Anal. Chem. 1999, 71, 5177-5184
Tunable-Column Selectivity and Time-of-Flight Detection for High-Speed GC/MS Carrie Leonard and Richard Sacks*
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109
A pressure-tunable ensemble of two capillary separation columns is combined with time-of-flight mass spectral detection for the high-speed characterization and analysis of volatile organic compounds. The detector can record up to 500 complete mass spectra per second and can obtain spectral deconvolution and thus characterization of severely overlapping and unknown chromatographic peaks. The tunable capillary column ensemble consists of the series combination of a nonpolar 5% phenyl poly(dimethylsiloxane) column and a polar poly(ethylene glycol) column. Adjustment of the pressure at the column junction point is achieved by the use of an electronic pressure controller. Changes in the junction pressure result in changes in the relative contribution that each of the columns makes to overall retention. This causes changes in the retention patterns, which can be used to enhance the utility of the TOFMS detector. The use of high spectral acquisition rates allows for the deconvolution of very closely spaced peaks. Adjustment of the column junction pressure often allows for the control of peak separation for critical component pairs. Since severely overlapping chromatographic peaks can be tolerated in many cases, the time axis of the chromatogram can be compressed, thus obtaining very fast GC/MS characterization of unknown mixtures. Tunable columns are very useful for facilitating time compression since significant control of relative peak positions can be achieved. Gas chromatography (GC) and GC with mass spectrometric detection (GC/MS) are the most widely used techniques for the characterization and analysis of volatile and semivolatile organic compounds. Strategies, instruments, and procedures are being developed for dramatically increasing the speed of capillary GC.1 In some cases, speed increases of 1-2 orders of magnitude can be achieved. This will make these methods far more useful for chemical process monitoring, general laboratory analysis, and environmental monitoring. Emerging technologies for high-speed GC (HSGC) include inlet systems capable of injecting very narrow vapor plugs,2,3 high-speed temperature programming,4,5 and col(1) Sacks, R.; Smith H.; Nowak, M. Anal. Chem. 1998, 70, A29. (2) Klemp, M.; Peters, A.; Sacks, R. J. Environ. Sci. Technol. 1994, 28, 369A. (3) van Es, A.; Janssen, J.; Bally, R.; Cramers, C.; Rijks, J. HRC&CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1987, 10, 273. (4) Ehrmann, E. U.; Dharmasena, H. P.; Carney, K.; Overton, E. B. J. Chromatogr. Sci. 1996, 34, 533. (5) MacDonald, J.; Wheeler, D. Int. Lab. 1998, 28, 6. 10.1021/ac990631f CCC: $18.00 Published on Web 10/09/1999
© 1999 American Chemical Society
umns with tunable and programmable selectivity.6-8 Recent successes with HSGC have accelerated interest in the potential for high-speed GC/MS. Mass spectrometry is unsurpassed in its ability to identify components separated by GC. The MS fragmentation pattern often can provide unambiguous component identification by comparisons with library spectra. When GC and MS are combined, the GC separation usually provides isomer specificity, and the MS detection provides compound class and homologue specificity. For characterization by comparison of fragmentation patterns with standard libraries, low-resolution quadrupole instruments typically are operated with spectral acquisition rates in the 1-2 spectra/s range and GC peak widths of several seconds. Methods typically require tens of minutes to over an hour depending in part on the complexity of the mixture. The long analysis times associated with GC/MS have emerged as a major limitation in numerous applications involving high sample throughput. This has greatly increased analysis cost and has resulted in severe bottlenecks in some applications. Scanning instruments also suffer from poorer powers of detection when complete mass spectra are recorded, since each mass resolution element is monitored for only a small fraction of the total scan time. With HSGC, elution peak widths typically are less than 1 s and may be less than 100 ms.9,10 Scanning MS instruments generally are not useful in this range unless operated in the selection-monitoring (SIM) mode. At low resolution using SIM, it may be difficult to obtain compound-class-specific and homologuespecific information. With high-resolution MS, the small differences from nominal fragment masses for different compound classes and different homologues can be used to enhance selectivity. In any case, SIM utilizes very little of the information potentially available with GC/MS. Clement and Tosine have reviewed these methods.11 Recently, time-of-flight (TOF) MS detection systems for HSGC have become available.12-14 With time-array detection, these instruments can obtain complete mass spectra (fragmentation (6) Sacks, R.; Akard, M. J. Environ. Sci. Technol. 1994, 28, 428A. (7) Smith, H.; Sacks, R. Anal. Chem. 1997, 69, 5159. (8) Smith, H.; Sacks, R. Anal. Chem. 1998, 70, 4960. (9) Peters, A.; Sacks, R. J Chromatogr. Sci. 1991, 29, 403. (10) Phillips, J.; Luu, D.; Lee, R. J. Chromatogr. Sci. 1986, 24, 396. (11) Clement, R.; Tosine, H. Mass Spectrom. Rev. 1988, 7, 593. (12) Watson, J.; Schultz, G.; Tecklenburg, R.; Allison, J. J. Chromatogr. 1990, 518, 283. (13) Wollnik, H.; Becker, R.; Gotz, H.; Kraft, A.; Jung, H.; Chen, C.; Van Yascker, P.; Janssen, H.; Snijders, H.; Leclercq, P.; Cramers, C. Int. J. Mass Spectrom. Ion Processes 1994, 130, L7.
