Patent Publication Number: US-6706535-B2

Title: Pulsed carrier gas flow modulation for selectivity enhancements with gas chromatography using series-coupled column ensembles

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/016,798 filed on Dec. 10, 2001. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to chemical mixture analysis and, more particularly, relates to a practical approach for achieving improved selectivity for capillary gas chromatography. 
     BACKGROUND OF THE INVENTION 
     Gas chromatography is an analytical technique for separating compounds from a mixture based on their volatilities and polarities. Gas chromatography thus provides both qualitative and quantitative information for the individual compounds present in a sample. These compounds move through a gas chromatography column as gases, either because the compounds are normally gases or because they have been heated and vaporized into a gaseous state. The compounds partition between a stationary phase, which can be either solid or liquid, and a mobile phase, which is typically gas. 
     As is known in the art, the introduction of the fused silica, wallcoated, capillary gas chromatography column in 1979 represented a paradigm shift in the analysis of volatile and semi-volatile organic compounds. However, with the introduction of the fused-silica capillary column, the emphasis gradually shifted towards the development of more universal columns with greater resolving power for a wider variety of compound mixtures. 
     Long capillary columns (generally greater than 20 m long) generally provide greater resolving power and, thus, the complete separation of more complex mixtures with analysis times comparable to those achieved with packed columns. 
     Briefly, as used herein, “selectivity” generally refers to the pattern of peaks produced from the gas chromatography column and the values of the retention factor ratios for all component pairs. Retention factor is equal to the difference between the retention time and the column void time ratioed to the void time. The greater the selectivity of the column for a specified component pair, the more separation between the corresponding peaks of the chromatogram can be achieved. 
     On the other hand, shorter capillary columns (generally 5-20 m long), which are operated at relatively high carrier gas flow rates, have been used to obtain faster mixture separations. However, in exchange for faster mixture separations, resolving power is sacrificed. As should be appreciated, if these techniques for high-speed gas chromatography are to be useful for more complex mixtures, selectivity or resolving power must be improved. 
     Accordingly, there exists a need in the relevant art to provide a method and apparatus whereby a wide variety of compounds may be quickly separated without significantly affecting the resolving power. Furthermore, there exists a need in the relevant art to provide a method and apparatus of capillary column gas chromatography that is capable of varying the selectivity during the course of the compound analysis to provide improved selectivity capability for various specific compound pairs. Still further, there exists a need in the relevant art to provide a method and apparatus of pulsed carrier gas flow modulation for improved selectivity of gas chromatography. Additionally, there exists a need in the relevant art to provide a method of capillary gas chromatography that overcomes the disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     According to the principles of the present invention, a gas chromatography system is provided having an advantageous construction and method of using the same. The gas chromatography system includes a computer-controlled venting valve system that delivers pressurized pulses to a column junction point of two series-coupled columns having different stationary-phase chemistries. Each pressurized pulse causes a differential change in the carrier gas velocities in the two columns, which lasts for the duration of the pressurized pulse. Whereby, the pressurized pulse selectively increases the separation of a component pair that exhibits separation at the exit of the first column, but otherwise co-elutes from the column ensemble. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a schematic drawing illustrating a gas chromatography system according to a first embodiment of the present invention; 
     FIG.  2 ( a ) is a graph illustrating application of a pressurized pulse to a pair of illustrative components prior to the components reaching a column junction point; 
     FIG.  2 ( b ) is a graph illustrating application of a pressurized pulse to the pair of illustrative components after one of the components has reached the column junction point; 
     FIG.  2 ( c ) is a graph illustrating application of a pressurized pulse to the pair of illustrative components after both components have reached the column junction point; 
     FIG.  2 ( d ) is an enlarged graph and chromatogram of box  1 ( d ) of FIG.  1 ( a ); 
     FIG.  2 ( e ) is an enlarged graph and chromatogram of box  1 ( e ) of FIG.  1 ( b ); 
     FIG.  2 ( f ) is an enlarged graph and chromatogram of box  1 ( f ) of FIG.  1 ( c ); 
     FIGS.  3 ( a )-( e ) is a series of chromatograms illustrating the result of application of an upward pressurized pulse applied at 10 s, 11 s, 13 s, 15.5 s, and 18 s, respectively, after injection; 
     FIGS.  4 ( a )-( f ) is a series of chromatograms illustrating the result of application of a downward pressurized pulse applied at 8.5 s, 9.2 s, 10.0 s, 11.2 s, 14.0 s, 16.0 s, respectively, after injection; 
     FIG. 5 a  illustrates an upward pressurized pulse applied to a column junction point, wherein the dashed line illustrates a nominal pulse shape; 
     FIG. 5 b  illustrates a downward pressurized pulse applied to the column junction point, wherein the dashed line illustrates a nominal pulse shape; 
     FIG. 6 is a graph illustrating the relationship between ensemble retention-time vs. pulse initiation-time for the pair of illustrative components for an upward pressurized pulse; 
