Anal. Chem. 1985, 57, 2275-2279
only be obtained by carrying out the experimentsat low mobile phase velocities.
2275
(4) Klrkland, J. J.; Rementer, S. W.; Yau, W. W. Anal. Chem. 1981, 53, 1730. (5) Klrkland, J. J.; Yau, W. W.; Doerner, W. A.; Grant, J. W. Anal. Chem. 1981, 53. 1730. (6) Klrkland, J. J.; Dilks, C. H., Jr.; Yau, W. W. J . Chromatogr. 1983, 255, 255. (7) Caldwell, K. D.; Nguyen, T. T.; Glddings, J. C.; Mazzone, H. M. J . Virol. Methods 1980, 7. 241. (5) Glddings, J. C.;Myers, M. N.; Caldwell, K. D.; Fisher, S. R. Methods Blochem. Anal. 1980, 26, 79. (9) Kirkland, J. J.; Yau, W. W. Science 1982, 278, 121. (10) Schallinger, L. E.; Yau, W. W.; Kirkland, J. J. Science 1984, 225, 434. (11) Schalllnger, L. E.; Gray, J. E.; Wagner, L. W.; Knowkon, S.; Kirkland, J. J. J . Chromatogr., Blomed. Appl., in press. (12) Dilks, C. H., Jr.; Yau, W. W.; Klrkland, J. J. J . Chromatogr. 1984, 375, 45. (13) Myers, M. N.; Giddlngs, J. C. Anal. Chem. 1982, 54, 2284.
ACKNOWLEDGMENT Special thanks are extended to W. H. Emerson for carrying out some of the experiments and to R. K. Iler who supplied the silica sol, silica-modified attapulgite, and sodium polysilicate plate samples. LITERATURE CITED (1) Giddings, J. C.; Yang, F. J. F.; Myers, M. N. Anal. Chem. 1974, 46, 1917. (2) Caldwell, K. D.; Karaiskakls, G.; Giddings, J. C. Colloids Surf. 1981, 3 , 223. (3) Glddings, J. C.; Karaiskakis, G.; Caldwell, K. D.; Myers, M. N. J . Colloid Interface Scl. 1983, 92, 66.
RECENED for review March 4,1985. Accepted May 24,1985.
Lightpipe Temperature and Other Factors Affecting Signal in Gas Chromatography/Fourier Transform Infrared Spectrometry Robert S. Brown, John R. Cooper, and Charles L. Wilkins* Department of Chemistry, University of California, Riverside, California 92521
Signal losses In GC/FTIR wlth Increased llghtpipe temperatures are examlned. Chopplng experiments performed on the normally unmodulated dc heat from the hot llghtplpe show It to be the malor cause of slgnal loss. A simple heat shleld which decreases slgnal loss from over 70 % to less than 25 % at 300 O C is demonstrated. Throughput Improvements of about 20% are accomplished through the use of low angle cones to collect otherwise scattered radiation at the exit of the lightpipe. Decreased system noise Is shown for the modified system.
Gas chromatography directly coupled with Fourier transform infrared spectrometry (GC/FTIR) is becoming increasingly popular. This fact stems from the progress made in the last decade in improving sensitivity, making possible its utilization with capillary columns. It has therefore become a powerful analytical tool for analysis of even extremely complex mixtures (1-5). Several comprehensivereviews have been published on this technique (6, 7). Most recently, FTIR used in conjunction with gas chromatography/mass spectrometry (GC/MS) in a combined linked system has been demonstrated (8, 9) and the potential of such systems continues to be developed. However, GC/FTIR remains overshadowed by the more popular GC/MS technique, due basically to its sensitivity limitations (low nanograms). This has precluded its application with the most efficient capillary columns (0.25 mm i.d. or less) and has made trace level determinations difficult because of the limited range (about 2 orders of magnitude) imposed by detector sensitivity and column capacity. In addition, lack of a large gas-phase infrared data base similar to that available for GC/MS (over 80 000 compounds)has precluded realization of GC/FTIR’s potential for identifying unknowns unambiguously. This latter problem is slowly being addressed as currently available data bases are expanded while research intended to improve both sensitivity and dynamic range is pursued. 0003-2700/85/0357-2275$01.50/0
Two basic approaches to the interfacing of gas chromatographs with FTIR have emerged. The earliest and still most popular method involves the use of a gold-coated glass “lightpipe” originally developed by Azarraga (10) through which the GC effluent passes while standard transmittance spectra are collected. Refinements in this area have pushed sensitivities to the low nanogram level (11, 12) for some compounds. A more recent innovation which offers substantial improvements in sensitivity for GC/FTIR (mid-picogram range) is the matrix isolation GC/FTIR interface (13). In this method GC effluents are trapped in an argon matrix (formed by using a He/Ar carrier gas and impinging effluent on a polished surface at low temperature). Subsequently, reflectance