US3607096A - Alkali flame ionization detector having cap means for changing the gas flow pattern - Google Patents

Abstract

A hollow flame jet is supported on a base member and surrounded by a cylindrical chimney. The outlet end of a chromatographic column extends into a mixing chamber in the base member where the effluent vapors from the column are mixed with hydrogen. This mixture issues from the flame jet and is burned in the presence of an alkali salt contained in a cup surrounding the tip of the jet. Air to support this combustion enters the chimney at the base of the flame jet. A pair of electrodes positioned near the jet are connected to a fixed voltage power supply and electrometer circuit in order to cause a current to flow in the interelectrode space and to measure this current. The magnitude of this current is proportional to the density of ions in the interelectrode space. Since nitrogen phosphorus-, sulfur-, and chlorine-containing hydrocarbon compounds produce relatively high currents compared with other hydrocarbon compounds, they can be reliably detected and quantified in the effluent stream.

Description

United States Patent [72] Inventor Charles H. Hartmann Primary Examiner-Morris O. Wolk 1048 Sanders Drive, Moraga, Calif. 94556 Assistant Examiner-R. M. Reese [2]] App]. No. 836,350 Attorneys-Stanley Z. Cole and Leon F. Herbert [22] Filed June 25, 1969 [45] Patented Sept. 21, 1971 ABSTRACT: A hollow flame jet is supported on a base member and surrounded by a cylindrical chimney. The outlet end of a chromatographic column extends into a mixing [54] ALKALI FLAME IONIZATION DETECTOR chamber in the base member where the effluent vapors from HAVING CAP MEANS FOR CHANGING THE GAS the column are mixed with hydrogen. This mixture issues from FLOW PATTERN the flame jet and is burned in the presence of an alkali salt 5 claimsz Drawing Figs contained in a cup surrounding the tip of the jet. Air to support this combustion enters the chimney at the base of the [52] US. Cl 23/254 E, flame jet- A pair of electrodes positioned near the jet are com 23/232 C nected to a fixed voltage power supply and electrometer cir- [51 Int. Cl ..G0ln 31/12 cuit in Order to cause a curl-em to flow in the interelecn-ode [50] Field of Search 23/254, Space and to measure this curl-em The magnitude f this cur 254 1 ,2 rent is proportional to the density ofions in the interelectrode space. Since nitrogen phosphorus-, sulfur-, andchlorine-con- [56] Reerences cued taining hydrocarbon compounds produce relatively high cur- UNITED STATES PATENTS rents compared with other hydrocarbon compounds, they can 3,423,181 1/1969 Dimick et al. 23/254 be reliabl detected and uantifled in the effluent stream. E 3' q RRRR 65 souRcE POWER SUPPLY A 65 ATTENUATOR 59 ,7

I RANGE RECORDER 57 55 I SWITCH 69 7| -55 I 15 T 42 54 ll HYDROGEN m PATENTED sERzT RN CURRENT SOURCE BUCKTNG R 2 FIG. 2 j? 28 59 55 57 He=|9m|/min 25 go "L I I 3: 6- n, 83. l/ I I l5 5 L T% 42 54 N E HYDROGEN IN g BACKGROUND ouRRENT Ys Ns TNNYTAzoBENzENa g RESPONSE CHARLES T'TT YT KRRNRNN n p B HYDROGEN FLOW m|/min vim 7-7 W ATTORNEY ALKALI FLAME IONIZATION DETECTOR HAVING CAP MEANS FOR CHANGING THE GAS FLOW PATTERN Unlike earlier detectors which permitted virtually unrestricted flow of air and combustion products out the top of the chimney, the detector described is provided with an airtight cap. Thus air entering the base of the chimney, instead of sweeping upwardly in a large amounts past the combustion region, forms a relatively static pool around the lower end of the flame jet. From this pool air sufficient to support com bustion diffuses upwardly to the combustion region where it is consumed. The products of combustion together with air in excess of that needed for combustion exits at the unsealed joint around the base of the chimney.

BACKGROUND OF THE INVENTION The present invention relates generally to thermionic detectors used in gas chromatographic analysis and in particular to such detectors equipped with a source of alkali salt adjacent the combustion region. Such detectors, known as alkali flame ionization detectors or AFID's, are especially useful in determining the presence and concentration of phosphorous, nitrogen-, sulfur-and chlorine-containing hydrocarbon compounds in the effluent stream from a gas chromatographic column because they exhibit enchanced sensitivity to these compounds as compared to other hydrocarbons present in the effluent stream. For a discussion of such detectors reference may be made to the following: U.S. Pat. No. 3,423,181, issued Jan. 21, 1969, an article in Nature," 201, 1,204 (Mar. 21, 1964) by Karmen and Guiffrida, article in Analytical Chemistry, 36 1,416 (July 1964) by Karmen.

