Anal. Chem. 1994,66, 3751-3756
Multidimensional Fast Gas Chromatography with Matrix I solation I nfrared Detection N. Ragunathan, Tania A. Sasakl, Kevin A. Krock, and Charles L. Wilkins' Department of Chemistry, Universiw of California, Riverside, California 9252 I
A study of multidimensionalgas chromatographyfor mixture analysis, coupling a fast second GC stage with a matrix isolation infrared spectrometer, is reported in this paper. It is demonstrated that separation, detection, and analysis of multicomponent mixtures can be carried out under the conditions of fast gas chromatographic separation linked to infrared instrumentation. Samples analyzed to characterizethe system are a Grob mix and partially separated eucalyptusand cascarilla bark essential oils. The essential oils are first separated on a polar precolumn, and portions for further analysis are cryogenically trapped. These heartcut samples are reinjected onto an analytical column by rapid resistive heating of the cryogenic trap. The effluent is trapped on a cryogenically cooled rotating disk for subsequent infrared analysis. In the present system configuration, the time required to complete the secondary separation has been reduced by an order of magnitude compared to standard GC-IR-MS techniques. An essential requirement for such an approach is that the compounds analyzed must show negligible decomposition due to rapid heating. That condition is satisfied in the present study. Complete analysis of complex mixtures containing several hundred components has proven to be an elusive goal due to a variety of factors.' Among those factors are high sample dynamic range, detection sensitivity limitations (e.g., IR), the possibility of incomplete separation, detector specificity limitations (e.g., FID), long analysis times, and difficulties in identification of components. This paper does not propose a universal solution to this complex problem but focuses on addressing one of these factors-improvement of analysis time by combining fast separation methods with information-rich spectral detection. There are many examples of combined systems where various separation techniques are interfaced with spectral detection (GC-MS, LC-MS, LC-IR, GC-IR, CE-MS, LC-NMR, TLC-IR, etc.). The novelty of the present approach is the use of higher speed GC separation methodology in such a combination. IJnfortunately, because of the mismatch between requirements of the techniques, operational hyphenated analysis systems, such as those mentioned above, often impose constraints on both the separation and detection systems. An example is the large gas flow necessary to provide the infrared light pipe detector with sufficient sample in an infrared gas chromatography system. Therefore, it is necessary to use a large bore capillary column (320 pm i.d. or larger). As a result, less than optimal separation resolution must be accepted, limiting such single stage GC systems to applications where sample complexity (1) Wilkins, C. L. Anal. Chem. 1987, 59, 571A. 0003-2700/94/0366-375 1$04.50/0 0 1994 American Chemical Society
and dynamic range do not require the separation power of high-performance capillary chromatography.2 This paper focuses on the problem of interfacing a fast secondary separation in a GC-GC instrument with an infrared detector. The type of sample matrices analyzed here were restricted to simple volatile mixtures, but multidimensional GC (MDGC) has been shown to be an effective method to separate complex volatile Implementation of MDGC can be performed either via thermal desorption or by use of the heartcutting technique. The former is a comprehensive multidimensional method, whereas the latter approaches multidimensionality, depending upon the experimental parameter^.^ The fast GC experiments described here utilize heartcutting for MDGC separation. In this method, eluents from a segment of the initial complex chromatogram are transferred to a cryogenically cooled trap. The trap is then heated to reinject the trapped materials onto an analytical column of different selectivity than the initial column for separation and subsequent detection. Although this method yields improved chromatographic resolution and more information about the mixture, the procedure of heartcutting and analyzing segments of a complex chromatogram is time consuming. One way to address this problem is to use several traps in parallel.12 In the present study, a different approach, involving a fast secondary separation, is implemented to reduce the experimental analysis time. The term fast rather than high-speed is used because the secondary separation takes place on the order of tens of seconds, rather than a few seconds.13-17 The tradeoffs involved in the choice of fast separations over slow separations are well known. The primary cost is decreased chromatographic resolution. However, this loss in resolution is compensated by an order of magnitude Ragunathan, N.; Krock, K. A.; Wilkins, C. L. And. Chem. 1993, 65, 1012. Himberg, H.; Sippola, E.; Riekkola, M. L. J. MicrocolumnSep. 