Anal. Chem. 2001, 73, 813-819
A Tandem Column Ensemble with an Atmospheric Pressure Junction-Point Vent for High-Speed GC with Selective Control of Peak-Pair Separation Tincuta Veriotti and Richard Sacks*
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109
A series-coupled (tandem) ensemble of two capillary GC columns using different stationary phases and a pneumatically actuated low-volume valve connecting the column junction point to an atmospheric-pressure vent line is used to adjust the ensemble separation of selected pairs of target compounds. The valve is normally closed, and the pressure at the column junction point assumes the value that would occur in the absence of any other connections. The valve can be opened for brief periods of time, thus producing pulses of atmospheric pressure at the column junction point. If a component pair is separated by the first column but coelutes from the column ensemble, the ensemble separation can be increased if a pulse occurs when one of the components has migrated across the column junction but the second component is still on the first column. All of the mixture components that are on the same column during the time that the valve is open (pulse duration) will be shifted to either larger or smaller retention times, but the pattern of peaks (elution order) for these components from the column ensemble will be relatively unaffected by the pressure pulse. Multiple pulses can be used to enhance the separation of different component pairs, which sequentially reach the column junction point. Performance of the valve-operated system is described. Time-of-flight mass spectrometry with time-array detection is used to examine the effects of pulse duration on the separation achieved for different component pairs. Series-coupled ensembles of capillary GC columns with different stationary phases are used for the purpose of achieving unique selectivity (pattern of eluting peaks) that can be tuned to minimize the separation time for a specified mixture.1-4 Recent work has reported the use of a computer-controlled system in which the pattern of peaks eluting from the column ensemble is software-adjustable.3-6 This is achieved by using an electronic pressure controller located at the junction point of the two columns (1) Purnell, J. H.; Williams, P. S. J. Chromatogr. 1984, 292, 197. (2) Ingraham, D. F.; Shoemaker, C. F.; Jennings, W. J. J. Chromatogr. 1982, 239, 39. (3) Sandra, P.; David, F.; Prood, M.; Diricks, G.; Verstappe, M.; Verzele, M. J. High Resolut. Chromatogr. Chromatogr. Commun. 1985, 8, 782. (4) Hinshaw, J. V.; Ettre, L. S. Chromatographia 1986, 21, 561. (5) Akard, M.; Sacks, R. Anal. Chem. 1994, 66, 3036. (6) Akard, M.; Sacks, R. Anal. Chem. 1995, 67, 2733. 10.1021/ac001028w CCC: $20.00 Published on Web 01/23/2001
© 2001 American Chemical Society
in the ensemble. A change in the junction-point pressure results in a change in the carrier-gas velocities in the two columns, with the average linear velocity in one of the columns increasing and the average linear velocity in the other column decreasing. This changes the ratio of carrier-gas holdup times for the two columns and affects the migration times of peaks eluting from the ensemble. Using a pressure controller with a 0.1-psi step size, up to several hundred software-selectable patterns can be obtained, depending on the inlet and outlet pressures of the column ensemble.4,7-9 Optimization of a pressure-tunable column ensemble refers to the selection of the peak pattern that will give the greatest resolution of the target compounds. Often, window diagram methods are used for the optimization.6,10-12 Usually, these methods examine the most difficult-to-separate peak pairs (critical pairs) and determine the column holdup-time ratio that will produce the best resolution of the critical pairs. Note that for a mixture containing more than a few components, any change in the junction-point pressure (holdup-time ratio) is likely to increase the resolution of some peak pairs while reducing the resolution of other peak pairs. The optimal holdup-time ratio is usually a compromise for the two component pairs that have the poorest resolution, and any departure from this optimal holdup-time ratio will result in a loss in resolution of one of the component pairs. As mixture complexity increases, it becomes less likely that any available ensemble peak pattern will provide adequate resolution of all critical pairs. The pressure at the column ensemble junction point can be adjusted on-the-fly during a separation in order to achieve programmable ensemble selectivity.13-15 The junction-point pressure initially is set to give adequate separation of the first group to reach the junction. After the last component of a subgroup has migrated across the junction into the second column, the junctionpoint pressure can be changed to obtain enhanced ensemble selectivity for the next subgroup to reach the column junction. (7) Sacks, R.; Smith, H.; Nowak, M. Anal. Chem. 1998, 70, 29A. (8) Sacks, R.; Coutant, C.; Veriotti, T.; Grall, A. J. High Resolut. Chromatogr. 2000, 23 (3), 225. (9) Smith, S.; Sacks, R. Anal. Chem. 1997, 69, 5159. (10) Laub, R. J.; Purnell, J. H. J. Chromatogr. 1975, 112, 71. (11) Laub, R. J.; Purnell, J. H. Anal. Chem. 1976, 48, 799. (12) Purnell, J. H.; Wattan, M. H. J Chromatogr. 1991, 555, 173. (13) Smith, S.; Sacks, R. Anal. Chem. 1998, 70, 4960. (14) Grall, A.; Zellers, E. T.; Sacks, R. Environ. Sci. Technol. 2001, 35 (1), 163. (15) Leonard, C.; Sacks, R. Anal. Chem. 1999, 71, 5501.