     FIG. 7 is a graph illustrating the relationship between peak separations vs. pulse initiation-time for the pair of illustrative components for an upward pressurized pulse; 
     FIG. 8 is a graph illustrating the relationship between ensemble retention-time vs. pulse initiation-time for the pair of illustrative components for a downward pressurized pulse; 
     FIG. 9 is a graph illustrating the relationship between peak separations vs. pulse initiation-time for the pair of illustrative components for a downward pressurized pulse; 
     FIG.  10 ( a ) is a graph illustrating the relationship between peak separation vs. pressurized pulse width for the pair of illustrative components; 
     FIG.  10 ( b ) is a graph illustrating the relationship between peak separations vs. pressurized pulse amplitude for the pair of illustrative components; 
     FIG. 11 is a schematic drawing illustrating a gas chromatography system according to a second embodiment of the present invention; 
     FIG. 12 is a graph illustrating solute band positioning at the column junction point and at the exit of the column ensemble for the case where no pressure pulse is present (e.g. valve closed); 
     FIGS.  13 ( a )-( g ) is a series of chromatograms illustrating the result of application of differing nominal pulse widths of 0 s (no pulse), 0.4 s, 0.8 s, 1.2 s, 1.8 s, 2.2 s, and 4.0 s, respectively; 
     FIG.  14 ( a ) is a graph illustrating the effects of pulse width on peak separation between component pairs  203 / 204 ,  204 / 205 , and  203 / 205 ; 
     FIG.  14 ( b ) is a graph illustrating the effects of pulse width on the relative peak area for components  203 ,  204 , and  205 ; 
     FIGS.  15 ( a )-( c ) is a series of chromatograms illustrating the result of application of a pulse applied at 30 s, 41 s, and 52 s, respectively, after injection; 
     FIG. 16 is a graph illustrating solute band positioning at the column junction point and at the exit of the column ensemble for the case where the vent valve is opened at three distinct times to create three distinct pulses; 
     FIG.  17 ( a ) is an analytic-ion chromatogram (AIC) illustrating the case where no pulse is applied; 
     FIG.  17 ( b ) is an extracted-ion chromatogram (EIC) illustrating the case where no pulse is applied; 
     FIG.  17 ( c ) is an analytic-ion chromatogram (AIC) illustrating the case where three pulses are applied; and 
     FIG.  17 ( d ) is an extracted-ion chromatogram (EIC) illustrating the case where three pulses are applied. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     As will be described in detail herein, the first embodiment of the present invention provides a method and apparatus for achieving improved selectivity for gas chromatography with capillary columns. This embodiment comprises a computer-controlled pressure controller that delivers pressurized pulses to a column junction point of two series-coupled columns having different stationary-phase chemistries. Each pressurized pulse causes a differential change in the carrier gas velocities in the two columns, which lasts for the duration of the pressurized pulse. The pressurized pulse selectively increases the separation of a component pair that exhibits separation at the exit of the first column, but otherwise co-elutes from the column ensemble. If both components are on the same column when the pressurized pulse is applied, the component separation remains essentially unchanged despite an overall shift in retention times. If one component of the pair is on the first column and the other component is on the second column, a pressurized pulse can result in a large change in the ensemble separation for the component pair, thereby increasing the peak separation in the chromatogram to aid in quantitation and identification. 
     A significant limitation of known prior art methods is the fact that a change in the pressure at the column junction point used to increase the separation of a particular component pair often results in reduced separation of one or more other component pairs. Thus, the selection of a pressure at the column junction point for a specified set of target compounds previously required a compromise. However, the present invention preferably employs a relatively short pressurized pulse to increase the separation of a specific component pair without significantly affecting the peak pattern and separation of the remaining components in the mixture. 
     Referring now to FIG. 1, a pressure-controlled, dual-column, gas chromatography system is schematically illustrated in accordance with a first embodiment of the present invention, generally indicated at  10 . Gas chromatography system  10  generally includes an inlet system  12 , a first gas chromatography separation capillary column  14 , a second gas chromatography capillary column  16 , a first detector (D 1 )  18 , a second detector (D 2 )  20 , and an electronic pressure controller  22 . 
     Inlet system  12  is configured generally to receive a sample “plug” of an analyte mixture for analysis within gas chromatography system  10 . More particularly, a stream of carrier gas from a carrier gas supply  24  entrains such analyte mixture during injection. This entrained analyte mixture and carrier gas composite is introduced into first column  14  through a passage  26 . Various configurations of inlet system  12  can be provided, such as mechanical injection valves, cryofocusing systems, and the like. 
     Similarly, first column  14  and second column  16  may take various forms, including columns having various types of liquid stationary phase materials. In addition, solid stationary phase-type columns using adsorbent materials, such as what is commonly known as porous layer open tubular (PLOT) columns or liquid crystal columns. Columns having polar or non-polar stationary phases may be used. However, for discussion purposes in the present disclosure, it is preferred that first column  14  is an approximately 10 m long, 0.25 mm diameter (id) column having a 0.25-μm thick bonded, stationary-phase, trifluoropropylmethyl polysiloxane film, while second column  16  is an approximately 10 m long, 0.25 mm diameter (id) column having a 0.25-μm thick bonded, stationary-phase, dimethyl polysiloxane film. 
