US3489498A

US3489498A - Flame photometric detector with improved specificity to sulfur and phosphorus

Info

Publication number: US3489498A
Application number: US506543A
Authority: US
Inventors: Samuel S Brody; John E Chaney
Original assignee: Melpar Inc
Current assignee: Melpar Inc
Priority date: 1965-11-05
Filing date: 1965-11-05
Publication date: 1970-01-13
Anticipated expiration: 1987-01-13
Also published as: BE807029Q

Description

Jan. 13, 1970 s. s. BRODY ET FLAME PHOTOMETRIC DETECTOR WITH IMPROVED SPECIFICITY TO SULFUR AND PHOSPHORUS Filed NOV. 5, 1965 2 Sheets-Sheet 1 HYDROGEN OXYGEN 22 SAMPLE EFFLUENT pHoibili'imc 'NJECTOR DETECTOR 27 NITROGEN PRECORDER l CHROMATOGRAHlC 2 F15 coLUMN L s RECORDER /-|6 I 1 IONI'ZATION 1G. 19 RECORDER '7 POWER SUPPLY EXHAUS GLASS PHOTOMULTIPLIER wmoow MIRROR 37 ,BURNER 4i HZIN FILTER 4 02 IN F162 \95 COLUMN INVENTORS EFFLUENT SAMUEL, SBR C'I JOHN BCHANEY ATTORNEYS Jan. 13, 1970 5, s, BRODY ET AL 3,489,498

FLAME PHOTOMETRIC DETECTOR WITH IMPROVED SPECIFICITY TO SULFUR AND FHOSPHORUS Filed. Nov. 5, 1965 2 Sheets-Sheet 2 FIG. 3

FILTER FOR PHOSPHORUS FLAME IONIZATION DETECTOR -70 P 58 INVENTORS "j SAMUEL s. BRODY a 57 57 JOHN E. CHANEX i=56 BY IZALL 55 ATTORNEYS United States Patent FLAME PHOTOME'I'RIC DETECTOR WITH IM- PROVED SPECIFICITY TO SULFUR AND PHOSPHORUS Samuel S. Brody, Springfield, and John E. Chaney, Vienna, Va., assignors to Melpar, Inc., Falls Church, Va., a corporation of Delaware Filed Nov. 5, 1965, Ser. No. 506,543 Int. Cl. G01j3/48;G01n 21/56, 21/58 US. Cl. 356--187 18 Claims ABSTRACT OF THE DISCLOSURE Gas analysis is performed utilizing flame photometry by burning a sample of the gas to be analyzed in a hydrogenrich flame in which combustion is supported by oxygen, restricting observable flame-induced emissions from materials in the sample tosubstantially only those Wavelengths of radiant energy emanating from a region above the cone of the flame, and selectively detecting wavelengths of radiant energy emanating from the region above the cone of the flame at the characteristic flame emission wavelength of sulfur or phosphorus, or both.

This invention relates to flame photometric detectors for gas chrom-atographs, and in particular, to a method which is specific to the detection of phosphorus and sulfur compounds present in the efliuent from a gas chromatographic column and apparatus therefor.

Heretofore, the accurate detection of phosphorus and sulfur by flame photometry in the art of gas chromatography had not been achieved due to the inability to provide a method or apparatus which had the required sensitivity and specificity to these elements. That is, flame photometric detectors of the prior art, if used for gas chromatography detection, would be responsive not only to the spectral emission of the burning phosphorus and sulfur compounds, but would respond also to the burning organic compounds and the hydrogen which might be used as fuel for combustion. Thus, the output of such a photometer could not provide a readout which could quantitatively analyze the amount of phosphorus or sulfur eluted from the column, since any response attributable thereto would be masked by the presence of the other compounds and the resulting noise.

Accordingly, it has been the general practice to use the technique of thermionic flame emission for the detection of phosphorus, which method however, is not highly specific thereto, and is responsive to the presence of halogenated compounds as well. Since sulfur compounds cannot be detected by the themionic emission technique, pyrolysis to H S or S0 is sometimes utilized in conjunction with a microcoulometric procedure for sulfur detection. Microcoulometry, however, is subject to interference and noise from the presence of other compounds and is consequently not as specific to sulfur as is desired. Likewise, microcoulometry might be applied for the detection of phosphorus compounds, however, its sensitivity to these compounds is less than its sensitivity to sulfur compounds.

