Flame Ionization Detection of Fluorine in Gas-Liquid Chromatography Effluents Arthur Karmenl and Eileen L. Kelly Department of Pathology, New York University Medical Center, 550 First Avenue, New York, N. Y . 10016
A four-stage, two-flame detector detects fluorine in organic compounds analyzed by GLC by: combustion of organofluorine to HF in a hydrogen flame; passage of the HF through a bed of calcium chloride crystals from which it releases HCI; reaction of the HCI with a strongly heated alkali metal salt; and detection of the resulting increased vaporization of alkali metal by hydrogen flame ionization. The detector responded to halogen-containing solutes, but not to other organic vapors. Omitting the calcium chloride eliminated the response to fluorine which permitted fluorine to be distinguished.
THEPOT~NTIALIMPORTANCE of a gas chromatography detector specifically sensitive t o fluorine, aside from its obvious usefulness in detecting organic compounds containing fluorine in the presence of other classes of compounds, arises from the stability and high volatility of fluorine-containing derivatives of a wide variety of compounds. Trifluoroacetyl and heptafluorobutyryl derivatives of alcohols and amines, for example, generally chromatograph better than the parent compounds and are often preferred for their analysis by gas-liquid chromatography (GLC). We, therefore, sought t o devise a specific fluorine detector. The hydrogen flame ionization detector can be sensitized t o chlorine, bromine, iodine, nitrogen, and phosphorus by heating an alkali metal salt in the flame ( I ) . This observation formed the basis of several flame ionization detectors which respond with different degrees of specificity t o compounds containing these elements (2-5). Unfortunately, all these devices are insensitive or only slightly sensitive to fluorine. The objective of the work described here was to develop a method for detecting fluorine based on a similar mechanism. The alkali flame ionization detectors respond to the different heteroatoms by different mechanisms (6). In those detector designs in which the products of combustion impinge directly on a heated source of alkali metal, chlorine, bromine, and iodine increase the volatility of the alkali metal. Since more alkali metal is present in the flame and is ionized, the ionization current increases. While the fraction of the alkali metal in the flame that is ionized varies somewhat, depending on the halogen present, this variation causes quantitatively less significant changes in ion current than does the increased volatility. On the other hand, organonitrogen compounds increase the ionization of the alkali metal markedly while affecting its volatility t o a much lesser extent. Because of these differences, it is possible to devise methods for disPresent address, Hospital of the Albert Einstein College of Medicine, 1825 Eastchester Road, Bronx, N.Y. 10461 (1) A. Karmen and L. Giuffrida, Nature, 201, 1204 (1964). (2) A. Karmen, ANAL.CHEM., 36, 1416 (1964). (3) A. Karmen, J . Gas Chromatogr., 3, 336 (1965). (4) W. A. Aue, C. W. Gehrke, R. C. Tindle, D. L. Stalling, and C. D. Ruyle, ibid., 5, 381 (1967). (5) A. Karmen, U S . Patent 3,425,806 (1969). (6) A. Karmen, J . Chromatogr. Sci., 7 , 541 (1969). 1992
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tinguishing compounds containing phosphorus and nitrogen from those containing halogens. While either of the two mechanisms could presumably provide a basis for detecting fluorine, the sensitivity achieved in exploratory experiments was very low. We, therefore, devised a n on-line method for exchanging fluorine for chlorine. The chlorine could then be detected by its effect on the volatility of the alkali metal. EXPERIMENTAL
Fluorine detection involved a sequence of reactions. First was the combustion of organofluorine t o inorganic fluoride. We chose to accomplish this by passing the column effluent through a hydrogen flame. This offered a method of monitoring the total organic compound i n the effluent by flame ionization as well as combustion that was close t o quantitative or at least reasonably reproducible. Second was the exchange of the inorganic fluoride for inorganic chloride. Study of this reaction required both a convenient method for passing hydrogen fluoride gas over the source of nonvolatile inorganic chloride, in which the exchange was t o take place, and a convenient chlorine detector to monitor the exchange. For chlorine detection in these preliminary studies we used a commercial Freon leak detector, composed of a propane-air flame passing through a combination copper and stainless steel disk (7) In this device, the color of the flame changes from blue to yellow to green upon the addition of increasing concentrations of HC1 at the air inlet. We studied the reaction of HF with various salts by filling 3-inch lengths of '/*-inch i.d. stainless steel tubing, fitted with nitrogen inlets and injection ports in the manner of gas chromatography columns, with the appropriate salt crystals. HF gas was delivered t o the tubing either by injecting samples of the head space over a n aqueous solution of H F or by passing the nitrogen inlet line over a solution of HF. The stainless steel tube was heated electrically to determine the effect of different temperatures on the exchange reaction. To study the mechanism of chlorine detection, we fitted a similar two-inch long, '/*-inch i.d. stainless steel tube with a n injection port, a gas inlet and outlet, provided it with electrical heating, and filled it with sodium sulfate crystals. A thermocouple was placed in the crystals, and the gas outlet was placed close to the air intake of a Bunsen burner. We determined the effect of adding HCl gas on the evolution of sodium vapor by crystals maintained at different temperatures. When these experiments indicated the approximate requirements for combustion, ion exchange, and chlorine detection, we adapted a two-flame ionization halogen detector for detection of fluorine as follows: Design of the Fluorine Detector. In the first stage of the detector, the column effluent was delivered to the 0.030411. i.d. flame jet of a hydrogen flame ionization detector where it was mixed with hydrogen and burned in a n air atmosphere. The electrical conductivity of the flame was recorded on a strip chart. (7) C. C. Anthus, U.S.Patent 2,779,666 (1957).