     First column  14  and second column  16  are coupled in series along a column junction point  28 . Electronic pressure controller  22 , which is capable of pressure step sizes of approximately 0.1 psi and pressure repeatability of ±0.01 psi, is fluidly coupled to column junction point  28 . An adjustable pressure source carrier gas supply  30  is provided in fluid connection with electronic pressure controller  22  to apply pressurized pulses to column junction point  28  in accordance with the principles of the present invention. Preferably, electronic pressure controller  22  is driven in response to a central processing unit  32 . Moreover, electronic pressure controller  22  is preferably coupled to column junction point  28  via an approximately 1.0 m long, 0.25 mm diameter (id) uncoated, fused-silica tube  34 , which is further vented to atmosphere at  36 . A restrictor member  38  may be used to restrict the bleed off of the pressurized pulse. Restrictor member  38  is used to reduce the pressure equilibration time for downward pressure changes and to prevent contamination of electronic pressure controller  22 . Preferably, carrier gas supplies  24  and  30  each consists of purified hydrogen, helium, air, or other gases. 
     Adjustment of this carrier gas pressure via electronic pressure controller  22  results in a differential change in the carrier gas velocities between first column  14  and second column  16 , which will result in output changes in the gas chromatogram. That is, an increase in the pressure at column junction point  28  results in reduced carrier gas velocity on first column  14 , yet increased velocity on second column  16 . If the void time of first column  14  increases and the void time of second column  16  decreases (due to an increase in the pressure at column junction point  28 ), the mixture components have increased residence time on first column  14  and decreased residence time on second column  16 . Consequently, this arrangement increases the influence of the stationary-phase chemistry of first column  14  and decreases the influence of the stationary-phase chemistry of second column  16 . Alternatively, a reduction in the pressure at column junction point  28  has the opposite effect. 
     First detector  18  is coupled to column junction point  28  to monitor a portion of the effluent from first column  14 . Similarly, second detector  20  is coupled to the output of second column  16  to monitor the output from second column  16  and, thus, column ensemble  14 ,  16 . Preferably, a 1.0 m long, 0.10 mm diameter (id), deactivated, fused-silica tube  40  is used to transport approximately 10% of the effluent from first column  14  to first detector  18 . However, any size tube may be used, although shorter tubes provided generally improved results. As will be described below, a chromatogram is obtained from first detector  18  to determine the time at which to initiate the pressurized pulse from electronic pressure controller  22  to column junction point  28 . Preferably, first and second detectors  18  and  20  are each two flame-ionization detectors. However, it should be appreciated that first detector  18  and second detector  20  may be any detector capable of providing sufficient results, such as but not limited to flame-ionization detectors, photo-ionization detectors, thermal-conductivity detectors, solid-state sensors, mass spectrometers, flame photometric detectors, flame thermionic detectors, electron capture detectors, pulse helium ionization detectors, and the like. First detector  18  and second detector  20  each output a resultant signal to central processing unit  32  via communication lines  42 . It should be understood that once the pressurized pulse initiation times are determined for a given mixture, first detector  18  may be eliminated, as the initiation time will remain the same for that mixture. 
     In order to obtain accurate void time values and retention time values for first column  14 , the gas transport time from column junction point  28  to first detector  18  along tube  40  is subtracted from the measured retention times outputted from first detector  18 . This gas transport time is computed using standard equations for gas flow in capillary tubes. It is further important to note that the gas transport time varies with the pressure at column junction point  28 . Retention factors for first column  14  are computed from these corrected retention time and void time values using central processing unit  32 . Void time and retention time values for second column  16  are further obtained by central processing unit  32  by subtracting the corresponding values for first column  14  from the ensemble values obtained from second detector  20 . 
     The retention factor values for individual columns as well as the column dimensions and pressures and the viscosity of the carrier gas at the column operating temperature are used as inputs to a band-trajectory model. This band-trajectory model uses spreadsheet calculations and takes into account the carrier gas flow velocity profile along the column axis, the change in retention for each mixture component as it crosses the junction from the first to second column  16 , and programmed changes in the pressure at column junction point  28 . 
     Finally, the pressure at column junction point  28  in the absence of the pressurized pulse, also referred to as the quiescent pressure, is further chosen to be equal to the pressure that would occur at column junction point  28  in the absences of any external connections to column junction point  28 , also referred to as the natural pressure. This natural pressure results in an ensemble void time that is near the minimum value achievable with the dual-column ensemble. 