The present invention provides an accurate and reliable method of detecting the presence of phosphorus and sulfur compounds in the efiluent from a gas chromatographic column by utilizing flame photometry and apparatus therefor which is rugged, accurate, and compact.

A carrier gas comprising nitrogen is fed through a sample injector apparatus. At a particular time, a quantity of the sample to be analyzed is injected into the carrier gas by the sample injector. The sample is separated as it moves through a gas chromatographic column and the 3,489,498 Patented Jan; 13, 1970 eflluent therefrom is fed to a single burner of a flamephotometer constructed in accordance with the present invention.

The column effluent, including the nitrogen carrier gas, is mixed with oxygen prior to reaching the burner, and hydrogen is then mixed therewith in the burner tip. The resulting gas mixture is hydrogen rich, i.e., in that the amount of hydrogen added is more than suflicient for complete combustion with the amount of oxygen present.

The total gas mixture is ignited at the burner tip, preferably utilizing an electrically heatable nichrome wire or other suitable heating element.

The flame and light emission qualities thereof thus produced are of a significant nature with respect to the construction of the present invention. Specifically, it is found that such a gas mixture produces light emission having a color which is characteristic of either sulfur or phosphorus or both, and that such light emission takes place primarily in the uppermost portion of the flame, above the inner cone. The lower portions of the flame are characteristic of the normal hydrogen-air flame. The characteristic colors produced by sulfur and phosphorus are blue and green, respectively, in a hydrogen-rich air supported flame,

The characteristic emission wavelengths of light most suitable for the purpose of identification of sulfur and phosporus have been found to be 394 and 526 millimicrons, respectively. Since there may be other compounds present, such as CO and hydrocarbons, generally, which have a broad band emission spectrum and thus also emit at both the 526 and 394 millimicron wavelengths, the total light emission from the flame is derived from all of these compounds. However, since it has been found that the emissions from compounds other than those containing sulfur and phosphorus take place almost exclusively in the lower portion of the flame, two features are provided, in accordance with the present invention, to obtain the required specificity and sensitivity to the sulfur and phosporus compounds and the quantitative determinations thereof.

First, there are provided two narrow band optical filters, the optical axis of each passing through the uppermost portion of the flame. One filter is transmissive to only light having a wavelength of substantially 526 millimicrons while the other is transmissive to only light of substantially 394 millimicrons. Responsive to the transmitted band from each respective filter is a photodetector, such as a photomultiplier tube or other suitable photoelectric or light sensitive device.

The electrical output of each photodetector is connected to an appropriate recording device for providing a graphic quantitative presentation of the presence of sulfur and/ or phosphorus in the sample.

The results so far discussed are materially improved in accuracy by a second feature in accordance with the invention which obviates deviations due to the aforementioned emission produced by the normal portion of the flame, and other flame components which have broad band characteristics and which would normally reach the photodetectors. In particular, this second feature includes the provision of a burner tip incorporating a shielding means for preventing the photodetectors from viewing anything but the upper portion of the flame. This, then provides for high specificity and for low background flame noise in the output of the photodetectors.

Additionally, a probe may be inserted above the burner tip and a potential applied between the probe and the tip in order to detect the ionization of the flame for a determination of the presence of all organic compounds, in a manner known to the art. Further, detection of thermionic flameemission may also be optionally provided by the use of an additional burner in the exhaust stream for a determination of the presence of phosphorus and halogen compounds. Thus, three analyses might be conducted concurrently, or separately, from a single burner flame fed from a suitable sample source, in addition to a fourth analysis utilizing an afterburner.

7 Thus, it is an object of the present invention to provide a method of and apparatus for obtaining quantitative results from a gas chromatographic column, where such method and apparatus is highly specific for, and highly sensitive to, sulfur and phosphorus compounds.

It is another object of the present invention to provide a means whereby sulfur and phosphorus compounds present in the effluent of a gas chromatographic column may be detected by utilizing flame photometry.

It is still another object of the present invention to provide a single unitary device for gas chromatographic detection having the capability of a multiplicity of simultaneous readouts by providing means for detecting hydrocarbons by flame ionization, and means for detecting sulfur and phosphorus compounds by flame-photometry, in addition to means for detecting phosphorus and halogen compounds by thermionic flame emission.

It is a further object of the present invention to accomplish the immediately aforesaid object by merely utilizing a single burner and without the necessity of splitting, or separating the effluent from the gas chromatographic column.

