example 2.

Figure 17.1-17.3 schematically illustrates the burner assembly for Example 2.

Figure 18 is a chromatogram of 5 μL of unleaded gasoline obtained on 10% OV-101. Column temperature was maintained at 55°C for 4 minutes and then ramped to 200°C over a period of 7 minutes.

Figure 19 shows the elution peaks obtained for various volumes of pentane in microliters. Column temperature, 90°C.

Figure 20 graphically illustrates peak height versus volume in microliters for dichloromethane. Figure 21 graphically illustrates peak height versus volume in microliters for trichlorotrifluoroethane.

Figure 22 graphically illustrates peak height versus volume in microliters for carbon tetrachloride.

Figure 23 graphically illustrates peak height versus volume in milliliters for carbon dioxide.

Figure 24 graphically illustrates peak height versus micromoles of compound injected obtained with a flame infrared emission detector for 1, carbon dioxide; 2, pentane; 3, 1,1,2-trichloro-1,2,2-trifluoroethane; 4, dichloromethane; 5, carbon tetrachloride.

Figure 25 graphically illustrates the logarithm of peak height versus logarithm of injection volume in microliters for pentane.

Figure 26 graphically illustrates the relative response of a flame infrared emission detector for methane, carbon monoxide and carbon dioxide. Figure 27 graphically illustrates the relative response of a flame infrared emission detector per mole of carbon for various compounds containing different numbers of carbons: BR-ETH, bromoethanol; DI-CL, dichloromethane; TRI-CL, tricloromethane; TRI-CL EN, trichloroethanol; TETRA-CL, carbon tetrachloride; TRI-CL-F, trichlorotrifluoroethane; N-PENT, n-pentane; N-HEX, n-hexane; N-HEPT, n-heptane; C-PENT, cyclopentane; C-HEX, cyclohexane; C-HEPT, cycloheptane, C-M-HEX, methylcyclohexane; C-OCT, cyclooctane.

Figure 28 graphically illustrates peak height versus carrier gas flow rate obtained with a flame infrared emission detector.

Figure 29 graphically illustrates peak area versus carrier gas flow rate obtained with a flame infrared emission detector. Figure 30 shows the chromatogram obtained isothermally at 50°C on an Apiezon-L column for a 5- μL injection of a 1:2:1:3 volume mixture: pentane (1); 1,1,2-trichloro-1,2,2-trifluoroethane (2); hexane (3); carbon tetrachloride (4).

Figure 31 schematically illustrates the experimental set up of the burner, mirror and Fourier Transform Interferometer for Experiment 3.

Figure 32 is a flame infrared emission spectra of carbon tetrachloride.

Figure 33 is a flame infrared emission spectra of methanesulfonyl fluoride.

Figure 34 is a flame infrared emission spectra of the H₂/air background at high gain, Figure 35 is a flame infrared emission spectra of methanol.

Figure 36 is a flame infrared emission spectra of trichloro-trifluoro ethane.

Figure 37 is a flame infrared emission spectra of tetramethylsilane.

Figure 38 schematically illustrates the apparatus for a combined infrared and flame ionization detector. SUMMARY OF THE INVENTION

The present invention relates to infrared emission detection means and method whereby the infrared emission of excited molecules of interest in a sample is used as a basis for detection of compounds.

In one aspect of the invention, infrared emission is observed as a means of detection for chromatography. Organic compounds introduced into a flame result in the production of carbon dioxide which allows observation of two strong emission bands over the wavelength from 1 to 5 μm.

Both total organic carbon (TOC) and total inorganic carbon (TIC) determinations in aqueous samples can be made. Total inorganic carbon and total organic carbon are important analytical parameters in the environmental characterization of water. Total organic carbon determinations are performed routinely as a non-specific measure of the organic content of water in pollution monitoring. Inorganic carbon exists in water as bicarbonate and carbonate ions and as dissolved carbon dioxide. The sum of these carbon species is called Total Inorganic Carbon (TIC). TIC is generally determined by acidification of the sample to convert bicarbonate and carbonate ions into dissolved CO₂, purging of the sample with gas to remove the dissolved CO₂, and measurement of the CO₂ (usually by infrared absorption). Total inorganic carbon determinations by flame infrared emission detection can be used in place of alkalinity titrations to determine the amount of inorganic carbonate present in a water sample.

Infrared emission detection can be used to monitor carbon impurities in electronic grade gases. Carbon/hydrogen characterization of compounds by infrared emission detection is possible by observing the two strong emission bands, one associated with carbon dioxide and one with both water and carbon dioxide. Heteroatoms can be observed by Fourier transform infrared emission spectroscopy. In that many biochemical reactions result in the release of carbon dioxide as a by-product, infrared emission detection can provide the basis for a variety of clinical and biochemical assays.

The flame ionization detector (FID) is probably the most currently used GC detector. It is surely the most sensitive for organic compounds. A combination infrared detector combining an FID and a flame infrared emission detector provides FID sensitivity and flame infrared emission detection of CO, CO₂ and other compounds which FID lacks and the superior quantitation of moles of carbon shown by flame infrared emission. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an infrared detection means and a method for detecting selected molecules of interest in a gaseous sample. The infrared detection means includes a detector means and a means for exciting molecules of interest in the sample to emit a characteristic infrared radiation pattern. In a one embodiment heating, preferably by a flame, is employed to excite the molecules of interest in the sample to produce vibrationally excited carbon dioxide which can emit infrared radiation. A pre-determined wavelength of infrared radiation emitted by the molecules of interest is observed by generating an electrical signal in response to the emission at the observed wavelength. The observation wavelength is preselected from the characteristic infrared radiation pattern of the molecule of interest. The means for isolating a preselected wavelength of infrared radiation is mounted between the exciting and detector means.

A factor necessary for successful implementation of IR Emission is to achieve a useful level of contrast between the source and the background, that is the source should be at a higher temperature than the surrounding background, and the temperature of the source should be greater than the temperature of the detector. For this reason, one would not expect to see infrared emission from a gas at room temperature if the background and the detector are also at room temperature. Therefore, the first requirement for the successful implementation of this technique is in most cases a means to heat the gas above room temperature. This does not necessarily mean that a flame is required. However, the hotter the gases, the greater the radiant emissivity, and the more sensitive the detection system. Since flames typically have temperatures on the order of 2000-3000 ^oK, they represent potentially good emission sources for this application if a low-background flame in the vicinity of the selected emission band can be found.

It should be stressed, however, that other means could be used to excite the gases so that they would emit infrared bands. If other means were used, the samples would have to be oxidized in order to detect the CO₂ emission band. Other non-thermal excitation means include: 1) excitation of carbon dioxide by electron impact in a gas discharge; 2) excitation of carbon dioxide by collisions of the second kind with vibrationally excited nitrogen (similar to the mechanism used in the carbon dioxide laser); and 3) photo-excitation with an appropriate source. Photoexcitation can be accomplished from the 000 level (resonance excitation) or from the 010 level which is appreciably populated even at room temperature (non-resonance excitation). Such infrared fluorescence could be conveniently excited with a carbon dioxide laser emitting radiation at 10.6 jam. Such radiation would cause the transition from 010 to 001.

A second requirement for high sensitivity is the avoidance of the use of any form of solid containment such as a sample cell. In order to see infrared emission, there must be some contrast between the infrared emitting source (i.e., the gas in this case) and the background as a result of a thermal gradient. When a heated gas is introduced into a solid sample container, however, the gas immediately begins to heat the container and the thermal gradient and the contrast gradually disappear with time as the system reaches thermal equilibrium. As a result of this process, the emission signal, which is initially present from the gas because it is hotter than the walls of the cavity, gradually fades into the background until it disappears. As a result of this phenomenon, previous experiments had to employ complicated recirculating systems so that hot gases and cold gases could be alternately admitted into the sample cell at a rate sufficiently great that the system would not have time to reach thermal equilibrium (i.e., use of thermal cycling). If no sample containment is used, as with a flame, there is no blackbody radiator to heat up in the vicinity of the emitting gases, and a steady-state emission signal can be observed. This emission signal is present as long as sample is introduced into the flame, and does not decay away with time, as would be the case with the previous systems where the background gradually increases. For this reason, thermal cycling is unnecessary with the present system.

Use of a high temperature sample cell is also beneficial in reducing the effects of atmospheric absorption (i.e., telluric absorption when observing CO₂ emission) due to carbon dioxide in the atmosphere. As a gas is heated, various upper level vibrational states become populated, and transitions between upper levels occur. Thus, the emission band observed for a hot gas contains components arising from upper level vibrational transitions (say, v = 4 to v = 3) and is therefore broadened and shifted somewhat to longer wavelengths as the temperature is increased. By contrast, carbon dioxide at room temperature is present mainly in the lowest vibrational level (v = 0). Since the emission band is shifted with respect to the absorption band (because the emitters are at a higher temperature than the absorbers), atmospheric absorption by carbon dioxide is not a significant problem. Finally, when CO₂ emission is being observed in a flame, a flame must be chosen which does not itself produce or contain carbon dioxide. This excludes the use of all carbon-containing fuels, and suggests the use of hydrogen-fueled flames. Two possibilities exist --the hydrogen/air flame and the hydrogen/oxygen flame. Of the two, the hydrogen/air flame is more convenient because it has a lower burning velocity which makes it easier to design a burner which will not flash-back (i.e., explode). The hydrogen/oxygen flame may produce a larger signal because of its higher temperature. Although one of the carbon dioxide emission bands is overlapped with. two water emission bands in the hydrogen/air flame (producing a composite band at 2.7μm), the region at 4.3 μm where the only other carbon dioxide emission occurs is clear of any other potentially interfering flame background. For these reasons, the hydrogen/air flame was selected as a sample cell for preliminary studies of flame infrared CO₂ emission.

Compared with other sources, the flame is particularly useful for the detection of organic compounds because combustion processes invloving hydrocarbons produce mainly CO, CO₂, and water vapor as combustion products. Because of their proximity to the combustion process (which is exothermic), the molecules produced are often in excited states with regard to vibrational and rotational transitions. Thus the carbon dioxide emission obtained from a combustion flame containing hydrocarbons may actually produce more intensity than an equivalent amount of carbon dioxide at the same temperature emerging from an oven where no combustion has occurred.

Many detectors sensitive in the infrared are also sensitive in the UV/visible, although they are generally not used in this region because they are considerably less sensitive than other available detectors such as the photomultiplier. The photomultiplier is based on the external photoelectric effect whereby a photon absorbed by a suitable photoemissive material is ejected from the material into the surrounding vacuum. To give the electron sufficient energy to escape the surface of the photoemitter, the photon absorbed must possess an energy greater than the sum of the energy bandgap and electron affinity of the photoemitter. Detectors based on photoemission of electrons as a result of the photoelectric effect can be made very sensitive by the use of electron multiplication with a dynode chain. Thus, photomultipliers can be made so sensitive that they are often limited by the fluctuation in the arrival rate of photons rather than fluctuations arising within the detector itself. Unfortunately, the energy requirement for external photoemission is sufficiently large that the use of photomultipliers is confined essentially to the UV/visible region of the spectrum. By reducing the electron affinity of certain photoemitters, photomultipliers that respond to longer wavelengths beyond the visible (out to about 950 nm) have been produced.