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Table 1. Compounds, Boiling Points, Retention Factors, and Masses Monitored for Test Mixture
Figure 1. Dual-column ensemble with polar column C1 and nonpolar column C2. CG, carrier-gas supply; I, autoinjector; V, pneumatically operated vent valve; MS, time-of-flight mass spectrometer for ensemble detection; FID, flame ionization detector for column C1 detection.
However, the selection of the optimal pressure for any subgroup represents finding compromise conditions for the two critical pairs in the subgroup. In a recent study of single-step pressure programming,16 it was concluded that significant changes in ensemble peak patterns occur only for components that are on different columns at the time of the pressure change. That is, if both components of a critical peak pair are in the same column when a pressure change is initiated, the ensemble separation of these components is relatively unaffected by the junction-point pressure change. However, a pressure change at the column junction point when one of the components is in the first column and the other component is in the second column will always result in an increased migration rate for one of the components and a decreased migration rate for the other component. This allows the use of relatively short pressure pulses to alter the separation of a specified peak pair without significantly changing the ensemble peak pattern (selectivity) for the other mixture components. This approach to achieving enhanced separation of a single component pair was demonstrated by using an electronic pressure controller to change the junction-point pressure for a few seconds and then returning the pressure to its original value.17 The pressure controller used for these studies has an internal volume of > 7 cm3, and this results in excessive pressure equilibration time, which precluded the use of pressure pulses less than ∼5 s in width. In the present study, the pressure controller is eliminated, and a low-volume gas valve is connected between the column junction point and an atmospheric-pressure vent line. This very simple system is used to achieve enhanced selectivity of specified component pairs by applying short pulses to atmospheric pressure. The use of multiple pulses to achieve enhanced separation of several component pairs is demonstrated. A time-of-flight mass spectrometer (TOFMS) is used for detection so that the compounds present in overlapping component peaks can be ascertained. The goal of this work is to achieve selective control of peak-pair separation for critical component pairs using a system that is simple when compared to those used previously. EXPERIMENTAL SECTION Apparatus. Figure 1 shows a diagram of the dual-column GC/ MS instrument with a valve-operated vent line to the atmosphere. A HP 6890 GC (Hewlett-Packard, Atlanta, GA) with electronic inlet pressure control is used as an experimental platform. Split injector 814 Analytical Chemistry, Vol. 73, No. 4, February 15, 2001
peak no.
compound
bp (°C)
kDB-200
kDB-5
m/z
1 2 3 4 5 6 7
nonane ethylbenzene 1-ethyl-3-methylbenzene decane 1,3,5-trimethylbenzene bicyclopentadiene 4-methylstyrene
151 136 158-159 174 162-164 170 170-175
0.68 0.80 1.42 1.14 1.50 1.74 2.14
1.45 1.18 2.47 3.12 2.60 3.98 3.27
43 91 105 43 105 66 117
I is connected to hydrogen carrier gas supply CG. Samples were injected by a HP 7683 autoinjector (Hewlett-Packard). Detection is obtained using a LECO model Pegasus II reflectron TOFMS instrument using time-array ion detection (LECO Corp., St. Joseph, MI). Instrument software provided completely automated peakfinding and spectral deconvolution of overlapping peaks from unknown, as well as known, mixtures. The series-coupled column ensemble consisted of columns C1 and C2, which are both 10 m long, 0.18 mm i.d. and use 0.18-µmthick bonded stationary-phase films of trifluoropropylmethyl polysiloxane (DB-200, J&W Scientific, Inc., Folsom, CA) and 5% phenyl dimethyl polysiloxane (DB-5, J&W Scientific), respectively. The vent valve, V, was a pneumatically operated, low-dead-volume device (model MOPV-1/50, SGE, Austin, TX), which was operated by a 50-55 psig compressed air source connected through an electronically actuated solenoid valve (model GH3412, Precision Dynamics, Phoenix, AZ). The valve was connected to the column junction point by means of a low-dead-volume stainless steel “T” (MT1C56, Valco Instruments, Houston, TX) and a 4.0-cm-long segment of 0.53-mm-i.d. deactivated fused-silica tube. A HP flame ionization detector (FID) was also connected to the column junction point by means of a second splitter and a 0.5-m-long, 0.05mm-i.d. deactivated fused-silica tube. On the basis of standard equations for gas flow through capillary tubes,18 ∼8% of the effluent from the first column was split to this detector. Materials and Procedures. Hydrogen carrier gas was purified by filters for water vapor, oxygen and hydrocarbons. A test mixture of the seven compounds listed in Table 1 was used for all studies. The neat liquids were mixed in equal volumes, and 0.5 µL of headspace vapor samples were injected with a 20:1 split ratio. All of the chromatograms were obtained isothermally at 80 °C. The inlet pressure was 15.0 psig. Natural-gas