US3372994A - Flame ionization detector - Google Patents

Description

March 12, 1968 L. E. GIUFFRIDA 3,

FLAME I ONI ZAT ION DETECTOR Filed Jan. 8, 1965 2 Sheets-Sheet l FROM 2 1 \I 'coLuMN l FIGJ PARATHION METHYL STEARATE INVENTOR FIG- 2 LAURA E.6IUFFRIDA ATTORNEY March 12, 1968 L. E. GIUFFRIDA FLAME IONI ZAT ION DETECTOR Filed Jan. 8, 1965 FIGS 2 Sheets-Sheet 2 RONNEL ETHION TRITHION FIG.5

DIAZINON PARATHION FIG.7

INVENTOR LAURA E.GIUFFR|DA ATTORNEY United States Patent Ofifice FLAME IONIZATION DETECTOR Laura E. Giutfrida, Arlington, Va., assignor to the United States of America as represented by the Secretary of the Department of Health, Education, and Welfare and the Secretary of the Department of Agriculture Filed Jan. 8, 1965, Ser. No. 424,443 4 Claims. (Cl. 23--254) ABSTRACT OF THE DISCLOSURE A hydrogen flame ionization detector, for use in gas chromatography, the heated electrode being coated with a fused alkali metal salt. Coating the electrode in this manner renders the detector selectively specific to phosphorus-containing organic compounds in mixtures, so that higher, more characteristic peaks are indicated when these mixtures are analyzed, while smaller peaks, or none at all, are indicated for the non-phosphorus-containing compo nents of the mixtures.

A non-exclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government, with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

This invention relates to a flame ionization detector for use in gas chromatography. More particularly, it relates to a novel thermionic detector having an exceptionally high selective sensitivity to phosphorus in an organic compound.

Gas chromatography has become an extremely valuable research tool for the detection, identification, and measurement of minute amounts of chemicals in mixtures and the like. Since about 1958, flame ionization detectors have been used in conjunction with gas chromatograph apparatus for this purpose. These detectors operate on the principle that, when an organic compound is burned in a hydrogen flame, the electrical conductivity of the latter increases. Thus, when a gas is passed through the chromatograph column it will elute substances which have been adsorbed on it and, when mixed with hydrogen and burned, the resulting flame will have a higher electrical conductivity than if the carrier gas alone is burned with the hydrogen. Different substances not only produce different changes in conductivity but also are eluted from the column at different rates. The time required to elute the particular material (referred to as the retention time), together with the relative increase in conductivity of the flame, can be used to determine the amount of and to identify the adsorbed material.

In general, the ionization detectors available prior to my invention, and currently in use, comprise a combustion chamber having an air-inlet to provide the necessary oxygen for combustion, a gas inlet which is connected to the outlet end of the chromatograph column, and an inlet for introducing hydrogen. The hydrogen inlet and the gas inlet are connected by means of ducts to a common duct which terminates in a jet within the combustion chamber. The mixture of hydrogen and carrier gas is ignited and the flame beats a chemically inert wire loop, usually made of platinum. If a voltage is impressed across the jet and the wire loop, the ions produced in the hydrogen flame will increase the electrical conductivity of the latter and thereby increase the current flow. The greater the number of ions the greater will be the conductivity of the flame. By connecting the detector to an electrometer, as is well known in the art, the differences in current can be detected and measured. Furthermore, by using a recording elec- 3,372,994 Patented Mar. 12, 1968 trometer, the changes in current can be traced on a moving tape so as to plot the current intensities against time. As succeeding substances are eluted from the chromato graph column by the carrier gas and the latter is burned in the flame detector, the traced curve will show peaks of varying height and sharpness. This apparatus is so sensitive that it can detect amounts as small as one or two micrograms.

In recent years, the use of organo-phosphorus insecticides has become quite extensive. However, while these substances are highly effective pesticides, they are also extremely toxic to warm-blooded animals, attacking the nervous system. For this reason, it is necessary not only to take great precaution in applying these insecticides, but also to insure that no residues remain on edible vegetables and fruits. Because of the great toxicity of even small amounts of these pesticides, gas chromatography has become a valuable technique in detecting them. However, while the presently available flame ionization detectors are generally suitable for this purpose, they are not sufficiently specific and will also record peaks for other substances which may be present in mixtures being analyzed. In some instances the peaks for the non-phosphorus-conraining components are far more well-defined than those for the compounds being sought and, often, depending on the nature of the phosphorus compound, the latter does not even produce a significant peak.