It is still a further object of the present invention to detect sulfur and phosphorus compounds by means of flame-photometry performed by utilizing a novel burner tip which enhances the specificity of the response to the above-named compounds.

Still another object of the present invention is the detection of sulfur compounds and phosphorus compounds by flame-photometry utilizing a hydrogen-rich flame.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a diagrammatic illustration of a gas chromatograph utilizing flame detection in accordance with the present invention;

FIGURE 2 is an elevational view, in section, of the flame-photometer in accordance with the present invention;

FIGURE 3 is a top view, in section, of the flame-photometer in accordance with the present invention; and

FIGURES 4a and 4b illustrate the top view and elevational cross-section, respectively, of the flame-photometer burner tip in accordance with the present invention.

Referring now to FIGURE 1 which shows in diagrammatic form a gas chromatograph with the flame photometric detector in accordance with the present invention, a supply of nitrogen carrier gas passes through the sample injector 11 and then through the chromatographic column 12 via

lines

24 and 22. At a particular time, a quantity of sample containing phosphorus and/ or sulfur compounds, as well as possibly halogen compounds is injected into the nitrogen carrier gas at the sample injector 1'1 and the components are separated as the sample moves through the column. These components, together with the nitrogen, are eluted from the column at 23 and flow to the flame photometric detector 13 where, the effluent is mixed with hydrogen 25 and oxygen 26 to produce a hydrogen-rich mixture. The hydrogen functions as fuel for the burner flame while the oxygen is provided to support the combustion. Although the supply of gas 26 has been referred to as oxygen, it is understood that a normal air supply may be substituted therefor and, as used herein, reference to oxygen contemplates the substitution of a supply of air. For detection of phosphorus and sulfur compounds, in accordance with the method of the present invention, it is found that for detection of the former it is necessary to maintain the gas mixture hydrogen-rich although for detection of the latter, the mixture can be maintained oxygen rich. The general combustion products, including water vapor, are exhausted at 27.

A plurality of independent electrical outputs are provided from the detector 13 which indicate the time required for each component to pass through the column 12 and also provide a quantitative measure of each component. The detector outputs 19, 20 and 21 are each electrically connected to recording

devices

16, 15, and 14, respectively, which recorders may be of any commercially available type. Each recorder and the detector 13 is provided with an appropriate electrical power source from power supply 17 through lead 18.

Referring now to FIGURES 2 and 3 which show the detailed construction of the detector 13 in order to illustrate a specific embodiment of the present invention, the column efiluent including the carrier gas passes through the entrance tube 45 where it is mixed with oxygen and/ or air from line 46. The mixture then flows to the center bore of the burner tip 42 where the effluentoxygen mixture is mixed with hydrogen supplied through four spaced circumferential ports, as will be more fully explained hereinafter. The gas mixture is electrically heated by a Nichrome resistance wire 40 which forms part of the igniter 39. Insulated lead 52 provides the electrical potential to one end of the heater wire 40, the other end being connected to the metal housing which is maintained at ground potential. The hydrogen-air flame heats the metal burner housing 36 and the exhaust gases and water vapor from combustion leave the burner assembly through common exhaust tube 48. The burner housing 36 is heated to a sufliciently high temperature by the flame that no condensation is formed thereon, thus obviating any problem of condensate drainage.

As shown in FIGURES 4a and 4b, the burner tip is constructed in such manner that the effluent-oxygen gas mixture flows up through the bore 55. The four ports 57 surrounding the center bore supply hydrogen for combustion which is mixed with the center-bore flow at 70. The flange 58 is adapted to provide a gas-tight seal to the main burner structure, shown in FIGURE 2. The hydrogen thus flows from entrance port 47, between the concentric cylinder walls 56 and the outer wall of the burner to the four ports 57 in the burner tip.

Shield 59 is provided about the combustion chamber depression at 70. The shield 59 extends about 0.2 inch from the upper surface of flange 58 and functions to substantially block the light emission from all but the uppermost portion of the flame. Reflecting baffles 60 are provided throughout the entire inner surface of the shield structure 59 and function to effectively'reflect the light emitted from the pilot flame toward the combustion chamber depression and away from the open end of the shield.

Referring back to FIGURE 2, a photodetector is shown comprising a photomultiplier tube 28, an optical filter 30, and a glass window 31 which serves as a heat filter. Each element of the photodetector is located on an optical axis viewing only the very upper portion of the flame and for all practical purposes the flame, itself, is not viewable by the photomultiplier tube 28.

