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(4) L. Mertz, "Transformations in Optics", John Wiley, New York, N.Y. 1965. (5) H. Bar-Lev, Infrared Phys., 7, 93 (1967). (6) M. J. D. Low, Anal, Lett., 1, 819 (1968). (7) P. R. Griffiths. Chemical Infrared Fourier Transform Spectroscopy", Wilev-Interscience. New York. N.Y.. 1975. ChaD. 7. (8) M. D. LOW and H. Mark, J . Paint Technol.,'43 (533),31 (1971). (9) P. R . Griffiths and J. 0. Lephardt, paper presented at the 20th Pittsburgh Conference on Analytical Chemistry and ~ p s p e~ ~clevehpj, ~ ~ ~ Ohio, 1969.
(10) L. Mertz, Infrared Phys., 7, 17 (1967). (1 1) S. Borrello, M. Kinch, and D. LaMont. Infrared Phys., 17, 121 (1977). (12) T. Hirschfeld, personal communication, 1977.
RECEIVEDfor review June 13, 1977. Accepted November 9, work was supported by the U.S. Environmental , Protection Agency under grant number R804333-01.
~1977. ~ ~This ~
On-Line Identification of Gas Chromatographic Effluents by Dual-Beam Fourier Transform Infrared Spectrometry Maria M. Gomez-Taylor and Peter
R. Griffiths"
Department of Chemistry, Ohio University, Athens, Ohio 4570 1
An infrared optical system was built around a commercial Fourier transform spectrometer primarily for the on-the-fly analysis of organic compounds eluting from a gas chromatograph. This system is based on the dual-beam or optical subtraction technique, whereby the dynamic range of the interferogram is reduced without decreasing the total energy flux reaching the detector. Therefore a more sensitive detector may be used without encountering digkization noise problems that occur when the signal-to-noise ratio of the interferogram exceeds the dynamic range of the analog-to-digital converter of the data system. The use of a mercury cadmium telluride photoconductive detector, a high temperature source and light-pipe gas-cells of high optical transmission contributed to the high sensitivity achieved by the system. Identifiable spectra of 100 ng or less of strong absorbers have been obtained with this dual-beam GC-IR system.
One factor limiting the sensitivity of conventional infrared Fourier transform spectrometers is the dynamic range of the analog-to-digital converter (ADC). For example, the signal-to-noise ratio (S/N) of an interferogram of an unattenuated incandescent source generated by a rapid-scanning interferometer and measured with a pyroelectric bolometer can be as high as 104:1,so that if the signal were digitized with a &bit ADC, the noise level would be less than two bits. If the S / N of the interferogram were much larger, the noise level would fall below the least significant bit of the ADC, and the noise level on the spectrum would be determined by the ADC (digitization noise) rather than by the detector (detector noise). In order to avoid inaccurate sampling of the interferogram, at least one bit should be used to sample detector noise. Under these circumstances, the full benefits of replacing the relatively insensitive triglycine sulfate (TGS) pyroelectric bolometer normally used for FT-IR spectrometers with the liquid nitrogen-cooled mercury cadmium telluride (MCT) photoconductive detector are not attained even though this detector is a t least 20 times more sensitive than the TGS detector. Several methods can be used to keep the S/Nof the interferogram from exceeding the dynamic range of the ADC when the MCT detector is used for GC-IR measurements. The temperature of the infrared source could be reduced; however, the S / N of the spectrum would be significantly degraded a t high frequencies. The dynamic range of the 0003-2700/78/0350-0422$01.00/0
interferogram could be reduced by scanning the moving mirror faster. However to reduce the S / N of the interferogram by a factor of X, the velocity must be increased by a factor of X'. As a result of this, the data rate may well be increased beyond the maximum allowed by the ADC or the disk-based data system. In addition, the duty cycle efficiency of the interferometer is usually lowered as the scan speed is increased. Azarraga ( I ) developed a technique to eliminate the dynamic range problem which involves the use of long and narrow light-pipes where reflection losses attenuate the signal across the complete spectrum. The decrease in sensitivity due to a smaller energy flux a t the detector is partially compensated by the increase in the absorbing pathlength of the cell. However, Griffiths (2) recently made some calculations showing that the S / N gained using Azarraga's method with an MCT detector is only about a factor of four better than the optimum value obtainable using a TGS detector. A technique that has been used to get around the dynamic range problem without decreasing the total energy