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patterns) for every ion pulse (spectral transient). The production rate of ion pulses and thus spectral transients is typically in the 5-10-kHz range. By summing a sequence of transients, high spectral acquisition rates and high signal-to-noise ratios can be obtained. These features are well suited for use with HSGC. An additional very attractive feature of TOFMS with time-array detection is the complete absence of concentration biasing. With scanning instruments, the concentration in the chromatographic peak and thus in the MS ion source changes for each mass during a scan. If the concentration change is significant, spectral deconvolution of unknown peaks with significant chromatographic overlap is often unreliable. Since all ion fragments from a TOF spectrum represent the same point on the chromatographic peak profile, there is no concentration biasing. This allows for reliable peak characterization by comparison with library spectra. Spectral deconvolution of partially overlapping and unknown chromatographic peaks then is straightforward if the fragmentation patterns for the overlapping components are significantly different. When deconvolution is possible, the resolution requirements of the separation may be greatly relaxed with the result that shorter columns, higher carrier gas flow rates, higher column temperatures, and/or higher temperature programming rates can be used to compress the time axis of the separation.15 The combination of HSGC technologies with TOFMS offers the potential for very fast GC/MS analysis of completely unknown and severely overlapping peaks. However, there are two important limitations. First, for the characterization of unknown mixtures, some chromatographic separation is required so that peak finding and deconvolution algorithms recognize the presence of two or more components within a single chromatographic feature. Second, the fragmentation patterns of isomers often are too similar for spectral identification and deconvolution based on comparisons with library spectra. Typically, sufficient peak separation requires that at least two mass spectra can be recorded between the peak centers. For high spectral acquisition rates, the required peak separation may be very small. However, file size per sample, data processing time, storage, and archiving become more significant issues at very high acquisition rates. These problems continue to be minimized by rapid advances in computer technology. If acquisition rate is reduced, file size will be reduced. However, greater peak separation is required for automated peak finding and spectral deconvolution. The ability to control peak positions and peak separations within a chromatogram would be very useful for GC/TOFMS. This can be achieved by the use of GC column ensembles with tunable selectivity.16-18 By combining in series two capillary GC columns of significantly different selectivity with electronic pressure control at the junction point between the columns, a wide range of retention patterns often can be achieved with very precise control of peak-pair separations.7 In this report, the use of a tunable column ensemble for enhancing the utility of TOFMS detection is described. Chromatographic peak position control is empha(14) Pack, B.; Broekaert, J.; Guzowski, J.; Poehlman, J.; Hieftje, G. Anal. Chem. 1998, 70, 3963. (15) Gankin, Y.; Gorshteyn, A.; Smarason, S.; Robbat, A., Jr. Anal. Chem. 1998, 70, 1655. (16) Akard, M.; Sacks, R. Anal.Chem. 1994, 66, 3036. (17) Hinshaw, J.; Ettre, L. Chromatographia 1986, 21, 561. (18) Purnell, J. H.; Jones, J.; Wattan, M. J. Chromatogr. 1984, 292, 197.