injections were used for holdup-time measurements for the first column using the FID. Holdup-time measurements for the column ensemble were made routinely by monitoring the nitrogen ion peak from the air in the sample headspace at m/z 28. Data processing for the TOFMS, including automated peak finding and spectral deconvolution of overlapping chromatographic peaks, was accomplished using software provided by the manufacturer. Data acquisition for the FID and control of the vent valve were obtained by using a 12-bit A/D board (DT-2801, Data Translation, Inc., Marlboro, MA). The interface board was controlled by means of Labtech Notebook software (Laboratory (16) Coutant, C.; Sacks, R. Anal. Chem. 2000, 72 (21), 5450. (17) Veriotti, T.; McGuigan, M.; Sacks, R. Anal. Chem. 2001, 73 (2), 279. (18) Grant, D. Capillary Gas Chromatography; Wiley: New York, 1996.
Figure 2. Plots of solute band position along the column ensemble vs time for an unretained component (U) and for the seven mixture components for the case in which the valve is closed for the entire separation. Peak numbers correspond to component numbers in Table 1. Chromatograms shown along the 10-m and 20-m coordinate lines are from the FID and the TOFMS, respectively.
Technologies, Inc., Wilmington, VA). Processing of the FID chromatograms was accomplished with Grams/32 software (Galactic Industries, Salem, NH). Excel spreadsheet calculations were used to obtain the plots of the solute-band position along the column ensemble vs time. RESULTS AND DISCUSSION Ensemble Retention Characteristics. Any change in the holdup-time ratio for the two columns in the ensemble results in a change in the residence times in the columns of all mixture components. For a mixture containing more than a few components, this change in residence times usually results in an increase in resolution for some component pairs and a decrease for other component pairs. This is the origin of the need for compromise conditions for two different component pairs. A significant advantage can be achieved by selectively increasing the separation of a targeted peak pair without degrading the resolution of other peak pairs. For any two mixture components when one component has greater retention on one of the columns and the other component has greater retention on the other column, a coelution will occur for some value of the holdup-time ratio for the two columns. A complex mixture may have many such component pairs. For any holdup-time ratio that results in a coelution, the two components often are separated by the first column in the ensemble, but the separation is reversed by the second column, and the result is that the components have very similar ensemble retention times. For these component pairs, a brief change in the carrier gas flow rate occurring when one of the components has migrated across the column junction and is in the second column, while the other component is still in the first column, will enhance the separation of these two components.17 Figure 2 shows the retention characteristics of the test mixture on the column ensemble for the case in which the valve is closed during the entire separation. The figure shows plots of the positions of the solute bands in the ensemble as a function of
time. These plots were obtained from a spreadsheet calculation using the retention factors for the mixture components on the individual columns (see Table 1), the lengths and diameters of the columns, the inlet and outlet pressure of each column, and the carrier gas viscosity at the column temperature. The algorithm used for these calculations has been described.15 For these plots, injection into the first (polar) column occurs at the lower-left corner of the figure. The horizontal line located at the 10-m coordinate corresponds to the column junction point, and the top horizontal line (20 m) indicates the downstream end of the ensemble. The plot marked U is for an unretained component and shows the carrier gas velocity profile along the column axis. Plot U shows no discontinuity across the junction, because the valve is closed. The plot for each mixture component does show a discontinuous change in slope (local migration rate) at the junction point due to the different retention factors on the two columns. The chromatogram shown along the horizontal junction-point line is from the FID that monitors a portion of the effluent from the first column. The extracted-ion chromatograms for the masses listed in Table 1 and shown along the top horizontal line in the figure are from the column ensemble. Peak numbers in the chromatograms correspond to the compound numbers in Table 1. These seven components were chosen to yield closely spaced groups of peaks in the ensemble chromatogram under conditions when the valve is closed during the entire separation. Note that components 1 and 2 are separated by the first column, but they coelute from the column ensemble. A similar situation occurs for components 6 and 7. Component 4 (n-decane) is well-separated from components 3 and 5 (aromatic compounds) by the first column, but an elution order change occurs for components 3 and 4 on the second column, with the result that peaks 3 and 4 are not completely separated by the column ensemble. Pressure