2356
Anal. Chem. 1987, 59, 2356-2361
Optimizing the Optical Configuration for Light-Pipe Gas Chromatography/Fourier Transform Infrared Spectrometry Interfaces David E. Henry, Aldo Giorgetti, Andrew M. Haefner, and Peter R. Griffiths*
Department of Chemistry, University of California, Riverside, California 92521 Donald F. Gurka
Quality Assurance and Methods Development Division, U S . Environmental Protection Agency, Environmental Systems Monitoring Laboratory, Las Vegas, Nevada 89109
Previous Investigators have predicted that by optimizing the optlcal COnffgMatlon of the NgM-pipe Interface between a gas chromatograph and a Fourier transform Infrared spectrometer and utHlzlng detectors wHh areas as small as 0.01 mm2, detection umils mlght be reduced to the subnanogram level. The results presented In thb report lndlcate that this Is presently not posslMe and suggest that the detection llmlts at ambient temperatures of a practlcal light-plpe interface can be improved by no more than 50% compared to contemporary systems. Three optical contlguratlons are evaluated for their ability to dlscrhnlnate against emission from the end of a hot Ilght-pipe. Of these, the most effectlve In reducing the slgnal loss normally encountered at elevated lightpipe temperatures utlllres an aperture at a focus between the light-pipe and detector. A loss In slgnal of only 20% Is observed when the temperature of the Ilght-plpe Is ralsed from ambient to 300 O C by uslng this optlcal conflguration.
The use of and continuing improvements in rapid-scanning Fourier transform infrared (FT-IR)spectrometers, gold-coated light pipes, and capillary gas chromatography (GC) columns have together played a major role in establishing capillary GC/FT-IR as a powerful technique for the on-line analysis of complex mixtures of volatile compounds. Since the development of the first commercial GC/FT-IR systems, improvements in the interface and surrounding instrumentation have allowed detection limits to be reduced by more than 2 orders of magnitude. By 1977, the injected quantity of material required for identification of “the industry standard”, isobutyl methacrylate, had been reduced to about 10 ng ( I ) . More typical numbers describing the detection limits of this technique as reported by numerous investigators (1-3) ranged from 10 to 20 ng for strongly infrared absorbing compounds to as much as 0.5-1 pg for weak absorbers. Under optimum circumstances, low nanogram and occasionally subnanogram detection limits may be achieved for light polar molecules eluting with capacity factors less than unity. It is significant that these numbers have stood firm for the better part of a decade with only minimal improvements being reported. While it is highly unlikely that another improvement of 2 orders in magnitude in sensitivity will be seen for GC/FT-IR systems employing light-pipes, previous observations have suggested that the instrumentation which allowed most GC/FT-IR systems to achieve their reported sensitivities may have been far from optimized. For instance, it has been shown (4)that for a beam entering a light-pipe, rays striking the walls at high angles of incidence are the most highly attenuated due to reflection losses. In this case the solid angle containing a majority of the energy emerging from a light-pipe is signifi-
cantly reduced relative to that of the entering beam. As a consequence, a slower collection optic at the end of the light-pipe could be employed which would result in a smaller focused image at the detector. Thus, detectors having areas smaller than those typically used for high-throughput FT-IR measurements could be employed beneficially, in view of the relationship between the noise equivalent power (NEP) of the detector, its area, AD, and specific detectivity, D*
NEP = AD1/2/D* The optimum optical configuration in GC/FT-IR provides the best trade-off in signal collection vs. detector NEP. The highest collection efficiency is achieved by collecting the radiation emerging from the light-pipe with the largest possible solid angle. However, focusing this radiation to a spot of the smallest possible area, so that a small area detector can be used to minimize the NEP, presents a severe problem in geometrical optics. To date, several authors (5, 6) have addressed this design compromise, primarily by means of theoretical models. In this paper, a more detailed study of GC/FT-IR design parameters is described. In the first part of this report, experimental findings are presented from which the optical configuration yielding spectra of the greatest signal-to-noise ratio (SNR) may be deduced. The second part of this paper addresses several means of optically overcoming the decrease in signal which is commonly observed at elevated light-pipe temperatures. This signal loss, which can be as high as 70% at light-pipe temperatures of 300 “C (7),has been identified as the result of unmodulated emission from the end of the hot light-pipe causing a nonlinear response of the detector and/or its associated preamplifer due to saturation (4). Brown et al. (8)have demonstrated a cold shield which eliminates much of the unmodulated emission (excluding that emitted from inside the light-pipe). This cold shield involves the use of a short length of light-pipe held close to the end of the hot GC/FT-IR light-pipe. The entrance of this second light-pipe is not held at a beam focus, so that the collection efficiency should be less than optimal. In this paper we discuss alternative optical configurations which are easy to implement and give equivalent or better results than any GC/FT-IR system which has been described previously.