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Figure 1. Instrumentation for tunable column selectivity and timeof-flight MS detection. GC, HP 6890 gas chromatograph; AI, HP 7683 autoinjector; MS, LECO Pegasus II TOFMS; I, heated interface line; PC, MKS pressure controller; R, vent line restrictor; C1, polar column; C2, nonpolar column.
sized, and the tradeoffs of spectral acquisition rate, analysis time, and file size are considered. The effects of acquisition rate and smoothing procedures on signal-to-noise ratios also are considered. The use of tunable columns for the generation of retention patterns that allow for significantly enhanced compression of the chromatogram time axis using TOFMS detection is illustrated. Note that the work performed in this study is for the case of unknown mixture characterization. The case of targeted analysis requires only the existence of unique ions and does not depend on the separation of the targeted analytes. EXPERIMENTAL SECTION Apparatus. Figure 1 shows a diagram of the apparatus used for these studies. An HP 6890 GC equipped with an HP 7683 autoinjector was used. The pressure-tunable column ensemble consists of a polar poly(ethylene glycol) column C1 (DB-Wax, J&W Scientific, Inc., Folsom, CA) and nonpolar 5% phenyl dimethyl polysiloxane column C2 (DB5, J&W Scientific, Inc., Folsom, CA). Both columns are 10 m long, 0.18 mm i.d. and use 0.18-µm-thick stationary phases. The pressure at the column junction point is controlled with an absolute-pressure capacitance manometer (MKS model 640A, MKS Instruments, Inc., Andover, MA). The pressure controller PC was located outside the GC oven. Hydrogen carrier gas is supplied at points CG. The controller was operated from a Gateway 2000 P5-75 computer. The controller operates over a range from 0 to 100 psia in 0.1 psi steps. The corresponding analog input voltage ranges from 0 to 5.000 V in 5-mV steps. The pressure set-point reproducibility is on the order of (0.01 psi. A gas vent line consisting of a 0.25-mm-i.d., 1.25-m-long fusedsilica tube R is connected between the pressure controller and the column junction point. The line is necessary to prevent contamination of the controller by sample when the set-point pressure is less than the pressure that would normally exist at the column junction point in the absence of the controller connection. The system can be operated without the vent line, but the range of junction pressures and thus the range of overall column ensemble selectivities is significantly reduced. Note that, for set-point pressures less than the normal junction-point pressure in the absence of any other connections, the effluent from the first column is split between the second column and the vent line. The split ratio depends on the set-point pressure and the ratio of the pneumatic restrictions of the second column and the vent-
Table 1. Compounds and Boiling Points for the Test Mixture no.
compound
bp
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
air n-pentane 2,2-dimethylbutane cyclopentane n-hexane ethyl alcohol cyclohexane 1,1,1-trichloroethane n-heptane benzene 1-propanol 1,2-dichloropropane n-octane toluene n-nonane ethylbenzene p-xylene o-xylene
36.1 49.7 49 69 78.3 80.7 74.2 98.4 80.1 97.2 96.8 126 110.6 150.8 136.2 138.3 144
line restrictor. Since the vent line vents to atmospheric pressure, the set-point pressure should not be less than 1 atm in order to prevent contamination of the controller. The effect of the vent line on quantitative results was not investigated for this report. The TOFMS detector was a LECO model Pegasus II instrument (LECO Corp., St. Joeseph, MI). The detector uses reflectron geometry19 with a total ion path length of about 1.5 m. The detector has a mass range of 5-1000 amu and a maximum spectral acquisition rate of 500 spectra/s. The detector is designed to generate 5000 spectral transients each second, and the spectral acquisition rate is controlled by the number of transients that are summed. Time-array detection20 using an integrating transient recorder21 is used to obtain a complete mass spectrum for each spectral transient. The electron-impact ion source has storage capabilities, and ion packets are formed using orthogonal