flux reaching the detector involves dual-beam (DB) or optical subtraction FT-IR, the theory of which is discussed elsewhere (3-5). The dual-beam technique has been used in the past for gas analysis by several authors with limited success. Bar-Lev (6) described a dual-beam interference spectrometer incorporating one source, two detectors, and a long pathlength cell. This system was used for the detection of gases a t low concentration. A nulling ratio (7) of 40:l was attained with this system. Low (8)described an experimental arrangement designed for the infrared identification of GC effluents, which consisted of one detector and two sources. A nulling ratio of 30:l was obtained with both gas cells a t room temperature and the nulling ratio decreased to 20:l after the cells were heated. Several years later, Low and Mark (9) described a system consisting of one source and two detectors, using which a nulling ratio of 1OO:l was achieved. Griffiths and Lephardt (10)designed an arrangement using a single source and a single detector for the purpose of measuring the infrared spectra of GC peaks. They attained a nulling ratio of 1OO:l even when the gas cells were hot. The main advantage of this system was that no pick-off mirrors were present in the beam path, thus increasing the optical throughput of the spectrometer. Surprisingly enough, the dual-beam technique has rarely been used under conditions when the single-beam spectrum would have been digitization noise limited (which is really the only time that this method is useful). As pointed out previously 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
I INTERFEROMETER
I
4
P A R A B O L O I AL
I I
/ /
MIRROR
I I
I
_ _I
I
DETECTOR
3 F F . AXIS ELLIPSOID
Figure 1. Optical layout of the dual-beam system for the on-line measurement of GC peaks
( 5 ) ,the application of the DB-FT-IR technique should allow a definite improvement in the sensitivity of GC-IR measurements now that the sensitive M C T detector is readily available. This paper describes a system designed around a Digilab Model FTS-14 Fourier Transform spectrometer for DB-FT-IR work using an M C T detector. The system has been applied to the on-the-fly measurement of infrared spectra of GC peaks. This system has both a high optical transmission and a high optical throughput and, consequently, a very high sensitivity should be achieved.
EXPERIMENTAL Instrumentation. The optical layout of the dual-beam GC-IR system is shown diagrammatically in Figure 1. The source is a modified Nernst glower (137 type from Perkin-Elmer Corp., Norwalk, Conn.). A 3-inch focal length 45' off axis paraboloidal mirror (Special Optics Corp., Little Falls, N.J.) was used to produce a beam of radiation 2 inches in diameter with a solid angle steradians as input into a Model 296 interferometer of 3.43 X (Digilab Inc., Cambridge, Mass.). The input beam to the interferometer is slightly skewed allowing both output beams to be collected by two plane mirrors which direct the radiation towards two off-axis paraboloids identical to the one mounted in the source unit. Each output beam is brought to a 3-mm diameter focus at the entrance to the light-pipes. Two light-pipes 30 cm in length and 4 mm x 4 mm in cross-section enclosed in an oven assembly (Norcon Instruments, Inc., S. Norwalk, Conn.) were used in this study. The two emerging beams from the light-pipes are focused onto a 2-mm square MCT detector (Texas Instruments, Dallas, Texas) by two off-axis ellipsoidal mirrors cut from a 12.5-inch diameter section with focal lengths of 4.3 and 10.3 inches (Special Optics Corp.). Chromatography. The gas chromatography was performed using a Perkin-Elmer Model 3920 gas chromatograph equipped with a thermal conductivity detector. For observing sample quantities of less than 1 Mg, a Gow-Mac Model 40-700 flame ionization detector (FID)/electrometer unit was adapted t o the chromatograph. This detector was used in conjunction with an effluent stream splitter whose measured split ratio was about 14:l. The temperature of the transfer line between the chromatograph and the light-pipes was monitored in several places to ensure that no cold spots were present.
423
Procedure. Interferograms were signal-averaged during the time that each peak was present in the light-pipe, and interferograms from successive GC peaks were stored in sequential arrays in the data system of the FTS-14 spectrometer. A 100-scan optically subtracted reference interferogram of the empty cells was subtracted digitally from each sample interferogram and the subtracted interferogram was then transformed using doubleprecision software (32 bits per word) to give the infrared spectrum. All spectra were measured at 8 cm-' resolution.