1989, I , 271. Liu, Z.; Zhang; M.; Phillips, J. B. J. Chromatogr. Sci. 1990, 28, 567. Krock, K. A.; Ragunathan, N.; Wilkins, C.L. Anal. Chem. 1994, 66, 425. Krock. K. A.; Ragunathan, N.; Klawun, C.; Sasaki, T.; Wilkins, C. L. Analyst (London), 1994, 119, 483. (7) Bertsch, W. In Multidimensional Chromatography Techniques and Applications; Cortes, H. J., Ed.; Chromatographic Science Serics 50; Marcel Dekker, Inc.: New York and Basel, 1990; pp 74-144. (8) Schomburg, G.; Kotter, H.; Stoffels, D.; Reissig, W. Chromatographia 1985, 19, 382. (9) Schomburg, G.; Weeke, F.; Muller, F.; Oreans, M. Chromarographia 1982, 16, 87. (10) Bertsch, W. J. High Resolut. Chromarogr. Chromatogr. Commun. 1978, I , 289. (11) Wilkins, C. L. Anal. Chem. 1994, 66, 295A. (12) Krock, K.A.; Ragunathan,N.; Wilkins,C. L. J. Chromatogr. 1993,645,153. (13) Desty, D. H.; Goldup, A. In Gas Chromatography; Scott, R. W. P., Ed.; Buttenvorth: London, 1960; 162. (14) Desty, D. H. A d a Chromatogr. 1965, I , 199. (15) Gasper, G.; Arpino, P.; Guiochon, G. J. Chromatogr. Sci. 1977, 15, 256. (16) Schutjes, C. P.; Vermeer, E. A.; Rijks, J. A,; Cramers, C. A. J. Chromatogr. 1982, 253, 1. (17) Ke, H.; Levine, S.P.; Berkeley, R. J. Air WasteManag.Assoc. 1992,42,1446.
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reduction (or more) in separation times. Furthermore, coupling of fast GC or any other separation method to spectroscopic techniques may partially compensate for the loss of chromatographic resolution by the improved information obtained for qualitative analysis of a multicomponent peak in a chromatogram. During the past 25 years, there have been several instrumentation advances associated with sample injection strategies in high-speed GC, such as use of a small sample loop with a mechanical valve,18 application of microchip technology,19the moving tube method,20and development of thermal desorption4 and resistive heating techniques.21 The motivation for developing specialized sample injection procedures is to minimize extracolumn peak broadening. By minimizing extraneous broadening, one hopes to minimize total elution time for each component. In high-speed GC, narrow peaks with full widths at half-height (fwhh) in the range of 20-50 ms have been demonstrated for early eluting components.22 Among the various sample injection methods, the resistive heating method is the easiest to implement. In this method, a high current pulse is applied to a metal tube having capillary dimensions which desorbs compounds trapped under cryogenic conditions. A 1-10 ms pulse width is necessary for obtaining a narrow injection bandwidth, and temperature changes associated with this pulse are of the order of 300 OC. However, as pointed out by Sacks and co-workers, such rapid heating may result in sample decompo~ition.~~ Several investigators have implemented fast multidimensional systems for analysis of complex mixtures. The recent applications by Lemmo and J ~ r g e n s o nand ~ ~ Phillips and coworker^^^^^ are notable. However, those investigators have not incorporated those improvements in analysis systems which include additional structure-specific detectors. Jorgenson et a1have implemented multidimensional separation by collecting sample loops from a liquid chromatograph and periodically reinjecting the collected effluents onto a capillary electrophoresis ~ y s t e m . A~ similar ~ , ~ ~ approach incorporates parallel traps in an MDGC system; in fact, implementation and usefulness of such a valve-based parallel trapping scheme for GC separation has been demon~trated.5,~J~ A similar method could also be utilized for fast MDGC-IR instrumentation if implemented differently. Although this is possible, the present study documents the feasibility of fast MDGC-IR with a single trap. Thus, there are several ways to attack the problem of complex mixture analysis, with the key idea being complementary use of multidimensionality in both separation and spectroscopy. Coupling fast or even high-speed GC with a rapid response detector such as time-of-flight mass spectrometry is attractive. On the other hand, a similar coupling with an inherently slower response detector, such as an infrared spectrometer, initially seems difficult. However, the present study involves just such (18) Jonkcr, R.; Poppe, H.; Huber, J. Anal. Cfiem. 1982, 54, 2447. (19) Lee, G.;Ray, C.; Seimers, R.; Moore, R. Am. Lab. 1985, 10, 124. (20) Tijsscn,R.;vandcnHoed,N.;vanKreveld,M.E. Anal. Cfiem.1987,59,1007. (21) Hopkins, B.; Prctorius, V. J. Chromatogr. 1978, 158, 645. (22) Peters, A.; Klemp, M.; Puig, L.; Rankin, C.; Sacks, R. Analyst (London)1991, 116, 1313. (23) Klemp, M.; Akard, M. L.; Sacks, R. Anal. Cfiem. 1993, 65, 2516. (24) Lemmo, A. V.;Jorgenson, J. W. Anal. Cfiem. 1993, 65, 1576. (25) Liu, 2.; Phillips, J. B. J. Chromafogr.Sci. 1991, 29, 227. (26) Bushey, M. M.; Jorgenson, J. W. Anal. Cfiem. 1990, 62, 978.