beam deflection techniques.22 The detector uses an electron-impact ionization source operated with electron kinetic energy of 70 eV. The ion source was operated at a temperature of 250 °C. The ion source is connected to the GC by means of a heated interface line, which was operated at a temperature of 250 °C. The pressure in the MS flight tube typically was 1 × 10-6 Torr. Ion detection is provided by a microchannel plate. The TOFMS software provides for completely automated peak finding and assignment. This feature dramatically reduces dataprocessing time. In addition, spectral deconvolution of partially overlapping unknown peaks is completely automated. This software was used for all studies reported here. Materials and Procedures. The GC inlet pressure was 49.7 psig for all experiments. Nominal split ratios of 20:1 and 100:1 were used. Hydrogen carrier gas was purified with filters for oxygen, water vapor, and hydrocarbons. A test mixture was made by mixing equal volumes of reagent grade compounds. Table 1 lists the compounds and their boiling points. Headspace vapor (1 (19) Cotter, R. Anal. Chem. 1992, 64, 1027A. (20) Erickson, E.; Enke, C.; Holland, J.; Watson, J. Anal. Chem. 1990, 62, 1079. (21) Holland, J.; Newcombe, B.; Tecklenburg, R.; Davenport, M.; Allison, J.; Watson, J.; Enke, C. Rev. Sci. Instrum. 1991, 62, 69. (22) Pinkston, J.; Rabb, M.; Watson, J.; Allison, J. Anal. Chem. 1990, 62, 1079.
Figure 2. Selected ion chromatograms at 50 °C showing the first 50 s of the separation using junction pressures of 20 (a) and 36 psia (b). Extracted ion chromatograms are shown for masses 40, 43, 45, 57, 59, 70, 71, 78, 84, 91, and 97. See Table 1 for peak identification.
µL/injection) was injected with the HP autoinjector. Holdup time from the tandem column ensemble was measured as the retention time for N2+ from the air in the headspace sample. Signal-to-noise ratios were evaluated for various spectral acquisition rates by injecting 1-µL samples of a 4.1 ppm solution of toluene in ethanol at a split ratio of 100:1. Data acquisition and processing were accomplished by use of the Pegasus II software, Version 1.1. Component identification was based on comparison with the NIST MS database. RESULTS AND DISCUSSION Tunable Selectivity. Figure 2 shows selected ion chromatograms obtained using junction (tuning) pressures of 20 (a) and 36 psia (b). These pressures correspond approximately to 34 and 66% contributions of the first (polar) column, respectively. These contributions can be found from holdup time values for the individual columns.16 Since only the final (overall) holdup time from the column ensemble was measured in the work reported here, these contributions could not be empirically determined. Values were estimated from the column dimensions, pressure drops, and carrier gas viscosity at the column temperature using standard equations for gas flow in capillary tubes.23 The column temperature was 50 °C, and the spectral acquisition rate was 50 spectra/s (Hz). The peak numbers in the chromatograms correspond to the label numbers in Table 1. The last four components15-18 elute after 50 s and are not shown in the figure. To show all 14 components in Figure 2, several extracted ion chromatograms (EIC) from mass-to-charge ratios of 40, 43, 45, 57, 59, 70, 71, 78, 84, 91, and 97 are superimposed in the figure. Note the dramatic differences in the retention patterns obtained at the two junction-point pressures. In chromatogram a, peaks 4 and 5 are baseline separated; while in chromatogram b, considerable overlap occurs. The elution order of 6-7-8-9 in (a) has changed to 7-9-8-6 in (b). Also note the dramatic differences in elution patterns for peaks 11-13. (23) Grant, D. W. Capillary Gas Chromatography; Wiley: New York, 1996; pp 16-20.
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Figure 3. Adjusted retention time vs junction pressure plots for all mixture components (a) and for components 1-10 on an expanded scale (b). The oven temperature was 50 °C. Plot numbers correspond to component numbers in Table 1.
Figure 4. Adjusted retention time vs junction pressure (a) and peak separation time (b) for 1,1,1-trichloroethane (A) and n-heptane (B) at an oven temperature of 50 °C.