The glass window 31 tends to block the long wavelength heat radiations, while the optical filter 30 transmits only the optical spectral band of interest. Assuming that where the photodetector is to be responsive to only sulfur compounds, filter 30 should be capable of selectively transmitting only a narrow band of wavelengths about 394 millimicrons, whereas if phosphorus compounds were to be detected the filter should pass wavelengths at or near approximately 526 millimicrons.

The filter 30 and window 31 are supported by a metal collar arrangement comprising inner and outer annular members 32 and 43. The collar 32 is arranged to fit at one end Within a circular bore in the burner housing 36, and at the other end within the outer member 43. The viewing aperture of the photomultiplier tube 28 is seated within the annulus of member 43 with the remainder of the tube being supported in a shock absorbing material 49 such as rubber, or other suitable material, encased within the outer metal housing 50. As shown in FIGURE 2, gasket rings are provided between the inner annular member 32 and the outer member 43 as well as between member 32 and the burner housing 36. Spacers 44 are provided for maintaining the filter 30 and the window 31 in a fixed position.

Also, centered on the optical axis of the photodetector is mirror 37 which serves to increase the effective illumination to the photodetector by reflection of the light rays thereto resulting in an increase in the sensitivity of the system.

Although the photodetector is shown as a photomultiplier tube, a solar cell or other photoelectric device might be used together with the appropriate electronic circuitry therefor.

In the embodiment of FIGURE 3, a dual detector as shown in which a second photodetector and mirror assembly is provided at right angles to the first detector for detection of the phosphorus compounds contained in the sample, continuing with the assumption that the first detector is to be used for detection of sulfur compounds. The structure of the phosphorus detector is similar to that of the sulfur detector already described. The glass window 34 for decreasing the heat transmission to the photodetector is similar to window 31 and the optical filter 33 is similar to the filter 30 with the exception that it is designed to only transmit light of a narrow band of wavelengths about a wavelength of 526 millimicrons, rather than 394 millimicrons. The photomultiplier tube 29 is identical to the tube 28. The leads 53 and 54 of the sulfur and phosphorus detectors, respectively, correspond to the

leads

20 and 21 in the system diagram of FIG- URE 1. As there shown, the output leads 53 and 54 are respectively connected to the

recorders

15 and 14. Mirror 38 coacts with the photomultiplier tube 29 and the phosphorus photodetector generally, in exactly the same manner as mirror 37 cooperates with similar components of the sulfur detection system. The single probe 62 (only shown in FIGURE 3), which extends above the flame is insulated from the burner housing 36 by means of an insulating sleeve 35. The probe 62 is electrically connected in series circuit relationship to a potential source, an amplifier and recording device 16 whereby a potential is applied across probe 62 and the burner tip 42. The amplifier may, of course, form a part of the recorder 16. The burner tip 42 is maintained at ground potential as is the housing 36. Probe 62 is preferably disposed concentric with the burner flame, although it is shown in an oflset position in FIGURE 3 for the purpose of illustration.

In the operation of the detector, the column efiluent and oxygen mixture entering the center bore 55 of the burner and mixing with the fuel entering through ports 57 burns substantially entirely within shield 59. The light emission characteristic of the presence of phosphorus and/ or sulfur would be present at a distance above the shield 59. That is, in the event of the presence of phosphorus or sulfur, or compounds thereof, the flame induced emission is physically above the burner tip, in-line with the optical axis of the photodetectors. The shield 59 prevents the photodetector from viewing the normal hydrogen-air flame, which as previously discussed, increases the specificity and reduces the flame background noise in the photomultiplier tube outputs. The presence of the light baflies 60 also provide an increase in the specificity because of the reduction in the available light emission not characteristic of phosphorous and sulfur compounds, as well as for the reasons aforementioned. Compounds such as carbon dioxide and hydrocarbon compounds may emit at both 526 mg and 394- m however, these emissions take place almost exclusively in the portion of the flame hidden from the photodetector, whereas the phosphorous and sulfur emissions may be in the hidden portion of the flame as well as above the hidden pilot flame in-line with the optical axis. It has been found, for example, that the specificity to phosphorus obtained with the present invention is 20,000 to 50,000/ 1 in the presence of chlorinated compounds, aromatic ketones, aromatic-aliphatic esters, and organonitrogen compounds.