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an interface. The problem of the mismatch between the speed of fast separation and the slow response of the IR spectrometer is solved by use of an efficient off-line interface. In this approach, matrix isolation methods are used to capture the eluents from the fast chromatogram. Subsequently, the matrix-isolated eluents can be analyzed by infrared spectrometry in a near real-time analysis. The experiments presented in this paper are the first to investigate the plausibility of interfacing fast GC with matrix isolation infrared instrumentation.
EXPERIMENTAL SECT1ON Instrumentationand Chromatography. Multidimensional chromatography was carried out using a modified commercially available Hewlett-Packard (HP) 5890 gas chromatograph coupled with a Mattson Cryolect 4800 FT-IR. A H P 5970B mass selective detector was used to monitor the first stage GC column effluents. Figure 1 is a schematic diagram of the instrument configuration used in this study. A 30 m long, 320 pm i.d. DB-Wax with a 1 pm film thickness was used as the first stage "precolumn" (J&W, Folsom, CA). The two different analytical columns used in this study were 250 pm i.d. Rtx-1701 (13 m, 0.5 pm film thickness; Restek, Bellefonte, PA) and 250 pm i.d. DB-5 (8 m, 0.25 pm film thickness; J&W, Folsom, CA) columns, respectively. Coupling between the precolumn and the analytical column was accomplished via an intermediate cryogenic trap. A fourport two-position valve (Valco, Houston, TX; 300 OC maximum temperature) directed precolumn effluent to either the mass spectrometer or the cold trap. The cold trap was housed in an external oven which was maintained at 200 OC. The analytical column output was connected to a Y press tight connector. The outputs of the Y connector were 100 pm i.d. and 220 pm i.d. transfer lines to the FID and the Cryolect, respectively. The Cryolect transfer line tip was positioned approximately 100 pm from the gold cryocollector disk, and the transfer line was maintained at 250 OC. The carrier gas for the precolumn separation was 99.999% He, and that for the secondary separation was 0.5% N2 in He with the nitrogen serving as the matrix. Secondary separations were conducted using a linear flow velocity of 100cm/s. Although hydrogen is the best choice of carrier gas for fast GC, for the present demonstration of FT-IR detection, helium is adequate. The FID electrometer or the associated data processing electronics were not modified to decrease response time, as this is an investigation focused on infrared spectral measurements of fast separation effluents. Other researchers have described in detail the modifications necessary to improve the electrometer and recorder from the combined standard response time of 100-300 ms to 5 ms or less.27.28 For normal measurements, deposition of effluents onto the gold Cryolect collector disk occurred at a constant cylinder velocity of 48 pm/s (corresponding to units of 135 steps/s for the stepper motor used to drive the cylinder). For fast GC studies, speeds of 6 times the normal speed were employed to separate components deposited upon the disk. Although spraying of effluents onto the disk significantly increases the
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(27) Villalob, R.; Annino, R. J. High Resolut. Cfiromatogr. Chromatogr. Commun. 1989, 12, 149. (28) J . Chromatogr. 1991, 556, 331.