Accordingly, one object of the present invention is to provide a flame ionization detector which is selectively specific to phosphorus-containing organic compounds. Another object is to provide such a detector which is more sensitive to phosphorus-containing organic compounds than conventional detectors and which will indicate higher, more characteristic peaks when these compounds are analyzed. Still another object is to provide a detector which gives smaller peaks, or none at all, for the nonphosphorus-containing components of mixtures. Other objects will become apparent to those skilled in the art from the description of my invention which follows.

In general, the foregoing objects are achieved by substituting, for the heated electrode of the conventional hydrogen flame detector one that has been coated with a fused alkali-metal salt. Although sodium sulfate produces the best results, and is preferred for that reason, other salts can also be used. Among the latter are lithium sulfate, sodium chloride, sodium phosphate, potassium sulfate, potassium chloride, potassium phosphate, potassium nitrate, rubidium sulfate, and cesium sulfate.

My modified flame ionization detector is used in the same manner as those previously available, as will be readily apparent to those skilled in the art.

In order that the invention may be better understood, reference is made to the following detailed description and to the accompanying drawings in which:

FIGURE 1 represents a partially schematic cross-sectional view of my novel detector;

FIGURE 2 shows the curve produced when a mixture containing lindane, parathion, and methyl stearate is analyzed by the conventional hydrogen flame ionization detector;

FIGURE 3 shows the curve produced when the same mixture is analyzed by my novel sodium thermionic detector;

FIGURE 4 shows the curve produced when .a first eluate of a crop extract containing a mixture of Diazinon, ronnel, parathion, ethion, and Trithion is analyzed by the conventional hydrogen flame ionization detector;

FIGURE 5 shows the curve produced when a duplicate first eluate of the above crop extract is analyzed by the sodium thermionic detector of my invention;

FIGURE 6 shows the curve produced when a second eluate of the same extract is analyzed by means of a conventional detector; and

FIGURE 7 shows the curve produced when a duplicate second eluate is analyzed by the sodium thermionic detector.

Referring to FIGURE 1 the device will be seen to resemble a conventional hydrogen flame ionization detecto' having a base or support 1, provided with air inlet 2, hydrogen inlet 3, and gas inlet 4. The latter is connected in the usual manner to the outlet of a conventional gas chromatograph column (not shown). As can be seen, inlets 3 and 4 are both connected to a tube 5 which terminates in a jet tip 6. One feature of my invention is the spiral electrode 7 which surrounds the flame 8 and is concentric with jet 6. This electrode is connected to conductor 9 by any suitable means, or it may be integral therewith, and may be maintained in place by winding several turns of conductor 9 into a coil 10 about tube 5. It will be apparent, however, that the precise manner of mounting electrode 7 is not important and that any alternative mechanical means for locating it and maintaining it around the flame would be satisfactory. The essential feature of the present invention resides in providing electrode 7 with a fused coating of sodium sulfate or of any of the other alkali-metal salts mentioned above.

A further feature of my invention resides in the electrode 11. This comprises a perforated, open-ended metal cylinder, preferably of platinum, which is rigidly mounted on a stiff wire conductor 12 and is concentric with electrode 7. Electrode 11 may be welded to conducting support 12 to provide maximum strength and rigidity and maximum electrical conductivity through the joint. Conductor 12 is secured to base 1 in any suitable manner as by providing a very snug fit through a hole in the base.

The manner of supporting electrode 11 is not critical and any other mechanical means to accomplish this would be satisfactory. Conductors 9 and 12 are both long enough to extend out of the device, as shown, so that they can be connected to an electrometer (not shown) in the usual manner. The electrodes and the jet are enclosed in housing 13 which is mounted on base 1 and is secured thereto by any appropriate means, such as brackets 14. Combustion products and other gases are exhausted from the interior of the device through vented cover 15.

The results obtained by the use of my novel detector are illustrated in the following examples:

Example 1 A mixture containing 1 microgram of lindane (a chlorinated cyclohexane), 1 microgram of parathion (an organo-phosphorus compound), and 2 micrograms of methyl stearate were adsorbed on a gas chromatograph column, eluted with nitrogen in the usual manner, and then analyzed by a commercially available hydrogen flame detector and also by the sodium thermionic detector of the present invention. FIGURE 2 shows the response of the hydrogen flame detector at an electrometer setting of 1O amperes full scale (A. FS). As can be seen from the figure, each of the three components of the mixture produced sharp, well-defined peaks. The curve in FIG- URE 3 shows the same sample analyzed by the novel sodium thermionic detector at a setting of 3 I A. FS. It was necessary to attenuate 300 times to keep the respouse for parathion on scale. In FIGURE 3, lindane is barely detected and methyl stearate does not appear at all. By comparing the records from both detectors the presence of a phosphorus-containing compound can be readily demonstrated and the amount determined in the known manner.