     Discussion of Results 
     In some cases, a pair of mixture components can co-elute from a tandem-column ensemble if 1) both components have very similar retention factor values on both columns, or 2) if one of the components has a larger retention factor on one of the columns and the other component has a larger retention factor on the other column. In the former case, adjustment of the pressure at column junction point  28  may not separate the two components. Therefore, it may be necessary to use a longer column ensemble, which should provide improved resolving power or an ensemble with greater selectivity for the specific component pair may provide the only solution. However, in the latter case, a change in the pressure at column junction point  28  often will result in adequate separation of the component pair. However as mentioned above, in conventional methods, a change in the pressure at column junction point  28  may result in the co-elution of other component pairs that are previously separated. 
     Referring now to FIGS.  2 ( a ) 14  ( c ), according to the principles of the present invention the application of a pressurized pulse to column junction point  28  at an appropriate time after injection of the analyte mixture can be used to separate a specific and discrete component pair without significantly affecting the pattern of peaks for other mixture components. In particular, for each of the three cases illustrated in FIGS.  2 ( a ) 14  ( c ), injection of the analyte mixture occurs at zero time in the lower left corner of the plots. A horizontal line A in the center of each figure corresponds to column junction point  28 . Thus, the area below horizontal line A represent first column  14  and the area above horizontal line A represent second column  16 . Therefore, elution from column ensemble  14 ,  16  occurs along the top horizontal line B at 20 m. A pair of vertical lines, generally indicated at C and D, in each of the three cases illustrates the time interval corresponding to the application of a 3 second wide (nominal) pressurized pulse from a quiescent value of 36.0 psia to 44.0 psia. It should be appreciated that the pressurized pulse may be of any duration necessary to facilitate separation of the component pair. A pair of band trajectories, generally indicated at E and F, represents a first component  100  and a second component  102  of the analyte mixture, respectively (see FIG.  1 ). For discussion purposes, first component  100  is n-octane and second component  102  is 1-pentanol. In each of FIGS.  2 ( a )-( c ), solid lines, E and F, illustrate the case where a pressurized pulse is used, while the broken line illustrates the case where a pressurized pulse is not used for reference, generally indicated at E′ and F′. 
     Still referring to FIGS.  2 ( a )-( c ), a band trajectory G, represented by a line having alternate dots and dashes, illustrates the trajectory of an unretained component along the column-ensemble axis for the case with the pressurized pulse. As can been seen from FIGS.  2 ( a )-( c ), this line for the unretained component illustrates no discontinuous slope change at column junction point  28 . This condition exists because the natural pressure at column junction point  28  is used in the absence of a pulse. 
     As seen in FIGS.  2 ( d )-( f ), an enlarged portion of FIGS.  2 ( a )-( c ) is illustrated, respectively. More particularly, FIGS.  2 ( d )-( f ) each illustrate the portion of the band trajectory plots in the broken-line box on an expanded time scale and the corresponding chromatograms. 
     As stated above, first column  14  includes a polar trifluoropropylmethyl polysiloxane stationary phase coating and, thus, first component  100 , being the polar 1-pentanol, migrates more slowly therethrough. Similarly, second column  16  includes a non-polar dimenthyl polysiloxane coating and, thus, second component  102 , being a non-polar n-octane, migrates more slowly therethrough. The result is that in the absence of a pressurized pulse, the band trajectory plots E and F for these components cross just prior to elution from column ensemble  14 ,  16 . The peak apex separation, generally indicated at H′, is less than 0.5 seconds, and only a single peak is observed in the chromatogram. 
     For the case illustrated in FIGS.  2 ( a ) and ( d ), the pressurized pulse, C a -D a , is applied while both components  100  and  102 , illustrated by band trajectories E a  and F a , are still on first column  14  (i.e. below horizontal line A). Since pressurized pulse C a -D a  results in an increase in pressure at column junction point  28 , the pressure drop along first column  14  is reduced, and the local migration velocities of both components  100  and  102  are reduced. This effect is illustrated by the shallower slope of band trajectories E a  and F a . This results in greater residence time on first column  14  and the ensemble retention times being shifted to greater values. However, as seen in FIG.  2 ( d ), peak-apex separation H a  is not significantly changed relative to non-pulse peak-apex separation H′. 
     For the case illustrated in FIGS.  2 ( b ) and ( e ), the pressurized pulse, C b -D b , is applied after first component  100  has crossed junction point line A, but before second component  102  has reached junction point line A. The larger pressure at column junction point  28  during pressurized pulse C b -D b  results in an increase in local carrier gas velocity on second column  16  (above junction point line A) and, thus, first component  100  accelerates as illustrated by the steeper slope of band trajectory E b . The result is a decrease in ensemble retention time for first component  100 . On the other hand, because second component  102  is still on first column  14  (below junction point line A) when pressurized pulse C b -D b  is applied, its migration rate decreases appreciably during pressurized pulse C b -D b . Therefore, band trajectory F b  arrives at junction point line A considerably later than without pressurized pulse C a -D a  (see band trajectory F′). This results in an increase in ensemble retention time. The overall result in this scenario, as seen in FIG.  2 ( e ), is a differential shift in individual retention times for first component  100  and second component  102  as illustrated by peak-apex separation H b . 