Pulse Width and Initiation Time Effects. The extracted-ion chromatograms obtained from the column ensemble in Figure 2 are for the case in which the vent valve is not opened during the analysis. Figure 3 shows extracted-ion chromatograms for components 3, 4, and 5 when the value is opened for various time intervals (nominal pulse width) beginning 41 s after injection. The numbers on the right edge of the figure give the nominal open time for the valve in seconds. When the valve is initially opened, component 4 has migrated across the junction and is in the second (nonpolar) column, while components 3 and 5 are still on the first column. When the valve is opened, the pressure at the column junction point falls from 21.0 psia to ambient pressure (14.2 psia). Note that the junction-point pressure is the outlet pressure for column C1 and the inlet pressure for C2. Thus, when the valve is open, the local carrier gas velocity at any point in C1 is increased, and the local carrier gas velocity in C2 is decreased. The result is an increased migration rate in C1 and a decreased rate in C2 for the interval when the valve is open. This shifts the ensemble retention time for component 4, which is in the C2 when the valve is open, to larger values and shifts the retention times for components 3 and 5 to smaller values. A nominal pulse width of 0.4 s results in an elution order change for components 4 and 5. A pulse width of 0.8 s results in baseline separation of all three components. Further increase in Analytical Chemistry, Vol. 73, No. 4, February 15, 2001
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Figure 3. Extracted-ion chromatograms for m/z 43 (peak 4) and 105 (peaks 3, 5) for different nominal pulse widths. Pulse widths in seconds are shown along the right edge of the figure.
Figure 4. Plots of peaks separation (a) and relative peak area (b) vs nominal pulse width for components 3, 4, and 5 from the chromatograms in Figure 3. Numbers near plots correspond to the peak numbers in Figure 3.
the pulse width to 1.2 s results in increased separation of components 4 and 5, but a decrease in area is observed for peak 3. For a nominal pulse width of 1.8 s, the area of peak 3 is reduced to ∼12% of the no-pulse value, and the area of peak 5 shows a 816
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significant decrease. The decreases in peak area are the result of venting some of the sample through the valve and the vent line. The amount of sample that is vented increases with pulse width until the width is sufficient that the residual sample crosses the junction, and the venting ceases. Note that component peak widths are decreased when venting occurs. This has been explained by a decrease in the elution peak width from the first column when the carrier gas velocity in the first column is increased during the pulse.16 The extracted-ion chromatograms in Figure 3 suggest that a nominal pulse width between 0.4 and 0.8 s will provide adequate separation of components 3, 4, and 5 with minimal component venting. The effects of pulse width on peak separation and peak area for components 3, 4, and 5 are shown in greater detail in Figure 4. Numbers by the plots give (a) the component pairs for the peak separation plots and (b) the components for the peak area plots. With no pulse (pulse width of 0 s), the separation of component pair 4/5 is -1.4 s and that of component pair 3/4 is just over 1 s. The negative value for the peak separation of component pair 4/5 with no pulse is the result of the ensemble elution order change when the valve is opened. Neither value is adequate for a complete (baseline) separation. For a nominal pulse width between 0.5 and 1.2 s, all component pairs have separations >2 s, which provides better than baseline separation. Note that the peak separations for component pairs 3/4 and 4/5 obtained with a nominal peak width of 0.2 s (smallest value used in this study) are substantially larger than what would be expected based on the change in separation with pulse width observed for the other data points. This suggests that the actual pulse widths are significantly larger than the nominal values. This may be the result of relatively slow valve closing, caused by the dead volume of the pneumatic system used to actuate the valve. If 1.3 s is added to each of the nominal pulse widths, the separation time data for peak pairs 3/4 and 4/5, including the values for the no-pulse case, can be fit to straight lines with correlation coefficients of 0.9918 and 0.9917, respectively, for the pulse-width range 0-1.8 s. The peak area plots show that for component 4, the peak area is independent of the pulse width. This is expected, because component 4 has crossed the junction before the pulse is initiated. For components 3 and 5, a sharp loss in peak area is observed if the corresponding components reach the column junction prior to the valve’s closing time. For component 5, which reaches the junction before component 3 (see Figure 2), the nominal pulse width should not exceed ∼0.5 s in order to obtain minimal component venting. Because a nominal pulse width of 0.5 s provides adequate separation of components 3, 4, and 5, this value was used for all further work reported here. Figure 5 shows the effects of pulse initiation time on retention times (a) and critical-pair peak separations (b) for all 7 mixture components using a nominal pulse width of 0.5 s. The numbers by the plots correspond to the component numbers in Table 1. For pulse initiation times < 24 s, all components are in the first column during the pulse, and all components show a shift of a few seconds to shorter retention times relative to the no-pulse values. However, no significant changes in the peak-pair separations are observed. If the pulse is initiated 31 s after injection, component 1 has crossed the junction, and its ensemble retention