Cold Nitrogen Gas
1 1
;I d r-a I"
Precolumn Injector
r-
----- ---
Precolumn
Analytical Column (AC)
I I
_1
Vent
V
Head Head Pressure for AC
1
1 V
Resistive Heating Connections
Figure 1. Schematic diagram of the MDGC-FT-IR system. Effluents from the precoiumn are directed by the four-port two-way valve to the HP-5970B MSD or to the cryogenic trap. The cryogenic trap, cooled by cold nitrogen gas at 90-100 K, is placed in an external oven maintained at 200 O C . IRD in the figure is the matrix isolation FT-IR utilized for infrared detection of secondary separations. Details on the matrix isolation FT-IR can be found in ref 28.
measured peak width in the Cryolect, the increased diskspeed allows separation of closely eluting components and leads to an apparent increase in IR chromatographic r e ~ o l u t i o n . ~ ~ A 0.5 mm X 0.5 mm MCT detector with a D* of 4.09 X 1Olo cm H z ' / ~ / Wat 10 kHz with 8.0 mA bias current was used for infrared measurement. A 1 mm infrared spot size diameter on the disk results in a 1.23 mm spot size diameter on the detector. The infrared reconstructed chromatogram was obtained with the use of standard Cryolect software incorporating a proprietary algorithm which is similar to the Gram-Schmidt reconstruction procedure.29 The reconstructed chromatogram was obtained with time resolution of 0.0025 min and 128 cm-' spectral resolution and 10 summed scans per reconstruction point. The disk speed utilized during sample deposition 248 pm/s was 6 times the normal speed, unless otherwise specified. Details of the disk speed programming can be found elsewhere.30 The cryogenic trap is a 10 cm long, 180 pm i.d. deactivated fused silica tube placed inside a 530 pm i.d. stainless steel tube. The steel tubing was cooled by placing it in a steel container and continuously cooling by cold nitrogen gas at 90-100 K. The input end of the trap was connected to a 320 pm i.d. transfer line from the valve and the output end to a Y press tight connector. The two outputs from the Y were to the analytical column and vent. The trapping of effluents is accomplished by directing the flow to the trap and simultaneously opening the vent. Prior to reinjection, the vent was closed and the valve switched to establish head pressure for the analytical column, and then the stainless steel tubing was heated by passing a current pulse through it. The design for electrical heating of the trap was similar to that demonstrated by Ewels and Sacks.31 Heating current was optimized to minimize sample decomposition but at the same time to reduce memory effects. The efficiency of trapping for (29) de Haseth, J. A.; Isenhour, T. L. Anal. Chem. 1977, 49, 1977. (30) Klawun, C.; Sasaki, T. A.; Wilkins, C. L.; Carter, D.; Dent, G.; Jackson, P.; Chalmers, J . Appl. Spectrosc. 1993, 47, 957. (31) Ewels, B.; Sacks, R. Anal. Chem. 1985, 57, 2774.
this system was not determined. Other experiments not described in detail here were attempted with deactivated fused silica-coated stainless steel capillary and deactivated fused silica traps with different inside diameters. Significant sample decomposition was observed with the fused silica-coated stainless steel traps and with fused silica traps with inside diameters larger than 180 pm. The results in this paper were obtained by using a 180 pm i.d. deactivated fused silica trap. The probable reason for negligible decomposition with small inside diameter fused silica is the high linear velocity through the trap, which results in rapidly flushing trapped effluents during reinjection. With the glass-coated stainless steel capillary, the decomposition may be attributed to rapid heating causing fractures in the fused silica lining, thereby exposing the samples to the metal surface. Samples. Mixtures analyzed with this system were a Grob test mix32and heartcuts from eucalyptus and cascarilla bark essential oils. The Grob mix was primarily utilized to check for system performance. The amount of sample injected in the case of the Grob mix (-400 ng/mL/component) was 0.4 pL, with a 1O:l split ratio. For the essential oil samples, only the earlier eluting hydrocarbons were investigated. The essential oils were diluted 1:10 in hexane, and a 0.2 p L aliquot was injected in a 1O:l split mode. The samples were injected from the cryotrap 1 min after establishing a matrix flow to the Cryolect.