Retention data for all mixture components were obtained over the tuning pressure range from 18 to 40 psia at 2-psi intervals. Note that with a pressure-step size of 0.1 psi, a total of 220 unique, computer-selected retention patterns can be obtained for this tuning-pressure range. Retention times were corrected for holdup time changes, which occur with tuning pressure changes.16 While a somewhat larger pressure range than that shown in Figure 3 can be used, overall holdup time increases rapidly for pressure values beyond the range considered. For the isothermal separations used here, overall analysis time is proportional to the overall holdup time of the column ensemble. In Figure 3a, adjusted retention times are plotted vs column junction (tuning) pressure for all 18 mixture components. In Figure 3b, the plots for the first 10 components are shown on an expanded retention-time scale. The curvature in the plots is a result of significant changes in the overall holdup time with changes in junction-point pressure. These plots can be linearized by dividing the adjusted retention times by the corresponding overall holdup times.16 This gives the overall retention factors for the compounds. However, the ability of the MS deconvolution algorithm to obtain spectral deconvolution of overlapping chromatographic peaks is based on differences in retention time and on the spectral acquisition rate. Thus, adjusted retention times are more useful than retention factors for the studies reported here. The shapes of the plots in Figure 3 depend on the retention of the specific compounds on the two columns. As the column junction pressure increases, the pressure drop along the first
(polar) column decreases with the result that all mixture components spend more time on the polar column and relatively less time on the nonpolar column. This increases the contribution that the polar column makes to the overall separation selectivity. For highly polar compounds such as ethyl alcohol (plot 6) and propyl alcohol (plot 11), greater contribution of the polar column results in a large increases in adjusted retention time. Note that for these components, the adjusted retention time more than doubles as the junction pressure is increased from 18 psia to 40 psia. For nonpolar compounds such as cyclohexane (plot 7), heptane, (plot 9), octane (plot 13), and nonane (plot 15) adjusted retention times decrease substantially with increasing junction pressure. These differences in the response of the adjusted retention times to changes in junction pressure for different compound classes result in a number of crossings of the plots at specified values of the junction pressure. At each of these crossing points, a pair of mixture components (critical pair) will overlap completely, and spectral deconvolution will not be possible for unknown mixture components. Figure 4 focuses on one critical component pair, which coelutes at a junction pressure of about 25 psia. In Figure 4a, adjusted retention time is plotted vs junction pressure for 1,1,1-trichloroethane (A) and n-heptane (B). In Figure 4b, the difference in adjusted retention times (peak separation) for these two components is plotted vs junction pressure. Note that, for both the right and left flanks of the plot in Figure 4b, the peak separation is a nonlinear function of the junction pressure. A series of plots for all critical component pairs (peak pairs that coelute for some
5180 Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
Table 2. Reproducibility Study adjusted retention time (s) day
n-heptane
1,1,1-trichloroethane
separation (s)
1 3 6 7
8.80 8.82 8.78 8.80
8.86 8.88 8.84 8.86
0.06 0.06 0.06 0.06
av std dev
8.80 (0.016
8.86 (0.016
0.06
junction-pressure value) defines all the junction-point pressure values which will result in at least one coelution and, thus, incomplete mixture characterization. Stability and repeatability of retention patterns and peak-pair separations is very important if tunable column ensembles are to have sufficient robustness for the applications described here. With electronic control of both the GC inlet pressure and the column junction pressure, retention patterns show good stability and reproducibility. This is confirmed in Table 2. These data are for a 1-week study in which a single injection was made 4 times over the 7-day period. The operating pressures and the split ratio were changed at various times between these injections and then reset just prior to the injections. The spectral acquisition rate was 50 Hz giving a time resolution of 20 ms. The range of retention time values was no more than 40 ms, and at this temporal resolution, no differences is peak separation were observed. While more detailed studies on long-term tuning stability are needed, these preliminary results are encouraging. Tuning Pressure and Required Spectral Acquisition