Figure 5. Plots of retention time (a) and critical-pair peaks separation (b) vs pulse initiation time for the seven mixture components using a nominal pulse width of 0.5 s. Plot numbers correspond to compound numbers in Table 1.
Figure 6. Extracted ion chromatograms for the masses listed in Table 1 with 0.5-s wide (nominal) pulses initiated (a) 30, (b) 41, and (c) 52 s after injection. Peak numbers correspond to compound numbers in Table 1.
time is shifted to a significantly larger value. This gives rise to a separation of ∼2.6 s for component pair 1/2. No significant changes in separation or retention times are observed for the other mixture components. For a 34-s pulse initiation time, both components 1 and 2 have crossed the column junction prior to the pulse, and both show an increase in ensemble retention time. However, their separation decreases to ∼0.2 s. Component 4 (n-decane) reaches the column junction 37 s after injection, and for pulse initiation times in the range 37 s to 46 s, enhanced separation of component pair 3/4 is observed. For a pulse initiation time of 41 s, a separation of ∼4.4 s is obtained. Significantly enhanced separation of component pair 4/5 also is observed, with a maximum separation of 1.8 s for a pulse initiation time of 41 s. Components 3 and 5 reach the column junction ∼46 s after injection, and pulses initiated after this time produce a shift of ensemble retention times of several seconds to later values but have relatively little effect on the ensemble peak pattern for these components relative to the nopulse case. Component 6 reaches the column junction 48 s after injection, and for pulse initiation times in the range from 48 to 57 s, a large increase in separation of component pair 6/7 is observed. A maximum separation of 3.0 s is obtained for a pulse initiation time of 52 s. Component 7 crosses the column junction 57 s after injection, and pulses initiated after this time result in increased ensemble retention times for both components but with little change in their separation. Note that components 1 and 2 elute from the column ensemble with retention times of ∼52 s and are completely unaffected by pulses delivered after this time. Figure 6 shows extracted-ion chromatograms from the complete mixture for a single 0.5-s wide pulse initiated (a) 30, (b) 40,
and (c) 52 s after injection. These pulse initiation times correspond to the broken vertical lines connecting Figure 5a,b. Note that in addition to the seven mixture components, several small impurity peaks also are observed in the extracted-ion chromatograms. When the pulse is initiated 30 s after injection (a), components 1 and 2 are well-separated, but the other two component groups show no significant change in the ensemble peak patterns relative to the no-pulse case. For a pulse initiation time of 40 s, a complete separation of components 3, 4, and 5 is achieved, but coelutions are observed for component pairs 1/2 and 6/7. For a pulse initiation time of 52 s, component pair 6/7 is well-separated, but the pattern of peaks for the other two component groups are essentially the same as for the no-pulse case. However, note that these component groups have shifted relative to each other. Multiple Pulses for Complete Mixture Separation. The test mixture used in this study results in three congested regions of the ensemble chromatogram. Figure 7 shows how the use of a sequence of three pulses can improve the separation of all three of the groups of mixture components. The broken vertical lines indicate the required pulse timing. Note that the required pulseinitiation times are altered by any previous pulses, because each previous pulse reduces the time required for subsequent component groups to reach the column junction. This was taken into account by the algorithm used to determine the band position vs time plots shown in Figure 7. Alternatively, the chromatograms obtained by FID monitoring the first column in the ensemble can be used to determine the appropriate pulse initiation times. For the spreadsheet algorithm used to calculate the solute band trajectories, a pulse width of 1.8 s was assumed. This value Analytical Chemistry, Vol. 73, No. 4, February 15, 2001
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Figure 7. Plots of solute band position along the column ensemble vs time for the seven mixture components for the case in which the valve is opened three times during the separation. Broken vertical lines indicate pulse timing. See text for details. Peak numbers correspond to component numbers in Table 1. Extracted ion chromatograms are shown along the 20-m coordinate line.