RESULTS AND DISCUSSION The dual-beam system for GC-IR work achieved a nulling ratio of 151when the light-pipes were heated. Even with this rather poor nulling, the reduction of the modulated signal of the interferometer was sufficient to reduce the S / N at zero path difference below the dynamic range of the ADC. T h e different variables affecting the sensitivity of any GC-FT-IR system have been discussed in detail previously ( 1 1 ) . These include the chromatographic conditions, the dimensions of the light-pipe gas-cell, the type of source, the type of detector, and the scan speed of the interferometer. The dual-beam FT-IR configuration for GC-IR measurements was designed with these variables taken into consideration. The improvement in sensitivity achieved by the system over an earlier GC-IR system based on the FTS-14 spectrometer (12) was derived from three main modifications: (1) the replacement of the nichrome wire source normally used with the FTS-14 spectrometer with a modified Nernst glower, ( 2 ) the installation of light-pipe gas-cells of dimensions 30 cm X 4 mm X 4 mm, and (3) the replacement of the standard TGS detector with an M C T detector. T h e replacement of the nichrome wire source with a modified Nernst glower permits operation a t higher temperatures, thus increasing the infrared energy reaching the detector especially in the fingerprint region, below 2000 cm-'. It was found that the use of a Nernst glower source allowed spectra t o be measured a t about two to three times greater sensitivity than that attainable with the nichrome wire source in this region. T h e S / N advantage decreases at higher frequencies since above 2000 cm-' the emissivity of the Nernst glower falls off quite rapidly. In order to obtain the maximum sensitivity in GC-IR, as much sample as possible should be present in the light-pipe during the measurement time. Griffiths (13) has made some theoretical calculations for the optimum dimensions of light-pipe gas-cells according to the chromatographic c m ditions and found that the optimum volume for a flow-through gas-cell is equal to the volume of the carrier gas between the half-width points of the GC peak. Sharp peaks are desired to obtain a greater S / N and the cell volume must be limited in order to avoid the possibility of having two different GC peaks in the cell simultaneously. The half-width of a sharp peak eluting from a gas chromatograph with a '/*-inch 0.d. packed column is typically about 5 mL, so that the volume of the light-pipes used, 4.8 mL, is therefore very nearly ideal for GC-IR work utilizing packed columns. T h e pathlength and cross-sectional area for this cell are close to the optimum calculated dimensions, although the measured transmittance of the tubes (20-25% j was only about half of the calculated value ( 50 70). An experimental comparison with respect t o the relative sensitivities of the M C T and T G S detectors is complicated by the nonlinearity of the MCT detector response. Kuehl and Griffiths (14) have found that a high d.c. level of radiation on the MCT detector caused the sensitivity advantage of an MCT detector to be smaller than expected. At high radiation levels the detector is apparently driven t o saturation which causes the nonlinearity in the observed response. As the energy reaching the detector is attenuated, the relative advantage of using a n MCT detector over a T G S detector in-
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
Table I. Experimental Detection Limits for a Group of Organic Compounds in the Dual-Beam System. The Experimental Detection Limits Obtained with a Single-Beam System Using Both a TGS and an MCT Detector Are Included for Comparison Detection limits, p g Compound Anisole Chlorobenzene Diethyl malonate Acetonitrile Benzonitrile Aldrin Perthane p,p'-DDT Heptachlor
Single-beam TGS MCT 0.8 1
Dual-beam 0.050 0.075 0.100
0.20
0.25
2 5 10
0.45
1.5 2.5 1.5 2.5 1.5
5 10
6 8
0.400
0.750 0.400
0.750 0.400 0.600
2.0
I
000
I
2000
CM'
~
800
Ordinate expanded spectrum of 100 ng of anisole between 2000 and 800 cm-' Flgure 2.
creases and, a t low radiation levels, the MCT detector response becomes linear. The S / N obtained with the dual-beam system for GC-IR measurements agrees well with the detector response calculations under conditions when the source signal is attenuated to 20-25%. Tabel I shows the detection limits obtained for several organic compounds measured on-the-fly using the dual-beam GC-IR system. Some sensitivity results obtained with a single-beam GC-IR system designed in our laboratory using the same source, interferometer, and light-pipes are included for comparison. The replacement of the TGS detector with a n MCT detector in the single-beam system led to an improvement in sensitivity of approximately a factor of four. The radiation from the source had to be attenuated somewhat to avoid digitization noise problems in this system. T h e dual-beam system gave an improvement in sensitivity of a factor of three compared to the single-beam system with identical optics. Consequently, this system is at least an order of magnitude more sensitive than the corresponding system utilizing a pyroelectric bolometer for infrared detection. Less than 100 ng of strong absorbers gave identifiable spectra for on-the-fly measurements with the dual-beam GC-IR system. Figure 2 shows a scale expanded spectrum of anisole obtained from 100 ng of injected sample between 2000 and 800 cm-'. All the strong bands in this region are still apparent in the spectrum. For all infrared measurements a dual-beam interferogram of the empty cells was subtracted digitally from the dual-beam sample interferogram to eliminate the residual background due both to the imperfect matching of the optics and to the presence of surface species on the beamsplitter. Figure 3 shows a spectrum of 100 ng of chlorobenzene between 2000 and 800 cm-' contrasting the effect of subtracting the sample
cM-
aoo
Ordinate expanded spectra of 100 ng chlorobenzene between 2000 and 800 cm-I (a)subtracting interferograms, (b) subtracting spectra
Flgure 3.