Rate. The greater the degree of peak overlap, the higher the spectral acquisition rate must be in order to ensure that at least two spectra can be obtained between the peak centers. While an acquisition rate of 500 Hz requires a peak separation of only slightly more than 6 ms, signal-to-noise ratios, file size, and data processing times become considerations. Figure 5 shows extracted ion chromatograms for masses 97 (A) and 57 (B). These are characteristic ions for 1,1,1-trichloroethane and n-heptane, respectively. The junction pressure was 25.0 psia for all cases. Spectral acquisition rates of 20 (a), 50 (b), and 200 Hz (c, d) were used. No smoothing was performed except for (d), which was subjected to a 20-point smoothing operation. A split ratio of 20:1 was used, which results in baseline peak widths of approximately 1 s. An acquisition rate of 20 Hz is inadequate to accurately define the chromatographic peak shapes, and the peaks have an angular appearance. Only about 20 spectra are obtained over the full peak width. It is apparent from the peak shapes that only one spectrum was obtained between the peak centers. Deconvolution was not successful. An increase in spectral acquisition rate to 50 Hz (b) results in a reduction in the intensity of the peaks (shown on the vertical axis). While the peak shape is better defined, the separation between the peak apexes is less than 50 ms. This was measured manually from the extracted-ion chromatograms and is not sufficient for deconvolution at the 50-Hz sampling frequency. For case c, using an acquisition rate of 200 Hz, the noise in the EIC profiles is apparent and the intensity of the peak is further reduced. Because of the larger noise signal, the peak centers are not accurately identified, and the peaks are not deconvoluted. By
Figure 5. Extracted ion chromatograms for 1,1,1-trichloroethane (A) and n-heptane (B) using a junction pressure of 25.0 psia for spectral acquisition rates of 20 (a), 50 (b), and 200 Hz (c, d). A 20point smoothing operation was used in case d.
Figure 6. Extracted ion chromatograms for 1,1,1-trichloroethane (A) and n-heptane (B) using junction pressures of 24.9 (a), 24.8 (b), 24.7 (c), and 24.6 psia (d). The oven temperature was 50 °C, and the split ratio was 100:1. The spectral acquisition rate was 500 Hz, and a 20-point smoothing operation was applied.
performing a 20-point smoothing operation (d), the signal-to-noise ratio is increased, and deconvolution is successful. These results highlight the ability to deconvolute severely overlapping peaks by using higher acquisition rates and, if needed, data smoothing. Figure 6 shows extracted ion chromatograms for m/e of 97 (A) and m/e of 57 (B) for junction pressure values of 24.9 (a), 24.8 (b), 24.7 (c), and 24.6 psia (d). Again, these are characteristic ions for 1,1,1-trichloroethane and n-heptane, respectively. The GC oven temperature was 50 °C, and the split ratio was 100:1. The higher split ratio produces narrower peaks with baseline peak widths of about 0.5 s. The spectral acquisition rate was 500 Hz, and a 20-point smoothing algorithm was applied to the data. For case a, the peak separation was 34 ms, and spectral deconvolution was successful. Since the peak separation is greater than the time required to obtain two spectra between peak apexes (6 ms), successful deconvolution was anticipated. When the junction pressure is decreased to 24.8 psia (b), peak overlap is Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
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Table 3. Signal-to-Noise Ratios and File Size for Toluene acquisition rate (Hz)
0
10 20 50 100 200 500
12.4 9.0 5.3 4.0 2.0 1.3
S/N for smoothing (pts) 5 20 50
11.7 8.6 3.9 2.8
15.6 11.1 7.8
23.0 13.6
file size (mb) 0.2 0.4 1.0 1.9 3.8 9.6
nearly complete and deconvolution is not successful. With a junction pressure of 24.7 psia (c), a reversal of elution order is observed but the separation is inadequate for spectral deconvolution. A further decrease in pressure to 24.6 psia results in a peak separation of 42 ms, and deconvolution was again successful. Note the power of the tunable column ensemble with electronic pressure control for precisely positioning the two peaks so that deconvolution can occur, but with a maximum peak overlap, and thus the maximum potential for time compression of the chromatogram. By further changing the junction-point pressure beyond the range used for Figure 6, greater peak separation is achieved, and spectral deconvolution can be obtained with lower spectral acquisition rates. For case d in Figure 5, successful deconvolution was obtained by applying a smoothing algorithm contained in the instrument software. The integrating transient recorder used for time-array detection sums spectral transients at a rate of 5 kHz. The number of transients summed per display point on the extracted ion chromatograms