measurements of the trapped species are made. Although superior sensitivity is thus achieved, this system is relatively expensive and suffers from the lack of any currently available matrix isolation reference spectral data base. Even though this approach may eventually become the method of choice for trace analysis, the less costly and mechanically simple lightpipe interface should remain competitive for many practical applications. Much work over the past few years has been directed toward improving the signal to noise ratio ( S I N ) of GC/FTIR measurements utilizing lightpipe interfaces. These include (a) improvements in coating procedures for GC lightpipes (14), (b) optimization of volume/length ratios of lightpipes to allow the best sensitivity with the least chromatographic degradation (15), (c) studies of various optical configurations (16)) (d) employment of smaller area detectors exhibiting lower noise characteristics (16,17),and (e) introduction of extremely rapid scanning interferometers for both improved time resolution and increased spectral averaging. Optical throughput is an important consideration in determining spectral SIN. This is especially true for the narrow bore low volume lightpipes employed in capillary GC/FTIR interfaces. It has been known for some time that signal decreases as the lightpipe interface is heated (3). Such heating is unavoidable, as it is necessary to ensure that compounds do not condense. A variety of 0 1985 American Chemical Society
2276
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985 2.1 mm H
Figure 1. Optical layout of the Nicolet 60SX GCIFTIR accessory.
explanations for this phenomenon have been proposed. These include changes in the reflectivity of the gold (18),out-of-phase thermal radiation destructively interfering with source radiation (3),sagging (or deformation) of the lightpipe, causing misalignment (most recently discussed by deHaseth (19),but originally proposed by Hirschfield), and detector saturation due to unmodulated radiation emitted from the lightpipe (16). NOwork has been published to date which presents experimental evidence for these proposed explanations of signal loss with temperature. The present paper demonstrates experimentally that the most significant contribution to signal loss is detector saturation caused by unmodulated lightpipe radiation. This causes a highly nonlinear response of the mercury cadmium telluride (MCT) detectors typically employed. Practical modifications to a commercially available system to minimize this effect have been investigated.
EXPERIMENTAL SECTION Experiments were performed on a Nicolet 60SX spectrometer equipped with a 16-bit AID converter, narrow band MCT (D* = 42.5 X lo9 cm Hz1/2W at 10 kHz) detector, triglycine sulfate (TGS)detedor, 80 Mbye storage module, and the standard Nicolet GC optics bench available for the 60SX as diagramed in Figure 1. The collimated beam from the interferometer enters the GC bench which is mounted vertically on the side of the main optical bench. The beam is focused on a 1mm i.d. X 15 cm long lightpipe by a 3.5 in. effective focal length (70° off-axis) paraboloid mirror and the beam exiting the lightpipe collected by a 9.3 in. effective focal length (60Doff-axis) paraboloid which passes the beam via a flat mirror to a second paraboloid of 2.5 in. effective focal length (90° off-axis), focusing the beam onto a 1 mm2 MCT detector. The lightpipe is enclosed in a standard Nicolet variable temperature mount which can be heated in excess of 300 OC and employs -1 mm thick NaCl windows. Infrared data were collected as 2048 point (8 cm-') interferograms and Fourier transformed employing one zerofill and a Happ-Genzel apodization function. This procedure produces a single beam spectrum which can be ratioed vs. a suitable reference spectrum to produce either an absorbance or transmittance spectrum. For all measurements, 16 scans were coadded per spectrum at a mirror velocity of 2.072 cm/s (rapid scan mode), requiring a 1.2 s total data acquisition time. After initial alignment at ambient temperature, no attempt at realignment was made as the temperature was increased. To perform the chopping experiments, a chopper was constructed. This consisted of a 22.8 cm circular piece of sheet metal with eight equally spaced 2.54-cm holes. This was mounted via a shaft to a variable speed rotary tool (Sears,Roebuck & Company,
CL
I ,
Figure 2. Water-cooled heat shield showing mounting on lightpipe holder, short lightpipe extension, and brass cone.
