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
Enhancement of nitrogen analyte response by background isobutane. Figure 6.
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
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ANALYTICAL CHEMISTRY, VOL. 60,
NO. 14, JULY 15. 19tI8
may result from the smaller size of current infrared data bases, rather than any inherent superiority of FT-IR for this purpose. T h e major impediment to the more widespread use of GC/FT-IR bas been its low sensitivity when compared with GC/MS analysis. Conventional GC/FT-IR analysis utilizes a low volume (-100 FL) gold-coated glass 'lightpipe" (22) through which the GC effluent passes while on-the-fly transmittance spectra are obtained. This method provides low nanogram (20-30 ng) sensitivity and precludes the use of narrow, high-efficiency capillary columns which are preferred for complex mixtures. Trace level analysis is also difficult, due to the limited dynamic range (-2 orders of magnitude) imposed by the column capacity and the sensitivity of the method. Much recent research has been concerned with improving the sensitivity of lightpipe interfaces and new commercially available GC/FT-IR instrumentation provides improved sensitivity. Even better performance may be possible through careful optimization of the GC/FT-IR system (22). However, without some additional major breakthrough only modest gains over currently available lightpipe instrument sensitivity are expected. An alternative to lightpipe interfaces is the eluate trapping technique. The most notable example is the matrix isolation interface (13,14). A commercial version called the Cryolect was introduced in 1984. Here, GC eluates are trapped at -12 K in an argon matrix on a rotating mirrored collector for subsequent recording of IR spectra. Sensitivity enhancements due to the concentration of the eluate, signal averaging, and peak sharpening induced by use of a cooled matrix result in the ability to obtain spectra for average absorbers (as opposed to isobutyl methacrylate) with quantities on the order of 200 pg. However, this is not achieved without a significant increase in cost. Another trapping GC/FT-IR interface used in conjunction with FT-IR microspectrometry has been developed (25, 26). An FT-IR microscope was used to examine GC eluates deposited on a movable cooled (-45 "C) infrared window. Spectra of quantities of samples between 1and 10 ng could be obtained. Although studies using this interface demonstrated that high S/N spectra could be obtained for low nanogram quantities of eluates without the need for a surrounding matrix, the loss of volatile samples (such as o-dichlorobenzene) from the cooled surface in a matter of minutes was such that it was concluded that additional cooling was required. Even though the excellent imaging capabilities of the FT-IR microscope are an advantage, its high cost and restricted working area are disadvantages for use as a GC/ FT-IR interface. The impressive sensitivities demonstrated with the Cryolect system suggested that it might he possible to obtain such improved sensitivity with a liquid-nitrogencooled collector. Rotatable liquid-helium-tilled cryostats have been reported previously for the study of chemical reactions at low temperature (27). With this approach, costs can be reduced substantially over those of the closed cycle heliumrefrigerated Cryolect, while still allowing high-quality "matrixless" FT-IR spectra of GC eluates. Additionally, use of xenon as the matrix gas, with its higher freezing point, can allow matrix-isolated spectra to be obtained. Thus, a liquid-nitrogen-cooled interface with sensitivity approaching that of GC/MS compatible with a linked GC/FT-IR/MS system of improved sensitivity should he possible.
EXPERIMENTAL SECTION The cryogenically cooled interface is diagramed in Figure 1. It utilizes a 330 L/s turbomolecular pump which produces background pressures of -6 X lo4 Torr (uncorrected ion gauge) with a GC flow rate of -1 mL/min. Base pressures without Torr. The vacuum chamber helium flow easily reach 2 X is constructed from 127 mm 0.d. X 123 mm i.d. stainless steel vacuum tubing. Conflat style vacuum flanges were welded to the
I R Wi"d0W
Figure 1. Block diagram of cryogenically cooled GCIFT-IR interface.