is inversely proportional to the spectral acquisition rate. The signal-to-noise ratio is proportional to the square root of the number of summed transients. Table 3 summarizes values of signal-to-noise ratio at varying sampling frequencies for an injection of 4.1 ppm toluene in ethyl alcohol. Data are also shown for the value of signal-to-noise ratio for these peaks after several different smoothing operations. These data show that while there is a significant loss in powers of detection at higher spectral acquisition rates, this loss is completely recovered by performing an appropriate smoothing operation. For example, an unsmoothed chromatogram acquired at 10 Hz shows approximately the same signal-to-noise ratio as a chromatogram acquired at 200 Hz and subjected to a 20-point smoothing operation. This is due to the integrating nature of the transient recorder. Table 3 also shows file sizes for a 60-s chromatogram scanning from 35 to 200 mass units at the varying acquisition rates. Note the linear increase in file size with increasing acquisition rate. The issue is more complex, however, since the use of higher spectral acquisition rates can result in the potential for further time compression of the chromatogram. This can be accomplished by increasing column temperature or temperature programming rate or by the use of shorter columns and higher carrier gas flow rates. This reduces the duration of the chromatogram, thus reducing file size. Further studies here are suggested. Figure 7 shows the range of junction pressures for which deconvolution was unsuccessful for different sampling (spectral acquisition) frequencies. In the figure, measured peak separations for 1,1,1-trichloroethane and n-heptane are plotted vs junction 5182 Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
Figure 7. Peak separation times vs junction pressure for 1,1,1trichloroethane and n-heptane using spectral acquisition rates of 10 (A), 20 (B), 50 (C), 200 (D), and 500 Hz (E). Split ratios were 20:1 except for (E), which used a split ratio of 100:1. A 20-point smoothing operation was used for plots D and E. Zero peak separation indicates only one peak was found.
pressure. Plots labeled A-E are for acquisition rates of 10, 20, 50, 200, and 500 Hz, respectively. The split ratio used was 20:1 for all cases except the 500-Hz experiments, which were performed at a split ratio of 100:1. The 200- and 500-Hz chromatograms were subjected to a 20-point smoothing algorithm. A zero peak separation indicates that the peak finding algorithm did not recognize the presence of two peaks. For an acquisition rate of 10 Hz (A), a separation of 0.30 s is required in order to deconvolute the peaks. At this frequency, the range of pressures for which deconvolution is not successful (exclusion region) spans 2.3 psi. By increasing the acquisition rate to 20 Hz (B), the exclusion region is reduced to 1.0 psi, and a separation of only 0.15 s is required to deconvolute the peaks. Further reduction in the size of the pressure exclusion region is shown at an acquisition rate of 50 Hz (C). With acquisition rates greater than 50 Hz, the peak locations are more uncertain because of the decreased signal, and smoothing is required to obtain reliable peak finding and deconvolution of these overlapping chromatographic peaks. Use of a sampling frequency of 200 Hz with a 20-point smoothing operation (D) results in an exclusion zone of only 0.3 psi. Higher acquisition rates give similar results for a peak width of 1 s. Increasing the split ratio reduces the peak width, and the exclusion region can be further reduced. This is shown by plot E, which is for an acquisition rate of 500 Hz and a split ratio of 100:1. In this case, only two pressure steps (24.7 and 24.8 psi) are excluded for this component pair. The details of plots like those in Figure 7 will vary with the sensitivity of peak separation to changes in junction pressure. This sensitivity depends on the relative polarities of the two compounds with respect to the two columns used in the tunable ensemble. Plots like those in Figure 7 can be constructed for every peak pair that is a critical (limiting) pair in a specified junction pressure range. A composite of all such plots for a specified spectral acquisition rate defines the pressure ranges (exclusion regions) which will result in unsuccessful peak finding and spectral deconvolution of one or more peak pairs. As the time axis of the chromatogram is compressed, eventually, these pressure exclusion regions from different critical pairs will coalesce, and no
Figure 8. Time-compressed chromatograms using a column junction pressure of 32.0 psia and an oven temperature of 80 °C: (a) total ion chromatogram; (b) extracted ion chromatograms for the complete mixture; (c), extracted ion chromatograms on expanded time scale. See Table 1 for peak identification.