Table I. Signal vs. Temperature-Standard Configuration temp, O C 25 55 100 150
200 250 300
interferogram centerburst, V
% of ambient
9.857 9.025 7.620 6.167 4.762 3.613 2.745
100 91.6 77.3 62.6 48.3 36.7 27.8
throughput
Chicago, IL) so as to chop the emerging beam directly after its exit from the lightpipe. The chopped (250 Hz) signals were then monitored with an oscilloscope. Short lightpipe extensions were cut from longer lightpipes, prepared as previously reported (15). The cooled holder (Figure 2) was machined from brass to easily mount on the outlet side of the lightpipe assembly, as were the various angular polished brass cones employed to collect scattered radiation.
RESULTS AND DISCUSSION Of the possibilities previously mentioned as the cause for the signal loss with temperature in GCIFTIR, sagging of the lightpipe and detector saturation could be examined most readily. Signal losses with temperature for the standard Nicolet 60SX configuration are shown in Table I. Interferogram centerburst. height was used as a measure of signal loss vs. lightpipe temperature. Signal losses were over 70% at 300 OC. This is similar t o losses previously reported for other systems (3). In order to test the theory that signal losses could be caused by deformation of the lightpipe during heating (Le., bending or sagging), the lightpipe was suspended from the inlet side via a graphite ferrule and held in place by the end-fitting to which the NaCl window was attached. This, coupled with the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
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Flgure 3. Chopping experiments showing qualitative comparisons of dc heat (+) and dc heat plus IR source (") vs. temperature for standard configuration: (a) MCT, (b) TGS.
vertical alignment of the lightpipe in the GC optics bench, removed the possibility of lightpipe sagging during heating and allowed for thermal expansion of the lightpipe without any bending. The signal losses with this configuration were identical with those measured with the standard arrangement, thus ruling out mechanical deformation as a significant cause of signal loss. Realignment in both bases after heating produced a small (- 2%),but relatively insignificant, improvement. It is well-known (20) that MCT detectors are easily driven to a nonlinear response as a result of too much radiation striking the detector. Because the output signal from the detector/preamplifier is only an indication of the modulated source radiation, constant dc sources (i.e., the hot lightpipe and holder) of IR radiation will go unnoticed but can still drive the detector to a point of nonlinear response. In order to study the amount of unmodulated heat striking the detector, a series of chopping experiments were performed. The chopper was placed directly beneath the beam exit from the lightpipe and operated at a chopping frequency of 250 Hz. In order to prevent saturation of the preamplifier, the detector output signal was measured with an oscilloscope after the first stage of amplification. Even a t this point, it was necessary to reduce the amplifier gain by half to ensure that the amplifier would not be saturated by the now modulated lightpipe/holder heat source. For these experiments, the interferometer scanning was disabled and the chopped signals from the normally unmodulated heat were examined with the IR source blocked. The sum of the now modulated heat and the IR source was also measured in a separate experiment with the IR source unblocked. These signals were examined as a function of temperature and are plotted in Figure 3a. The lower line (+) is the amount of normally unmodulated radiation reaching the detector as the lightpipe
is heated from room temperature to 300 "C. Figure 4 represents the theoretical output of a blackbody radiator as a function of temperature (aPwhere T i s temperature; arbitrary scaling). The response of the detector can be seen to be far from the ideal blackbody curve, indicating severe detector nonlinearity when the lightpipe is heated. The upper solid line (asterisk) in Figure 3a is the combined response of the IR source radiation and the heat from the hot lightpipelholder. The difference between the two solid lines can be seen to mimic the throughput loss due to heating (Table I). This rules out destructive interference of out-of-phase thermal radiation as the cause of signal loss with temperature. Although not conclusively eliminating changes in lightpipe transmittance, this does suggest that the most likely cause of the signal losses lies in the highly nonlinear response of the MCT detector when swamped by a large amount of unmodulated heat. In order to show that changes in lightpipe transmittance vs. temperature were not the cause of signal loss, a more linear (but less sensitive) TGS detector was substituted for the MCT detector and the above experiments were repeated. Figure 3b shows that the response for the chopped heat and chopped heat plus IR source radiation now are parallel within the accuracy of the oscilloscope (with no throughput loss) and that they also approximatethe response expected from a blackbody source. This shows that the dominant reason for signal losses with increased temperature is the high unmodulated heat flux