     For the case illustrated in FIGS.  2 ( c ) and ( f ), the pressurized pulse, C c -D c , is applied after both components  100  and  102  have migrated across junction point line A into second column  16 . Since pressurized pulse C c -D c  results in an increase in pressure at column junction point  28 , the pressure drop along second column  16  is increased, and the local migration velocities of both components  100  and  102  are sharply increased. This effect is illustrated by the steeper slope of band trajectories E c  and F c . This results in reduced residence time on second column  16  and the ensemble retention times being shifted to smaller values. However, as seen in FIG.  2 ( f ), peak-apex separation H c  is not significantly changed relative to non-pulse peak-apex separation H′. 
     These results illustrated in FIGS.  2 ( a )-( f ) illustrate that ensemble values of peak-apex separation can be significantly increased for components that are on opposite sides of junction point line A when the pressurized pulse C-D is applied. For components that are on the same side of junction point line A during the pressurized pulse C-D, significant retention time shifts occur, but peak-apex separations and thus the pattern of peaks eluting from column ensemble  14 ,  16  are not significantly changed. 
     Referring now to FIGS.  3 ( a )-( e ) and  4 ( a )-( f ), chromatograms obtained with upward and downward pressurized pulses, respectively, initiated at various times after the sample injection are illustrated. Each of the figures is recorded at various pulse-initiation-time-after-injection times. For reference in each of these figures, the peak-apex retention time for a single peak containing both first component  100  and second component  102  is approximately 29.5 seconds, without using a pressurized pulse, as indicated by vertical line Z. As seen in FIGS.  5 ( a ) and ( b ), the actual pulse shapes are illustrated as an upward pressurized pulse and a downward pressurized pulse, respectively. It may be important to note that an upward pulse (to higher pressure at column junction point  28 ) results in lower carrier gas velocity on first column  14  and higher velocity on second column  16 . The opposite situation occurs for a downward pressurized pulse. 
     Referring in particular to FIG.  3 ( a ), a pulse initiation time of 10.0 seconds after analyte mixture injection results in the entire pulse J occurring while both components  100  and  102  are on first column  14 . This shifts the apex of peak J to a retention time of approximately 31.8 seconds, but has very little change in the shape of the peak. Referring to FIG.  3 ( b ), when the pressurized pulse initiation time is delayed until 11.0 seconds after injection, both components  100  and  102  are on first column  14  at the beginning of the pressurized pulse, but part of first component  100  crosses column junction point  28  before the end of the pressurized pulse. This results is a broad, distorted double peak K and L. 
     As seen in FIG.  3 ( c ), with a pulse initiation time of 13.0 seconds, first component  100  has completely crossed column junction point  28  prior to the pressurized pulse and second component  102  remains on first column  14  during the entire duration of the pressurized pulse. This results in a large shift of peak M to lower ensemble retention time for first component  100  and no significant change in retention time for second component  102 , illustrated by peak N, relative to the 10.0 second pulse initiation time. The peak-apex separation for the 13.0 second delay case is 3.8 seconds. The peak tailing observed for second component  102  in FIG.  3 ( c ) is often seen for alcohols and is observed even when only this example compound is injected and no pressurized pulse is used. 
     Referring now to FIG.  3 ( d ), with a 15.5 second pulse initiation time, second component  102  reaches column junction point  28  before the pressurized pulse is complete. Consequently, a very broad, low-amplitude feature P is observed in the chromatogram. As seen in FIG.  3 ( e ), with an 18.0 second pulse initiation time, both components have migrated across column junction point  28  prior to pulse initiation and only a single peak Q is seen in the chromatogram. However, the peak apex retention time has shifted by 1.5 seconds relative to the case with no pressurized pulse. 
     The same general trends are observed for the downward pressurized pulse illustrated in FIGS.  4 ( a )-( f ), but the retention-time shifts relative to the no-pressure-pulse case (broken vertical line Z) are in the opposite direction. In addition, the maximum peak separation is only 1.8 seconds, compared to 3.8 seconds for the upward pressurized pulse case above. For the no-pressurized-pulse case, the elution times for first column  14  for components  100  and  102  differ by about 4.9 seconds. 
     FIG. 6 illustrates a graph of ensemble retention-time vs. pulse initiation-time for first component  100  and second component  102  for an upward pressurized pulse with a 3.0 seconds nominal pulse width. A pair of broken horizontal lines R and seconds illustrate the calculated ensemble retention times for the case of no pressurized pulse for first component  100  and second component  102 , respectively. Line T represents first component  100 , while line U represents second component  102 . 