The flame ionization detector operates in a manner well known in the art. That is, the presence of organic compounds, hydrocarbons for example, changes the conductivity of the flame. Since the electrode 62 is in electrical contact with one portion of the flame, the other portion being in contact with the burner tip, the resistance between these two points is a measure of the ionization of the flame and hence, the corresponding amount of organic compounds present. This measurement may be provided in permanent recorded form by the utilization of an electronic recording device 16 such as is well known in the art. It has been found, however, that by using a hydrogenrich flame for ionization detection, the absolute response to the presence of halogenated compounds is enhanced while the response to the presence of non-halogenated compounds remains unchanged as compared to conventional ionization detection utilizing an oxygen-rich flame. Thus, in accordance with another feature of the invention, the sensitivity to halogenated compounds is increased by using a hydrogen-rich flame in conjunction with the flame ionization detector.

Thus, the detector, in accordance with the present invention, may provide three analyses concurrently or any one separately, as shown in the system of FIGURE 1. Photomultiplier tube 28, being responsive to the presence of sulfur in the column eflluent, produces a record in recorder 15 while the photomultiplier tube 29, being responsive to the presence of phosphorus in the column efiiuent, produces a record in the recorder 14. The measurement or the organic, or hydrocarbon, component content is measured, or may be measured, with the flame ionization detector producing a recorded output in recorder 16.

Additionally, thermionic emission might be detected by the inclusion of an additional electrode near the flame of an after-burner, as previously described, where said electrode is coated with a sodium salt, such thermionic emission detection being known in the art.

The particular burner construction may have a variety of different physical arrangements of orifice sizes or dimensions, etc. However, it is of significance with respect to the present invention to shield the pilot flame from the photodetector to prevent the same from responding to the spectral emission therefrom.

We claim:

1. A flame photometric detector comprising burner means;

means for supplying a sample to be identified, a fuel gas, and a combustion supporting gas to said burner means wherein said gases are mixed and a flame is produced therefrom;

photodetector means having an optical axis in line with the uppermost portion of said flame; said photodetector means responsive to a given wavelength of light emission from the uppermost portion of said flame, said given wavelength of light being characteristic of the presence of an element selected from the group consisting of phosphorus and sulfur;

said burner means comprising a shielding means for preventing the light emission from all but the uppermost portion of the flame from entering the optical path of said photodetector means, such that light emitted at said given wavelength by elements other than said selected element is not detected.

2. The apparatus of claim 1 wherein said shielding 7 means includes means for reflecting the light emission from all but the uppermost portion of the flame in a direction opposite to said uppermost portion.

3. The apparatus of claim 1 further comprising another photodetector means having an optical axis in line with said uppermost portion of said flame;

said another photodetector means responsive to another given wavelength of light emission from said flame;

said another given wavelength of light being characteristic of the presence of the other elements from said group whereby both elements of said group may be detected concurrently.

4. The apparatus of claim 1 wherein said photodetector comprises a photoelectric device and an optical filter along said optical axis, said optical filter selectively transmitting substantially only the emitted light at said given wavelength.

5. The apparatus of claim 1 further comprising means for reflecting the light emission from the uppermost portion of said fiame toward said photodetector means along said optical axis whereby the sensitivity to said element is increased.

6. A flame photometric detector for gas chromatography comprising burner means;

means for supplying column effluent, a fuel gas, and

a combustion supporting gas to said burner means wherein said gases are mixed and a flame is produced therefrom;

photodetector means only responsive to a given wavelength of light emission from the uppermost portion of said flame characteristic of the presence of an element selected from the group consisting of phosphorus and sulfur; and

means including an opaque shield about the cone of said flame, for preventing the light emission at said given wavelength by elements other than said selected element from being detected by said photodetector.

7. The method of detecting phosphorus and sulfur compounds is gas chromatography comprising the steps of injecting the sample to be identified into a stream of carrier gas;

passing the sample and carrier gas through a chromatographic column for separating the sample into its components;

mixing the effluent from the column with hydrogen and oxygen, burning the resulting gas mixture producing a flame therefrom;

detecting the presence of a given wavelength of light emission from said flame characteristic of an element selected from the group consisting of phosphorous and sulfur; and

shielding said flame such that such light emissions at said given wavelength caused by elements other than said selected element are substantially restricted to a portion of the flame from which detection is prevented, whereby the specificity of said method to said selected element is increased.

8. The method of claim 7 wherein said step of shielding includes reflecting the light emission caused by said other element in a direction opposite to the upper most portion of said flame.