stainless steel tubing for bolting to the turbo and a high vacuum rotating feedthroughutilizing Teflon V-rings, and a bored Conflat blank-off flange were constructed. V-ring feedthroughs are also pumped by the mechanical pump which hacks up the turbomolecular pump to remove any air which leaks in during rotation of the cryocollector. A double-necked Dewar (850 mL) was constructed from stainless steel materials. The inner neck connected to the main reservoir is 1.27 cm o.d., while the outer neck, which remains a t ambient temperature, is 2.54 em 0.d. and was polished smooth to facilitate ita turning within the V-rings of the feedthrough. Inner and outer walls are positioned by three nylon set screws which produce minimal thermal transfer. Connection of an auxiliary mechanical pump to the Dewar exhaust can also he accomplished to further reduce the temperature of the Dewar to -65 K, the triple point of nitrogen, by pumping (-100 mTorr) on the liquid nitrogen, which then freezes. Vacuum regulation is provided by means of a needle valve. A 2 5 0 0 0 step/revolution stepper motor (Compumotor, Petaluma, CAI is coupled via 4 1 gear reduction to the Dewar neck which also turns on a corkscrew thread of 3 mm/revolution. The stepper motor is controlled by a Model 2100 Compumotor controller which allows external control via either an RS-232 interface with a computer or manual control of functions via thumhwbeel switches on the controller's front panel. Minimum disk rotation speed is 8 rm/s The cryocollector is a 2.54 cm thick by 10.16 em wide OFHC copper disk with the outer circumference single-point diamondmachined to a mirrored surface. A hardened gold coating was applied to the entire disk. The disk was mounted on the hottom of the Dewar after removing much of its inner mass, to minimize the thermal mass of the collector. Several windows were incorporated into the vacuum system. Two windows (33 mm in diameter) for the infrared beam to enter and exit are made of KC1. Two additional windows for observing the positioning of the fused silica transfer line are made of plate glass. A binocular stereoviewing microscope provides 10-40X magnification. A Bayard-Alpert ionization gauge and Granville Phillips Model 270 controller are used to monitor pressure in the vacuum system. The entire interface is mounted on an external optical table and has an airtight cover attached to allow for purging of the optical path. Connection between the spectrometer and
ANALYTICAL CHEMISTRY, VOL. 60. NO. 14. JULY 15, 1988
ll
1485
m
Figure 3. High vacuum rotating feedthrough for heated fused silica transfer line.
Figure 2. Optical configuration of GCIFT-IR interface.
column evacuation and to reduce sample expansion, prior to deposition. The restrictor is manually positioned within approximately 75-100 pm of the collector surface by means of a screw mechanism and a high-vacuum feedthrough incorporating Teflon V-rings (Figure 3). The test sample consisted of equal quantities (0.5 pg/pL) of n-nonane, benzaldehyde, phenol, l,Z-dichlombenzene, rr-terpinene, nitrobenzene, undecane, 2,6-dimethylphenol,Z,&dimethylphenol, and naphthalene dissolved in chloroform. All chemicals were used as received from Aldrich Chemical Co.
the external bench is made via 10 an i.d. flexible hose which also provides the IR beam path. The optical configuration is shown in Figure 2. The beam from a Nicolet 60SX FT-IR spectrometer is directed via a pair of beam-steering flat mirrors to a 6.60 cm effective focal length (EFL), 90' off-axis parabolic reflector (Pichell, Temecula, CA) which focuses the beam to a small spot on the collector, An identical mirror collects the reflected beam and passes it, via a flat mirror, to a third parabolic reflector, identical with the first two parabolic reflectors. This mirror focuses the beam onto a 0.5 mm X 0.5 mm MCT detector (D*= 4.9 X 10'O cm H Z ' / ~W a t 10 kHz; A,, = 14.9 pm). These optics provide a 3.21 demagnification of the variable aperture of the 60SX onto the cryocollector followed by 1:l imaging onto the detector. Thus the smallest ''ideal" spot that can be irradiated by the IR beam is -0.3 mm wide (the minimum aperture of the 60SX is -1 mm). All spectra were collected with a Nicolet 60SX FT-IR spectrometer utilizing the standard software. Infrared data are obtained as 2048-point (8 cm-') interferograms and Fourier transformed employing one zero-fill and a Happ-Genzel apodization function. The resulting single beam spectrum is ratioed versus a suitable reference to produce the resulting transmittance spectrum which would also be converted to absorbance units. For all spectra (unless noted in the text) 500 scans were collected and coadded for both the sample and reference. Acquisition time for 500 scans is 1.5 min (0.733 cm/s mirror velocity). Gas chromatographic equipment consists of a Hewlett-Packard Model 5880A equipped with a 0.25 mm X 30 m fused silica column coated with