Anal. Chem. 1908, 6 0 , 1487-1483
isomers (6). I t was postulated that this trend may be instrument related. Since the recoveries of all internal standards in the series are similar (see above), and approximately the same amount of all eight standards was spiked into the samples, the relative peak areas for the molecular ion of each standard will be directly proportional to their absolute sensitivities. Thus, the signal to noise ratio for these ions will decrease in going from tetrachlorinated to octachlorinated analytes, and the error in determining the area under the molecular ion peak will be greater for the higher chlorinated isomers. This trend is not found for R(-) values. In this case, the larger error in determining R(-) may mask this effect.
CONCLUSIONS The use of injection standards is an effective technique for increasing the reliability of recovery data obtained by GC/MS analysis. It is particularly useful for analyses that requires handling and injecting very small volumes of sample (L
-
l
1
1 1 ,
0
I000 2000 3000 4000 dCL‘5
Flgure 2. Nitrogen background luminescence vs applied voltage.
EXPERIMENTAL SECTION The photometric detector used in this study is similar to one described earlier (19). Briefly, it is a modified Shimadzu flame photometric detector, in which the flame as the source of energy is replaced by an axially inserted electrode carrying, typically, a potential of +4000 V at a distance of 10 mm opposite the grounded detector jet tip, and by a small sliver of 6 3 N i / Afoil. ~ In addition, the detector is kept airtight and at a slight overpressure to minimize the influx of atmospheric nitrogen. Nitrogen is introduced to “prepurified” grade argon (99.998% minimum) by direct injection (for larger amounts and for confirmation) or by exponential dilution flask (for lower amounts or for establishing a slowly changing background). Both introduction techniques are combined when the behavior of the analyte onlagainst a particular background is of interest. A Jarrel-Ash quarter-meter grating spectrometer provides spectral information, while the conventional R-374 Hamamatsu photomultiplier tube semes without optical discrimination (similar to a filterless flame photometric detector arrangement) for analytical measurements. RESULTS AND DISCUSSION Figure 1 shows the spectrum of the detector luminescence emitted close to the polarizing electrode (where the concentration of argon metastables is highest). A bluish glow is easily visible to the dark-adapted eye. Most of the emission lies in the ultraviolet region and belongs to the second positive system of molecular nitrogen (20). Figure 2 shows the level of luminescence produced by about 300 ppm N2,as it rises with voltage applied across the 10-mm interelectrode gap. At the typical drive voltage of 4000 V, the current is about 20 nA. Operation a t significantly higher voltages is limited by sparking (and by a 5-MO resistor in the voltage feed line).
100
I
I
I
I
1
200 400 800 1600 3200 NITROGEN ppm
Flgure 4. Response of nitrogen analyte on a nitrogen background.
Figure 3 shows the calibration curve, established by injecting nitrogen into the exponential dilution flask that forms part of the argon supply line. The upper regions of the curve were confirmed by injecting directly into a gas chromatography (GC) port located between flask and detector. At a signalto-noise ratio of 2, the minimum detectable volume-per-volume concentration of added nitrogen is about 0.3 ppm. The linear range stretches over more than 3 orders of magnitude. This performance compares well with that of the literature methods (and it proved more than adequate for solving our own problem). Still, there is little doubt that it could be further improved by, for instance, optimizing the light-gathering setup. The new photometric nitrogen detector can and has been used both with and without a gas chromatographic column. Particularly when a large chromatographic system offers several potential routes for the undesired entry of atmospheric nitrogen, though, the question arises of how far such a background could depress the nitrogen response proper. This question is valid not only for nitrogen but also for other permanent gases that might be present in the sample stream or intrude from the outside (e.g. atmospheric oxygen). Figure 4 shows the response of 2 p L of nitrogen gas (corresponding to ca. 1000 ppm at peak apex) that is repeatedly injected into a packed column while background nitrogen is introduced simukaneously via the exponential dilution flask. Figure 5 shows the results of a similar experiment involving background oxygen or hydrogen. (This quenching effect can, incidentally, serve to detect a variety of permanent gases separated by the chromatographic column, if a background of nitrogen luminescence is deliberately established in the detector.) For the detection of nitrogen in high-purity argon, however, these quenching effects are too small to represent a noticeable
Anal. Chem. 1988, 60, 1483-1488 40
effect might be considered an opportunity to improve the detectability of nitrogen. However, this presumes the absence of any nitrogen contamination from the instrumental background, which would otherwise be also amplified and would commensurately raise the noise level. The determination of trace concentrations of molecular nitrogen in a high-purity argon stream-with or without chromatography-is thus easily achieved if a flame photometric detector happens to be available in the laboratory for modification. We assume that an argon ionization detector could serve a similar function if fitted with a photomultiplier tube (or a light guide and photomultiplier tube if other purposes forced its occasional operation at high temperature). However, we did not investigate the latter possibility. Registry No. N P ,7727-37-9;Ar, 7440-37-1;02, 7782-44-7;H,, 1333-74-0;isobutane, 75-28-5.
in
c C 3
P 30
f 2W (3
g
1483
20
t z
0 + w ln 10 z
a 0 rn W
LL
0 0
4 0 0 800 1200 1600 2000 2400 WENCHER. ppm
LITERATURE CITED
.q
Figure 5. Quenching of nitrogen analyte response by background oxygen and hydrogen.