reaching the detector, driving it into a nonlinear response range. The net result is a depression of the expected response to the entire signal (unmodulated heat and IR source) and particularly the detector's response to the decreasing proportion of the total infrared radiation coming from the source. Understanding the cause of the signal losses allows steps to be taken to minimize the unmodulated heat focused on the detector. Several methods of blocking unmodulated heat while minimizing IR source radiation loss were examined. A variable aperture was placed over the exit from the lightpipe and decreased to an opening slightly larger than that causing throughput loss at room temperature. Here, reduction to 50% (as compared to 37% normally) of the room temperature throughput at 250 "C was observed. The inability to place the aperture close enough to the lightpipe window to allow good blockage of unmodulated heat without blocking the diverging IR beam and heating of the aperture through conduction probably prevented further improvements. Following this experiment, a more effective heat shield was constructed. The final version of the heat shield combined several features to improve overall performance. A short lightpipe was chosen to serve as a beam block and mounted against the exit window via the holder (Figure 2). This ensured blocking at the point of the smallest emerging IR beam and allowed the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
2278
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Table 11. Signal vs. Temperature-with Shield temp, O C 25 55
100 150 200 250 300
interferogram centerburst, V 9.402 9.329 9.111 8.681 8.234 7.653 7.292
% of ambient
-
0031
throughput 100 99.2 96.9 92.3 87.6 81.4 77.6
narrowest lightpipe extension to be employed, thus blocking as much of the unmodulated heat as possible. To ensure that the extension and holder would not be heated through contact with the heat source, cooling water was run through the mounting block. The length of the extension was made sufficient (35 mm) to allow for cooling the end facing the detector optics through contact with the cooled mounting while allowing the opposite end to heat up through contact, thus preventing condensation on the outlet window. The cooled mounting was insulated from direct contact with the heater block. The internal diameter (i.d.) of the extension was found to be critical in order to effectively block the unmodulated heat without significantly blocking the IR beam. The final internal diameter employed was 1.47 mm. This choice passed over 95% of the IR beam and substantially blocked a large portion of the unmodulated heat. To examine the effectiveness of this heat block, chopping experiments were again performed as previously described. Figure 5a shows both the normally unmodulated heat (+) and unmodulated heat and IR source radiation (asterisk) vs. temperature. As can be seen, a dramatic decrease in unmodulated heat is obtained with minimal IR source throughput loss. The unmodulated heat now can be seen to follow the
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Wavelength dependence of increased throughput with 10" brass cone. Figure 7.
theoretical blackbody response (Figure 4) much more closely. Table I1 presents the actual interferogram height vs. temperature data. Even at 300 "C, almost 80% signal is maintained as compared to less than 30% with the standard configuration. During construction of various heat blocks, it was noted that a significant increase in throughput was obtained when a conical brass end fitting (Figure 2) was employed at the exit of either the lightpipe extension or the NaCl window at all temperatures. This effect is attributed to the refocusing of scattered radiation exiting the lightpipe. This phenomenon appears to be related only to the lightpipe and not to the NaCl window since it is observed to approximately the same extent with or without the window present. A study of the effect of the angle of the cone was performed and correct choice of cone angle was found to be important to obtain maximum improvement of optical throughput. Figure 6 shows a plot of cone angle vs. maximum signal throughput at ambient temperature using the unmodified system. A gradual increase can be seen until loo,after which the signal drops. This is approximately the angle of the diverging beam as it exits the lightpipe. A variable aperture closed to the diameter just causing signal loss at the detector was used at two distances from the lightpipe end and a beam angle of -13" was determined. While an improvement of -20% in throughput is achieved, as measured by the interferogram centerburst,
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
Table 111. Signal vs. Temperature-with Shield and Cone temp,
"C
25 55 100 150 200 250 300
interferogram centerburst, V
% of ambient
11.084 10.967 10.659 10.236 9.624 8.841 7.870
100 98.9 96.2 92.3 86.8 79.8 71.0
throughput
Table IV. System Noise vs. Temperature temp,
"C
25 55 100 150 200 250 300
I"
RMS noised IIb
111'
3.29 3.46 4.05 5.10 6.29 7.99 10.33
3.57 3.54 3.69 3.63 3.73 4.26 4.40
2.90 3.02 3.08 3.31 3.44 3.87 4.06
a S t a n d a r d configuration. L i g h t p i p e extension. L i g h t p i p e extention 10" cone. dAbsorbance X10-4 (16 scans coadded).