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HALOGEN DETECTOR
HALOGEN DETECTOR COIL COLD F-BENZENE TOLUENE ( 2 0 ~ ) CI-BENZENE
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Figure 1. The upper flame of the twoflame fluorine detector, minus the calcium chloride, and with the helix of the chlorine detector not heated, did not respond to chlorine or fluorine containing organic compounds, or to other organic compounds. The response of the lower, or conventional hydrogen flame detector to 20-pg quantities of fluorobenzene, toluene, and chlorobenzene is shown on the upper graph; the lack of response of the upper flame on the lower graph
Figure 2. The fluorine detector recorded as in Figure 1, minus calcium chloride crystals, but with the helix heated, responded to 20 pg of chlorobenzene but not to trifluorotoluene. The upper flame response (lower record) to the solvent peak was minimal
The second stage of the detector consisted of a platinum wire mesh screen placed 2 cm above the flame jet. This distance was sufficient to prevent the screen from being heated to incandescence when the flame was small (30 ml/min of hydrogen; 40 ml/min of nitrogen). The screen was covered with 2-mm diameter calcium chloride crystals, the kind usually employed in drying tubes. The chlorine detector consisted of an electrically heated alkali metal salt and a hydrogen flame ionization detector for sensing the vapor pressure of the alkali metal. The source of alkali metal was a 3-mm diameter helix of 0.030-in. diameter nichrome wire, 2 cm long, formed into a semicircle, and provided with copper leads for electrical heating. Droplets of sodium sulfate solution were deposited and dried on the helix. The flame burned at a 0.030-i.d. stainless steel jet mounted 0.5 cm above the center of the semicircle of the helix. A platinum wire electrode suspended above the flame provided both the polarizing voltage and the electrometer lead for recording the electrical conductivity of the flame (between the electrode and the flame jet and helix). For detecting fluorine, the heated helix of this device was mounted 1 cm above the platinum screen supporting the CaClzcrystals. Only two operating parameters were semicritical : the height of the lower flame was adjusted by controlling the hydrogen flow so that the screen above it was not incandescent. Voltage was applied to the nichrome wire helix from the secondary winding of a filament transformer (120 volt primary, 11 volts, 10 amperes secondary) sufficient to cause the helix to glow dull red. RESULTS
Exchange of Fluoride for Chloride. The “Freon leak detector” responded quite sensitively to the introduction of HCl vapor into its air line but did not respond to H F vapor. Passing the H F vapor through a 4-inch length of sodium
chloride crystals released sufficient HCl as to yield a yellowgreen flame test. However, the reactivity of the sodium chloride crystals did not persist for more than 20 or 30 minutes and was inconsistent. Heating the crystals shortened the duration of the effect. Although it seemed likely that moisture was required for the exchange to take place, the addition of water vapor to the gas restored the effect only temporarily. The exchange was consistent and reproducible with calcium chloride crystals. Mild heating had little or no effect. Heating the tube to dull red for many minutes caused evolution of chloride. The responses to equal concentrations of H F and HCI were approximately equal if the gas flowed through as little as 1 cm of calcium chloride crystals, indicating that close to quantitative exchange was feasible. This observation suggested the use of a platinum screen covered with calcium chloride crystals above the lower flame of the ionization detector as the source of chloride. The screen distributed the heat and combustion products of the lower flame more evenly and also formed the electrostatic limit of the lower flame ionization chamber. The problem of detecting the chloride released remained to be solved. Chlorine Detection. When nitrogen was passed through electrically heated sodium sulfate crystals to the bunsen flame, there was no detectable evolution of sodium vapor unless the crystals were heated to almost dull red. At the temperature just sufficient to cause detectable evolution of sodium, injection of HCI vapor increased the evolution markedly. If the temperature was reduced slightly until the sodium light became undetectable, the HCI vapor produced only a somewhat smaller effect. The signal-to-noise ratio for chlorine detection was quite high under these circumstances because of the low background sodium emission. However,
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
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F-DETECTOR
F-DETECTOR (C0Cl2I
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nitropropane or to the solvent peak
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Figure 3. The detector, in Figure 2, but with calcium chloride crystals added, responded to fluorobenzene but not to toluene and only minimally to the solvent peak