EXPERIMENTAL SECTION Two different FT-IR spectrometers were used to collect the data in these studies. Measurements employing the optical system shown in Figure 5 (vide infra) were made with a Digilab (Cambridge, MA) Model 296 interferometer and source optics removed from a Digilab FTS-11 spectrometer. The source optics and interferometer were removed from the original spectrometer and mounted on a separate optical bench along with the transfer optics, light-pipe, and detector. The remainder of the measurements reported in this paper were performed with a Digilab FTS-60 spectrometer using the external collimated beam. The transfer
0003-2700/87/0359-2356$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
Table I. Mercury-Cadmium-Telluride Detectors Used in This Investigation
Table 11. Theoretical Image Demagnification for Different Object and Image Distances
i09D*, cm
detector length, width, mm A
B C D E
2.0, 2.0 1.0, 1.0 0.5. 0.5 0.2, 0.2 0.1, 0.1
range
Hz' 2/ Wa
medium narrow medium medium medium
23 33 47 18 23
Specific detectivity as determined at a modulation frequency of 1 kHz at its wavelength of maximum response as measured by the manufacturer (Infrared Associates). a
optics, light-pipe, and detector were mounted on a separate optical bench which was rigidly attached to the FTS-60 spectrometer. Light-pipes used in this study were constructed according to the method of Yang et al. (9). The mirror used for focusing the collimated beam from each interferometer into the light-pipe was a 90" off-axis paraboloid having an effective focal length of 65 mm and a clear aperture of approximately 50 mm (Special Optics, Little Falls, NJ). Three different mirrors were used to collect the radiation passing through the light-pipe and refocus it onto either an aperture or an MCT detector. The f i t was a 90" off-axis ellipsoidal mirror having effective focal lengths of 250 mm and 42 mm and a 60-mm clear aperture. The second was a spherical mirror having a diameter of 100 mm and focal length of 113 mm (radius = 226 mm). The final mirror was a 30" off-axis, 50 mm by 50 mm square toroid having a focal length of 195 mm. The lens used in these studies was a 50 mm f / l biconvex KCl lens (50 mm in diameter and having a 50 mm focal length) purchased from Janos Technology, Inc. (Townshend, VT). All mercury cadmium telluride (MCT) detectors used were manufactured by Infrared Associates, Inc. (New Brunswick, NJ), and are listed in Table I. The D* values listed in this table were taken from the manufacturer's specification sheet. On-line GC/FT-IR measurements were made with a Digilab FTS-60 FT-IR spectrometer coupled to a Varian 3700 gas chromatograph via an interface built within our lab. The interface contained a 1.06 mm i.d. light-pipe 15 cm in length (135-pL volume) having a measured transmittance of 37% relative to an aperture of the same size. The method of measurement was identical with that described by Yang et al. (9),except that the mirror used to collect the collimated beam from the interferometer and focus it into the end of the light-pipe was the one described in this paper. A 25-mm section of deactivated 0.32 mm i.d. fused silica was glued into a small groove cut into each end of the light-pipe. The capillary column was directly connected to one of these small sections of fused silica, and a 60-cm section of deactivated fused silica was used to connect the other to a flame ionization detector (FID). Zero dead volume butt connectors (Supelco, Inc., Bellefonte, PA) were used to make the connections. NaCl windows were glued onto the end of the light-pipe with a high-temperature silicone rubber sealant (RTV-106, General Electric; Waterford, NY), and the same glue was used to glue the transfer lines into place. The light-pipe and transfer lines were heated within an oven constructed from solid aluminum within our laboratory. The modulated IR radiation emerging from the light-pipe was collected and reimaged a t a 1.06 mm diameter aperture by the toroidal mirror described above. The radiation passing through the aperture was then collected and focused onto detector C (see Table I) using the ellipsoidal mirror. All optics were mounted on a 3 ft X 4 ft optical breadboard (Newport Corp., Fountain Valley, CA). Separations were performed on a 60 m X 0.32 mm i.d. DB-5 fused silica GC column (J&W Scientific; Rancho Cordoba, CA) containing a 1 pm thick film of the liquid stationary phase. A 1.8 pg/pL solution of isobutyl methacrylate was prepared by pipetting 0.2 mL of the analyte into a 100-mL volumetric flask followed by dilution with methylene chloride. Splitless injections of 0.4 pL were made into a Grob-type glass injector. The linear flow velocity of the helium carrier gas was 29 cm/s. The injector temperature was held constant at 220 O C , the transfer lines and light-pipe a t 280 "C, and the FID a t 300 "C. For all separations, the GC column oven temperature was initially held at 60 O C for
2357
mm
Y,* mm
image demagnification
150 100 75
1:2 1:l 2: 1 3: 1 4: 1
75 100 150 200
66.7
250
62.5
" x is the distance between the lens and the light-pipe. b y is the distance between the lens and MCT detector.