RESULTS AND DISCUSSION According to Gasper, the type of experiment detailed in this paper is a fast GC analysis.28 Nevertheless, it is worthwhile to explore coupling of this method with infrared detection, if only to establish feasibility of the approach. Only modest technological improvements would be required in the future to permit routine fast or even high-speed analysis using a GC-FT-IR-MS system. Figures 2a and 2b show the flame ionization detector (FID) and infrared reconstructed chromatograms for separation of (32) Grob, K.; Grob, G.; Grob, K. J . Chromafogr. 1978, 156, 1.
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Flgure 2. (a) I R reconstructed chromatogram of Grob mix obtained in an Rtx-1701 column. (b) FID trace. The separation condltlons are 85 O C for 1.5 mln, 60 deg/mln to 175 O C , linear velocity at 85 O C is 105 cm/s, with the sample injected at 1 mln. Peak numbers refer to components In Table 1.
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Table 1. Compounds Identlfled by Infrared Spectroscopy In Flgures 2-5 peak no. Figure 2 Figure 3 Figure 4 Figure 5 1 2 3 4 5 6 I 8 9 10 11 12 13 14
water polysiloxane 2,3-butanediol decane undecane octanol nonanal ethylhexanoic acid 2,6-dimethylphenol 2,6-dimethylaniline C10 methyl ester dicyclohexylamine C11 methyl ester C12 methyl ester
water a-thuyene a-pinene camphene @-pinene @-furncsene a-phenllandrene a-terpenene limonene @-phellandrene
water a-pinene camphene sabenene @-pinene a-phellandrene a-terpenene
water a-thuyene a-pinene camphene @-pinene @-fumesene a-terpenene limonene
y-terpene
theGrobmixusing theDB-1701 (midpolar) analytical column. Table 1 identifies the compounds in the Grob mix as well as the components of the other mixtures characterized in the study. The sample is injected 1 min after initiation of cryocollectordisk motion, and the separation is complete within 2.3 min after injection. The reason for injecting the sample 1 min after beginning collector disk motion is to deposit the analyte away from a "blob" of frozen nitrogen matrix on the cry0 disk. This occurs because the matrix gas continuously flows through the analytical column, except during the trapping period, and condenses onto the cold cry0 disk. The first component observed is water, eluting at 1.2 min. This can serve as a convenient internal standard for estimating the mobile phase linear velocity. Elimination of water was attempted by cooling the gas lines with cold nitrogen gas. However, this strategy met with limited success. There was no apparent overlap of mixture components with water, as indicated by the absence of infrared features corresponding to any organic functionality. This observation suggests that effluents did not decompose to yield lower molecular weight products. The only other component, eluting after 1.2 min but before the first eluted Grob mix component (Le., 2,3butanediol, 1.5 min) was polysiloxane (1.3 min). The origin of this compound is not known. Other data presented in this 3754
Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
paper do not show the polysiloxane peak in the infrared reconstructed chromatogram. Additionally, in the separation of the Grob mixture shown in Figure 2, only ethyl hexanoic acid (peak 8) and 2,6-dimethylphenol are unresolved in the FID trace. However, in the IR reconstructed chromatogram, these two components are partially resolved and can each be identified from their infrared spectra. A possible explanation for the partial resolution observed with the infrared but not with the FID may be an insufficiently rapid response time for the FID or a lack of sensitivity toward a change in component molecular structure between the acid and the phenol compared to the infrared detector. Another important observation is that the valve utilized for directing the effluents does not significantly adsorb or react with any of the components. The presence of a large dicyclohexylamine peak in the FID trace supports this observation. In addition, related experiments (utilizing slow MDGC-FT-IR-MS) carried out in our laboratory with 40 different components, including the Grob mix, have demonstrated that in a valve-based parallel trapping scheme, the valves do not significantly adsorb or react with components. In fact, in those experiments, only dicyclohexylamine showed any significant adsorption, probably as a consequence of its higher basicity. The measured peak widths (fwhh) in the FID trace for the Grob mix range from 0.6 s for decane to 1.1 s for the C12 methyl ester. If hydrogen, rather than helium, were used as carrier gas, the peak widths would be expected to decrease by about a factor of 2 at the velocity used for this separation. In the IR reconstructed chromatogram, the peak widths were constant over the length of the experiment, with an average peak fwhh of 1.15 s. This consistency in peak width is due to spraying the effluents on the cryocollector disk and seems to be independent of