the magnitude of response to HC1 was less than at the minimally higher temperature sufficient to cause some evolution of sodium vapor in the absence of HC1. This temperature was also more easily set by inspection of the flame. When the helix was not heated, the flame ionization detector above the platinum screen did not respond to chlorine in the lower flame (Figure 1). This was taken to indicate that all surfaces exposed to the products of combustion of the lower flame were at a temperature lower than that necessary for the halogen-alkali metal reaction. Effort was directed toward operating the device under these conditions. In previous devices, in which the combustion of the effluent in the lower flame heated the alkali metal source, changes in flame temperature such as those produced when a large quantity of organic material reached the flame (such as when the solvent front emerged) increased the evolution of the alkali metal by a direct temperature effect. These large concentrations of combustible materials were thus indistinguishable from smaller concentrations of halogen-containing compounds. The present means for detecting halogen thus offered increased specificity. As an alternative chlorine detector, a coil of wire dipped in sodium sulfate solution mounted on the upper flame jet close to the middle of the flame sensitized the upper flame to chlorine in the lower flame, but the sensitivity was poor; the response to chlorobenzene was only about l / g ~ that of the lower flame. With the two-flame halogen detector described previously, the responses to chlorobenzene in the upper and lower flames were similar in magnitude. The electricallyheated helix mounted above the calcium chloride crystals provided better geometric opportunity for collision of the chlorine with the sodium salt and much improved sensitivity. When no current was passed through the helix, the flame above it responded to neither chlorine nor fluorine in the lower flame gas. With the helix heated, the response to chlorobenzene was approximately equal to that of the lower flame. Fluorine Detection. Without calcium chloride present, the detector did not respond to fluorotoluene (Figure 2). 1994
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With calcium chloride present, the response per molecule of fluorine in the upper flame was comparable to the response to that of chlorine. This indicated that the exchange of fluorine for chlorine was almost quantitative (Figures 3, 4, and 5 ) . The upper flame also responded to chlorine, bromine, and iodine in the lower flame. It did not respond to organic nitrogen compounds. It responded to a lesser extent to organophosphorus compounds than the two-flame detector described previously (2). The response to organic vapors containing no heteroatoms was appreciably less ; the solvent peak, which regularly caused a large response in the upper or halogen sensitive flame of the previous detector, produced only a small response in the present design.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
v
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Figure 5. The fluorine detector (lower record, as in Figure 1) responded to fluorine and chlorine in compounds in the lower flame with almost equal sensitivity
DISCUSSION
The approaches we considered for detecting fluorine included: flame photometry; ion exchange in conjunction with colorimetry; and the possible reaction of hydrogen fluoride with heated metal salts which might produce changes in their volatility or their ionization or excitation in a flame. Flame photometric detection of fluorine has been based on the excitation of the band emission of CaF?, SrF2, and BaF? (8). The fluoride is generally added to a solution of the metal salt following which the solution is aspirated into the flame. Detection of fluorine in a GLC effluent is possible with this approach if, for example, the column effluent were first combusted, thus converting the fluorine in organofluorine residues into hydrogen fluoride; the HF could then be collected in a solution of a calcium salt which is continuously aspjrated into the flame. The effect of fluorine on the volatility of alkali and alkaline earth metal salts was small under the same conditions used successfully for the detection of other halogens. HF, of course, is known to react with silicon to produce volatile silicon fluoride. The silicon can then be excited to emission. Ion exchange is the basis of a colorimetric analysis of fluorides. The sample is stirred with a suspension of thorium chloranilate. The fluoride ion exchanges with the chloranilate, precipitating as thorium fluoride, and releasing highlycolored chloranilate ion into solution where it can be measured colorimetrically. This reaction can serve as a basis for a GLC detector for volatile fluorine-containing compounds. We constructed a device in which a gas stream containing H F was bubbled con(8) K. Fuwa, J. Chem. Soc. Japan, Pure Chem. Sect., 75, 1257 (1954), quoted by J. Dean, “Flame Photometry,” McGraw Hill Co., New York, N.Y., 1960, p 264.
tinuously through an aqueous suspension of thorium chloranilate, a device similar to that described by Popjak er al., for collecting radioactive compounds from a GLC column effluent (9). The clear colored solution of chloranilate ion was pumped through the flow cell of a colorimeter continuously, through a fine filter. Although the response of this system was slower than could be desired, because the reaction did not reach instantaneous equilibrium, the response could be made adequately fast with a moderate sacrifice of sensitivity by replacing the aqueous solution at a constant rate. The fluorine in HF exchanged more readily and consistently with the chlorine in CaCL than with that in NaCI. If the insolubility of the CaF2 was responsible, one might predict similar reactions with salts of other cations which form insoluble fluorides. The almost equal molar responses to fluorine and chlorine indicated almost quantitative exchange was achieved, The response of the detector to compounds containing the other halogens could not be distinguished from that to compounds containing fluorine in the detector described. Should greater specificity be required, it should be possible to split the sample between a detector for the other halogens and fluorine detector and detect the fluorine-containing compounds by the differential response of the two. It may also be possible to absorb the other halogens selectively. RECEIVED for review July 19. 1971. Accepted September 7, 1971. Work supported by National Institutes of Health Grant G M 16745. (9) G. Popjak, A. E. Lowe, D. Moore, L. Brown, and F. A. Smith, J. Lipid Res., 1, 29 (1959).
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