Detectors which respond to infrared radiation can be classified into two basic categories: thermal detectors and quantum detectors. Thermal detectors respond to the heating effect of the infrared radiation and include thermocouples, thermistors, and pyroelectric detectors. Quantum detectors make use of the internal photoelectric effect whereby an electron is promoted from the valence band to the conduction band but is not ejected from the material. As a result, these detectors respond out to wavelengths whose energies correspond to the semiconductor bandgap. These detectors include photovoltaic and photoconductive detectors. Neither category of detector discussed above employs any form of internal amplification similar to the photomultiplier and for this reason, IR detectors are less sensitive than detectors commonly used for the detection of UV/visible radiation. Since the major source of noise with these detectors originates within the detector itself and is not due to fluctuations in the radiation field, these detectors are frequently cooled to reduce detector noise, and spectrometers which employ them are termed detector-noise limited.

Of the various infrared detectors which are available, quantum detectors generally have a higher specific detectivity than thermal detectors, although thermal detectors have the advantage of flat response over a wide wavelength range. For the applications under consideration, a detector which did not require cooling to dry ice or liquid nitrogen temperatures was desired. For the wavelength region under consideration (2-5 μm) , the lead selenide and indium antimonide detectors were two possibilities. Of the two, the indium antimonide had the higher specific detectivity but generally required cooling. For this reason, the PbSe detector was selected as the most appropriate for preliminary studies on the basis of spectral response and cost. Even with the PbSe detector, however, some thermoelectric cooling may be beneficial to shift the maximum response of the detector to longer wavelengths.

To detect the 4.3μm emission band from carbon dioxide without interference from other emission bands (such as the one at 2.7 μm), some form of wavelength descrimination or isolation was needed. Because the radiation throughput of a conventional grating monochromator is relatively small, these systems have not been completely satisfactory in the infrared region, and for this reason, Fourier-transform methods are preferred. However, for single molecule response a filter is satisfactory, and therefore a bandpass filter which would transmit the desired band was selected. In this way, the desired band was isolated without reducing the radiation throughput to the detector.

In the visible region of the spectrum, the advantage of emission measurements over absorption measurements is well known among spectroscopists. In view of this, it is surprising that no one thought to exploit this advantage in the infrared. The reason, no doubt, lies in the intuitive, but completely erroneous, notion prevalent among chemists that emission is less sensitive than absorption because emission is based on monitoring excited state populations whereas absorption employs ground state populations. Since everyone knows that the ground state is more populated than any excited states, it follows, ergo, that absorption must be more sensitive than emission. The great fallacy in this argument is the fact that it is irrelevant.

In fact, the reason that absorption measurements are less sensitive than the corresponding emission experiment is due to the fact that as the detection limit is approached the absorbance, which is the logarithm of the ratio of the incident beam intensity to the transmitted beam intensity, approaches zero. This means that the magnitude of the transmitted beam intensity approaches the magnitude of the incident beam intensity. The question of detection then revolves around whether it is possible statistically to tell the difference between these two large numbers. It is well known from statistics, that differences between two large numbers, which are close to one another in magnitude and which fluctuate, are often not significant. In emission, by contrast, the detection limit occurs when the signal cannot be statistically distinguished from the background. Since the background is hopefully small, this situation is equivalent to the difference between two small numbers which is statistically more reliable. In addition, emission measurements are usually linear over a much wider range of concentrations. Thus, use of emission measurements rather than absorption measurements is based on sound analytical reasoning and is not simply an alternative way of accomplishing the same thing.

It is instructive to point out that, if a flame is used to combust and convert the sample into carbon dioxide, it is very unlikely that this carbon dioxide could be monitored by absorption measurements. Again, the reason is based on an understanding of Kirchoff's law. If the carbon dioxide produced by the flame were to be monitored by absorption measurements, a blackbody emission source hotter than the temperature of the flame gases would be required. Since the flame temperature is about 2300 ^oK, finding a solid blackbody source which is hotter than the flame is highly unlikely to say the least.

Process gases are those gases used in manufacturing and electronic-grade gases in particular are those gases used in the maunufacture of electronic devices. Impurities such as CO, CO₂, and trace hydrocarbons in gases used in the manufacture of semiconductor devices must be controlled to assure reliable manufacturing conditions. Concentrations of impurities as low as 0.5 to 1 ppmv in nitrogen and argon have been reported to cause difficulties (Whitlock, W. H. et al., Microcontamination, May, 1988) . Currently, determination of the above impurities is carried out using a flame-ionization detector (FID) using instruments based on gas chromatography. Because the FID does not respond to either CO or CO₂, a catalyst system using nickel as the catalyst must be employed to convert the CO and CO₂ to methane. The need for the catalytic methanator and complex valving makes the current technology less than ideal.

By contrast, the infrared emission detector has good sensitivity to CO, CO₂, and the light gaseous hydrocarbons. Since the catalytic methanator is not needed, problems with the methanation catalyst and complex valving are not encountered. Furthermore, a continuous-monitoring technique is possible with the infrared emission system. In such a system, three gas streams can be analyzed simultaneously. One stream is fed into a flame directly. This stream measures the total impurity concentration (i.e., CO, CO₂ , and hydrocarbons) [S₁]. A second stream passes first through a bed of a CO absorbent prior to entering a second infrared emission detector. This stream measures the sum of the CO₂ and hydrocarbons (S₂). A third stream passes first through a bed of CO₂ absorber (Ascarite) prior to entering a third infrared emission detector. This stream measures the sum of the CO and hydrocarbons (S₃). The signals from the three streams relate to the impurity concentrations as follows:

S₂ + S₃ - S₁ = hydrocarbons S₁ - S₃ = CO₂

S₁ - S₂ = CO By measuring combinations of components simultaneously, the infrared emission system has a multiplex advantage over systems which measure one component at a time.

Another major application of infrared emission technology is for water analysis. This includes drinking water (potable water), environmental samples, wastewater, and even clay-based drilling muds used in petroleum production (i.e., oil rigs). This application falls into two major categories: total inorganic carbonate- and organic carbon determinations.

The presence of carbonates in water and other fluids often has a deleterious effect on the use of these liquids. For example, the presence of dissolved carbonates along with dissolved calcium can lead to scale formation in both domestic and industrial plumbing systems. Scale formation in plumbing systems not only impedes fluid flow but reduces the heat transfer efficiency of the fluid when used for cooling purposes. In petroleum production, the presence of carbonates adversely affects the performance of deflocculated clay-based drilling muds (Garrett, R.L., J. Pet. Tech., June, 1978; p. 860.)

In current water technology practice, the carbonate concentration is determined indirectly by means of alkalinity titrations. The alkalinity of a water sample is determined from the proton condition of the solution as

C_B - C_Α = [Alk] = [HCO₃-] + 2[CO₃ ^2-] + [OH-] - [H⁺]

[Alk] =α₁C_τ + 2α₂C_τ + [OH-] - [H⁺] where

C_A = concentration of strong acid

C_B = concentration of strong base

C_T = [H₂CO₃] + [HCO₃-] + [CO₃ ^2-]

D = ( [H⁺] ² + K₁ [H^÷] + K₁K₂ ) α₁ =K₁ [H⁺]/D α 2 = K₁K₂/D and K₁ and K₂ are the first and second dissociation constants of carbonic acid. The alkalinity of a water sample is also a measure of the acid neutralizing capacity of the solution. By titrating a water sample with strong acid (H₂SO₄) to a methyl orange endpoint, the total alkalinity of a water sample is determined. The total inorganic carbonate or C_T is determined from a knowledge of the alkalinity by rearranging the alkalinity relationship: C_T = ([Alk] - [OH-] + [H⁺])/(α₁ + 2α₂)

C_T~ [Alk]/(α₁ + 2 α₂) Thus as long as there are no other alkaline materials in the water besides carbonate the alkalinity and the pH of the solution are all that is needed to determine C_T. In certain determinations, however, such as drilling muds, there are appreciable amounts of other alkaline materials present which make alkalinity titration data unreliable as a means of determining C_T. Even in the absence of other interferences, the alkalinity titration is difficult to perform because the indicator endpoint is not sharp. Even when potentiometric titrations are performed, the endpoint is still not sharp because the sample is being titrated back to carbonic acid.

By contrast, the infrared emission TIC determination is simple, convenient and direct. By direct, it is meant that the infrared emission measures C_T directly rather than a property which is related to C_T (i.e., the acid neutralizing capacity). The use of a direct measurement technique leads to more reliable data on the actual parameter of interest. The direct measurement does not require a knowledge of the dissociation constants for carbonic acid to calculate the desired parameter from the measured quantity.

In the infrared emission procedure, a water sample (2 mL) is introduced into a fritted sparging tube where it is subsequently acidified with sulfuric acid (0.5 mL) to release carbon dioxide gas which is flushed with helium into a hydrogen/air flame. The infrared emission from carbon dioxide is measured with an infrared emission detector as described previously. A calibration curve prepared from standard carbonate solutions is used to determine the total inorganic concentration in the sample.

Infrared emission detection is also useful in determining the carbon dioxide content in carbonated beverages such as soft drinks and beer.

Organic materials in water samples may arise from naturally occurring compounds produced by living organisms or from anthropogenic sources. The sum of the naturally occurring organic materials and the synthetic organic materials is referred to as the total organic carbon in the water sample. Thus, the total organic carbon content of a sample is a non-specific (i.e., doesn't determine the actual individual compounds present) measure of the organic content of the sample. Total organic carbon (TOC) determinations are performed on a wide range of samples, including ground water, drinking water, semiconductor process water, municipal wastewater, and industrial wastewater (Small, R.A. et al., International Laboratory, May, 1986). Industrial applications of TOC determinations include determination of organic contamination in mineral products such as acids, -caustic solutions, as well as aluminum-, nickel-, and cobalt chlorides. Power generation plants use TOC measurements to determine organic contaminants in cooling water and steam-generation water. Even small amounts of formic- and acetic acid can cause corrosion of turbine blades and heat exchanger equipment (Bernard, B.B., "A Summary of TOC Developments", 0.1. Corporation College Station, Texas, 1985). Since an increase in the organic content of water can be an indication of pollution, TOC determinations have been used to monitor surface water, ground water (i.e., wells). and other water sources for wastewater contamination and industrial effluents.