In general an AF ID detector provides means for mixing the effluent gases from a chromatographic column with a combustible gas such as hydrogen and burning the mixture in the presence of one of the salts of an alkali or alkaline earth metal. A pair of electrodes disposed in the vicinity of the flame are connected to a constant voltage DC source and the ionic current flowing between the electrodes is measured with an electrometer and plotted as a function of time. The flame jet and electrodes are typically enclosed within a cylindrical tower or chimney which shields the combustion region from the ambient atmosphere. Air to support combustion is introduced under pressure at the base of the chimney and flows upwardly to the combustion region and out the top which is typically partially closed by a loosely fitted lid which restricts the flow of ambient air to the combustion region.

For reasons that are not clearly understood the products of combustion of nitrogen-, phosphorous-, sulfur-, and chlorinecontaining hydrocarbon compounds cause a dramatic change in the ionic density and hence in the current in the interelectrode space. Typically the current increases in the presence of one of these compounds. However sulfur and chlorine sometimes cause a decrease. The alkali flame ionization detector (AFID) exhibits a markedly enhanced sensitivity to the nitrogen-, phosphorus-, sulfurand chlorine-containing hydrocarbon compounds in comparison with the sensitivity to other hydrocarbon compounds. Thus, in theory, very small concentrations of these compounds can be measured in the effluent of the chromatographic column even when the effluent contains substantial concentrations of other hydrocarbon compounds.

Unfortunately the sensitivities which have been realized in practice have not been as high as expected largely because of excessive noise. In addition the background current, which is the current existing under quiescent conditions when no sample is present, has been excessively high when the air and hydrogen flows were properly adjusted to give adequate sensitivity and selectivity for the hydrocarbon compounds of interest.

Since it appeared that the flow pattern of air within the detector chimney might be responsible for some of the noise encountered with prior art detectors work on controlling the flow pattern was commenced, resulting in the present invention.

SUMMARY OF THE INVENTION According to the present invention the flow of air within the detector can be controlled in such a way as to reduce background current and noise while enhancing sensitivity to the compounds under investigation by providing a removable sealed top for the chimney and an exhaust passage for the products of combustion and for excess air at the base of the chimney. Thus in operation the air instead of flowing in quantity past the combustion region and out the top, diffuses from the base region of the chimney into the combustion region only in sufficient quantities to be virtually consumed by the flame. Any excess air is exhausted together with combustion products through the exhaust passage near the base of the chimney. The result is that the deleterious effects of the unrestricted upward flow of air in the prior art detectors are substantially eliminated.

The principal object of the present invention is to provide an improved alkali flame ionization detector having enhanced sensitivity to organic vapors of compounds of interest while substantially reducing background current and noise.

Another object is to control the flow of air within the chimney of such a detector in such a way that air enters the combustion region only in sufficient quantity to support combustion.

Another object of the present invention is to provide such a detector in which air in excess of that needed for combustion is exhausted from the chimney via a passage which is located so that currents of air adjacent the combustion region are eliminated.

A further object is to provide an alkali flame ionization de' tector in which air sufficient for combustion is transported to the combustion region solely by the forces of gaseous diffusion.

These and other objects, features and advantages of the present invention will become more apparent upon reading the following detailed description and examining the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of an improved AFID incorporating the means for altering the flow pattern in accordance with the present invention; and

FIG. 2 is a graph illustrating the effect of hydrogen flow on the performance of the AFID.

In FIG. 1, the improved AFID 1 according to the present invention includes a base 3, chimney 5 and a cap 7. The base 3 and chimney 5 may be made of stainless steel while the cap 7 may be of aluminum, for example. Each of these parts may be cylindrical about a vertical axis in the drawing.

Mounted on base 3 is a jet assembly 9 from which emerge the gases to be burned. A connector assembly 11 provides a removable sealed coupling means for connecting the outlet end portion of a gas chromatograph column 13 in gas flow communication with jet assembly 9. A collector electrode assembly 15 and an igniter-polarizing voltage electrode assembly 17 project through the wall of chimney 5 into the interior thereof.