a 0.25-pm film of DB-5 (5% phenyl methyl siloxane) from J&W Scientific (Folsom,CA). Helium is used as the carrier gas a t a flow rate of 1mL/min. Samples are introduced via the split injection technique with typical split ratios of 501. A Valco manual three-port switching valve is mounted in the oven and connected between the column and the interface transfer line to allow diversion of the GC effluent to waste. This prevents excess solvent from freezing between the transfer line & cryocollector and prevents material from collecting when the cryocollector is stationary. Alternately, the effluent can he passed to a standard detector, although this was not done in the current experiments. The transfer line for directing GC effluent to the interface utilizes 0.4 m of 0.25 mm i.d. fused silica tubing within a heating block. A 5 em piece of 50 s m i.d. fused silica tubing is attached to its end via high-temperature epoxy and acts as a restrictor to prevent
RESULTS AND DISCUSSION Initial work centered upon characterizing the Dewar hold time. In its final form, the Dewar could be maintained at 77 K for at least 16 h with a single fill of liquid nitrogen. Initial cool down and filling required approximately 2.5 L of liquid nitrogen. However, once the Dewar was cold, it required only about 1.0 L to refill. The disk is large enough to collect about 7.5 h of chromatographic effluent before the disk needs to be warmed to room temperature to remove trapped materials. This is accomplished by blowing dry air into the Dewar, or alternately, utilizing a heating cartridge dropped down the neck of the Dewar. Increased collection time can be achieved by changing the corkscrew gear from the 3 mm/revolution to 1-1.5 mm/revolution, without different tracks overlapping, as deposition areas are on the order of 0.4 mm or less. This provides two to three times the collection capacity. The disk can be cooled further (to 65 K) by attaching an auxiliary mechanical pump to the Dewar exhaust and maintaining a modest vacuum (5100 mTorr). The increased boil off of Nzcauses additional cooling until the Nz triple point is reached. For a temperature of 65 K, hold times for the Dewar are decreased to approximately 12 h. This feature is utilized for matrix isolation experiments with xenon. Comparisons of the Dewar's boil-off rate (at atmospheric pressure) with the liquid-nitrogen-cooled MCT detector's boil-off rate indicate the latter is approximately half as much. Therefore, improvements in hold time probably can be achieved. Although interface sensitivity can he enhanced by utilizing slower rotational speeds of the cryocollector (to the extreme of not moving it a t all), which would increase the eluate concentration per unit area, it was decided that a more realistic approach was to determine the maximum allowable time slice of the chromatogram that the detector should "see" (assuming the detector size is the limiting aperture) and to calculate the appropriate collector velocity. Because 1:l imaging opties are employed, the detector ideally sees a 0.5 mm X 0.5 mm area of the collector. Capillary gas chromatographic peaks for narrow-bore columns are typically 3-10 s wide (although this depends heavily on K'values); therefore a compromise time
Gas
I
I, 2 . 6 " E . , . L Po'orl-an,,
Chromatograph
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
slice of 5 s was chosen to present as much of the material as possible to the detector without substantially degrading chromatographic performance. This corresponds to a velocity of 0.1 mm/s and was used for all subsequent work. This value is -1.3-2 times faster than that used by other workers (15, 16) and depends upon the detector and IR beam size employed. If a 0.25 mm X 0.25 mm detector had been used, a velocity of 0.05 mm/s could have been employed, enhancing the concentration of eluate, while maintaining the same time resolution. This assumes eluate deposits can be made small enough that they are not the limiting factor. Estimated spot sizes for nonmoving deposits are -0.2-0.3 mm with the restrictor placed 100 pm from the surface. When the IR spot was the limiting factor (at minimum aperture -0.3 mm wide) actual chromatographic time resolution obtainable was improved from 5 to 3 s. Thus, sensitivity could have been improved by slowing down the cryocollector. This trade-off ultimately depends upon the time resolution required for a particular sample mixture. Initial difficulty with the alignment of the optics (in particular, overlap of sample deposit and IR spot) presented some problems. A straightforward alignment procedure was eventually developed and will be presented briefly. Initial alignment of the optics is accomplished by the use of a visible light source which produces a collimated output beam. A collector blank, identical with the mirrored collector in dimensions, was machined out of aluminum. With nonreflecting white tape placed upon