Table 111. Theoretical Image Diameter" vs. Measured Full-Width of the Beam Focus at 2570, 5070, and 75% of the Profile Height distance of lens from light-pipe, mm I5
100 150
200 250
theoretical 2.12 1.06 0.53 0.35 0.27
image diameter, m m experimental 25%
50%
75%
0.72
1.24 0.90 0.49 0.37 0.29
2.06 1.31 0.76 0.55
0.55 0.30 0.25 0.19
0.46
"Theoretical image diameter = 1.06y/x. 4 min and then ramped at 10 "C/min to 220 "C. The collection of GC/IR spectra was performed with a Digilab 3260 data station equipped with a 72-Mbyte hard disk. In each experiment, four single-sided interferograms (8 cm-', 2048 data points) were collected per second a t a data acquisition rate of 20 kHz, coadded, and stored as one scan set. The single-beam spectrum of each scan set was then computed by using triangular apodization, and ratioed against a single-beam reference (16 coadditions), and the absorbance spectrum was calculated. Each reported absorbance value is the result of coadding the spectra from three consecutive scan sets (12 interferograms).
RESULTS AND DISCUSSION
A. Optimization of Optics. Relative to mirrors, lenses have the advantage of being easily employed in on-axis optical configurations which virtually eliminate coma. In addition, they are generally more easily aligned than mirrors or other reflectors. Although lenses do exhibit chromatic aberrations, it was felt that a lens employed as the collection optic would be the best way in which to perform an experiment requiring a considerable number of optical configurations and alignments in terms of cost and time. The conditions under which the lens was used are listed in Table I1 and have been calculated from the simple lens equation based on the fact that the lens to be employed had a 50-cm focal length. To test the quality of the image produced by the lens, and thus the feasibility of this work, the image produced when the lens was held at distances of 75, 100, 150, 200 and 250 m m away from the end of the light-pipe was characterized. The profile of the image was measured by translating a 0.1-mm MCT detector horizontally across the center of the beam at its focus. The results of the experiment are tabulated in Table 111. The diameters of the locus of the image where the signal has been reduced to 50% of the maximum intensity of the profile of the beam focus are within 10% of the theoretically predicted diameters for four of the five distances at which the work was to be done. Only when x = 75 mm, corresponding to image magnification rather than demagnification, was the full width at half-height significantly different from the value calculated from the object and images distances. Since all practical optical configurations for GC/FT-IR involve a demagnification of the image of the light-pipe at the detector,
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
Table V. Theoretical Relative SNR’s Achievable with the Use of Smaller Detectors”
SNR (detector size, mm)
collection optic
fll
f10.87
100 (1) 130 (0.67) 117 (0.50) 131 (0.33) 89 (0.29) 79 (0.25) 73 (0.22) 70 (0.20)
143 (0.7) 149 (0.58) 135 (0.43) 151 (0.29) 120 (0.25) 90 (0.21) 83 (0.19) 80 (0.17)
‘Based on the use of a 1 mm i.d. light-pipe.