and reference interferograms before performing the F F T on the resultant interferogram and subtracting the spectra after performing the FFT on each individual interferogram. A fairly flat baseline was obtained when the interferograms were subtracted but a less flat baseline was obtained when spectra were subtracted. This result is the opposite of the result that was found for longer measurements using condensed phase samples, and is presumably related to the lack of repeatability of the dual-beam interferograms from scan to scan. I t may be noted that the water vapor absorption is also better compensated when interferograms are subtracted, resulting in a slightly higher S / N between 1900 and 1300 cm-'. The detrimental effects of uncompensated water vapor was one of the main problems during the measurements, although it may be noted that all results were found using an unpurged spectrometer. It is possible that the width of the GC peaks may become broadened during the transit time from the GC column to the light-pipes. Such an effect would result in undesirable degradation of GC resolution. As a check on the amount of peak broadening in our system, cyclohexanone was injected into the chromatograph and the IR absorption profile was determined from the variation of the absorbance of the carbonyl stretching mode during consecutive scans; the IR absorption profile was then compared with the GC profile (see Figure 4). With a helium gas flow rate of 30 mL/min, the half-width of the GC peak was 10 s which corresponds to a peak volume of 5 mL, while the IR profile shows a half-width of approximately 14 s. Peak broadening due to the integration of the sample in the light-pipe will always occur, and it has been shown (13) that for a peak volume approximately equal to the light-pipe volume (as is the case here), the IR profile should be 1.5 times broader than the GC profile. These results therefore suggest that there is little peak diffusion occurring in the transfer line. The profile was also monitored by placing
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
0
425
used for quantitative analysis. At very low concentrations the S / N observed is slightly less than that expected from the results a t higher concentrations, which may indicate a small amount of error in the data collection timing due to the "hit-or-miss" nature of the measurements. This effect may also be due to sample loss somewhere in the system but we were unable to detect any cold spots between the detector and the light-pipes. It is likely that the detection limits observed in these studies will be further reduced if the chromatographic conditions are optimized, for example through the use of support-coated open-tubular columns. It should also be noted that these measurements were taken on a three-year-old spectrometer and the performance of commercial FT-IR instrumentation has been improved over the past three years. If all chromatographic and spectroscopic parameters were optimized, it is very likely that the detection limits of GC-IR measurements will be reduced below 10 ng for strong absorbers.
GC-IR PEAK
LITERATURE CITED i
1 0
20
30
40
SO
bb
IO
TIME(~~coNDs) Figure 4. I R absorption profile vs. GC profile. Carrier flow rate: 30
mL/min
a flame ionization detector a t the exit of the light-pipe. A peak broadening of about 25% was observed at this point but this value is relatively insignificant if the GC peaks are resolved in the chromatogram. The feasibility of performing quantitative GC-IR analysis has also been investigated, using the chlorobenzene band located at 1080 cm-' (away from the water vapor region). Different amounts of chlorobenzene were injected on the chromatograph and the GC-IR spectra were measured onthe-fly. A plot of the S / N obtained for the 1080 cm-' band vs. the amount of injected sample is linear except at very low sample concentration indicating that the technique can be
(1) L. V. Azarraga, paper presented at the 5th Annual Conference on the Analytical Chemistry of Pollutants, Jekyll Island, Ga., 1975. (2) P. R. ctifcrths, Chapter 10 in "Fouriec Transform IR: Applications to Chemical Systems", J. R. Ferraro and L. J. Basile, Ed.. Academic Press, New York, N.Y., in press, 1977. (3) W. J. Burroughs and J. Chamberlain, Infrared Phys., 11. 1 (1971). (4) J. 0. Lephardt, Ph.D. Dissertation, University of Maryland, 1972. (5) P. R. Griffiths, "Chemical Infrared Fourier Transform Spectroscopy", Wiley-Interscience, New York, N.Y., 1975, Chapter 7. (6) H. Bar-Lev, InfraredPhys., 7 , 93 (1967). (7) P. R. Griffiths, Ref. 5, p. 175. (8) M. J. D. Low, Anal. Lett., 1, 819 (1968). (9) M. J. D. Low and H. Mark, J . Paint Techno/., 43 (No. 553), 31 (1971). (10) P. R. Griffiths and J. 0. Lephardt, paper presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 1969. (11) P. R. Griffiths, Ref. 5, Chapter 10. (12) K. L. Kizer, Am. Lab., 5 (6), 40 (1973). (13) P. R. Griffiths. Appl. Spectrosc., 31, 284 (1977). (14) D. Kuehl and P. R. Griffiths, Anal. Chem., this issue.
RECEIVED for review June 13,1977. Accepted August 22,1977. This work was supported by the U.S. Environmental Protection Agency under grant number R804333-01.