Currently, TOC determinations are performed by first oxidizing the organic material to carbon dioxide by a variety of methods (Small, R.A. et al., International

Laboratory May, 1986). The carbon dioxide gas generated by this oxidation is then determined by non-dispersive infrared (NDIR) absorption spectrophotometry. Although a variety of NDIR procedures have been employed, a primary problem with absorption measurements is the concomitant absorption by water vapor and acid gases. Partial elimination of the water vapor interference has been achieved through humidity control of the reaction gases flushed into the infrared absorption cell. Because measurements are made in absorption, all TOC analyzers require an infrared emission source to produce the infrared radiation.

With the infrared system, carbon dioxide is produced by the same methods currently used to oxidize organic materials to CO₂. These oxidation methods include one or a combination of the following: chemical methods such as the use of peroxydisulfate, heating such as in a furnace with copper oxide and the use of UV radiation. After oxidation the CO₂ is flushed out of the sample to the infrared emission detector, Alternatively, for some compounds if the infrared emission detector uses a flame for exciting the molecules of interest in the sample, the sample may be combusted directly in the flame to generate CO₂. By utilizing infrared emission instead of absorption of CO₂ the interference by other concomitants produced by the oxidation process is avoided. Since the infrared emission detector is not affected by water vapor and acid gases, these intereferences are absent with the infrared emission TOC analyzer. Since the infrared emission system employs a filter, it falls in the category of non-dispersive infrared analysis (only in emission rather than absorption). Since two strong emission bands are observed in the flame, one corresponding to the asymmetric stretching vibration of carbon dioxide and the other to water and carbon dioxide combination bands, carbon/hydrogen characterization of compounds is possible by using both bands. In the complete combustion of any organic material, both carbon dioxide and water vapor are produced. The presence of both of these components alters the intensities of the water band at 2.9 um in addition to the carbon dioxide band at 4.4 um. Thus, a chromatography infrared emission detector monitoring both of these bands provides additional information about the compound beyond that available with an FID or thermal conductivity detector. Although this system may not be able to determine the carbon/hydrogen ratio with the same precision as conventional combustion analysis, it distinguishes different alkanes, alkenes, aromatics, et cetera. A carbon to hydrogen ratio instrument is useful for combustion monitoring, as in smoke stack and rocket engine firing monitoring.

The infrared emission is useful as a detector in conventional carbon/hydrogen analyses (as opposed to using it as a detector in chromatography). As with the TOC analyses, the infrared emission system is used as the detection means in the analysis. Thus, an instrumental carbon hydrogen analyzer can use conventional combustion tube techniques to transform the organic material into water and carbon dioxide. The infrared emission is then used to detect the amounts of these materials which have been generated.

Various other emission bands are emitted when organic compounds containing hetero-atoms are introduced into the hydrogen/air flame. Fluorine and chlorine containing compounds produce characteristic emission spectra of HCl and HF. Silicon and sulfur containing compounds produce emission characteristic of Si-O vibrations and SO₂ vibrations. Freon 114 produces a very interesting emission spectrum with HF, HCl, CO₂, H₂O, and a number of other bands attributed to C-F vibrations. Many biochemical reactions release carbon dioxide as a by-produce of the reaction (for example, fermentation of sugar by yeast). Infrared emission is therefore useful for a variety of clinical and biochemical assays involving carbon dioxide, such as the clinical determination of carbon dioxide in blood.

The following experimental embodiments are illustrative.

EXPERIMENT 1

Figure 1 shows the experimental arrangement used for the initial non-dispersive studies using thermistor detection. A Varian Techtron burner assembly 10 was used as a nebulizer and to provide the sample introduction system. A Meker burner head producing a flame 11 with a diameter of 1.5 cm was designed and fitted to the burner assembly. Initial studies of the infrared emission from a hydrogen/air flame 11 were made using various thermoflake thermistors 12 (Thermometrics Inc., Edison, NJ) mounted in a detector head/housing unit on a test setup facing the burner. For these non-dispersive studies, a metal tube was attached to the detector housing to limit the field of view of the thermistor flake. The thermistors 12 and 13 were incorporated into a Wheatstone bridge circuit and a Model 3120TX Bascom Turner digital data aquisition unit (Bascom Turner Instruments, Norwood, MA) was employed to record the output. Sample introduction was accomplished by means of a teflon injection device 14 consisting of a T-coupler with one passage capped to accept a septum. The teflon injector unit 14 described above was employed with this test setup to study the direct injection of organic compounds into the flame 11 and to simulate sample introduction from the chromatograph. Using this arrangement, samples up to 50μL could be injected by means of a Hamilton microsyringe. Figure 2 shows the experimental arrangement employed for the dispersive wavelength-selective studies using PbSe detector 24. The IR detection system used in this study was assembled using devices and equipment from various manufacturers as listed in Table I. Table I. Equipment Used

Burner Varian Techtron burner/nebulizer with Meker burner head

Dispersion System Spex Model 1870 0.5-m Czerny-Turner monochromator with 150 groove/mm Bausch and

Lomb grating blazed for 4 jam

Entrance slit 3 mm wide, 3 cm high

Detector Hamamsatsu Model PbSe Model P2038-01

Amplifier Princeton Applied Research Model 128A lock-in amplifier with PAR Model 125A variable speed chopper

Readout Varian Aerograph chart recorder Flow meters Brooks Instrument Division calibrated flow meters

Chromatograph Varian Model 5000 HPLC with MCH-5 column A Spex 0.5-m Czerny-Turner monochromator (Spex Industries, Inc., Metuchen, NJ) is used as the primary wavelength dispersive device or isolation means. The monochromator is equipped with a 150 groove/mm grating blazed for 4μm. A 3-mm entrance slit was used. The wavelength scale of the Spex 1870 monochromator was calibrated by the manufacturer for a 1200 groove/mm grating. To determine the wavelength settings with the 150 grove/mm grating, measurements of grating rotation versus counter setting were made. These measurements combined with the geometric arrangement of the mirrors enabled the wavelength corresponding to a given counter setting to be calculated by means of the grating equation,

mλ = d[sin (r - θ₄) + sin (r + θ₂) ]

where m is the order, λis the wavelength, d is the grating constant, and r is the rotation angle of the grating. In this equation, θ₂ is the angle of reflection of the chief ray from the collimating mirror and θ₄ is the angle of incidence of the chief ray on the focusing mirror. For a given optical layout, θ₂ and θ₄ are constants. These calculations were checked by observing the 670.7 nm line from a lithium hollow cathode in the third order and comparing the actual counter setting with the observed counter setting.

A Hamamatsu lead-selenide photoconductive cell 24 with integral thermo-electric cooling (P2038-01, Hamamatsu Corp., San Jose, CA) was employed as the infrared detector. Table II lists the specifications of this particular device.

Table II. Performance Characteristics of P2038-01 Hamamatsu PbSe Detector

Sensitive Size: 1 × 3 mm

Peak Responsivity: 3.8 μm

IR Cutoff Wavelength: 4.85 μm

Dark Resistance: 0.6 Mohm

Recommended Load Resistance: 0.5 Mohm

Specific Detectivity (500 K source, 600 Hz chopping frequency, 1 Hz bandwidth): 1.2 × 10 ⁸ cm Hz^½ w^-1 A housing/mounting assembly was fabricated so that the detector, 24 in Figure 2, could be mounted in the focal plane of the monochromator with the preamplifier electronics in close proximity. A 30.0 volt regulated power supply was used initially as the power supply for the detector 24. The preamplifier circuit, consisting of a BIFET operational amplifier 46 and associated components, is shown in Figure 4. The amplified signal from the preamplifier was applied to the input of a Princeton Applied Research Model 128A lock- in amplifier (Princeton Applied Research, Princeton, NJ), not shown. The radiation from the flame was modulated with a Princeton Applied Research Model 125A variable speed chopper (Princeton Applied Research, Princeton, NJ) at a chopping frequency of 86 Hz. Output from the lock-in amplifier was displayed on a Varian Aerograph stripchart recorder. The PbSe detector 24 was used in all studies utilizing the monochromator.

Wavelength-selective studies were also conducted with a high-pass filter in conjunction with the PbSe detector 24 in Figure 2 to isolate the 4.3 μm emission band. In this embodiment, a high-pass filter (Corion Corp., Holliston, MA) with a short wavelength cutoff of 3.5 μm was mounted in a housing in front of the PbSe detector 24. Since the long wavelength response of the detector 24 only extends out to about 5 μm (See Table II), this arrangement effectively isolates the 4.3 μm band without the need for a monochromator.

The Varian Techtron burner assembly 20 (Varian Instruments, Palo Alto, CA) used for the non-dispersive studies is also used as the sample introduction system in the dispersive studies. A flame shield is constructed of sheet stainless steel and attached to the burner assembly to minimize the effect of drafts on the flame. Flow of combustion and other gases to the burner 20 was controlled using Brooks Instrument gauges and flow meters (Brooks Instrument Division, Emerson Electric Co, Hatfield, PA). Three fuel/oxidant mixtures are employed in this study: hydrogen/air, acetylene/air, and hydrogen/20% oxygen- 80% argon.

The liquid chromatograph used in this study was a Varian Model 5000 (Varian Instruments, Palo Alto, CA), equipped with a MCH-5 reverse-phase column. The interfacing of the Model 5000 and the Varian burner assembly 20 was accomplished initially by simply attaching a polycarbonate tube of similar diameter to the stainless steel outlet tube of the MCH-5 column by means of a zero volume stainless steel coupler. After initial experimentation and testing, a telfon T-coupler 26 was added. This allowed other solvents to be mixed with the chromatographic effluent prior to aspiration by the burner 20. The purpose of the coupler 26 is to improve sample aspiration by the burner/nebulizer 20 by mixing the chromatographic effluent with water prior to nebulization; alternatively, the same device permitted the direct introduction of column effluent into the burner 20 without prior mixing with water if desired. All chromatographic results reported are obtained using the T-coupler 26.

Methanol and water are used as the solvents in all chromatographic runs. The methanol is HPLC grade and the water is triply deionized. All standard compounds are reagent grade.

Chromatographic runs were done using various mixtures of methanol and water as well as pure water as the eluent. Injection loops of either 10 or 50 microliter volume were employed in all runs. Samples were prepared from mixtures of pure compounds and were loaded into the sample loop using a 1-mL syringe. Samples were introduced onto the column by means of the conventional rotary valve. All runs were done using water as the make-up solvent 27 in the T-coupler 26 prior to aspiration by the burner assembly 20.

Initial non-dispersive studies aimed at determining the feasibility of monitoring infrared emission from a combustion flame were conducted using thermistor detectors. The flake thermistors used in this study fall in the category of thermal detectors as described in Putley (Putley, E.H. In Optical and Infrared Detectors, Keyes, R.J., Ed.; Springer-Verlag: Berlin, 1980, Chapter 3) and are fabricated so that the actual detector mass is kept as low as possible, thereby producing a faster response time (75 ms) and increasing the relative response by keeping the heat capacity of the detector low.