     For an upward pressurized pulse, pulse initiation times less than 9 seconds result generally in a constant shift in retention times of about 1.6 seconds for both components since the pressurized pulse is complete before either component reaches column junction point  28 . This is illustrated by the relative shift of lines R and T and lines seconds and U. For pulse initiation times in the range of about 9-11 seconds, first component  100  reaches column junction point  28  before completion of the pressurized pulse, and the retention time for first component  100  sharply decreases at Point V. For pulse initiation times in the range of about 11-14 seconds, the entire pressurized pulse occurs while first component  100  is on second column  16  and second component  102  is on first column  14 . Here, a separation of 2.3-2.4 seconds is observed with the higher values occurring near the end of this time interval because of carrier gas acceleration. If the pressurized pulse initiation time is in the range of about 16-24 seconds, both components  100  and  102  are on second column  16  during the entire pressurized pulse and the resultant peaks illustrate nearly equal shifts to shorter retention times. As the pressurized pulse initiation time is further increased, one or both peaks elute from column ensemble  14 ,  16  before the pressurized pulse is completed and the retention times return to their no-pulse values. 
     It may be important to note that in FIG. 6, lines T and U cross for a pulse initiation time of about 10 seconds. This is explained by reference to the band migration trajectory plots in FIGS.  2 ( a )-( f ). Without a pressurized pulse, the band trajectories E and F cross just prior to elution from column ensemble  14 ,  16  with the result that second component  102  elutes from column ensemble  14 ,  16  about 0.4 seconds before first component  100  as indicated by H′. With an upward pressurized pulse beginning 10 seconds after injection, first component  100  reaches column junction point  28  before the end of the pressurized pulse and its ensemble retention time is shifted lower by about 0.4 seconds. This results in complete co-elution of the two components at point W. Further delay in the start of the pressurized pulse by 1 second results in a larger shift for first component  100 , resulting in an elution-order change and a relatively large separation. 
     Similarly, FIG. 7 illustrates a graph of ensemble peak separation vs. pulse initiation-time for first component  100  and second component  102  for an upward pressurized pulse with a 3.0 seconds nominal pulse width. Peak separation with no pressurized pulse is about 0.4 seconds as shown by the separation of lines R and seconds in FIG.  6 . The nearly flat regions, generally indicated at AA and BB, are expected from the retention time plots in FIG.  6 . The maximum predicted peak separation for the upward pressurized pulse illustrated in FIG. 7 is about 2.4 seconds, while measured peak separations are about 3.8 seconds. 
     FIG. 8 is similar to FIG. 6 except the data is for the case of a downward pressurized pulse with a nominal width of 3.0 seconds, rather than an upward pressurized pulse. Line CC represents first component  100 , while line DD represents second component  102 . For the downward pressurized pulse with initiation times less than about 4 seconds, the entire pulse is complete before either components  100  or  102  reaches column junction point  28  and the retention times decrease only slightly in this interval with increasing delay in the pressurized pulse initiation. For pulse initiation times in the range of about 4-10 seconds, first component  100  reaches column junction point  28  prior to completion of the pressurized pulse. Consequently, its ensemble retention time increases with increasing initiation time due to the lower carrier gas velocity on second column  16  during the pressurized pulse. Second component  102  reaches column junction point  28  in about 10 seconds, and for pulse initiation times greater than about 10 seconds, the ensemble retention time for second component  102  increases rapidly with increasing pulse initiation time until about 16 seconds when second component  102  crosses column junction point  28 . Further increases in pulse initiation time have only a minor effect on retention times until about 26 seconds when components  100  and  102  begin to elute from column ensemble  14 ,  16  before completion of the pressurized pulse. For pulse initiation times greater than about 30 seconds, both components elute prior to the start of the pressurized pulse and the retention times are the same as the no-pulse values. 
     As can be seen in FIG. 8, lines CC and DD do not cross as in the case illustrated in FIG.  6 . In other words, a downward pressurized pulse causes the retention time for first component  100  to shift to larger values for pulse initiation times greater than five seconds. There is no change in elution order and thus lines CC and DD do not cross. It is interesting to note that the shift in ensemble retention times for first component  100  occurs for initiation times greater than 8 seconds for the upward pressurized pulse (see FIG. 6) and for times greater than 5 seconds for the downward pulse (see FIG.  8 ). This is the result of the lower carrier gas velocity on first column  14  during an upward pulse and the higher carrier gas velocity on first column  14  during a downward pulse. 
     Referring now to FIG. 9, the triangularly shaped peak-separation vs. pulse-initiation-time plot for the downward pressurized pulse is the result of the increased carrier gas velocity on first column  14  during the pressurized pulse. This causes second component  102  to begin shifting to larger retention time values for pulse initiation times greater than 10 seconds; while component  2020100  has not completely shifted to higher values for initiation times less than 11 seconds. For the downward pressurized pulse, the maximum predicted peak separation as seen in FIG. 9 is about 3.3 s, while measured peak separations is about 1.7 seconds. These differences between the predicted and observed maximum peak separation values are due in part to the pressurized pulse shapes illustrated in FIGS.  4 ( a ) and ( b ). 
     FIGS.  10 ( a ) and ( b ) illustrates the effect on peak separation caused by changes in the pressurized pulse width Δt and amplitude ΔP for the case of an upward pressurized pulse, respectively. With particular reference to FIG.  10 ( a ), ΔP is held constant at 8.0 psi. Similarly, with reference to FIG. 10 ( b ), Δt is held constant at 3.0 seconds. For both FIGS.  10 ( a ) and ( b ), the pressurized pulse initiation time is 12 seconds such that first component  100  has crossed column junction point  28  but second component  102  is still on first column  14  at the start of the pressurized pulse. 