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Flgure 8. 100% lines at 300 'C for (A) standard configuration, (6) lightpipe extension, and (C) lightpipe extension and 10" cone. Ail are to the same scale with appropriate offsets to A and C.
it is wavelength dependent. Figure 7 shows the single beam spectrum with and without addition of the cone. More throughput is achieved at longer wavelengths, possibly due to imperfections in the machined cone which would tend to scatter shorter wavelengths. This possibly might be corrected by gold-coating the cones as is done for lightpipes. A 10' polished brass cone whose dimensions are shown in Figure 2 was constructed to mount against the lightpipe ex-
-
2279
tension on the beam block previously described. Table I11 shows the throughput as a function of temperature. Although percent of ambient throughput is slightly poorer for this system than that without the cone, the total throughput is higher, especially at the longer wavelengths (i.e., the fingerprint region). The reason for this drop in relative throughput can be seen in the results of the chopping experiments presented in Figure 5b. The cone increases not only the modulated IR source radiation reaching the detector but also the unmodulated heat. This drives the detector into a more nonlinear region and produces a correspondingdrop in observed signal. It should be noted here that the addition of the exit cone actually produces an increase in the throughput of IR radiation and not just an increase in observed detector signal as in the case of the heat block. Achievement of the goal of improving SIN is demonstrated in Figure 8. Here, 100% lines at 300 "C for the three configurations described are plotted. As expected, both modified systems showed decreased noise over the standard system. Root mean squared (RMS) noise values calculated from 4000 to 800 cm-l as a function of temperature are presented in Table IV for each system. The noise is roughly proportional to throughput, as expected.
LITERATURE CITED Cooper, J. R.; Taylor, L. T. Appl. Specfrosc. 1984, 38, 366-370. Purceil, J. M.; Magidman, P. Appl. Specfrosc. 1984, 38, 181-184. Gurka, D. F.; Laska, P. R.; Titus, R. J. Chromatogr. Sci. 1982, 20, 145-154. Smith, S. L.; Garlock, S.E.; Adams, G. E. Appl. Spectrosc. 1883, 3 7 , 192-196. Gurka, D. F.; Betowski, L. D. Anal. Chem. 1982, 54, 1819-1824. Erickson, M. D. Appl. Spectrosc. Rev. 1979, 75,261-325. Griffiths, P. R.; deHaseth, J. A.; Azarraga, L. V. Anal. Chem. 1983, 55,1361A-1387A. Laude, D. A.; Brissey, G. M.; Ijames, C. F.; Brown, R. S.; Wilkins, C. L. Anal. Chem. 1984, 56, 1163-1 168. Wiikins, C. L.; Giss, G. N.; White, R. L.; Brissey, G. M.; Onyiriuka, E. C. Anal. Chem. 1882, 54,2260-2264. Azarraga, L. V. Appl. Spectrosc. 1980, 3 4 , 224-225. Cooper, J. R.; Taylor, L. T. Anal. Chem. 1984, 56, 1989-1993. Kuehl, D.; Lemeny, G. J.; Griffiths, P. R. Appl. Spectrosc. 1980, 3 4 , 222-224. Reedy, G. T.; Bourne, S.; Cunningham, P. T. Anal. Chem. 1979, 57, 1535-1540. Yang, P. W.; Ethridge, E. L.; Lane, J. L.; Griffiths, P. R. Appl. SpectrOSC. 1984, 38, 813-816. Giss, G. N.; Wilkins, C. L. Appl. Specfrosc. 1984, 38, 17-20. Yang, P. W.; Grifflths, P. R. Appl. Spectrosc. 1984, 38, 816-821. Hirschfeld, T. Appl. Spectrosc. 1977, 37,471-472. Hlrschfeld, T. Presented at FACSS V; Boston, MA; November, 1978; Paper No. 148. deHaseth, J. A. Presented at 23rd Eastern Analytical Symposium; New York, NY; November 1984; Paper No. 112. Chase, D. E. Appl. Specfrosc. 1984, 38, 491-494.
RECEIVED for review March 28, 1985. Accepted June 7,1985. We gratefully acknowledge support of this research by the National Science Foundation under Grant CHE-82-08073. Partial support from the United States Environmental Protection Agency under cooperative agreement CR-811730-01 is also acknowledged.