Figure 1. Optical configuration used in this work to measure the amount of energy contained within a given solid angle of the beam emerging from a light-pipe. Collimated radiation from the interferometer is focused by paraboloidal mirror M 1 (90’ off-axis, 65 mm e.f.1.)onto the light-pipe (1.06 mm i.d.). The emerging radiation is collected and refocused onto a 2-mm MCT detector (D) by mirrors M 2 and M3 which are identical with mirror M1. An aperture (A) is used to control the solid angle of radiation allowed to reach the detector. Table IV. Measured Interferogram Centerburst as a Function of Collection Aperturea$b
centerburst collection aperture
fll f11.5 f12 f/3 fl3.5 f,’4 fl4.5 fi5
(peak-to-peak, V)
% of f / l
3.461 3.001 2.025 1.033 0.852 0.676 0.562 0.482
100 86.7 58.5 43.8 29.8 19.5 16.2 13.9
*As measured usin8 a 1.06 mm i.d. light-pipe. ‘Here, collection aperture is defined as the ratio of the distance between the aperture and light-pipe and the aperture diameter.
we believe that the results of this study are directly relevant to the optimization of these interfaces. The idea of employing smaller detectors in a GC/FT-IR system assumes that the solid angle of radiation emitted from the light-pipe is reduced by reflection losses of the off-axis rays within the light-pipe. In order to determine what percentage of radiation could be collected by a 50 mm diameter collection optic located a t a distance, x, from the end of a light-pipe, an optical configuration was employed in which a pair of matched paraboloidal mirrors, identical with the one focusing the beam into the light-pipe, replaced the lens. These mirrors were used to collect and reimage the beam emerging from the light-pipe onto a 2-mm MCT detector. In this configuration, which is illustrated in Figure 1, a variable aperture was located 25 mm from the end of the light-pipe such that the solid angle of radiation being emitted could be controlled. The peak-to-peak values of the centerburst of the interferogram as measured a t various f stops are listed in Table IV. The solid angle of radiation collected was controlled by the aperture. The percentage of beam collected relative to the total being emitted from the end of the light-pipe is also listed in this table. From these results it appears as though the amount of radiation which can be lost by placing the collection optic a large distance from the exit aperture of the
light-pipe is considerably greater than that which had been originally forecast by earlier beam profile studies ( 4 )in which a detector was translated across the diverging beam at various distances, x, from the exit aperture of the light-pipe. Therefore, some of our early estimates obtained by integrating the profile graphically in two-dimensional space (5, I O ) are in conflict with the results presented here. It is now apparent just how much radiation is located in the attenuated “wings” of the profile, and the intensity of this fraction of the beam can be so low as to be very difficult to observe using an MCT detector, without the appropriate focusing optics. Knowing the exact amount of energy collected by an optic of given f/number provides the information necessary to determine the theoretical benefits of using smaller detectors with the appropriate collection and detector optics. From knowing the focal ratios of both the collection optic and detector foreoptic, it is possible to calculate the degree of demagnification of the image a t the detector (demagnification = (f/number of collection optic)/(f/number of detector foreoptic)). Assuming the size of the detector is matched to the size of the reduced image, the SNR of the spectrum is proportional to the product of this ratio of flnumbers and the percentage of beam collected. A series of relative SNRs was calculated for theoretical systems employing f/0.87 and f / l detector foreoptics. This first number was chosen as being appropriate for a configuration where the maximum solid angle of the beam entering a detector is limited by the field of view of the detector. The highest acceptance angle of most detectors is 60°, which corresponds to an flnumber of 0.87. The second number ( f / l ) was selected as being closer to the practical limit of most beam focusing optics. As shown in Table V, for the best possible case, detectors ranging from 0.7 to about 0.3 mm in diameter should theoretically yield not more than a 50% improvement over the older conventional systems employing 1:1 imaging optical configurations together with a 1 mm i.d. light-pipe and a 1-mm detector. Unfortunately this seems very small gain for the chromatographer/ spectroscopist when compared to the highly optimistic 10-fold increase in sensitivity projected by previous investigators (5). To determine whether or not even the marginal theoretical benefits of using smaller detectors might be realized in practice, a series of measurements was made in which detectors ranging from 2 to 0.1 mm were employed in a system where the position of the collection optic could be varied between 7 5 and 250 mm away from the end of a light-pipe. The light-pipe was 15 cm in length and had an internal diameter of 1.06 mm. The results are summarized in Figures 2 and 3. Figure 2 shows a normalized plot of measured signal recorded as a function of the distance between the exit aperture of the light-pipe (which is related to the lens collection efficiency) for detectors of several different sizes. Here it can be seen that the optimum signal is strongly dependent on not only the amount of radiation collected by the detector but also