     Referring now to FIG.  10 ( a ), for pulse widths up to about 9.0 seconds, the peak separation is nearly linear with varying pulse width. For pulse widths greater than 9.0 seconds, second component  102  crosses column junction point  28  prior to the end of the pressurized pulse and, thus, has little effect on peak separation. 
     Referring now to FIG.  10 ( b ), it can be seen that peak separation vs. pulse amplitude remains generally linear for the amplitude range 2-10 psi. Therefore, because of the peak separation being proportional to the pressurized pulse amplitude and width, the peak separation should be proportional to the pressurized pulse area (ΔPΔt). 
     According to the principles of the present invention, a gas chromatography system is provided that employs relatively narrow pressurized pulses to control the separation of specific or discrete pair of components eluting from a column ensemble  14 ,  16 . For mixture components that are on the same column during the pressurized pulse, little change in peak pattern and peak-pair separation occurs in the ensemble chromatogram. However, relatively large peak separation increases are observed only when one of the pair of mixture components is on second column  16  and the other of the pair of mixture components is still on first column  14  during at least a portion of the pressurized pulse. The principles of the present invention are particularly useful for complex mixtures where most components are adequately separated, yet it is necessary to obtain enhanced separations for a relatively few component pairs or small groups without loosing separation quality for the rest of the mixture components. This approach also may be more useful than pressure (selectivity) tuning and programming since for most of the duration of the analysis the quiescent pressure at column junction point  28  can be adjusted to give minimum ensemble void time. It is important to note that instead of applying a set pressure at the column junction point, which would invariably cause other previously separated component pairs to now co-elute, the present invention applies a brief pressurized pulse at an optimal time to cause a targeted pair of components to elute separately without adversely effecting the remaining components in the mixture. 
     Furthermore, it has been shown that upward pressurized pulses may be more useful than downward pulses for several reasons. First, an upward pressurized pulse results in a decrease in the carrier gas velocity on first column  14 , and this increases the available time for the pressurized pulse to be completed before the second component reaches the junction. Second, a downward pressurized pulse is more likely to result in sample loss through the vent line if the pressure during the pressurized pulse falls below the pressure that would exist at column junction point  28  in the absence of the pressure controller. Third, the pressure equilibration time is smaller for an upward pulse for any specified quiescent-pressure value. However, downward pressurized pulses do have some useful applications. 
     According to a second embodiment of the present invention, a method and apparatus are provided for achieving improved selectivity for gas chromatography with capillary columns. This second embodiment replaces electronic pressure controller  22  of the first embodiment with a low-volume gas valve coupled between column junction point  28  and an atmospheric pressure vent line. 
     As described in detail above, electronic pressure controller  22  of gas chromatography system  10  is used to change the pressure at column junction point  28  for a predetermined time interval and then return the pressure to its original value. This pressure change, when timed correctly, enhanced the ensemble separation of first component  100  and second component  102 . However, electronic pressure controller  22  may not afford sufficiently rapid pressure equilibration time. This can be seen in FIGS. 5 a  and  5   b  in the curves inability to follow the ideal square pressure pulse. That is, electronic pressure controller  22  of the first embodiment defines an internal volume of greater than approximately 7 cm 3 . Therefore, this internal volume may result in an excessive equilibration time, which may preclude pressure pulses of less than about 5 seconds in width. This may not be of importance in some applications, however it may be useful to use pressure pulses of less than 5 seconds in certain separation applications. 
     According to the principles of the second embodiment, electronic pressure controller  22  and carrier gas supply  30  are eliminated and replaced with a lower cost, low-volume gas valve coupled between a column junction point  128  and an atmospheric pressure vent line  136 . With particular reference to FIG. 11, a pressure-controlled, dual-column, gas chromatography system is schematically illustrated in accordance with a second embodiment of the present invention, generally indicated at  110 . Gas chromatography system  110  generally includes an inlet system  112 , a first gas chromatography separation capillary column  114 , a second gas chromatography capillary column  116 , a first detector (D 1 )  118 , a second detector (D 2 )  120 , and an vent valve  122 . 
     Inlet system  112  is configured generally to receive a sample “plug” of an analyte mixture for analysis within gas chromatography system  110 . More particularly, a stream of carrier gas from a carrier gas supply  124  entrains such analyte mixture during injection. This entrained analyte mixture and carrier gas composite is introduced into first column  114  through a passage  126 . Various configurations of inlet system  112  can be provided, such as mechanical injection valves, cryofocusing systems, and the like. 