Jet assembly 9 comprises a hollow jet 19 which has near the lower end a conical sealing plug 21 which is joined to jet 19 as by swagging for example. Plug 21 is received in a similarly conical recess in base 3 and forced into good sealing engagement with base 3 by an externally threaded clamping nut 23. When nut 23 has been tightened there still remains an annular recess 25 which interconnects an air flow passage 27 and a series of bores 28 equispaced around the axis of nut 23. Passage 27 is connected to a source of air under pressure (not shown) in order to supply air to the interior of chimney 5 for combustion.

A salt cup 29 is mounted on the end of jet 19 and stored in place for example by brazing. A duct 31 is aligned with the central bore in jet 19. A salt of an alkali metal or alkaline earth metal 33, rubidium sulfate (Rb S0,) for example, is pressed into an annular recess in cup 29 under high pressure to form a solid block surrounding duct 31.

Chromatographic column 13 projects into a mixing chamber 34 and is sealingly supported concentrically therein by an internally threaded sleeve 35 and a conical plug 37. A cylindrical adapter 39 is received within a counterbored recess in base 3 and may be sealed to base 3 by swagging for example. A hydrogen flow passage 41 interconnects mixing chamber 34 and a source of hydrogen gas under pressure (not shown). A short elbow-shaped duct 42 interconnects mixing chamber 34 and hollow jet l9.

Electrode assembly 17 comprises an igniter electrode 43 in the form of a coiled resistance heater element disposed adjacent the salt cup 29, a pair of insulators 45 which enclose the leads for electrode 43, a shell 47 which is fastened within an aperture in chimney for example by brazing, and a two-terminal coaxial connector 49 which is insulatingly mounted in the shell 47.

Collector electrode assembly is similar except that only a single insulator and lead are needed to connect to a ringshaped collector electrode 51 disposed above the jet assembly 9. Although electrode 51 is actually disposed with the plane of the ring approximately perpendicular to the axis of symmetry of the chimney 5, it has been illustrated in the drawing as if twisted about the axis of its electrical lead in order to display the circular configuration of the electrode.

Cap 7 comprises a plug 53 dimensioned to fit loosely within the bore of chimney 5 and having an O-ring 55 disposed in a circumferential groove in plug 53 and providing a seal within chimney 5. A rod 57 disposed, for example by press fitting, within a bore in plug 53 and a knob 59 of phenolic for example, on the end of rod 57 form a convenient handle for removal ofthe plug which becomes very hot in use.

In operation gaseous samples emerge serially from column 13 into mixing chamber 34. A controlled flow of hydrogen also enters chamber 34 as previously noted and flows into the annular space surrounding the portion of column 13 within chamber 34. The hydrogen then flows axially within this annular space to the end of column 13 as shown by the arrows where it mixes with the effluent gaseous samples from column 13. The mixture then flows through elbow-shaped duct 42, into jet 19 and emerges from the duct 31 in salt cup 29.

Air to support the combustion of the gaseous mixture enters through air flow passage 27, flows into annular recess and upwardly through bores 28. The air then emerges from the bores 28 as shown by the arrows into the annular space surrounding jet 19.

With cap 7 removed to prevent explosion of the mixture of air and hydrogen, igniter electrode 43 is momentarily energized by a low voltage established between the leads from a power supply 61 to cause electrode 43 to glow producing ignition of the combustible mixture. At this point all of the air entering chimney 5 flows upwardly past the combustion region and out the open top of chimney 5 together with combustion products as in the prior art detectors such as the one shown in the aforementioned US. Pat. No. 3,423,181.

With combustion underway the low voltage is removed from the leads to element 43 and cap 7 is replaced, sealing the open end of chimney 7. At this point the flow pattern of air and combustion products changes radically. Since the top end of chimney 5 is this all gases must leave the detector via the unsealed joint between chimney 5 and base 3. The mating surfaces at this point are not microscopically fiat and chimney 5 may be held in place only by its own weight or, if desired, under light pressure from a clamping device (not shown) such that small interconnected voids inevitably exist at the interface between chimney 5 and base 3. These voids together comprise a radial flow passage of relatively small cross-sectional area around the base of chimney 5 through which combustion products and air in excess of that necessary for combustion are vented to the outside atmosphere. Alternatively, one or more small passages could be formed at the base of the chimney 5. If this were done, base 3 and chimney 5 could be formed as one piece, while such passages could be used to vary gas conductance by constricting or closing one or more of them. Typically it has been found that a small pressure on the order of 0.1-0.2 inches of water exists within the interior of AFID l in operation due to the constricting effect of the small area radial flow passage described in the preceding paragraph. A pool or static mass of air is believed to exist in the regions surrounding the top of nut 23 such that as air under pressure enters this pool through bores 28, some air leaves through the above-described radial flow passage while the remaining air travels upwardly to the combustion zone above salt cup 29. As air is consumed by the combustion of gases, the partial pressure of oxygen in the region of the flame is reduced so that more air diffuses into the combustion zone from the pool of air below as shown by the arrows in the drawing.

