Alkali Flame Ionization Detector Attachment for Conventional Hydrogen Flame Ionization Detectors Arthur Karmen and Harold Haut Department of Laboratory Medicine, Albert Einstein College of Medicine, Bronx, New
In a two-flame, alkali-flame ionization detector (AFID), halogens increase the volatility of alkali metal on a solid matrix, while phosphorus increases the ionization of alkali metal and only secondarily affects its volatility. These reactions serve as bases for a number of different GLC detcctor configurations with differential sensitivities to the different heteroatoms (1). These detectors can generally be constructed from conventional flame ionization detectors by installing a properly located probe coated with an alkali metal salt close to the flame jet. However, although the probe generally alters the sensitivity of the detector to other kinds of organic compounds, it does not eliminate it, resulting in less than specific detection of the heteroatom of interest. In addition, simultaneous recording of the unaltered FID response generally provides useful information. While the two-flame AFID described previously provided both an essentially unaltered flame response and greater specificity toward heteroatoms, its use has generally required modification of the entire detector and detector oven. Because of the potential advantages offered by a halogen detector that could be added to a conventional detector when required, and as easily removed, the design of a selfcontained AFID attachment to a conventional FID was undertaken. ( 1 ) A.
Karmen,J Chrornatogr SCI , 7, 541 (1969)
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
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EXPERIMENTAL The FID served the dual purpose of detecting organic compounds and burning them to carbon dioxide and water, thus rendering them undetectable by further flame ionization. The AFID consisted of a 22-gauge platinum wire helix coated with an alkali metal salt, and a FID to monitor the release of alkali metal vapor from it. The helix was heated electrically so that the effect of halogen on it was independent of the flame temperature of either FID. In order to study the position of the upper detector elements relative to the lower flame that provided maximal delivery of combustion products of the lower flame to the heated helix, and the relationship between the salt-coated helix and the upper flame jet for maximal detection of sodium vapor, a two-flame detector model was constructed in a 4-in. length of 3/4-in. i.d. quartz tubing closed a t the lower end by a No. 4 rubber stopper and opened at the upper end. Two 3-in. lengths of 0.060-in. i.d. stainless steel tubing were inserted through the stopper, one centrally and one off center, to serve as lower flame jet and air inlet respectively (Figure 1 ) . A mixture of H2 and N2 was delivered to the flame jet. The nitrogen stream was first passed through a glass T fitted with rubber stoppers to permit injection of vapors into the gas stream. The upper flame jet was a 0.060-in. i.d. stainless steel tube suspended within the quartz tubing from above. The alkalimetal source was a 25-cm length of No. 28 AWG nichrome wire wound into a 4-mm diameter helix which in turn, was bent into a 1.3-cm diameter circle surrounding the jet. The ends of the wire were brazed to rigidly mounted No. 12 AWG copper wires. The helix was connected to the secondary winding of an 11-volt, 10-A filament transformer, the primary winding of which was supplied with current from the secondary of a variable autotransformer. The helix was coated by inserting it into a concentrated aqueous solution of sodium sulfate and heating it electrically to bright red. The dry salt crystalized as the water evaporated, melted, and coated the helix evenly. The position of the upper flame jet above the helix was adjusted for maximal emission of sodium light when the helix was hot and for minimal or absent emission when the heater current was turned off. Placing the helix sufficiently close to the flame to be heated was avoided in order to improve the reproduction of helix temperatures. The position of the helix above the lower (FID) flame was adjusted for maximal response to halogen introduced into the lower flame and minimal heating of the helix by the lower flame as shown by minimal sodium emission in the upper flame in the absence of electrical heating of the helix. An AFID attachment was constructed for a Packard Model 811 FID mounted in a Packard Model 802 column oven (Figure 2). The helix was a 25-cm length of No. 22 AWG platinum wire wound on a 3-mm diameter form, and shaped into a 1.3-cm 0.d. circle, Its leads were brazed to a two-pronged connector which could easily be removed for replacement or retreatment of the helix. The helix was treated with sodium sulfate as described above. The electrical conductivity of the flame was measured between an electrode consisting of a 6-cm length of No. 20 AWG platinum wire bent into a 2-cm diameter circle and mounted 0.8 cm above the flame jet, and the flame jet itself which was maintained a t 200-2000 volts above ground. The lower FID was operated in the usual manner. The outputs of both electrometers were recorded on separate strip chart recorders. The response of the AFID to the different heteroatoms was tested by injecting graded quantities of butyl bromide, iodobutane, butyl chloroacetate and triethyl phosphate into a 6', 4 mm. i.d. GLC column packed with 10% ethylene glycol adipate polyester on chromosorb W operated a t 150 "C. The response in coulombs/ mole of heteroatom was calculated from the respective peak areas.
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SIDE VIEW
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Sectional views of the AFID attachment
RESULTS AFID Geometry. The emission of sodium light in the AFID flame was maximal when the flame was mounted from 0.25 to 2.5 cm above the heated helix. It decreased only gradually as the flame was raised further. When the flame jet was 0.5 cm or more above the helix, the flame did not heat the helix sufficiently for reaction with halogen when the heater current was turned off. Introduction of halogen vapor into the lower (FID) flame produced a maximal response in the upper flame when the helix and the AFID flame, clamped together, were fixed directly above and concentric with the axis of the lower flame, from 3 to 5 cm above the flame jet. Effect of Helix Temperature. With other variables held constant, the base-line current and the response of the AFID to halogen and phosphorus increased with increase in heater current corresponding to temperatures from 400 to 1000 "C (Figure 3). The responses increased roughly in parallel with one another. The sensitivity to phosphorus was greater than to chlorine, bromine, and iodine, in that order, over the entire range of heater temperatures and alkali metal vapor concentrations tested. Effect of Polarizing VoltaFe. The AFID response also increased with increase in polarizing voltage (Figure 4). The current increased with increasing voltage through an ionization chamber plateau to a region of ion multiplication. The relative responses of the detector to different heteroatoms were not appreciably affected by different polarizing voltages.
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ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 4 , A P R I L 1973
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Linearity. The AFID response was proportional to sample size when the polarizing voltage was in the midrange of the ionization chamber plateau and when it was sufficient to cause ion multiplication. The response a t the higher voltage setting was not quite so reproducible. Sensitivity. The ultimate sensitivity of the detector was limited by the level of base-line current and fluctuations in it. As little as 5 x 10-12 gram of P, C1, and Br produced responses twice the noise level (Figure 5 ) . The response of the AFID to organic compounds other than those containing P, C1, Br, I, was minimal. Solvent peaks in the FID produced barely detectable responses in the AFID. The response of the AFID does not recover quite so rapidly as the response of the FID following passage of a peak concentration of halogen, suggesting that the halogen is adsorbed in the alkali metal salt to some degree. The response is somewhat more rapid at higher helix temperatures, which is consistent with this hypothesis. CONCLUSIONS The performance of a self-contained AFID attachment to a conventional FID was comparable to that of the twoflame AFID described previously. Electrical heating of the alkali metal source produced an increase in specificity, as shown by a smaller response to solvent, presumably because the temperature of the alkali metal source was less affected by the increase in FID flame temperature during the passage of the solvent peak than it would have been if
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the FID flame were the major source of heating of the helix. Any losses in sensitivity caused by heating parts of the helix that were not exposed to the halogen vapor were minimal. ACKNOWLEDGMENT The authors express their gratitude to Ellen Sullivan for assistance in preparing the manuscript. Received for review December 7, 1972. Accepted January 22, 1973. This work was supported by NIH grant GM 19478.
