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Response of the Flame-Photometric Detector to Ammonia William K.Fowler Southern Research Institute, P.O.Box 55305, Birmingham, Alabama 35255-5305 Although many different kinds of analytical methods are used for determining ammonia in different sample matrices, ammonia is frequently determined by gas chromatography (GC) in either of the following two general situations: (a) when other gases and vapors must also be determined at the same time and (b) when interferences from chemically similar substances, e.g., simple amines, must be avoided. In the first situation, the analyst must use a GC detector that responds to all analytes, such as the thermal conductivity detector (TCD) (I, 2). If all analytes happen to contain nitrogen, it is possible to use one of the chemiluminescent detectors (CLDs) (3-5), or the Hall electrolytic conductivity detector (HECD) (61,or one of the thermionic nitrogen-phosphorus detectors (NPDs) (6). These nitrogen-specific detectors are also useful for minimizing interferences when ammonia must be determined by itself in complex matrices. But because the flame-photometric detector (FPD) is relatively inexpensive, widely available, and easy to use, attempts have been made to modify the FPD to obtain a sensitive response to nitrogen compounds. For example, one such study (7)was based on monitoring the flame emissions of the cyanogen radical (CN) at wavelengths of 311,357,385, and 525 nm. Selectivity for nitrogen compounds was said to be high, but detection limits were typically about 1 pg. Our laboratory attempted to detect ammonia and other nitrogen compounds with the FPD by using a notched interference filter with the band-pass centered at the wavelength of maximum emission intensity of the NH radical (336 nm). A somewhat similar approach has been taken previously to the determination of ammonium-nitrogen (8). But the detection limit of our modified FPD for ammonia was only about 1 pg. Because the photomultiplier tube of the FPD is prevented from viewing the flame directly, it was suspected that the NH emission does not occur efficiently in regions far enough above the flame to be observed by the photomultiplier tube. Moreover, the optical system of the FPD may not efficiently accomodate radiation of such a low wavelength. However, the combustion of ammonia in a fuel-rich hydrogen flame also produces a broad emission band whose wavelength of maximum intensity can vary from 425 to 575 nm (8). For fuel-rich hydrogen flames, the optical emission due to ammonia in this wavelength region apparently has been attributed both to NH2 (ammonia-a) emission (8) and to N O 4 continuum emission (9). The close correspondence of this band to the wavelength of maximum transmission of the phosphorus-selective interference filter of the FPD (525 nm) suggested that an evaluation of the P-selective FPDs response to ammonia might be worthwhile. Consequently, a brief, preliminary evaluation was conducted. This evaluation disclosed that the P-selective FPD apparently does yield a potentially useful response to ammonia. I could find no literature reports of a strong FPD response to other nitrogen compounds, which suggests that such compounds may, indeed, not yield a sensitive FPD response. Hence, the FPD may prove to offer superior discrimination against background substances in certain applications involving ammonia determinations.
EXPERIMENTAL SECTION This work was performed with a Hewlett-Packard (Palo Alto, CA) Model 5890 gas chromatograph equipped by HewlettrPackard with an FPD. The chromatographic column was a 15-m by 0.53-pm-i.d. DB-1 fused silica capillary column with 5.0-pm
stationary phase thickness (J & W Scientific, Inc., Folsom, CA). Except where stated otherwise, the injection port and FPD were operated at 200 OC, the column was maintained at 60 *C, and the flow rates of air, hydrogen, and carrier gas were 120,55, and 20 mL/min, respectively. Moreover, the integrator/recorder chart speed was 1.0 cm/min. All injections of ammonia into the gas chromatograph were performed with a 10-pL gastight syringe with a Teflon plunger tip. Between each injection of ammonia, the syringe was cleaned thoroughly with acetone, and an air blank was analyzed to demonstrate the absence of ammonia carryover. Prior to its injection into the gas chromatograph, the ammonia was frequently diluted with air by injection, through a Teflon-lined septum, of a 3.0-mL aliquot of ammonia vapor into a 250-mL gas sampling flask that contained air at atmospheric pressure. This flask was made of borceilicate glass and fitted with Teflon stopcocks. Blank samples were drawn from the flask prior to its use and were found to yield only a slight negative response in the gas chromatograph at the retention time of ammonia. This small negative dip was observed systematically in blank (air) samples, and it appeared to be proportional to the injected sample volume. It should be understood that ammonia was essentially unretained on the GC column under the conditions employed in this work. That is, the column was used merely as a convenient means of transporting ammonia vapor samples from the injection port into the detector, so that detector response could be evaluated. To ascertain whether the efficiency of ammonia transmission through the injection port and column was suitably high, an experiment was performed in which the downstream end of the column was routed outside the column oven and into the inlet end of a colorimetric ammonia detector tube (Matheson Gas Products, Inc., Morrow, GA) that sampled room air at 100 mL/min. Syringe injections of 1-pL amounts of gaseous ammonia into the GC injection port yielded essentially the same lengthof-stain response in the detector tubes as did comparable injections introduced directly into the inlets of the tubes. Thus, the experiment demonstrated that the transmission of a 1-pL quantity of gaseous ammonia through the injection port and column was essentially quantitative. Except where indicated otherwise, all ammonia used in this work was taken from a metal cylinder of anhydrous ammonia (Matheson Gas Products, Inc.) in the gas phase. The purity of this material was stated by the manufacturer to be at least 99.99 w t %. The ammonium hydroxide (approximately 58 w t %) (Mallinckrodt, Inc., St. Louis, MO), n-butylamine (Eastman Organic Chemicals, Rochester, NY),and methanol (Malliickrodt, Inc.) were of reagent grade.