a portion of the surface, the visible beam can be positioned. Reflecting aluminized Mylar tape covers another section of the blank collector and can be rotated to pass the beam onto the detector. A third section was covered by the reflecting tape and then overlaid with the nonreflecting tape which had a 1-mm hole. This produced a small area reflector which is used to align the invisible IR beam and, when rotated opposite the fused silica transfer line, also allows it to be positioned properly. Finally, when the mirrored collector is used a t cryogenic temperatures, the alignment is further fine tuned to correct for thermal contraction. This final alignment is accomplished by using the smallest IR source aperture and rotating the collector until half of the IR signal is lost. At this point, the center of the IR spot is exactly on the edge of the collector. By rotating the collector back the predetermined distance, one can now align the transfer line with the collector's edge. The finetuning procedure is very effective and can be accomplished by rotating the holder, while right and left positioning is performed by rotating the disk. Sample volatility at 77 K was determined. Benzaldehyde was chosen as a test compound based upon its high absorptivity and large number of IR absorption bands. It is also reasonably representative of the relative vapor pressures of "typical" GC sample components. Five nanograms of benzaldehyde was injected onto the GC and collected on the cryodisk. Infrared spectra were recorded immediately after deposition and after 9 h. Total absorbance of the most intense peak in the latter spectrum had decreased to 85% of its initial value. Some of this loss may result from alignment changes due to vibrations of the turbomolecular vacuum pump, as no attempt at realignment was made between spectra. Therefore, this loss represents the worst case for sample loss due to volatility. Similar results were observed for nonane. Benzaldehyde was also used to test the linearity of absorbance as a function of the quantity of sample injected. The maximum absorbance when plotted as a function of concentration in the range 1-30 ng was linear. Several points should be made about these results. It was later found that linearity is highly dependent upon the fused silica transfer line placement, both with respect t o distance from the surface as
-
'1 m
0
BENZflLOEHYDE 5NG
Flgure 4. (a)Reference Akirich neat library spectrum of benzaklehyde. (b) GC/FT-IR spectrum of benzaldehyde (5 ng) (77 K).
Table I match
match
library
no.
value
entry
1031
1290
1859 2529 2536 2576 2584 2892
1299
615 1301 1291 1471
1296
compound name benzaldehyde (chlorine free), 98% 3-chlorobenzaldehyde, 97 % acrolein, 97% p-tolualdehyde, 97% o-tolualdehyde 2-acetylpyridine,98% m-tolualdehyde,97%
well as alignment with the optical beam. Linearity is excellent when measurements are made following successive injections without moving the transfer line between injections. If the transfer line is repositioned between runs, the linearity is much poorer. In practice, the transfer line is retracted after each chromatographic separation and prior to collection of spectra to prevent possible damage to the fused silica or collector during subsequent positioning. In early work, when larger IR focused beams were required in order to ensure overlap of the IR beam and sample, 10 ng of benzaldehyde produced a maximum of -0.008 absorbance unit. With improved alignment procedures, this was increased to -0.02 absorbance unit for the same amount of material, utilizing an IR beam focused to -0.4 mm. A spectrum of 5 ng of benzaldehyde collected during the early development of the interface is shown in Figure 4b. For comparison, Figure 4a contains the library spectrum of benzaldehyde from the Aldrich neat compound library. Although relative band intensities are different between the two spectra (not surprising given their different physical states) most of the bands can be matched readily. Indeed, by use of the conventional Nicolet search software (absolute difference metric), the closest of the top seven match results obtained was benzaldehyde (Table I). Not only was benzaldehyde identified correctly, but six of the top seven matches were aldehydes and the seventh was a ketone. This demonstrates the class identification potential for GC/FT-IR data, even if the spectrum of the actual compound were not present in the library. Figure 5 shows the results obtained from a combination of improvements in both optical alignment and deposition
ANALYTICAL CHEMISTRY, VOL. c
60, NO. 14, JULY 15, 1988
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Figure 6. F I D chromatogram of model mixture: nonane, benzaldehyde, phenol, 1,2dichlorobenzene,y-terpinene, nitrobenzene, undecane, 2,6dimethylphenol, 2,5-dimethylphenol,and naphthalene: temperature programmed, held at 85 OC for 8 min followed by increase of 4 'Clmin to 125 OC.