the FID trace fwhh or molar absorptivity. However, the peakseparation quality is similar to that observed in the FID chromatogram. In addition, the measured peak widths in the IR chromatogram are considered apparent because the response of the IRdetector at lower concentrations, Le., on the shoulders, may be n ~ n l i n e a r .This ~ ~ could result in a smaller than actual measured fwhh. A significant result is the clear separation in the IR chromatogram of dicyclohexylamine and C11 methyl ester, whose peaks are 2 s apart in the FID trace. In a previous study on the utility of cyro disk speed programming, it was noted that increasing the disk speed reduces infrared detection s e n ~ i t i v i t y .This ~ ~ is a result of spraying the effluents on the disk moving at a high speed. Using simple assumptions, it was noted that, for an infinitely sharp GC peak, the diameter of the spot size on the disk would be 400 pm for a 200 pm tip diameter located 100 pm from the disk and spraying at a 4 5 O angle. With increasing speed, this circular spot would elongate into an ellipse with a longer major axis. At 3 times the regular speed, the major axis of this spot size would be greater than the detector dimension and would create a blank area having the detector dimension. This blank area would then define a valley between two peaks. Three times the normal speed would resolve two components separated by 8 s. For smaller separation times between components, a higher diskspeed is necessary. The requirement of higher speed results from thechoice of a 200 pm tip diameter
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Figure 3. Secondary separation (a) I R reconstructedchromatogram of a segment of the cascarilla bark 011 separation using the DB-1701 column and (b) FID trace. The precoiumn separation was done in a DB-Wax column. The secondarySeparationwas carriedout isothermally at 95 O C with a linear velocity of 90 cm/s. (c) Total ion chromatogram section of this 011 obtained with a 30 m long, 320 pm ID, 1 pm film thickness Rtx-1701 column at a linear velocity of 30 cm/s with temperature programming. Peak numbersrefer to componentsin Table 1.
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I\
Figure 5. Secondary separation (a) IR reconstructedchromatogram of cascarilla bark oil separated using the DB-5 column and (b) FID trace. The section analyzed in this figure is identicalto that in Figure 3. The precoiumn Separation was done in a DB-Wax column. The secondary separation was carried out isothermally at 60 O C with a linear velocity of 140 cm/s.
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y-axis to show the weakly absorbing or low-quantity components clearly. The inset in Figure 3 shows the separation of earlier eluting components of cascarilla oil in DB-1701 column (30 m, 320 pm i.d., 1 pm film thickness, 0.1 p L neat, splitless injection, 30 cm/s linear velocity) measured with a slow separation speed. A comparison of the two separations reveals that components analyzed in the fast separation by rapid desorption do not show evidenceof sample decomposition. For the fast separation, the major component, eluting at 14.5 m in the inset chromatogram, was not cut. However, the first eight components in the slow separation (-9.8-14.1 min) are identical to those detected in the fast separation, where they are sufficiently resolved for spectral measurement. It also should be noted that the sample quantity injected in the faster separation is approximately 100 times lower than that for the separation shown in the inset chromatogram in Figure 3. Unfortunately, injection of large quantities of sample (-400500 ng) from the cryotrap under the conditions of rapid heating does cause sample decomposition. Thus, even under controlled rapid heating conditions, there might not be sufficient dynamic range to analyze minor and major components simultaneously. Figure 5 shows the secondary separation of the heartcut identical to that in Figure 3 using an 8 m DB-5 (nonpolar) analytical column. Here, the linear velocity used is 140 cm/s. The focus of this experiment is to determine whether a faster separation can be accommodated by the IR detection system. A second issue is whether it is possible to design experiments where analysis of the secondary separation could be carried out in two or more parallel analytical columns. The disk speed utilized for this separation was 6 times the normal disk speed and identical to that in the previous separations. It is clear from the IR chromatogram that, even at this relatively high linear velocity and disk speed, a reasonable separation can be carried out. Furthermore, spectral measurements at the shoulders of the second peak (- 1.3 min.) in the IR reconstructed chromatogram shown in Figure 5 result in twodistinct spectra. Other examples of the qualitative
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1.10
1.20
1.30
1.40
1.50 1.60 1.70 1.8 Retention time [minutes!