available junction pressure will result in complete mixture characterization at a specified spectral acquisition rate. Time-Compressed Chromatograms. Chromatograms for Figures 2-7 were all obtained with an oven temperature of 50 °C. To compress the time axis of the chromatograms, experiments were conducted at higher oven temperatures. The upper temperature limit was determined by the need for baseline separation of ethylbenzene and p-xylene (see plots 16 and 17 in Figure 3) which cannot be deconvoluted because of similarities in their fragmentation patterns. This upper limit was found to be about 80 °C. Figure 8 shows time-compressed chromatograms obtained with an oven temperature of 80 °C and a junction pressure of 32.0 psia. The total ion chromatogram is shown in Figure 8a, and a composite of extracted ion chromatograms is shown in Figure 8b. The last component elutes in about 42 s. The very congested portion of the chromatogram between 16 and 24 s is shown using an expanded time scale in Figure 8c. The vertical lines show the locations of the peaks identified with the detector software. The numbers by the peaks refer to the label numbers in Table 1. Note that the total ion chromatogram provides very little useful information about the early-eluting components (peaks 1-8). Also note that ethylbenzene (16) and p-xylene (17) are completely separated. No peak is identified for n-heptane (component 9) because it coelutes with ethyl alcohol (peak 6). For Figure 9, the junction pressure was reduced to 28.0 psia. Only relatively small changes are observed in the total ion chromatogram compared to Figure 8, but a large change in the
Figure 9. Time compressed chromatograms using a column junction pressure of 28.0 psia and an oven temperature of 80 °C: (a) total ion chromatogram; (b) extracted ion chromatograms for the complete mixture; (c), extracted ion chromatograms on expanded time scale. See Table 1 for peak identification.
retention pattern for the early-eluting peaks is revealed in the extracted ion chromatograms. Most significantly, peak 9 no longer coelutes with peak 6, and all mixture components are identified. Again note the power of column ensembles with pressure-tunable selectivity for enhancing the utility of TOFMS with time-array detection for high-speed GC/MS. CONCLUSIONS Time-of-flight MS with time-array detection clearly is very powerful for the characterization of mixtures separated by highspeed GC. Since complete fragmentation patterns are obtained at very high spectral acquisition rates with no concentration biasing, peak characterization by comparison with spectral libraries is straightforward. This allows for spectral deconvolution of severely overlapping unknown peaks. Higher acquisition rates have the advantages of more accurately characterizing narrow peaks and enabling deconvolution of unknown peaks with more severe chromatographic overlap. In many cases, the time axis of the chromatogram can be dramatically compressed, thus providing very rapid mixture characterization. Pressure-tunable column ensembles can provide very precise peak position control. This can be used to significantly enhance the power of high-speed GC/TOFMS. In cases where a pair of components completely coelute, a simple change in junction-point pressure often can provide adequate peak separation for spectral deconvolution. By adjusting peak separation, the minimum Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
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spectral acquisition rate required for reliable quantitation can be used, thus minimizing file size. Alternatively, tunable selectivity can be used to optimize peak positions for time-compressed chromatography. This will significantly decrease analysis times, resulting in increased throughput and decreased analysis costs. The studies described in this report consider the effects of tuning pressure and spectral acquisition rate on peak finding and spectral deconvolution for only one critical component pair. Optimization algorithms need to be developed that will consider all critical pairs in a mixture over the available range of tuning pressures. The goal of an optimization algorithm is the determination of the column junction-point pressure that will allow for the maximum time compression of the chromatogram while obtaining complete characterization. A useful algorithm also must take into account the need for greater separation of isomers.
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The work reported here used isothermal GC. Studies in progress involve the use of tunable and programmable selectivity for high-speed, temperature-programmed GC/TOFMS. The use of fast temperature programming with on-the-fly selectivity changes during a separation has been demonstrated for HSGC using FID detection.8 This technology also should be very useful with TOFMS detection for high-speed characterization and analysis of complex, wide-boiling-point range mixtures. ACKNOWLEDGMENT The authors gratefully acknowledge the LECO Corp., St Joseph, MI, for the use of the Pegasus II TOFMS instrument. Received for review June 11, 1999. Accepted August 26, 1999. AC990631F