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Figure 2. Relative interferogram centerburst intensity measured as a function of distance between a lens (50 mm diameter) collecting the beam emerging from a light-pipe (1.06 mm i.d.). The exact positions of the lens and detector are given in Table I I as is the calculated image size. Each plot was obtained by using the following detectors: (A) 2.0 mm; (B) 1.0 mm; (C) 0.5 mm; (D) 0.2 mm; (E) 0.1 mm. the position of the collection optic and thus the solid angle of energy collected. As would be expected, when smaller detectors are used, the collection efficiency of the detector becomes increasingly important relative to the collection efficiency of the foreoptic. Of course, if the demagnification of the optic were to be increased or a smaller light-pipe were to be employed in the system, the size of the image would be reduced correspondingly and a smaller detector (with a concomitantly lower NEP) could be employed. In this case, all maxima for detectors smaller than the image would shift to values corresponding to higher efficiency collection optics. The 100% lines measured under the conditions represented by each of the maxima in Figure 2 are shown in Figure 3. Each plot is the result of ratioing 16 coadded spectra against another 16-scan spectrum measured immediately thereafter. Singlesided interferograms were collected a t a resolution of 8 cm-' (2048 data points) with a data aquisition rate of 20 kHz. Single beam spectra were calculated by using triangular apodization. Along with each plot is the peak-to-peak noise level measured in the region between 2250 and 2050 cm-' for five individual experiments. As can be seen, under the conditions we were using, the best results were obtained by using a 0.5-mm detector in conjunction with the 50-mm collection optic located a t a distance of 100-150 mm from the end of the light-pipe. It may be noted that at least one manufacturer, Digilab, employs a 0.5-mm MCT detector in their current GC/FT-IR system. The results obtained in this study with the 2-mm detector are far better than would be normally expected while those obtained by using the 0.2-mm detector are worse than those predicted. These abnormalities are best explained by noting that from previous experience with these two detectors, the
WAVENUMBERS (cm
)
Figure 3. 100% lines measured at ambient temperature under the conditions represented by each of the maxima in Figure 2. Each plot is the result of ratioing 16 coadded spectra against 16 coadded spectra collected at a resolution of 8 cml. The peak-to-peak noise b e l (% T ) taken over the region between 2250 and 2050 cm-' is as follows: (A) 0.050; (B) 0.087; (C) 0.042; (D)0.140; (E)0.103. particular 2-mm detector has consistently proven to be easily the most sensitive in our laboratory when compared to others of equivalent size and specifications while the 0.2-mm detector has never really seemed to perform up to its specifications. Overall then, it seems that these results are in adequate agreement with our predictions as well as with the results of other investigators. Finally, we wished to check into the possibility that the apparent absorbance of the sample might be reduced by collecting a smaller solid angle of radiation, since, although attenuated by reflectance losses, the off-axis rays would also be more attenuated because of Beer-Lambert absorption by the sample due to the increased path length. To check the magnitude of this effect, the absorbance of the 1155-cm-' band in the spectrum of isobutyl methacrylate was monitored as the distance between the light-pipe and collection optic was increased. The effluent passed through the light-pipe to an FID, and small differences in sample introduction were corrected for by noting differences in the FID response. The results are illustrated in Figure 4 and show that the absorbance of the analytical band is independent of the position of the collection optic. This finding indicates that the effect of the marginal increase in path length of the off-axis rays is offset by their low intensity. Little can be gained, therefore, by collecting the off-axis rays. B. Discrimination of Radiation from Light-Pipe. As already noted, a large decrease in signal a t high light-pipe temperatures is a common observation in GC/FT-IR interfaces employing MCT detectors. This effect results from high levels of unmodulated infrared radiation emitted from the hot glass surrounding the bore of the light-pipe, which saturates the detector and/or its associated preamplifier. A cold shield positioned at the end of the light-pipe is one means of eliminating the emission from the end of the hot light-pipe (8).
2360
ANALYTICAL CHEMISTRY, VOL. 59. NO. 19. OCTOBER 1. 1987
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