1.90
2.00
2.10
2.20
Flgure 4. Secondary separation (a) IR reconstructedchromatogram of a segment of the eucalyptusoil separationusingthe Rtx-1701column and (b) FID trace. The secondary separation conditions are identical to those described In the Figure 3 caption. Peak numbers refer to components In Table 1.
and a relatively large area detector. This problem could be solved by modifying the tip to a narrower bore, e.g., 50 or 100 pm i.d., and by reducing the size of the infrared detector from its present dimension of 0.5 mm X 0.5 mm to 0.1 mm X 0.1 mm. However, decreasing the detector area would also require decreasing the infrared beam spot size diameter from the current 1 mm to 0.25 mm to maintain sufficient radiation throughput. These modifications of the tip, detector, and infrared beam spot size could lead to significant improvements in the sensitivity and peak separations. In addition, such modifications could reduce the disk speed necessary for observing closely eluting components. Figures 3 and 4 show the IR and FID analytical column chromatograms of heartcuts of cascarilla and eucalyptus essential oils analyzed in the DB- 1701 midpolar column. The infrared reconstructed chromatograms are enlarged along the
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Analytical Chemistry, Vol. 66,No. 21, November 1, 1994
3755
0.040
A b S
0.035
0.030
0
r b a
0.025
0.020
n C
0.015
e 0.010
0.005
0.000
4000
3500
3000
2500
2000
1500
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Wavenumber Figure 8. Infrared spectrum of the a-pinene peak (labeled 3 in Figure 4). The spectral acquisition conditions are 128 scans at 4 cm-I resolution with a mirror velocity of 1.27 cmls. The time for acquiring this spectrum is 120 s. The bands at -3700 and 1600 cm-I are due to water.
value of this approach are found in the essential oil separations, where even minor components are detected with sufficient IR spectral quality to permit their identification. For example, a-pinene in eucalyptus essential oil is seen to elute at 1.53 min in Figure 4. The infrared spectral measurement for this material is shown in Figure 6 . The quality of this spectrum is sufficient for its identification by a spectral library search. These examples demonstrate that the linkage of fast GC with IR spectral detection is feasible. More work will be required to optimize such methodology. Excluding the carrier gas as a parameter, in the present studies, other possible sources of peak broadening include the use of press tight connectors and the coupling of a 180 hm trap to a 250 pm analytical column. It is possible that the press tight connectors used in this study may not have provided zero dead volume connections. For the essential oil separations, the measured peak widths at half-height for the components average about 1 s. In those separations, about 200 plates/s were generated. A significantly higher number of plates per second were generated in the Grob mix separations. However, those measurements included a temperature ramp and cannot be used for direct comparison. The question now is whether the low plate generation per unit time is due to the choice of carrier gas or to the connections. This will be the subject of further investigation.
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CONCLUSIONS A successful demonstration of fast multidimensional GC linked with a matrix isolation FT-IR has been achieved. The feasibility of linking fast GC with FT-IR detection, coupled
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with the usefulness of parallel cryogenic trapping MDGC with parallel analytical columns, suggests a number of additional experimental designs with analytical potential. For example, one could use parallel traps, each linked to an analytical column of different selectivity. Identical segments from the precolumn could be trapped in parallel and subsequently reinjected into each of the analytical columns sequentially. This would provide additional spectroscopicand retention index information in a much shorter time. Another alternative might be to use a double oven, which could permit trapping and desorption of precolumn segments separated by a few minutes. The necessity for double ovens arises from the different temperature requirements of fast secondary separations, which are usually carried out isothermally. A double oven configuration would permit trapping 1 min segments of the precolumn chromatogram every 3 min, with each trapping followed by injection of that segment to the analytical column. In order to analyze the complete precolumn chromatogram in this manner, more than one precolumn sample injection would be required. This approach is conceptually similar to those of Phillips4 and J o r g e n ~ o n ' s * ~multidimensional .~~ methods; however, the implementation of the concept is different. ACKNOWLEDGMENT We gratefully acknowledge support from the National Science Foundation (Grant CHE-92-01277). Received for review April 11, 1994. Accepted July 6, 1994." Abstract published in Aduance ACS Abstracts. September 1, 1994.