RESULTS AND DISCUSSION The response of the FPD to ammonia was optimized with respect to hydrogen flow rate by systematically varying the flow rate of hydrogen while the air flow rate was maintained a t 120 mL/min. To test the response at each hydrogen flow-rate setting, an appropriate gas mixture was prepared by diluting a 3-mL aliquot of gaseous ammonia with 250 mL of room air, as described above. From the resulting gas mixture, a 5.0 pL aliquot was withdrawn by syringe and injected into the gas chromatograph after each adjustment of hydrogen flow rate. The response data from this experiment are plotted as a function of hydrogen flow rate in Figure 1. Note that the response varied only by a factor of about 2 for flow rates ranging from 40 to 100 mL/min. Note also that there appear to be two closely spaced maxima in the response profile; I can offer no plausible explanation for this phenomenon. Because the maximum at 55 mL/min was somewhat higher than the one at 62 mL/min, all subsequent work was carried out at a
0003-2700/91/0363-27~8$02.50/0 @ 1991 Americen Chemlcel Society
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Figure 1. Plot of FPD response to ammonia versus the hydrogen flow rate into the detector.
hydrogen flow rate of 55 mL/min. An attempt was also made to optimize the response with respect to detector temperature by varying the detector temperature while the gas chromatograph was challenged with 5-pL injections of the test-gas mixture described above. But the average of duplicate responses measured at each of the tested detector temperatures (175,200, and 225 “C) fell within 7% of the overall average response at all three temperatures. In addition, the relative standard deviation of the pooled responses for all three temperatures was 8%. Because this level of variability was typical of that encountered in all previous work with ammonia at one detector temperature, the instrument was deemed to have yielded essentially the same response to ammonia at all three detector temperatures. Next, the linearity of the response was evaluated over an order-of-magnitude range of ammonia quantities. Contrary to the usual practice, however, the volume, rather than the concentration, of the ammonia injected into the gas chromatograph was varied over the required range. This procedure was adopted primarily because it was suspected that the strong wall adsorption of ammonia (IO) would preclude the preparation of accurately known ammonia-vapor concentrations in the form of simple static test atmospheres. Moreover, resources were not available for preparing dynamic test atmospheres of ammonia vapor. The test gas was again prepared as described above, i.e., by injecting 3.0 mL of gaseous ammonia into a 250-mL gas sampling flask containing air at atmospheric pressure. In the absence of wall adsorption, this procedure would have been expected to result in the preparation of an ammonia-vapor concentration of 8.3 pg/mL. A linear-regression analysis of FPD responses ( Y , peak height in millimeters) versus the volume (X, in microliters) of test gas injected (1.0, 3.0, 5.0, 7.0, and 10.0 pL) yielded a slope of 4.80 mm/pL, a Y intercept of 9.5 mm, and a correlation coefficient of 0.99673. Thus, the response of the FPD to ammonia was at least approximately linear, although the Y intercept was, for unknown reasons, rather high (ie., about 40% of the response obtained for a 1.0-pL injection of the test gas). The chromatogram that resulted from the 1.0-pL injection of the test gas in the above study is shown in Figure 2, along with a typical response to 10.0 WLof ambient air, i.e., a blank sample. Note that the blank signal displays the slight negative
Figure 2. Typical chromatogaphic response to 1.0 pL of a gas W e that was prepared by injecting 1.0 ml of ammonia vapor into 250 mL of air (A). A typical response to 10 pL of air (Le.,a blank) is also shown (B).