Although flake thermistors are more sensitive than their nonflake counterparts, they were not found to be sensitive enough to detect the levels of infrared energy present when the radiation from the flame was dispersed by the 0.5-m monochromator. This lack of response in the dispersive mode is attributed, at least in part, to the relatively high dispersion obtained with the 0.5-m monochromator. Used in a non-dispersive mode (i.e., with the thermistor placed about 30 cm from the flame), however, these detectors are easily able to monitor changes in infrared energy emitted from the flame as a result of introducing microliter amounts of carbon-containing compounds into the flame.

In a preferred embodiment shown in Figure 3, the effluent from a chromatograph is analyzed by fast Fourier Transform interferometery. A burner/nebulizer 30 is used for vaporizing and exciting liquid chromatograph effluent sample. The burner assembly 30 may be either the burner previously discussed with respect to Figure 2, or the burner assembly illustrated in Figures 16 and 17, as will be hereinafter discussed in detail with respect to Experiment 2. After the sample is introduced into the flame, the emitted infrared radiation is focused by a three mirror assembly 32 through lens 39 and thereby directed into an interferometer 34 intrinsically having an amplifier and infrared detector. The distance between lens 39 and interferometer 34 is about 10 cm. The output of the interferometer 34 is connected to a computer 38 to analyze the collected data by a Fast Fourier Transform to obtain the spectral response of the sample and the molecules of interest contained therein.

Using the apparatus illustrated in Fig. 1, samples were introduced into the hydrogen/air flame by injection using a sample injection device similar to that described earlier, (Busch, K. W.; Howell, N. G.; Morrison, G. H.; Anal. Chem. 1974, 46, 1231). Figure 5 shows the signal profile obtained as a function of time for a 50 μL inj ection of toluene. Figure 6 shows a plot of signal (i.e., peak height) as a function of injection volume in microliters. Similar plots were obtained for injections of various volumes of methanol. These results suggest that it is possible to observe infrared emission from the hydrogen/air flame as a result of the combustion of small amounts of organic compounds. The results also establish that the emission observed is a function of the amount of sample introduced into the flame. Subsequently, a series of studies was conducted to determine the effect of various flame parameters on the infrared emission observed. Figure 7 shows the effect of observation height in the flame on the signal obtained when pure ethanol was aspirated into the flame. These results were obtained non-dispersively with the thermistor detector located 25 cm from the flame. To limit the field of view seen by the detector, a stainless steel tube 10 cm long with a 1.2 cm diameter is attached to the detector housing. Using this arrangement, maximum emission was found to occur just above the tip of the visible portion of the secondary combustion zone. Studies of the effect of fuel-to-oxidant ratio with this system revealed that flame stoichiometry had less of an effect on the signal than observation height in the flame. Over the range of flame stoichiometries available, the maximum signal was always observed just above the tip of the flame and did not appear to be greatly affected by flame stoichiometry. A fuel-to-oxidant ratio of 0.53 was used in all subsequent studies because it gave a flame of convenient size (i.e., not too tall) that was not greatly affected by drafts. Since a stoichiometric hydrogen/air flame corresponds to a fuel-to-oxidant ration of 0.4, the flame used in these studies was slightly fuel rich.

Studies on the influence of flame parameters on the signal observed are consistent with the hypothesis that the majority of the infrared emission observed is due to emission from the excited combustion products. Since complete combustion of organic samples to carbon dioxide and water is likely to be achieved only relatively high in the flame, if at all, the concentration of CO₂, for example, is likely to be greatest relatively high in the flame. The increase in the concentration of CO₂ with increasing distance from the burner is counteracted by a decrease in temperature as well as dilution effects from entrained air. The combination of these factors would then be expected to result in a maximum signal at some point above the burner. From the results obtained, the point at the tip of the visible portion of the secondary combustion zone corresponds to the maximum concentration of excited combustion products. Since this maximum is located at the tip of the flame, it is not surprising that the signal observed is not greatly affected by the fuel-to-oxidant ratio.

A series of studies was conducted to determine the effect of compound structure on the signal observed. Three homologous series of organic compounds were selected on the basis of availability: alcohols, cycloalkanes, and aromatics. Figure 8 shows the signal per mole of carbon as a function of the number of carbon atoms in the molecule. If all the compounds introduced into the flame burned completely to carbon dioxide, a horizontal plot would be obtained. Figure 8 shows that the signal obtained per mole of carbon in the compound depends on the number of carbons in the compound -as well as the compound type. The decrease in response observed with the longer chain compounds is probably a result of incomplete combustion of the compound to carbon dioxide. Likewise, the difference in response between saturated and aromatic compounds is undoubtedly a result of differences in the ease and extent of combustion. Regardless of the actual mechanism of signal production, it is clear that the response of the system is compound dependent. As a result, quantitative determiniation is possible using individual calibration curves.

To improve the sensitivity of the system to the point where it would be possible to use the monochromator for wavelength studies, a PbSe photconductive detector was evaluated using the apparatus illustrated in Fig. 2. These detectors are about two orders of magnitude more sensitive than the thermistor and respond over the wavelength range from 1 to 5 μm. Other detectors suitable for flame infrared emission detection are indium antimonide and mercury cadmium telluride. For this embodiment, the lead selenide detector was selected on the basis of the cost effectiveness. Since PbSe detectors are intrinsic semiconductors (i.e., not doped), the long wavelength response cutoff is determined by the inherent energy gap between the valence band and the conduction band. (Boyd, R. W.; Radiometry and the Detection of Optical Radiation; John Wiley: New York, 1983, Chapter 10). As a result, the detector response drops off rapidly as the photon energies approach the bandgap energy. When operated at room temperature peak response occurs at 3.8 μm. When the device is operated with cooling, the peak response is shifted to longer wavelengths. In addition to the shift in the wavelength of peak response, the dark resistance and time constant also change with cooling. For example, according to manufacturer's specifications, cooling causes the dark resistance to increase by 2.5%/°C and the time constant to increase by 5.3%/°C. Cooling with dry ice (-77°C) or liquid nitrogen (-196°C) increases the detectivity (D) by an order of magnitude. The device used in this study was provided with a thermoelectric cooling system which could maintain the detector at -10°C.

In studying the influence of operating conditions on the performance of the PbSe detector, two factors were investigated: the effect of applied voltage and the effect of chopping frequency. Figure 9 shows the effect of bias voltage on the signal observed at 4.3 μm when a 10% methanol/water mixture was pumped into the burner from the liquid chromatograph at a constant rate of 2 mL/minute. From the figure, it can be seen that a threshhold voltage of 10 volts is required to produce a minimum measurable signal. For bias voltages above 15 volts, the signal increases almost linearly with increasing bias voltage. A bias voltage of 30 volts was employed in all subsequent studies in Experiment 1.

Throughout all of the above experiments, it was observed that the noise on the signal appeared to be constant. In an effort to determine potential sources of noise in the system, the filtered dc power supply which was used to provide the bias voltage was replaced with a 30 volt battery. This substitution decreased the observed noise by a factor of five. For this reason, the 30 volt battery was used in place of the dc power supply to provide the bias voltage in all subsequent experiments in Experiment 1.

The other factor which was studied was the effect of chopping frequency on the signal observed when pure ethanol was aspirated at a steady rate into the flame. Figure 10 shows the results of this study. From the figure, it can be seen that the maximum signal is obtained for a chopping frequency of about 90 Hz. On the basis of this study, a chopping frequency of 86 Hz was employed in all studies in Experiment 1 with the PbSe detector.

The 0.5-m monochromator used in this study was selected solely on the basis of its availability in the laboratory and not on the basis of optical considerations. In fact, a shorter focal length dispersion system would have been preferable. When equipped with a 150 groove/mm grating, the system had a reciprocal linear dispersion of 13 nm/mm which is much lower than is necessary for infrared work of the type described here. Furthermore, since the entrance slit was wider (3 mm) than the width of the PbSe detector (1 mm), the dispersion system was characterized by a trapezoidal slit function with a base equivalent to 0.04 μm and a flat peak equivalent to 0 . 01 μm. In spite of the non-ideal slit function, the effective spectral bandwidth of the system was much less than the halfwidth of the molecular emission bands (~0.4 μm) being studied, so instrumental distortion of the band shapes was not a problem. Since high resolution is not required, a shorter focal length system with a correspondingly higher value for the reciprocal linear dispersion would have permitted more energy from the infrared bands to have been focused on the detector rather than dispersing it on either side as occurs with the current system. Measuring the integrated band intensity with a lower resolution system would therefore be expected to increase the sensitivity of the system significantly.

A hydrogen/air flame was selected because of its low background in the vicinity of the 4.3 μm CO₂ band. Figure 11 shows a comparison of the spectra obtained with the system for an hydrogen/air flame and an acetylene/air flame using the PbSe detector. The spectrum obtained with the acetylene/air flame is typical of the results obtained when a carbon-containing fuel is used and is in agreement with the spectrum obtained by Plyler (Plyler, E.K.; J. Res. Nat. Bur. Stand. 1948, 40, 113) for a Bunsen flame. Since the 4.3 μm emission band is due solely to excited carbon dioxide, it is not present to any great extent in the hydrogen/air flame. The band at 2.7μm, on the other hand, is due to both water and carbon dioxide and is therefore present in both flames. Compared with the hydrogen/air flame, no significant difference in the spectrum was observed when the hydrogen/20% oxygen-80% argon flame was used. It was anticipated that the use of the hydrogen/20% oxygen-80% argon flame might produce a lower background in the 4.3 μm region because, compared with air, this gas mixture did not contain carbon dioxide. Since a reduced background was not observed, use of the 20% oxygen-80% argon gas mixture was discontinued. To prove that the emission observed at 4.3 μm when organic compounds were introduced into the hydrogen/air flame was indeed due to carbon dioxide emission, the spectrum of carbon dioxide gas introduced into the flame was compared with the spectrum obtained when ethanol was introduced into the flame. Figure 12 shows that identical spectra are produced regardless of whether an organic compound is burned or carbon dioxide is introduced directly into the flame.

Figure 13 shows the signal obtained at 4.3 μm as a function of the flow rate of carbon dioxide introduced into the flame. Figure 14 shows the signal obtained when various volumes of ethanol were eluted from the liquid chromatograph. The zero offset observed with the liquid chromatograph was attributed to the background signal due to the methanol in the eluting solvent. Both experiments produced calibration curves which curved upwards in contrast to the results obtained in the non-dispersive studies with the thermistors. This upward bending of the growth curve is reminiscent of the effect caused by ionization when alkali metals are introduced into the flame. Whatever the explanation, the effect clearly involves only carbon dioxide since it is not observed when both the carbon dioxide and water bands are monitored simultaneously.

Only two intense bands (2.7 μm and 4.3 μm) were observed over the wavelength interval accessible by the PbSe detector. Of the two bands, the 4.3 μm band was deemed the most useful analytically since it arises solely from the presence of carbon dioxide. Since it did not appear necessary to vary the wavelength, the monochromator was replaced by a high-pass optical filter in an effort to increase sensitivity by increasing optical throughput. By this means, the wavelength response of the system was limited at lower wavelengths by the short-wavelength cutoff of the filter and at higher wavelengths by the response of the detector itself. The substitution of the filter for the monochromator resulted in an increase in sensitivity of about 2-3 orders of magnitude.