Flgure 8.
techniques. The spectrum shown is that of 10 ng of benzaldehyde deposited as the collector moved at a rate of 0.1 mm/s. This spectrum compares more favorably with the library spectrum in Figure 4b, with the exception of the water band between 3600 and 3200 cm-'. This water band is due to trace water in the carrier gas which appeared only after helium suppliers were changed and despite elaborate attempts to eliminate it including using water scrubbers and liquid nitrogen cold trapping. Earlier spectra did not reveal this problem (see Figure 4a) and it is expected to be eliminated by using an improved grade of helium. A final possibility is a leak, either in the injector of transfer line connections, allowing small amounts of air (and therefore water) to enter the carrier gas stream. The water band could be subtracted by using a background spectrum, except that there is a shift in absorption frequency due to association with the sample. When subtraction is attempted, a derivative-shaped band which interferes worse than the water band is produced. The IR spot size utilized was approximately 0.4 mm in diameter. Absorbance could be nearly doubled by reducing the spot size to its minimum value of 0.3 mm in diameter, but concurrent reduced optical throughout causes the S / N to be approximately the same. Faster focusing optics and a smaller area detector would offer improved performance. Figure 6 shows a chromatogram of the separation of the model ten-component mixture which was used to evaluate the interface (FID, flame ionization detection). Benzaldehyde was the second compound eluted (after the solvent) and repre-
sentative spectra have already been discussed. Spectra of two of the other components in the mixture, nonane and nitrobenzene, are contained in Figures 7 and 8, respectively. These spectra represent 5 ng of each component injected. The major peaks in the nitrobenzene spectrum also compare favorably with a neat spectrum of nitrobenzene. The absence of a water band in the nonane spectrum is due to the lack of association between the two, which permits water to be ratioed out by proper choice of the background spectrum. The nonane spectrum, although similar to the neat library spectrum of nonane except for the sharpness of bands, appears to be a closer match for a vapor-phase spectrum. A spectrum obtained from 1 ng of nonane injected, using a minimum aperture setting, is shown in Figure 9. The high S/Nlevel suggests that subnanogram quantities could be detected and identified. Finally, an example of the use of this interface to collect matrix-isolated spectra of GC effluents is shown in Figure 10. One percent xenon was added postcolumn via a low volume tee where it mixed with the GC effluent and was deposited on the cryocollector for subsequent IR analysis. At 66 K, the vapor pressure of xenon is still high enough that approximately 50% is lost after 3 h. This is enough time to measure spectra of GC eluates but could pose a problem for complex mixtures where detailed analysis of many components may be required. The spectra in Figure 10 compare the relative absorbance of matrix-isolated benzaldehyde (10 ng) at 2 cm-' resolution with non-matrix-isolated benzaldehyde (20 ng) at 8 cm-' resolution.
3i:cc
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,
-
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,
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.-5c
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spectrum of nitrobenzene (5 ng) (77 K)
1488
ANALYTICAL CHEMISTRY, VOL. ! \ - \2W
Y
00,NO. 14, JULY 15, 1988
?T
1
to those available in present condensed-phase libraries, may be more suitable for GC/FT-IR/MS applications, perhaps eliminating the need for compiling new libraries of matrixisolated spectra. This could be important, because it has been convincingly demonstrated that matrix-isolation spectra collected at 10 K for a variety of organic compounds are much different than condensed or vapor-phase spectra (18-20).In addition, the problem of spectral changes with concentration which can be expected in samples with larger compound dynamic ranges (Le. varying concentration), due to the necessary compromise between the amount of matrix employed for high concentration components and minimizing the dilution of minor components, will be avoided by elimination of the matrix. Conversely, at higher concentrations, base-line shifts due to scattering may become important in the non-matrixisolation interface. Although sensitivities as high as those reported for the Cryolect system have not been demonstrated with the present interface, it is anticipated that improvements in design, with particular emphasis on optimizing the optics and detector size, will produce sensitivities in the range obtainable with the Cryolect, because use of the matrix appears to yield signal increases of no more than a factor of 2 or 3.