N> Response
/I
Background
L-
0
I
40
00
1 5 0 - BUTANE.
120
ppm
Figure 6. Enhancement of nitrogen analyte response by background
isobutane. interference. Even with a significant (but constant) background, the calibration curve for nitrogen would be simply shifted into a slightly higher concentration range. It should be noted in this context that contaminants with ionization potentials below 11.7 eV-such as most organic compounds-will not depress but rather will enhance the nitrogen signal (19). This is demonstrated in Figure 6. The
(1) Morisako, 1.; Kato, T.; Ino, Y.; Schaefer, K. I n t . J . Mass Spectrom. I o n fhys. 1983, 48, 19. (2) Koprio, J. A.; Gaug, H.; Eppler, H. I n t . J. Mass Spectrom. Ion fhys. 1983, 48, 23. (3) Annu. Book ASTM Stand. 1987, 10.03, C 997-83. (4) Bourke, P. J.; Dawson, R. W.; Denton, W. H. J. Chromatogr. 1984, 14, 367. ( 5 ) Jeffery, P. G.;Kipping, P. J. Gas Analysis by Gas Chromatography, 2nd ed.; Pergamon: Oxford, 1972, p 102. (6) Fay, H.; Mohr, H.; Cook, G. A. Anal. Chem. 1962, 3 4 , 1254. (7) Smith, R. E. Report BDX-613-2774; US. Department of Energy: Washington, DC, 1982, p12; Anal. Abstr. 45(5),528, 5J7. (8) Sheppard, D. S.;Truesdeil, A. H. Chromatographia 1985, 20, 681. (9) Wright, A. N.; Wlnkler, C. A. Active Nitrogen; Academic: New York, 1968. (10) Northway, S. J.; Brown, R. M.; Fry, R. C. Appi. Spectrosc. 1980, 3 4 , 338. (11) Hughes, S.K.; Brown, R. M.; Fry, R. C. Appi. Spectrosc. 1981, 3 5 , 396. (12) McKenna, M.; Marr, I.L.; Cresser. M. S.;Lam, E. Spectrochim , Acta, f a t i S 1986, 4 1 8 , 669. (13) Freeman, J. E.; Hieftje, G. M. Appl. Spectrosc. 1985, 3 9 , 211. (14) McCormack. A. J.; Tong, S. C.; Cooke, W. D. Anal. Chem. 1985, 3 7 , 1470. (15) Korolev, V. V.; Timofeev, E. F. Zavod. Lab. 1973, 3 9 , 1150; Chem. Abstr. 1974, 80, 66364e. (16) Bochkova, 0.P.; Gardashnikov, L. E.; Mikhailov, S. K.; Turkin, Yu. I. Spektrosk., Tr. Slb. Soveshch ., 6th, 1968 (1973),89; Chem. Abstr. 1974, 8 0 , 66429e. (17) Lovelock, J. E. J. Chromatogr. 1958. 1 , 35. (18) Tang, Y.2.; Aue, W. A. J. Chromatogr. 1987, 409, 243. (19) Tang, Y.-2.; Aue, W. A. J. Chromatogr. 1987, 409, 125. (20) Pearse, R. W.; Gaydon, A. G. The Identification of Molecular Spectra; Chapman and Hall: London, 1976, p 219.
RECEIVED for review November 18,1987. Accepted February 22, 1988. This study was supported by NSERC Operating Grant A-9604. Material was taken from the doctoral thesis of Y.-Z. Tang, Dalhousie University, 1987.
Cryogenically Cooled Interface for Gas Chromatography/Fourier Transform Infrared Spectrometry Robert S . Brown Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Charles L . Wilkins*
Department of Chemistry, University of California, Riverside, Riverside, California 92521 The capabilities of gas chromatography/Fourier transform infrared spectrometry (GC/FT-IR) for complex mixture analysis have been well documented (1-5). Its ability to provide complementary information when used in conjunction with gas chromatography/mass spectrometry (GC/MS) has been discussed in depth, as has the enormous potential of 0003-2700/86/0360-1463$01.50/0
directly linked GC/FT-IR/MS systems (6-10). GC/FT-IR data often appear to provide better computer spectral search identifications of components than GC/MS, as well as compound class identification when spectra of unknowns are not present in the spectral data base. Compound class identifications are more difficult to obtain from GC/MS data. This 0 1988 American Chemical Society