9. The apparatus according to claim 1 further comprising means for measuring the ionization characteristic of said flame wherein said flame is burned hydrogen-rich 1-0. A flame photometric detector having high specificity to phosphorus and sulfur, said detector comprising:

a burner,

means for supplying a sample to be tested for presence of sulfur and/or phosphorus to said burner,

means for mixing hydrogen gas and oxygen gas with the sample to be supplied to said burner wherein the amount of hydrogen gas is in excess of that required for complete combustion with the a ount f oxygen gas present such that a hydrogen-rich flame may be produced,

means associated with said burner for igniting said mixture to produce said flame at said burner,

a shield associated with said burner and encompassing at least the lower portion of said flame to inhibit flame-induced emissions occurring within said shielded portion of said flame, including emissions at wavelengths about the respective emission Wavelengths of sulfur and phosphorus, from emanating beyond said shielded portion of said flame, and

photometric detection means for selectively detecting one or more of said emission wavelengths emanating from a region above the shielded portion of said flame.

11. The invention according to claim 10 wherein said detection means includes a first photometric detector for phosphorus and a second photometric detector for sulfur.

12. The invention according to claim 11 wherein each of said detectors has an optical axis oriented for viewing the same region above said shielded portion of said flame.

13. In a gas analyzer utilizing flame photometry,

means for burning a sample of the gas to be analyzed in a hydrogen-rich flame in which combustion is supported by oxygen,

means associated with said burning means for restricting observable flame-induced emissions from materials in said sample to substantially only those wavelengths of radiant energy emanating from a region above the cone of said flame, and

photometric detector means for selectively detecting wavelengths of radiant energy emanating from said region at the characteristic wavelength of an element selected from the group consisting of sulfur and phosphorus.

14. The invention according to claim 13 wherein said region under observation for detection of said wavelengths is the same for both sulfur and phosphorus.

15. In a method of analyzing gas by use of flame photometry,

burning a sample of the gas to be analyzed in a hydrogen-rich flame in which combustion is supported by oxygen,

shielding at least the lower portion of the flame from direct observation to restrict observable flame-induced emissions from materials in the sample to substantially only those wavelengths of radiant energy emitted from a region above the cone of the flame, and

selectively detecting emissions at a wavelength corresponding to the emission wavelength of one of the elements from the group consisting of sulfur and phosphorus, from said region above said cone.

16. The method according to claim 15 wherein is included concurrently and selectively detecting emissions at wavelengths corresponding to the emission wavelengths of both sulfur and phosphorus.

17. The method according to claim 15 wherein said shielding includes reflecting the radiant energy emitted from materials within the cone of the flame back toward said cone.

18. A flame photometer for concurrent detection of sulfur and of phosphorus or of compounds of sulfur and of phosphorus in a sample of a medium under test, comprising:

burner means,

means for supplying said sample together with a fuel gas and combustion-supporting gas in mixture to said burner means,

ignition means associated with said burner means for igniting said mixture to produce a flame therefrom, shielding means further associated with said burner means for restricting observation of said flame along optical paths transverse to the axis of said flame to a region above the cone of said flame and for suppressing radiant energy emissions from the cone of said flame into said region, whereby to substantially prevent observation of emissions from other materials that may be present in said mixture at wavelengths interfering with the sulfur and phosphorus emission wavelengths to be detected along one or more of said optical paths; and

first and second photodetector means optically aligned with separate ones of said optical paths for selectively detecting light at said emission wavelengths of sulfur and phosphorus, respectively, emanating from said region.

References Cited UNITED STATES PATENTS 1 0 3,211,050 10/1965 Pelavin. 3,213,747 10/1965 Vander Smissen.

OTHER REFERENCES A Modulator for the Automatic Subtraction of Continuous Background in Optical Spectrometers, G. M. Svishchev, Optics & Spectroscopy, v01. XVI, N0. 2, pp. 184-186, February 1964.

Specific Detection of Halogens and Phosphorus by Flame Ionization, A. Karmen, Analytical Chemistry, vol. 36, No. 8, pp. 1416-1421, July 1964.

A Sensitive For Flame Emission R. N. Kniseley, Analytical Chemistry, vol. 35, No. 7, pp. 910- 911, June 1963.

RONALD L. WIBERT, Primary Examiner R. J. WEBSTER, Assistant Examiner U.S. Cl. X.R.