LITERATURE CITED
Figure 10. (A) Benzaldehyde ( I O ng) with xenon matrix. (B) Benzaldehyde (20 ng) without xenon matrix.
The number of scans (16 and 100, respectively) were selected to make collection times roughly equivalent. When concentration differences and peak heights are normalized, the matrix-isolated spectrum is approximately 3 times as intense as the non-matrix-isolatedspectrum. The bands are also much narrower due to the use of a matrix. The matrixless spectrum would not be sharpened significantly even at higher instrument resolution, as most of the bands are on the order of 16 cm-' wide. The higher noise level of the matrix-isolated spectrum results from its higher bandwidth as well as the use of less sighal averaging. Scattering of radiation by the matrix also contributes slightly by reducing optical throughout.
CONCLUSIONS The interface described is a workable alternative to the more costly commercially available Cryolect GCIFT-IR interface. It is expected that the present interface, with its capability for allowing measurement of spectra quite similar
(1) Cooper, J. R.; Taylor, L. T. Appl. Spectrosc. 1984, 38, 366-370. (2) Gurka, D. F.; Betwoski, L. D. Anal. Chem. 1982, 5 4 , 1819-1824. (3) Smith, S. L.; Garlock, S. E.: Adams, G. E. Appl. Spectrosc. 1983, 37, 192- 196. (4) Purcell, J. M.; Magldman, P. Appl. Spectrosc. 1984, 38, 181-184. (5) Griffths, P. R.; deHaseth. J. A.; Azarraga, L. V. Anal. Chem. 1983, 55, 1361A-1387A. (6) Wilkins, C. L.; Giss, G. N.; Brissey, G. M.; Steiner, S. Anal. Chem. 1981, 53, 113-117. (7) Wllklns, C. L.; Giss, G. N.; Whlte, R. L.; Brissey, G. M.; Onyiriuka, E. C. Ana/. Chem. 1882, 54, 2260-2264. (8) Laude, D. A.: Brissey, G. M.; Ijames, C. F.; Brown, R. S.; Wilkins, C. L. Anal. Chem. 1984, 56, 1163-1168. (9) Cooper, J. R.; Bowater, 1. C.; Wilkins, C. L. Anal. Chem. 1988, 58, 2791-2796. (10) Gurka, D. F.; Titus, R. Anal. Chem. 1988, 58, 2189-2194. (11) Azarraga, L. V. Appl. Spectrosc. 1980, 3 4 , 224-225. (12) HirschfeM, T. Appl. Spectrosc. 1985, 39, 1086-1087. (13) Reedy, G. T.; Bourne, S.; Cunningham, P. T. Anal. Chem. 1985, 39, 1086- 1087. (14) R&y, G. T.; Ettinger, D. G.; Schnelder, J. F. Anal. Chem. 1985, 5 7 , 1602-1 609. (15) Fuoco, R.; Shafer, K. H.; Griffiths, P. R. Anal. Chem. 1986, 58, 3249-3254. (16) Griffiths, P. R.; Pentoney, S. L.; Giorgetti, A,: Shafer, K . H. Anal. &em. 1988, 58, 1349A-1366A. (17) Thomas, A. Trans. Faraday SOC. 1981, 5 7 , 1679-1685. (18) Coleman. W. M., 111; Gordon, B. M. Appl. Spectrosc. 1987, 4 7 , 1159-1162. (19) Coleman, W. M., 111; Gordon, 8. M. Appl. Spectrosc. 1987, 4 7 , 1163-1169. (20) Coleman, W. M., 111; Gordon, 8. M. Appl. Spectrosc. 1987, 4 7 , 1169-1172.
RECEIVED for review November 6,1987. Accepted February 24,1988. Support of the National Science Foundation under Grant CHE-85-19087 is gratefully acknowledged.