To demonstrate the potential of infrared emission as a means of detection of organic compounds which have been introduced into the flame, a mixture of methanol, ethanol, and propanol was separated by means of liquid chromatography and detected by means of infrared emission using the PbSe filter arrangement. Figure 15 shows the results obtained for a 50 μL injection of an equivolume mixture of the three components.

The infrared emission at 4.3 μm provides a sensitive means of detecting small amounts of organic samples introduced into the flame. Since the emission wavelength does not vary, a relatively low-cost filter instrument can be constructed to monitor the desired emission. The detector is suitable for application to both liquid and gas chromatography. The use of the detector with a gas chromatograph is in some respects easier than with a liquid chromatograph because of the absence of the background signal from the eluent which is present when methanol/water mixtures are used.

The experiments reported in this application were conducted with equipment available in the laboratory. The burner assembly used was selected on the basis of its availability in the laboratory, and is not necessarily an ideal system for chromatographic detection because the flame produced with the burner is much larger than is actually necessary. A more appropriate burner for this application is described below under Experiment 2. EXPERIMENT 2

The experimental arrangement used in this second study is shown in Figure 16. A Hamamatsu lead selenide photoconductive cell (P2038-01, Hamamatsu Corp., San Jose, CA) was employed as the infrared detector, and was positioned to view a hydrogen/air flame maintained on a specially designed burner 160 described below. A high-pass filter 168 (Corion Corp . , Holliston , MA) with a short wavelength cutoff of 3.5 μm was mounted in a housing in front of the PbSe detector 164 as described in Experiment 1 to give a detection system with a response from 3.5μm to about 5 μm. The power supply and pre-amplifier circut used in this study for the detector were also as described in Experiment 1. Radiation from the flame was modulated at 90 Hz by a chopper 165 which was constructed in the laboratory. The modulated signal was applied to the input of a Model 128A Princeton Applied Research lock-in amplifier (Princeton Applied Research, Princeton, NJ), and the amplified signal was displayed on a Varian Aerograph stripchart recorder. A Model 3120TX Bascom-Turner digital data acquisition system (Bascom Turner Instruments, Norwood, MA) was used to store and display data for some experiments where peak area measurements were made.

The oven and injection port of a Model 705 Varian Aerograph gas chromatograph (Varian Instruments, Palo Alto, CA) were used in conjunction with a 1/4" stainless steel column packed with 10% OV-101 on Chromasorb W-HP to evaluate the performance of the flame infrared emission detection system for gas chromatography. A vent in the side of the gas chromatograph was utilized to connect the GC column to the detection system. The interface between the 1/4" GC column and the 0.10" OD stainless steel capillary from the burner system was made by means of an adapter machined from brass. Mounts and detector shields were fabricated from aluminum sheet metal stock, and were used to isolate the detector from the heat produced by the GC oven and from drafts of air or other heat sources (the detection system is sensitive enough to detect the heat from persons moving in the room). The sides of the baffle surfaces facing the detector were painted flat black to avoid reflection of IR energy from the surroundings into the aperture of the detector unit.

Column carrier gas flow and pressure were monitored with Brooks Instrument Division flow meters 169 of Figure 16 (Brooks Instrument Division, Emerson Electric Co., Hatfield, PA) calibrated for helium flow. A metering valve and separate shut-off valve were installed in the gas line of the helium supply. All carrier gas metering was done upstream of the GC and column.

All experimental runs were made after a 30 minute warm-up time to allow the environment of the detector to reach thermal equilibrium. The radiation from the flame was chopped at 90 Hz, and a detector bias voltage of 30 volts, obtained from a bank of batteries, was employed. Injections were made using standard 10 and 50 microliter syringes (Hamilton Co., Reno, Nevada) for liquid samples, and a 500 microliter gas syringe (Hamilton Co., Reno, Nevada) for gas samples. All chromatograms were obtained with a carrier gas flow rate of 40 mL/min. unless stated otherwise. All compounds used were reagent or spectroscopic grade with the exception of pentane which was Eastman 98%.

In all experiments in which detector response for different carrier gas flow rates was being measured, the temperature of the GC oven was maintained at a sufficiently high value to avoid sample interactions with the stationary liquid phase as the sample passed down the column. This procedure minimized any column effects which might alter the readings obtained in these studies.

For injections smaller than 0.5 microliter, solutions of the compound of interest were prepared in a solvent with a higher boiling point than the sample which could be easily separated from the sample by the column. The temperature of the oven would then be held above the boiling point of the compound of interest, but below that of the solvent in order to maximize separation and minimize column effects on the sample.

Small quantities of various gaseous samples (Matheson Gas Products, Secaucus, NJ) were collected over water from lecture bottles. Chromatograms of these gas samples were obtained by injecting 500μL of the gas into the chromatograph with a gas syringe.

The chromatogram obtained for unleaded gasoline was obtained by holding the column temperature at 55°C for 4 minutes and then ramping the temperature up to 200°C over a period of 7 minutes.

The detector system used in this study was described above with respect to Fig. 2, and the same conditions for operation of the system were found to be satisfactory for this work. Due to space considerations, the commercial chopper used in the previous study was replaced with a smaller unit which was fabricated in the laboratory and used a synchronous AC motor. Since no optical components to focus the radiation on the detector were employed, it was important to place the detector/chopper combination in close proximity with the flame, and this could be accomplished only with a small chopper. Background signal from a variety of sources was minimized by minimizing the field of view of the detector and shielding the small area viewed by the detector with sheet metal baffles whose interior surfaces were painted flat black to minimize reflections. This shielding also kept air drafts that would affect the flame to a minimum. The use of a 10-cm diameter spherical mirror used as the focusing element in the previous study was rejected as this would have increased the area requiring shielding to an unacceptable level.

The atomic absorption burner unit used in the previous study was replaced by a smaller specially fabricated pre-mixed burner (Figure 17) because the flame produced by the atomic absorption burner was much larger than necessary.

A special burner shown schematically in Figure 17, was designed to produce a small hydrogen/air pre-mixed flame. The burner was machined from a block of aluminum, and consisted of a burner body 170 with a mixing chamber 170a and a burner head 171 held in the body 170 by a rubber o-ring seal 172. Openings 173a and 173b in the sides of the burner body permitted the burner to be connected to the combustion gas supplies with Swagelok (Crawford Fitting Co., Solon, OH) tube fittings.

The burner system described in this application produces a stable flame of small size by means of an array of stainless steel capillary tubes 174. These tubes 174 were cut to a length of approximately 2.5 cm and permitted the use of very low hydrogen/air support gas flow rates without problems from flashback. The combustion gases issued from the burner head through the circular array (Figure 17.1) of six stainless steel capillaries of 0.10" OD and 0.06" ID which were cemented in a small hole in the burner head by means of epoxy cement. In the burner design implemented in this application, six capillaries arranged in a circular array were used to form the orifice for the combustion gases.

The circular array of capillaries surrounded a central capillary 175 through which flowed the column effluent directly into the center of the hydrogen/air flame and which, therefore served as a connection between the burner and the gas chromatograph. The central capillary 175 was bent at a right angle to exit the burner body through a side port 176, and was held in place by a rubber seal in a tubing fitting.

The new burner design has several important advantages which should be emphasized. Because the rate of sample addition to the flame is determined solely by the carrier gas flow rate and not by the combustion gas flow rates, the rate of sample addition to the flame may be varied independently of the combustion gas flow rates, thereby avoiding changes in the flame size or stoichiometry. By introducing the sample directly into the flame from the central capillary, peak broadening associated with mixing chambers is avoided. Since the capillary has a small internal diameter (0.06" ID), post-column volume can be kept to a minimum (0.5 mL/30 cm length of tubing). Finally, use of a narrow-bore capillary leads to a high linear velocity gas jet which travels up the center of the flame. (A carrier gas flow rate of 40 mL/min., for example, results in a linear velocity in the capillary of 40 cm/s.) At velocities of 40 cm/s, the transit time for the eluted sample through the 30 cm length of exposed capillary, which runs from the end of the GC column to the flame, is about 750 ms. The linear velocity appears to be sufficient to avoid sample condensation on the walls of the exposed capillary, even in the absence of insulation or heating, for the largest sample volumes investigated in this study. Initially, it was felt that the effluent capillary should be insulated for the short length that it was exposed to the ambient air outside of the GC oven or the burner body to avoid condensation of the chromatographic effluent. The possibility of having to heat this section of the effluent capillary was also considered. Neither of these options were found to be necessary with any of the samples studied in this experiment regardless of sample size. It is believed that the carrier gas flow rates used in this study (which are typical carrier gas flows for packed GC columns) resulted in a gas velocity in the effluent capillary which was so high that the sample components did not have sufficient time to condense on the walls. Although typical carrier gas flow rates of 30 to 40 mL/min. were used, flows as low as 10 mL/min. were employed without any problems.

Combustion gas flow rates of 300 mL/min. and 800 mL/min. were used for hydrogen and air, respectively. This combination of flow rates gave a flame that was approximately 20 mm high and 3 mm wide and produced the smallest flame that the given combination of metering devices and capillary tube size would permit without the flame pulsating or burning out. The fuel-to-oxidant ratio used corresponds to a nearly stoichiometric mixture, and gave the smallest possible flame consistent with good signal conditions. The best signal-to-noise ratio was obtained when the detector was positioned to view the upper portion of the secondary combustion zone of the flame. In aligning the detector with the proper flame zone, a problem was encountered because the flame itself is invisible. To make the flame visible for alignment purposes, a small amount of 1 M sodium chloride was applied to the outside surfaces of the burner capillaries to produce a visible sodium (yellow) emission. This sodium signal lasted for quite some time after the system was aligned, but did not affect the detector signal as seen in the chromatogram of several compounds taken while the flame was still yellow. One advantage of the flame infrared emission detection system is that it does not respond to visible emission from the flame or other ambient sources such as the room illumination. Another advantage of the flame infrared emission detection system which was observed was that nitrogen could be used as a carrier gas in place of helium without any change in detector response.

A warm-up period of approximately 30 minutes was found to be necessary because the signal from the PbSe detector varies with temperature changes in the surrounding area. When all the components of the burner/detector combination reached thermal equilibrium (about 30 minutes), a steady baseline was obtained from the recorder. The baffles surrounding the flame had the added effect of insulating the flame area from the infrared emission from the chromatograph oven, thereby decreasing the background signal from the detector. Prior to the installation of the baffles, changes in the GC oven temperature caused baseline drift which necessitated a stabilization period before further chromatography could be conducted. Shielding almost completely eliminated the effect of GC oven temperature on the detector signal except for a slight baseline change which was thought to be due to changes in the temperature of the carrier gas introduced into the flame. This slight baseline shift can be seen in Figure 18 shich shows a temperature-programmed chromatogram obtained with a 5jal injection of unleaded gasoline. The chromatogram shown in Figure 18 also demonstrated that the flame infrared emission detector could be employed successfully as a detector in an actual gas chromatographic separation. Since the purpose of this study was to demonstrate the performance of the detector, no effort was made in optimizing or improving the separation conditions for the gasoline sample.