As the mixture of hydrogen gas and the column effluent gases is burned, the salt in salt cup 29 is heated. The effluent vapors of hydrocarbon compounds burning in the presence of this heated salt mass form a conductive plasma of ions in the interelectrode space between electrodes 43 and 51. The presence of these positive and negative ions in the interelectrode space lowers the resistance of this space. A polarizing voltage of, for example, 300 volts DC is applied in the series circuit loop which includes power supply 61, the input of an electrometer amplifier 63, collector electrode 51, igniterpolarizing electrode 43 and the associated leads, with the igniter-polarizing electrode typically being negatively polarized, and the collector electrode being at or near ground. The current which flows in the circuit is proportional to the density of ions in the interelectrode space.

When the effluent gas from column 13 consists ofonly pure carrier gas without any of the gaseous components to be detected, a relatively steady background current exists in the series circuit. Since this background current contains no information about the samples of interest it is desirable to, in effect, subtract it from the current to be measured in amplifier 63 and the remaining circuitry. For this purpose a bucking current source 65 passes an adjustable current through the input of amplifier 63 in opposition to the main current flowing from the interelectrode space. Source 65 can be adjusted to null any background current so that only current indicative of a sample gas will be registered.

After amplification in amplifier 63 the signal output is connected, through an attenuator 67, to a recorder 69 which is typically of the strip chart type. A range switch 71, which may consist of a bank of selectable feedback resistors, is connected from output to input of amplifier 63 in order to adjust the amplifier again.

Although a particular electrometer circuit has been discussed in the foregoing, it should be understood that other electrometers could be used as well without affecting the performance of the AFID. See, for example, Littlewood, Gas Chromatography, pages 289-292, Academic Press, 1962.

Turning now to FIG. 2 a graph summarizing performance factors and illustrating the optimization of operating conditions is shown. The AFID used to 12 this data was as shown in FIG. 1 and had the following approximate dimensions: Chimney 5 height2 inches; inside noise, of chimney 5-seveneighths inch; height of collector electrode 51 above top of salt cup 29five-eighths inch; loop diameter of collector electrode 5l--one-fourth inch; distance from top of salt cup 29 to top of base 3-eleven-sixteenths inch. The ignitor electrode 43 was held at a potential of 300 volts below collector electrode 51 potential and the salt used in salt cup 29 was Rb S0,. The carrier gas flowing through column 13 was helium.

As noted in FIG. 2, an air flow of 230 ml./min. was chosen since investigations have shown that flow rates between 200 and 270 ml./min. yield optimum results with this detector. Then the hydrogen flow was adjusted over a range of to 20 ml./min. starting at 85 ml./min.

The solid line of FIG. 2 indicates that background current reached a maximum of somewhat more than 5X10 amps at approximately 43 ml./min. of hydrogen. Since high background currents correspond to rapid consumption of alkali salt and to excessive noise, operation of the detector at 43 ml./min. of hydrogen is plainly undesirable in this case.

The response curve which is plotted only qualitatively with respect to the ordinate scale passes through a maximum at about the same hydrogen flow of 42-43 ml./min. However response which may be measured in coulombs of electric charge crossing the interelectrode space per gram of sample which, when burned, produces this charge transfer, does not take into account the noise level.

Hence in determining the optimum hydrogen flow, sensitivity" which is equivalent to signal-to-noise ratio should under most circumstances be maximized. FIG. 2 shows that sensitivity, which may be defined as the minimum sample flow rate in grams per second which can be reliably detected, reaches a maximum at approximately 37 mL/min. of hydrogen. At this hydrogen flow, background current is only 50 percent of the maximum value. Qualitatively similar results have been produced using a variety of AFlDs constructed according to FIG. 1 although the optimum air and hydrogen flows have been found to vary slightly according to the fit of the chimney 5 to the base 3 and the exact positioning of the electrodes. In general then the procedure for establishing optimum operating conditions is to choose an air flow in the range of 200 to 270 ml./min. and to scan hydrogen fiow in step changes from 85 to 20 ml./min. to produce a curve of background current similar to FIG. 2. The hydrogen flow should then be set at the value corresponding to 50 percent of maximum background current on the rising portion of the background current curve.