corresponds to the 0.5-s nominal pulse width plus the 1.3-s correction obtained from the data in Figure 4a. No direct junctionpoint pressure measurements were made in this study, because the available pressure gauges have considerable dead volume and, thus, could alter the junction-point equilibration time when the valve is opened or closed. Figure 8 shows a direct comparison of the analytical ion chromatograms (AIC) a and c and extracted-ion chromatograms b and d for the no-pulse case a and b and for the case when three sequential 0.5-s (nominal) pulses are used as described in Figure 7. The AIC is obtained by summing all the extracted ion chromatograms that exceed a user-defined signal-to-noise ratio for the found peaks. The AIC provides information that is similar to that obtained in a total-ion chromatogram but at substantially increased signal-to-noise ratios. For this work, the AIC is used to simulate the use of a single-channel detector, such as an FID. For the no-pulse case, the AIC shows only five peaks, because peak pairs 1/2 and 6/7 almost completely overlap, and only a single peak is observed for each component pair. In addition, peaks 3 and 4 show excessive overlap for accurate peak-area calculations. The TOFMS instrument used for these studies has software for the automated finding and spectral deconvolution of overlapping peaks from unknown mixture components if the peak apex separation is sufficient to obtain two complete mass spectra between the peak apexes and if the mass spectra from the two components are sufficiently different. For the no-pulse case, component pairs 1/2 and 6/7 require minimal spectral acquisition rates of 15 and 7 spectra/s, respectively, for automated peakfinding and -deconvolution. When three pulses are used, the AIC shows that a complete separation is achieved with at least baseline resolution for all adjacent peak pairs. CONCLUSIONS The use of a low-dead-volume valve and an atmosphericpressure vent can be very effective in enhancing the ensemble 818 Analytical Chemistry, Vol. 73, No. 4, February 15, 2001
Figure 8. Analytical-ion chromatograms, a and c, and extractedion chromatograms, b and d, for the seven-component mixture without any valve opening for a and b and with the valve opened three times during the analysis for c and d.
separation of specified groups of mixture components. The technology described here is an extremely simple approach to control the selectivity in relatively small, specified regions of the ensemble chromatogram. The main point is that all of the mixture components that are on the same column during the time that the valve is open (pulse duration) will be shifted to larger or smaller retention times, but the pattern of peaks for these components from the column ensemble will be relatively unaffected by the pressure pulse. If two mixture components are on different columns during the pulse, then one will be shifted to a smaller ensemble retention time, and the other will be shifted to a larger retention time, with the result that a relativity large change in their separation will occur. The apparatus used for this work is simpler, less expensive, more flexible, and has a faster response than previously described tandem column ensembles using electronic pressure control at the column junction point. For cases in which more than one coelution occurs, multiple pulses can be used with each pulse, timed to facilitate the separation of a specified component group. The initiation times of the second and subsequent pulses, however, must be adjusted to take into account the reduced component migration times on the first column that are caused by each successive pulse. The work reported here used isothermal GC. However, multiple-pulse techniques should be very effective with temperature-programmed GC, because the higher-boiling-point mixture components migrate very slowly during the early, low-temperature portion of the analysis, and thus, will be relatively unaffected by pulses that are used to facilitate the separation of the lower-boiling-point components. An important advantage of the use of short pressure pulses which target specific component groups is that, for most
of the analysis time, the column ensemble is operated with the valve closed, and the column junction-point pressure is the value that would occur without any additional connection to the junction point. This can substantially increase analysis speed and ensemble efficiency relative to a pressure-tunable column ensemble where the best separation may occur for a junction-point pressure producing larger holdup time and reduced resolving power.
ACKNOWLEDGMENT The authors gratefully acknowledge LECO Instruments, St. Joseph, MI, for use of the Pegasus II TOFMS instrument. Received for review August 28, 2000. Accepted December 15, 2000. AC001028W
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