Figure 19 shows the shape of typical peaks obtained from the system when various volumes of pentane were injected into the chromatograph as neat samples. To study the response of the detector itself to various compounds without the influence of column effects, the GC oven was maintained at a temperature above the boiling point of the compound under investigation. This caused the sample to be carried through the column with very little interaction with the stationary phase as indicated by the similarity in the retention times (i.e., within a few seconds of one another) for diverse compounds. This procedure also resulted in peaks with high symmetry as shown in Figure 19. Such peaks were necessary to enable a comparison of response to be performed' on the basis of peak height as opposed to peak area. Since integration of the peaks was not possible with the experimental setup used throughout most of this work, peaks with high symmetry were desired because these peaks showed the best correlation between peak height and the amount of compound present (i.e., peak area). To obtain peaks of similar shape, one carrier gas flow rate of 40 mL/min. was used throughout. The use of peak height as a measure of the amount of compound present was justified by the goodness of fit of the data obtained for the calibration curves.

Figures 20, 21 and 22 show plots of peak height versus injection volume for dichloromethane, trichlorotrifluoroethane and carbon tetrachloride. These compounds were chosen on the basis of availability and to investigate the ability of the flame to combust them to carbon dioxide. While it was obvious that hydrocarbons and other compounds such as aromatics and cycloalkanes would actually become a fuel in the flame, it was not clear if other compounds such as the halocarbons utilized above would combust sufficiently to give a signal. From the figures, it can be seen that the response is reproducible and is a linear or almost-linear function of the amount injected. This was the case for all of the compounds studied even for those cases where complete combustion to carbon dioxide was somewhat questionable. The calibration curve for carbon dioxide shown in Figure 23 was prepared to confirm that the phenomenon that was being observed by the detector was, in fact, the emission from carbon dioxide, and also to show that the signal obtained varied linearly with the amount of carbon dioxide introduced. It should be noted that the data shown in Figure 23 indicate a linear relationship between peak height and sample volume in contrast to the results obtained in the previous study using the atomic absorption burner. It' is felt that the linear relationship obtained in the present study is due to the use of the smaller flame, the smaller amounts of carbon dioxide injected, and the method of sample introduction.

Figure 24 shows five calibration curves obtained for carbon dioxide, pentane, 1,1,2-trichloro-1,2,2-trifluoroethane, dichloromethane, and carbon tetrachloride. In Figure 24, points represent single injections; average reproducibility 2.75%. These curves were all obtained under identical conditions and are plotted in terms of moles of compound injected in order to facilitate cross comparison of detector response. While it was obvious that such compounds as hydrocarbons, aromatics, and cycloalkanes would act as a fuel in the flame, it was not clear whether halocarbons would combust sufficiently to give a signal. (Flame ionization detectors give notoriously poor response to such compounds). From Figure 24, it can be seen that the flame infrared emission system responds well to the three halogenated compounds, although carbon dioxide produced the greatest response (i.e., largest slope). Since detector response should be proportional to the number of moles of carbon dioxide present in the flame, these curves clearly indicate that none of the organic compounds plotted in Figure 24 are completely oxidized to carbon dioxide in the flame.

It is interesting that the calibration curve for propane (not shown) has a slope that is almost exactly 3 times that obtained for carbon dioxide, indicating essentially complete combustion in the flame. A comparison of the response obtained for the four organic compounds shown in Figure 24 with the values expected for complete combustion (i.e., 5 times the response for carbon dioxide in the case of pentane, 2 times the response for carbon dioxide in the case of 1,1,2-trichloro-1,2,2-trifluoroethane, etc.) suggests that all four are combusted to only about 7-11%. Thus, while the flame infrared emission detector does show some compound-dependent response (like the FID), many compounds appear to be combusted to roughly the same extent. This conclusion is further supported by a preliminary study of 15 substituted n-alkanes and cycloalkanes, which produced roughly the same signal per mole of carbon.

Table III shows the reproducibility of response in terms of peak height obtained for a series of 1- uL injections of four different compounds. Each entry in the table represents the peak height obtained for a single injection. The average relative standard deviation in peak height observed for all four compounds was 2.75%.

The linear dynamic range of the detection system was studied using pentane as a test compound. Figure 25 shows a log-log plot of chromatographic peak height versus sample volume of pentane introduced into the chromatograph for sample volumes ranging from 0.02 up to 50 μL. Sample volumes greater than 0.5 μL were introduced into the chromatograph by direct injection of the neat sample. Signals corresponding to sample volumes less than 0.5 μL were obtained by injecting appropriate amounts of pentane dissolved in hexane. To obtain data over this range of sample volumes, different degrees of amplification were required which necessitated changes in the gain setting of the lock-in amplifier. The accuracy of these gain changes was checked by comparing the signal obtained for 1-μL injections of hexane under different amplifier settings. Figure 25 shows the response obtained from the lowest level up to injections of approximately 20- to 30-μL volume, where some downward curving takes place. This downward curving of the calibration curve could be due to several factors including the use of peak height instead of peak area as a measure of the signal, column flooding as a result of injection of large samples, and self absorption of the radiation in the flame. If the downward curving were due to self absorption, the expected slope of the log-log growth curve would be one-half. As expected, the measured slope in the linear portion of the log-log plot was one, and, while there were not a large number of data points to work with in the curving region, the slope did not appear to go to one-half. As a result, it was postulated that the signal drop off was due to column effects, a supposition which was backed up by the observation of tailing in the recorder tracings of the larger peaks.

From the data presented in Figure 25, the dynamic range of the flame infrared emission detector was estimated to be on the order of 10 4. From this data, if the smallest volume of pentane which could be seen was taken as 0.02 μL, this would correspond to a detection limit of about 4.6 x 10⁴ mg/s of pentane. This is actually a very conservative estimate of the detection limit because signals obtained for 0.02μL were readily measurable above background.

To determine the effect of compound structure on the response of the detector, response data were collected for several series of compounds available in the laboratory and compatible with the GC column used. The compounds used were classified into several categories on the basis of their structure, and included substituted methanes and ethanes as well as n-alkanes and cycloalkanes. Gaseous compounds such as methane, carbon monoxide and carbon dioxide were studied by injecting 500 μL of the compound with a gas syringe. The response for various liquid compounds was obtained by injecting 1 μL of the neat liquid. For each liquid compound injected, the relative response per mole of carbon was calculated from the injection volume, the density of the liquid, the formula weight of the compound and the number of carbons in the compound.

Figure 26 shows the relative response obtained for equal volumes (i.e., equal moles) of one-carbon gases. If both methane and carbon monoxide were completely combusted to carbon dioxide in the flame, their response would be expected to equal that obtained for an equivalent amount of carbon dioxide. Since the data shown in Figure 26 appear to be equal within experimental error, it suggests that the hypothesis of complete combustion for these gases is valid.

Figure 27 shows the relative response obtained per mole of carbon as a function of the number of carbons present in 15 compounds . Table IV shows the actual data obtained for 1-μL injections of the 15 compounds as well as the calculated signal per mole of carbon.

With the exception of cycloheptane and cyclooctane, Figure 27 shows that roughly the same relative response was obtained regardless of the number of carbons in the compound out to carbon numbers of about seven. This suggests that, if not completely combusted to carbon dioxide, all of the compounds studied with the exception of the two mentioned above are combusted to about the same extent with very little influence of carbon number or compound structure. The exceptions to this rule obtained for cycloheptane and cyclooctane may be more apparent than real because the peak shapes obtained for these compounds were not symmetric. As a result, the use of peak height as a quantitative measure of the signal produced may not have been a reliable indication of the actual peak area. Neglecting the data for cycloheptane. and cyclooctane, the average signal per mole of carbon in Table IV is 1 x 10⁶ units which corresponds to about 50% of the signal per mole of carbon estimated for carbon dioxide. This comparison suggests that the liquid samples were not completely converted to carbon dioxide in the flame.

The influence of carier gas flow rate on the chromatographic peak area was studied in an effort to determine whether the detection system behaved as a concentration-dependent detector or a mass/flow rate-dependent detector. These two different detector response categories can be distinguished from one another on the basis of the effect of carrier gas flow rate on the chromatographic peak area obtained for a given sample size. In the case of the concentration-dependent detector, the chromatographic peak area will vary inversely with carrier gas flow rate, whereas with a mass/flow rate detector the chromatographic peak area will be independent of carrier gas flow rate (McNair, H. M., Bonelli, E. J., Basic Gas Chromatography, 5th ed., Varian Instrument Division, Palo Alto, CA, 1969, pp. 81-5.). Figures 28 and 29 show the effect of carrier gas flow rate on chromatographic peak height and peak area for a series of 0.500 mL injections (22.3 μmol at STP) of carbon dioxide. Each point represents a single injection of carbon dioxide; reproducibility is about 2.8%. Because the flame infrared emission detector consumes the sample in the process of producing a signal, the system is expected to behave in a mass-flow rate manner similar to the FID (flame ionization detector). In the case of the flame infrared emission detector, the signal will be proportional to the instantaneous concentration of carbon dioxide present in the flame. The net amount of carbon dioxide present in the flame at any instant will depend on the difference between the rate at which sample is introduced into the flame (and converted into carbon dioxide by combustion) and the rate at which combustion products are removed from the observation zone by transport of flame gases. For a given set of fuel and oxidant flow rates, the rate of removal of combustion products from the observation zones should be fixed. Therefore, the mass-flow rate detector model predicts that peak signal will increase directly with carrier gas flow rate and the integrated peak area will be independent of carrier gas flow rate.

Figure 28 shows that peak height does indeed increase with carrier gas flow rate, although the relationship is not strictly linear. Figure 29 shows that for carrier gas flow rates above 30 mL min^-1, chromatographic peak area varies only slightly with increasing carrier gas flow rate. Apparent deviations from mass-flow rate behavior can be attributed to flame cooling, incomplete mixing, and dilution. As the carrier gas flow rate increases, introduction of increasing amounts of helium into the flame will lead to a small decrease in flame temperature as well as a dilution of flame gases. Both of these factors will contribute to a reduction in detector response. In addition, sample mixing with flame gases may become less complete at higher flow rates, leading to incomplete combustion.