dip at the retention time of ammonia that was mentioned previously in this paper. The signal-to-noise ratio (SIN) of the response to ammonia in Figure 2 is 13.3; thus, a linear extrapolation to an S I N of 3 suggests a detection limit for ammonia of 1.9 ng. But the presence of a high Y intercept implies that this extrapolation is likely to be erroneous. Moreover, the strong affinity of ammonia vapor for the walls of its container leads to the conclusion that the actual concentration of ammonia in the test gas almost certainly was significantly less than 8.3 pg/mL. But because the injected test gas could not have contained more than 8.3 ng of ammonia, it is clear that the detection limit of the phosphorus-selective FPD for ammonia was on the order of a few nanograms, which is roughly in line with the reported ammonia detection capabilities of the TCD (2) and the CLD (4). It should be noted that the manufacturer’s stated purity of the anhydrous ammonia (99.99 wt 7%) appears to preclude the possibility that the observed FPD response is due to an impurity in the ammonia, rather than to ammonia itself. That is, a 10-ng sample of ammonia would be expected to contain no more than 1 pg of impurities, which is at or below the detection limit of most FPDs for phosphorus. But because the means were not avilable in our laboratory to confirm the manufacturer’s stated purity, it was desired to at least confirm the FPD response to ammonia with an independent source of ammonia. Accordingly, a 3.0-pL sample of the head space from a bottle of concentrated ammonium hydroxide was injected into the gas chromatograph. This test elicited a very large “ammonia” response from the FPD that was roughly equivalent to the signal that would have been expected from 500 ng of the anhydrous ammonia vapor. This result appeared to lend further credence to the premise that the FPD was, indeed, responding to ammonia, as the probability seemed low that the impurities in both ammonia sources would be capable of producing a strong response in the FPD. Any lack of response of the FPD to nitrogen compounds other than ammonia could be advantageous in applications where such compounds are prevalent background constituents.
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As far as is I know, no instances of significant FPD sensitivity to nitrogen compounds have been reported. Indeed, secondary amines have even been derivatized with a sulfur-bearing reagent to permit them to be detected by the FPD (11). Because sensitive FPD responses to other nitrogen compounds would be of major significance to FPD users as potential sources of interference, the apparent absence of reports of such responses in the literature strongly implies that the FPD does not respond sensitively to most nitrogen compounds. But to confirm this lack of response to, e.g., a simple amine, a 1.0-pL aliquot of a 100-c(g/mL solution of n-butylamine (in methanol) was injected into the gas chromatograph under the conditions given above for ammonia determinations. No response other than the solvent peak was observed in this test. To demonstrate that the system would respond to n-butylamine in sufficient quantity, a 10-pL sample of head space gas from the n-butylamine container was injected. A large chromatographic peak, well-removed from the retention time of methanol and presumably due to n-butylamine, resulted from this challenge. Accordingly, the lack of a response to the 100-ng injection of n-butylamine (in solution form) implied that the FPD detection limit for n-butylamine was greater than 100 ng. If this tentative result is later found to hold true for all amines, the FPD could find some use in discriminating against amines during ammonia determinations. The implication that ammonia yields a sensitive FPD response, whereas amines do not, is somewhat surprising. Although several potential explanations exist for this observation, one possibility is that the flame combustion of the hydrocarbon moiety of an amine effectively quenches the emission process. Therefore, the possibility of hydrocarbon quenching obviously should be investigated before any analytical use is made of the ammonia-emission phenomenon. CONCLUSIONS It was concluded that the phosphorus-selective FPD pro-
vides a potentially useful response to ammonia in amounts greater than a few nanograms. Moreover, the FPD may prove to be the only commonly available GC detector other than the mass spectrometer that responds sensitively to ammonia without also responding sensitively to other nitrogen compounds. Finally, the FPD possibly could be rendered sensitive to most other nitrogen compounds, as well as to phosphorus compounds, by installing a reductive pyrolyzer ahead of it to convert the nitrogen compounds to ammonia (as is now done in the HECD) and the phosphorus compounds to phosphine. This approach might offer the further advantage of making the FPD respond to essentially all nitrogen and phosphorus compounds in proportion to their N or P content, so that responses to all such compounds could be calibrated with a single nitrogen compound and a single phosphorus compound. Registry No. Ammonia, 7664-41-7.
LITERATURE CITED (1) Bethea, R. M.; Meador, M. C. J. chrometogr. Scl. 1989, 7 . 655-664. (2) Hecker, W. C.; Bell, A. T. Anal. Chem. 1981, 53, 817-820. (3) McNamara, E. A.; Montzka. S. A.; Barkley, R. M.; Sievers, R. E. J. ChrOmatOgr. 1988, 452, 75-83. (4) Kashihira, N.; Makino, K.: Kim, K.; Watanabe, Y. J . Chrometog. 1982, 239, 617-624. (5) Hutte. R. S.: Sievers. R. E.: Birks. J. W. J. Chromatcar. Sci. 1988. 24. 499-505. (6) Sene, F. J.; Croce, L. C. J. C h f m t o g r . Sci. 1982. 2 0 , 575-578. (7) Sevcik, J. Chromatographla 1971, 4 , 195-197. (8) Butcher, J. M. S.; Kirkbright, G. F. Analyst 1978, 103, 1104-1115. (9) Belcher. R.; Bogdanski, S. L.: Calokerinos, A. C.; Townshend, A. Ana&St 1981, 106, 625-635. (10) Kuessner, A. Chromatographla 1982, 16, 207-210. (11) Hamano, T.; Hasegawa, A.; Tanaka, K.; Matsuki, Y. J. Chrometog. 1979. 779, 346-350.
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RECEIVED for review May 10,1991. Accepted September 12, 1991.