Example 2 In this example, Diazinon and ronnel, 0.05 part per million (ppm) each, and 0.1 ppm. each of parathion, ethion, and Trithion were added to a 100 gram sample of broccoli. All are organic phosphorus-containing substances. The sample was then prepared for gas chromatography by the method described by Mills of al. in the Journal of the A.O.A.C., vol. 46, page 186 (1963). FIG- URE 4 shows the analysis of an aliquot of the first eluate of the sample, representing 1 gram of broccoli, by the hydrogen flame detector at l() A. PS. As can be seen, there was practically no response. When a duplicate aliquot of the first eluate was analyzed by the sodium thermionic detector at a sensitivity of 3X10 A. FS, sharp peaks due to ronnel, ethion, and Trithion appeared. These are shown in FIGURE 5.

Diazinon and parathion are recovered from the chromatograph column in the second eluate. When the latter was analyzed by the hydrogen flame detector at an electrometer setting of lO A. PS, the two major peaks shown in FIGURE 6 represented organic compounds that did not contain phosphorus. However, when a duplicate analysis of the second eluate was made, using the sodium thermionic detector at a setting of 3X10 A. FS, Diazinon and parathion gave the sharp responses shown in FIGURE 7. Nitrogen was used as the eluting gas in this example.

As previously indicated, my invention resides in the novel detector. This was made by modifying a hydrogen flame detector of basic conventional design which was provided with an electrode assembly that permitted electrodes to be readily interchanged. Although the invention has been described using a coated spiral as the positive electrode, the latter can also be in the form of a ring or circular gauze. In each instance, the electrode is preferably of platinum and carries a coating of the sodium salt fused on the surface. A diameter of about 5 mm. was found satisfactory. While larger electrodes can be used, they would require larger flames. Similarly, smaller electrodes can be used, but these would require more critical centering about the jet. The electrode is positioned approximately 2 mm. above the jet. The shape of the electrode does not appear to be critical, as responses from all three are comparable. However, the spiral wire is preferred as being the most convenient to use.

The electrodes are prepared by placing a drop of saturated aqueous solution of the alkali-metal salt on the wire and heating it gently in a flame to dry. This can be repeated a sufiicient number of times (e.g., three times) until a layer of the desired thickness is formed. The wire is then heated to a temperature high enough (e.g., red heat) to cause the salt to fuse and distribute itself over the surface. When cool, the electrode can be assembled in the detector.

The novel detector can be used with any commerciallyavailable gas chromatography equipment. Operating conditions, techniques, and calibration of the apparatus are well known to those skilled in the art and need not be described in detail here.

Having described my invention, what I claim is as follows:

1. A flame ionization detector for use in gas chromatography comprising:

(a) a base member and a housing mounted thereon;

(b) gas-burning means mounted on the base within said housing;

(c) means connected to said gas-burning means for conducting a stream of hydrogen thereto;

(d) means also connected to the gas-burning means for conducting a stream of carrier gas and eluate from a chromatograph column to said gas-burning means;

(e) means for introducing air into the interior of the housing;

(f) a first electrode, having a coating of a-fused alkalimetal salt selected from the group consisting of so dium sulfate, lithium sulfate, sodium chloride, sodium phosphate, potassium sulfate, potassium chloride, potassium phosphate, potassium nitrate, rubidium sulfate, and cesium sulfate, mounted in said housing in proximity to said gas-burning means;

(g) a second electrode mounted in said housing in proximity to the first electrode; and (h) electrically conductive means connected to each of said electrodes. 2. The detector of claim 1 wherein the first electrode has a coating of fused sodium sulfate thereon.

3. A flame ionization detector for use in gas chromatography comprising:

(a) a base member and a housing mounted thereon;

(b) gas-burning means mounted on the base within 10 said housing;

(c) means for introducing a stream of hydrogen into said gas-burning means;

(d) means for also introducing a stream of carrier gas and eluate from a chromatograph column to said 10 gas-burning means;

(e) means for introducing air into the interior of the housing;

(f) a first electrode, having a coating of a fused alkalimetal salt selected from the group consisting of sodium sulfate, lithium sulfate, sodium chloride, so-

dium phosphate, potassium sulfate, potassium chloride, potassium phosphate, potassium nitrate, rubidium sulfate, and cesium sulfate, mounted in said housing and disposed concentrically about said gasburning means;

(g) a second electrode mounted in said housing disposed concentrically about said first electrode; and (h) electrically conductive means connected to each of said electrodes. 4. The detector of claim 3 wherein the first electrode has a coating of fused sodium sulfate thereon.

References Cited UNITED STATES PATENTS 3,129,062 4/1964 Ongkiehong et a1. 23-232 C MORRIS O. WOLK, Primary Examiner. JOSEPH SCROVRONEK, Examiner. R. M. REESE, Assistant Examiner.

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