At very low carrier gas flow rates (less that 30 mL min^-1), the reproducibility in peak area measurements degrades severely (Figure 29) while chromatographic peak height tends to approach a limiting value. Under these low flow rate conditions, sample removal is the dominant process so the net amount of carbon dioxide present in the flame at any instant is small, resulting in a small signal. In addition, low carrier gas flow rates imply a longer retention time and a correspondingly wider peak bandwidth. As the peak bandwidth increases, the concentration of sample introduced into the flame at any given instant decreases. Since a minimum concentration of carbon dioxide is required to produce a measurable signal with the flame infrared emission detector, reducing the carrier gas flow rate, while favoring increased sample mixing and combustion, will eventually lead to C0_ levels in the flame below the limit of detectability.

At these flow rates, both response and reproducibility should decrease in the manner observed.

Before reporting the detection limit obtained for pentane by using the flame infrared emission detector, it is worthwhile to discuss how detection limits are determined from chromatographic measurements. Because the flame infrared emission detector responds in a mass flow manner, the detection limit will depend on the limiting base-line noise and the response of the detector. The response, R, of the detector is determined from the slope of the calibration curve obtained by plotting signal, S, versus mass flow rate,

M_f, into the detector. R = ΔS / ΔM_f (1)

Mass flow rate, expressed as mg s^-1, is determined by dividing the total mass of injected sample (mg) by the bandwidth (s) of the resulting chromatographic peak. If the calibration curve obtained by plotting signal versus mass flow rate into the detector (i.e., S = RM_f) is extrapolated back to smaller mass flow rate values, a point will be reached where the signal can no longer be distinguished from the base-line noise of the chromatogram. If this point is taken as a signal equal to twice the root-mean-square (rms) base-line noise, the detection limit will be given by

S = RM_f = 2 (rms base-line noise) (2)

Solving eq 2 for M_f gives

(M_f)_d1 = 2 (rms base-line noise) /R (3)

where (M_t)_d1 is the minimum detectable mass flow rate into the detector in mg s^-1

To compare the performance of a mass flow rate detector like flame infrared emission with a concentration-dependent detector like the thermal conductivity detector (TCD), the minimum detectable mass flow rate is divided by the carrier gas flow rate to give

C_d1 (mg mL^-1) = 2 (rms base-line noise) /RF (4) where F is the carrier gas flow rate in mL s and C,, is the lowest concentration that the detector can sense. Since the rms base-line noise observed is dependent on the amplifier time constant, it is important to specify the time constant of the system when reporting the detection limit.

With eq 3, the detection limit for pentane was determined from detector response measurements and estimates of the rms base-line noise to be 4.6 x 10^-4 mg s for an amplifier time constant of 3 s. The rms noise was estimated as one-fifth of the peak-to-peak base-line noise. In terms of concentration of sample entering the detector for a 40 mL min carrier gas flow rate, the minimum detectable concentration of pentane was determined from eq 4 to be

7 × 10^-4 mg mL or 1 x 10^-8 mol cm of pentane. Assuming standard temperature and pressure, the concentration of pentane reported above corresponds to 224 ppm on a volume basis.

By comparison, detection limits for similar compounds obtained with a flame ionization detector (FID) are on the order of 10 mg s and for a thermal conductivity detector (TCD), the detection limit is typically considered to be on the order of 10 ^-6-10^-7 mol cm^-3 (Karger, B.L. et al

An Introduction to Seperation Science, Wiley, New York 1973 pp. 232-236) depending on operating conditions. From the above discussion, it can be seen that the flame infrared emission detector is more sensitive than a TCD, but considerably less sensitive than an FID. Therefore, it is clear that significant increases in sensitivity will be required before the flame infrared emission detector is competitive with the FID. Nevertheless, the detector described in Experiment 2 is only a prototype, and significant improvements in sensitivity can be expected as the detection system is refined and limiting noise sources are identified.

Figure 30 shows the performance of the flame infrared emission system under isothernal conditions for a synthetic sample consisting of a 1:2:1:3 volume mixture of pentane, 1,1,2-trichloro-1,2,2-trifluoroethane, hexane, and carbon tetrachloride. This chromatogram was obtained from a 5-μL injection of this mixture with an Apiezon L column maintained at 50°C. Since the PbSe detector can respond to intensity variations in the kilohertz range, the flame infrared emission detector has no difficulty in following the relatively slow intensity variations produced during elution of components from the gas chromatograph.

The flame infrared emission detection system has been shown to be a relatively simple, inexpensive detector for gas chromatography. Compared with other detection systems currently employed, the use of infrared emission has a number of advantages. Since the system is not based on thermal conductivity, nitrogen can be used as a carrier gas in place of the more expensive helium required with a thermal conductivity detector. The system described in this application has been shown to exhibit a wide dynamic range characteristic of emission measurements. The detector has a relatively fast response time which is a potential asset in being able to detect narrow chromatographic peaks as might be obtained with capillary column gas chromatography.

Although the analytical applications of infrared emission described above centered on the measurement of the infrared emission from carbon dioxide, other combustion products such as oxides of nitrogen and sulfur should produce infrared emission at other wavelengths. Thus, a detection system having several detector/filter combinations could be produced which would respond not only to the presence of carbon but to nitrogen and sulfur as well.

The detection systems described above appear to respond to carbon dioxide produced by the combustion of compounds introduced into the flame, and do not appear to be greatly affected by the structural nature of the samples.

They also respond to certain gases such as carbon monoxide and carbon dioxide which do not respond well with the flame ionization detector.

EXPERIMENT 3

The minature capillary-head burner of Experiment 2 was modified for use with liquid samples. As the previously designed burner was intended to admit a gas stream from the gas chromatograph to the center of the burner-head, the burner was modified for nebulized liquid samples. The central sample injection capillary was removed, and the number of small-bore capillary tubes in the burner-head was increased from 6 to 19 (the internal diameter of the capillary tubes was 0.6 mm). The overall diameter of the burner orfice was 0.5 cm. The capillary-head burner was fitted with a Jarrell-Ash model X-88 atomic absorption cross-flow nebulizer and a 3 cm long x 4 cm diameter teflon spray chamber. The nebulizer and spray chamber were coupled to the burner body by boring a one inch hole in the side of the burner body (perpendicular to the capillary-head) and press fitting the spray chamber/nebulizer assembly to the burner.

A 1:1 hydrogen/air flame stoichiometry was used for all measurements, and the fuel and oxidant flow-rate were maintained at 200 ml/min. A 1:1 fuel/oxidant mixture resulted in a stable flame approximately 4 cm in height by 1 cm in width. The infrared emissions were observed over a 1 cm vertical segment centered at a height of 1.5 cm from the burner top. The reagent grade liquid samples were introduced into the flame via aspiration by the nebulizer.

All flame infrared emission spectra were acquired on an unpurged Mattson Cygnus 100 Fourier transform spectrometer. Fourier transform infrared emission spectroscopy allows multiwavelength nonmetal analysis. The Fourier transform spectrometer, by virtue of the multiplex nature of the data acquisition, is a multichannel instrument and can therefore monitor all infrared wavelengths simultaneously. Since nonmetal molecular emission occurs in the infrared spectral region, any standard, commercially available Fourier Transform-Interferometer can be utilized without the need for special optics, beamsplitters, or detectors. The Fourier transform instrument also provides several advantages for infrared emission spectroscopy. These advantages include: a single instrument for both elemental and molecular analysis, high optical throughput, good spectral resolution, accurate wavelength recording due to the reference laser, the ability to signal average by coaddition, and the capability of performing spectral subtraction.

Figure 31 schematically shows the arrangement of the burner 310, mirror 312 and Fourier Transform-Interferometer 314 for Experiment 3. A 5-cm-focal-length, 10-cm-diameter aluminum mirror 312 was used to collect and collimate the infrared emissions from the flame. It should be noted that the infrared collection mirror 312 was placed off the optical axis by approximately 30 degrees. No significant abberational defects were observed.

A room temperature, triglycine sulfate (TGS) detector (D* = 2 × 10 ⁹ W H^1/2) and KBr beamsplitter were employed in the Fourier Transform Interferometer 314. All spectra were acquired with 4 cm resolution at a mirror velocity of 0.32 cm/s. A Beer-Norton medium (F2) apodization function was used with 1X zero filling and, due to the discrete line nature of the emission spectra, phase correction was not applied. Instead, the single-beam power spectra were calculated and plotted, and none of the spectra in Figures 32-37 have been corrected for instrumental response.

The hydrogen/air flame was chosen to excite the molecules of interest in order to eliminate carbon dioxide emissions from the fuel gases. Otherwise, the determination of carbon, as carbon dioxide, would be significantly impaired.

Figures 32-37 are characteristic infrared emission spectra for carbon tetrachloride, methanesulfonyl fluoride, the H₂/Air flame background, methanol, trichloro-trifluoro ethane and tetramethylsilane. These spectra clearly show that bands other than those from H₂O and CO₂ can be observed in the flame.

In a further embodiment, a flame infrared emission detector is combined with a flame ionization detector wherein the same flame is used to simultaneously conduct both types of detection. The flame infrared detector provides better quantitation of moles of carbon present in the compounds while the flame ionization detector provides higher sensitivity to extremely small amounts of hydrocarbons.

Additionally the flame infrared emission detector is able to detect compounds not observed by the flame ionization detector such as carbon monoxide and carbon dioxide. An experimental schematic for a combined flame infrared emission flame ionization detector is shown in Figure 38. The burner body 380 is the same as used for Experiment 2. Hydrogen/Air is used as the fuel/oxidant mixture being supplied to the capillary tubes of the burner through a Swagelok T 383a,

383b. The sample is supplied through the central capillary

386. The flame ionization detector utilizes two electrodes in an electrode assembly 382 where a potential of approximately 300 V DC is regulated between the electrodes by a power supply. An electrometer measures the ion current accross the flame. The infrared emission is simultaneously detected by a PbSe detector 384. Radiation from the flame is modulated by an optical chopper 385. The infrared detector 384 must not "see" the electrodes (due to blackbody emission background), therefore an aperture device is mounted on the infrared detector unit.

Although the invention has been described by reference to some preferred embodiments, it is not intended that the novel infrared detection means and method be limited thereby but various modifications are intended to be included as falling within the spirit and broad scope of the foregoing disclosure, the attached drawings and the following claims.

Claims

IN THE CLAIMS :

1. Infrared detection means for detecting selected molecules of interest in a gaseous sample, said means comprising:

(a) means for exciting any molecules of interest in said sample to emit a characteristic infrared radiation pattern,

(b) infrared detector means for detecting a selected wavelength of infrared radiation emitted by said molecules of interest, said detector means generating an electrical signal in response to the emission of said preselected wavelength,

(c) means for isolating the preselected wavelength of infrared radiation, said wavelength being selected from the characteristic infrared radiation pattern of the molecule of interest, said means being mounted between said means for exciting and said detector means.

2. Infrared detection means as claimed in Claim 1 which further includes means for vaporizing a liquid sample, said means positioned to vaporize said sample into said means for exciting the molecules of interest in said sample.

3. Infrared detection means as claimed in Claim 1 or 2, wherein said means for exciting the molecules of interest in said sample further includes a flame generated by a torch which burns either hydrogen/air or hydrogen/oxygen.