However, it has also been discovered that higher air flows yield higher maximum background currents and higher noise levels as a result. As a general rule maximum background currents in excess of approximately 3x10 amp are an indication that a lower air flow should be chosen in order to minimize noise level.

The performance of the AFlD according to FIG. 1 has been evaluated using as alkali salts KBr, RbBr, CsBr, K 50, and Rb,SO Each required different H and air flows for optimum performance. With each of these salts the AFID exhibited very high sensitivity to nitrogenand phosphorous-containing hydrocarbon compounds compared to other hydrocarbon compounds i.e. the selectivity for the nitrogen and phosphorous compounds was excellent such that they could be reliably detected and quantified even in the presence of other hydrocarbon compounds in the effluent gases from the column. The AFlD also exhibited good selectivity for sulfurand chlorine-containing compounds.

When the AFlD according to the present invention was compared with the detector according to U.S. Pat. No. 3,423,181, it was discovered that background current had been decreased from about 3X10" amp to 3X10 amp or a factor of l00. Similarly noise level had dropped from ZXIO" amp to 2X10 amp. At the same time the sensitivity to phosphorous-containing compounds had improved by approximately 20 times.

What is claimed is:

1. A chromatographic detector for analyzing the gaseous components in the effluent gas emerging from a chromatographic column comprising in combination: an enclosure defining at one end thereof an open portion; a cap means for sealingly closing said open portion; a hollow jet member extending into said enclosure from one wall thereof, said jet member having an open end within said enclosure; said enclosure defining a first gas flow passage in communication with said jet and with the exterior of said enclosure for connection to said chromatographic column; means in communication with said passage for mixing said effluent gases with a combustible gas to form a combustible mixture; said enclosure defining a second gas flow passage therethroug'h for conducting an oxygen-containing gas mixture to the interior of said envelope from a source of said oxygen-containing gas mixture; means disposed adjacent said open end of said jet member for producing ions in response to the combustion of said combustible gas mixture; said enclosure defining a third gas flow passage extending from the exterior of said enclosure to portrons of the interior of said enclosure distal said open portion thereof for exhausting products of combustion from the interior of said enclosure; first and second electrodes insulatingly extending through a wall of said enclosure and defining between the ends thereof within the interior of said enclosure an interelectrode gap; means for causing an electrical current to flow across said interelectrode gap and means for measuring the magnitude of said electrical current.

2. A detector according to claim 1 wherein said enclosure includes a base portion defining on one surface thereof a recess, and a chimney portion having one end thereof received in nesting relation within said recess and wherein said third gas flow passage is defined between the mating surfaces of said base portion and said chimney portion.

3. A detector according to claim 1 wherein said cap means comprises a plug dimensioned to be insertable within said open portion of said enclosure and having a resilient gasket extending around the periphery thereof.

4. The detector according to claim 1 wherein one of said electrodes incorporates means for igniting said combustible gas mixture.

5. The detector according to claim 1 wherein said means for producing ions comprises a cup member surrounding said open end of said jet member, said cup member containing an alkali salt.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION September 21, 1971 Patent No. 3 607 096 Dated Inventor(s) Charles H. Harmann It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

After the name and address of the inventor --Assignee: Varian Associates Palo Alto, California-.

Insert:

Signed and sealed this 13th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks

Claims (4)

1. 2. A detector according to claim 1 wherein said enclosure includes a base portion defining on one surface thereof a recess, and a chimney portion having one end thereof received in nesting relation within said recess and wherein said third gas flow passage is defined between the mating surfaces of said base portion and said chimney portion.
2. 3. A detector according to claim 1 wherein said cap means comprises a plug dimensioned to be insertable within said open portion of said enclosure and having a resilient gasket extending around the periphery thereof.
3. 4. The detector according to claim 1 wherein one of said electrodes incorporates means for igniting said combustible gas mixture.
4. 5. The detector according to claim 1 wherein said means for producing ions comprises a cup member surrounding said open end of said jet Member, said cup member containing an alkali salt.

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