4. Infrared detection means as claimed in Claim 3, wherein said detector means is a lead selenide detector.

5. Infrared detection means as claimed in Claim 3 wherein said detector means is an indium antimonide detector.

6. Infrared detection means as claimed in Claim 3 wherein said detector means is a mercury cadmium telluride detector.

7. Infrared detection means as claimed in Claim 3, wherein said means for isolating a preselected wavelength is a monochromator.

8. Infrared detection means as claimed in Claim 3, wherein said means for isolating a preselected wavelength is an infrared filter.

9. Infrared detection means as claimed in Claim 3, wherein said means for isolating a preselected wavelength is an infrared interferometer, and said detector means includes a computing means which applies a fourier-transform to the electrical signal.

10. Infrared detection means as claimed in Claim 3 which further includes baffle means surrounding said means for heating and said infrared detector means to thereby limit ambient infrared radiation received by said detector means.

11. Infrared detection means as claimed in Claim 3 in which the molecule of interest generates a CO₂ molecule when oxidized by said flame, and the characteristic infrared radiation pattern includes peaks at 4.3 um and 2.7 um.

12. Infrared detection means as claimed in Claim 2 which further includes:

(a) means for acidifying the liquid sample prior to vaporization to generate CO₂ from any carbonates or calcinates contained therein,

(b) a flame generating means within said means for exciting the vaporized sample.

13. A method of detecting selected molecules of interest in a gaseous sample by detecting characteristic infrared emission, said method comprising:

(a) exciting any molecules of interest in the gaseous sample, and thereby emitting a characteristic infrared radiation pattern. (b) isolating a preselected wavelength of the infrared radiation emitted by the excited molecules of interest, said wavelength being selected from the characteristic infrared radiation pattern of the molecule of interest,

(c) detecting the presence of said preselected wavelength of infrared radiation when emitted by said molecules of interest, and

(d) generating an electrical signal when said preselected wavelength has been detected.

14. A method of detecting selected molecules of interest as claimed in Claim 13, which further includes the step of vaporizing a liquid sample prior to said exciting step.

15. A method of detecting selected molecules of interest as claimed in Claim 13 or 14, wherein said exciting step includes the step of combusting the sample in a flame fueled with either hydrogen/air or hydrogen/oxygen.

16. A method of detecting selected molecules of interest as claimed in Claim 15, wherein said detecting step includes the use of a lead selenide detector.

17. A method of detecting selected molecules of interest as claimed in Claim 15, wherein said detecting step includes the use of an indium antimonide detector.

18. A method of detecting selected molecules of interest as claimed in Claim 15, wherein said detecting step includes the use of a mercury cadium telluride detector.

19. A method of detecting selected molecules of interest as claimed in Claim 15, wherein said isolating step includes the use of an infrared filter.

20. A method of detecting selected molecules of interest as claimed in Claim 15, wherein said isolating step includes the use of a monochromator.

21. A method of detecting selected materials of interest as claimed in Claim 15, wherein the isolation and detection steps include the use of a fourier-transform infrared interferometer.

22. A method of detecting selected molecules of interest as claimed in Claim 15, wherein said method includes the step of shielding the detector from ambient infrared radiation.

23. A method of detecting selected molecules of interest as claimed in Claim 15, wherein the molecule of interest is CO₂ and the characteristic infrared radiation pattern includes peaks at 4.3 um and 2.7 μm.

24. A method of detecting selected molecules of interest as claimed in Claim 23, which further includes the step of acidifying the sample to generate CO₂ from any calcinates or carbonates contained in said sample.

25. A method of detecting selected molecules of interest as claimed in Claim 15, wherein said gaseous sample is a process gas and said molecules of interest are generated by the combustion of contaminants within said gaseous sample.

26. A method of detecting selected molecules of interest as claimed in Claim 25 wherein said molecule of interest is CO₂ and said contaminants are CO, CO₂ and hydrocarbons.

27. A method of detecting selected molecules of interest as claimed in Claim 13, wherein said molecules of interest are excited by electron impact in a gas discharge.

28. A method of detecting selected molecules of interest as claimed in Claim 13, wherein said molecules of interest are excited by collisions of the second kind with vibrationally excited nitrogen.

29. A method of detecting selected molecules of interest as claimed in Claim 13, wherein said molecules of interest are excited by photo-excitation.

30. Infrared detection means for detecting selected molecules of interest in a sample, said means comprising:

(a) a flame or plasma generating means ,

(b) means for injecting a sample into said flame or plasma to thereby excite any molecules of interest contained therein, said molecules thereby emitting a characteristic infrared radiation pattern,

(c) means for focusing said emitted infrared radiation onto an infrared radiation interferometer,

(d) computer means for conducting a fourier-transform on the output of said interferometer to detect said characteristic radiation pattern,

(e) output means responsive to said computing means for providing an output indication of presence and quantity of said selected molecules of interest.

31. Infrared detection means as claimed in Claim

30, wherein the selected molecule of interest is CO₂ and the characteristic infrared pattern includes peaks at 2.7 um and 4.3 μm.

32. Infrared detection means as claimed in Claim

31, wherein the selected molecule of interest is a hydrocarbon, said hydrocarbon being oxidized by said flame generating means to generate CO₂ as a by-product of said oxidation.

33. Infrared detection means as claimed in Claim

32, which further includes means for vaporizing a liquid sample prior to injection into said flame.

34. Infrared detection means as claimed in Claim 31, wherein said means further includes means for acidifying the sample to generate CO₂ from any calcinates or carbonates contained therein.

35. Infrared detection means as claimed in Claim 30 wherein said means for isolating, a preselected wavelength of infrared radiation comprises a three mirror assembly for focusing the emitted infrared radiation through a lens to the infrared detector means.

36. Infrared detection means as claimed in Claim

30 wherein said molecules of interest include CO₂ and H₂O and said output means provides an output ratio indication when said fourier-transform indicates characteristic radiation peaks at 4.4 um and 2.9 um.

37. A method of detecting selected molecules of interest in a sample by detecting characteristic infrared emission, said method comprising:

(a) exciting any molecules of interest in the sample to cause emission of a characteristic infrared radiation pattern,

(b) detecting said infrared radiation with an interferometer,

(c) conducting a fourier-transform on the output of the interferometer to detect said characteristic infrared radiation pattern,

(d) generating an electrical signal when said characteristic infrared pattern has been detected.

38. A method of detecting selected molecules of interest as claimed in Claim 37 wherein said exciting step includes combustion of the sample to excite CO₂ molecules formed by oxidation of a selected molecule of interest.

39. A method of detecting selected molecules of interest as claimed in Claim 37 wherein the sample is a biochemical sample and the molecules of interest are dissolved CO₂.

40. A method of detecting selected molecules of interest as claimed in Claim 37 which further includes the step of acidifying the sample to generate CO₂ from any calcinates or carbonates contained in said sample.

41. A method of detecting selected molecules of interest as claimed in Claim 40 wherein said sample of interest is a water sample.

42. A method of detecting selected molecules of interest as claimed in Claim 38 wherein said sample is a water sample.

43. A method of detecting selected molecules of interest as claimed in Claim 38 wherein said sample is an industrial effluent.

44. A method of detecting selected molecules of interest as claimed in Claim 37 which further includes the step of oxidizing the sample to generate CO₂ from any organic compounds contained in said sample.

45. A method of detecting selected molecules of interest as claimed in Claim 44 wherein said sample is a water sample.

46. A method of detecting selected molecules of interest as claimed in Claim 44 wherein said sample is an industrial effluent.

47. A method of detecting selected molecules of interest as claimed in Claim 38 wherein said sample is a halo organic compound.

48. A combined infrared and flame ionization detector for detecting selected molecules of interest in a sample, said means comprising: (a) means for introducing a sample which may contain molecules of interest into a flame to excite any molecules of interest therein,

(b) infrared detection means for detecting infrared radiation emitted by said molecules of interest when excited by said flame, said detector means generating a first electrical signal in response to said infrared radiation;

(c) first means to create a potential across the flame,

(d) second means for detecting any ion current across the flame said second means generating a second electrical signal in response to the presence of one ion current,

(e) means responsive to said first or said second electrical signal to indicate the presence of said molecules of interest.

49. A combined infrared and flame ionization detector as claimed in Claim 48 which further includes means for isolating a preselected wavelength of said infrared radiation, said wavelength being selected from a characteristic infrared radiation pattern emitted by the molecule of interest, said means being mounted between said flame and said infrared detector means.

50. A combined infrared and flame ionization detector as claimed in Claim 48 which further includes means for vaporizing a liquid sample, said means positioned to vaporize said sample into said flame.

51. A combined infrared and flame ionization detector as claimed in Claim 49 or 50 wherein said detector means is selected from a group of detectors, said group consisting of a lead selenide detector, an indium antimonide detector, and a mercury cadmium telluride detector.

52. A combined infrared and flame ionization detector as claimed in Claim 49 wherein said means for isolating a preselected wavelength is a monochromator.

53. A combined infrared and flame ionization detector as claimed in Claim 49 wherein said means for isolating a preselected wavelength is an infrared filter.

54. A combined infrared and flame ionization detector as claimed in Claim 49 wherein said means for isolating a preselected wavelength is an infrared interferometer, and said infrared detector means includes a computing means which applies a fourier-transform to the electrical signal.

55. A combined infrared and flame ionization detector as claimed in Claim 48 wherein said means for detecting the ion current is an electrometer.

56. A combined infrared and flame ionization detector as claimed in Claim 49 in which one of the molecules of interest generates a CO₂ molecule when oxidized by said flame and the characteristic infrared radiation pattern includes peaks at 4.3 jam and 2.7 jam.

57. A combined infrared and flame ionization detector as claimed in Claim 49 which further includes a first baffle means surrounding said infrared detector means and a second baffle means surrounding said combined infrared and flame ionization detector to thereby limit ambient infrared radiation received by said infrared detector means.

58. Infrared detection means for detecting a plurality of molecules of interest in a sample, said means comprising: (a) means for introducing said sample into at least a first and a second supply means,

(b) first means for introducing a portion of said sample from said first supply means into a first means for generating and exciting selected molecules of interest, said selected molecules emitting a characteristic infrared radiation pattern when excited,

(c) second means for introducing a portion of said sample from said second supply means into a second means for exciting selected molecules of interest, said molecules emitting a characteristic infrared radiation pattern when excited,

(d) first and second radiation detector means for detecting infrared radiation from said first and second means, respectively said first and said second radiation detectors generating first and second electrical signals in response to said infrared radiation,

(e) adsorber means for removing said selected molecules of interest from the sample in the first supply line.

59. Infrared detection means as claimed in Claim 58 wherein said detection includes a third supply means, a third means for exciting molecules of interest provided from said third supply means, a third infrared detector for generating a third electrical signal in response to infrared radiation emitted by said selected molecules of interest when excited by said third means and means for removing CO from said third supply means.