     First column  114  and second column  116  are coupled in series along column junction point  128 . Vent valve  122  is fluidly coupled to column junction point  128 . Vent valve  122  is preferably a pneumatically operated, low-dead-volume device, which is operated by a 50-55 psig compressed air source connected through an electronically actuated solenoid valve. Atmospheric pressure vent line  136  is provided in fluid connection with vent valve  122  to relieve fluid pressure at column junction point  128  in accordance with the principles of the present invention. Preferably, vent valve  122  is driven in response to a central processing unit  132 . Moreover, vent valve  122  is preferably coupled to column junction point  128  via an approximately 4.0 cm long, 0.53 mm diameter (id) deactivated, fused-silica tube  134 . 
     Referring now to FIG. 12, the retention characteristics of a test mixture on gas chromatography system  110  is illustrated for a case in which vent valve  122  is closed during the entire separation sequence. This figure illustrates plots of the positions of the solute bands in the ensemble as a function of time, wherein line A 1  represents the solute band position at column junction point  128  and line B 1  represents the resultant solute band position at the exit of second column  116 . 
     The seven components, as indicated by  201 ,  202 ,  203 ,  204 ,  205 ,  206 , and  207  were chosen for purposed of discussion to yield closely spaced groups of peaks in the ensemble chromatogram under conditions when vent valve  122  is closed during the entire separation. It is important to note that components  201  and  202 , and  206  and  207  are separated by first column  114 , but coelute from second column  116 . On the other hand, component  204  is well-separated from components  203  and  205  by first column  114 , but an elution order change occurs for components  203  and  204  on second column  116 , with the result that peaks  203  and  204  are not completely separated by second column  116 . 
     Referring now to FIGS.  13 ( a ) 14  ( g ), a chromatogram is illustrated for components  203 ,  204 , and  205  when vent valve  122  is opened for various time intervals (nominal pulse width) beginning 41 seconds after injection. The values to the right of each figure give the nominal open time for vent valve  122  in seconds. When the valve is initially opened, component  204  has migrated across column junction  128  and is in second column  116 , while components  203  and  205  are still on first column  114 . When vent valve  122  is opened, the pressure at column junction point  128  falls from 21.0 psia to ambient pressure (14.2 psia). As one skilled in the art will understand, the pressure at column junction point  128  is the outlet pressure for first column  114  and the inlet pressure for second column  116 . Thus, when vent valve  122  is open, the local carrier gas velocity at any point in first column  114  is increased, and the local carrier gas velocity in second column  116  is decreased. The result is an increased migration rate in first column  114  and a decreased rate in second column  116  for the interval when vent valve  122  is open. This shifts the ensemble retention time for component  204 , which is in second column  116  when vent valve  122  is open, to larger values and shifts the retention times for components  203  and  205  to smaller values. 
     With particular reference to FIG.  13 ( b ), a nominal pulse width of 0.4 seconds results in an elution order change for components  204  and  205 . While, a pulse width of 0.8 seconds results in baseline separation of all components  203 ,  204 , and  205 , as seen in FIG.  13 ( c ). Further increase in the pulse width to 1.2 seconds results in increased separation of components  204  and  205 , but a decrease in area is observed for peak  203 . For a nominal pulse width of 1.8 seconds, the area of peak  203  is reduced to approximately 12% of the no-pulse value, and the area of peak  205  illustrates a significant decrease. The decreases in peak area are the result of venting some of the sample through vent valve  122  and atmospheric vent line  136 . 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, as can be seen by the decrease in component peak widths when venting occurs. This has been explained by a decrease in the elution peak width from first column  114  when the carrier gas velocity in first column  114  is increased during the pulse. 
     Still referring to FIGS.  13 ( a )-( g ), the chromatograms suggest that a nominal pulse width between 0.4 and 0.8 seconds will provide adequate separation of components  203 ,  204 , and  205  with minimal component venting. The effects of pulse width on peak separation and peak area for components  203 ,  204 , and  205  are shown in greater detail in FIGS.  14 ( a ) and ( b ). That is, with no pulse (pulse width of 0 seconds), the peak separation of component pair  204 / 205  is approximately —1.4 seconds and that of component pair  203 / 204  is just over about 1 second. The negative value for the peak separation of component pair  204 / 205  with no pulse is the result of the ensemble elution order change when vent valve  122  is opened. Neither value is adequate for a complete baseline separation. For a nominal pulse width between 0.5 and 1.2 seconds, all component pairs have separations greater than 2 seconds, which provides better than baseline separation. 
     Referring now to FIG.  14 ( b ), the relative peak area plots show that for component  204 , the peak area is independent of the pulse width. This is expected, because component  204  has crossed the junction before the pulse is initiated. For components  203  and  205 , a sharp loss in peak area is observed if the corresponding components reach column junction  128  prior to the time that vent valve  122  closes. For component  205 , which reaches column junction  128  before component  203  (see FIG.  12 ), the nominal pulse width should not exceed approximately 0.5 seconds in order to obtain minimal component venting. Because a nominal pulse width of 0.5 seconds provides adequate separation of components  203 ,  204 , and  205 , this value was used below for discussion purposes. 
     FIGS.  15 ( a )-( c ) illustrates extracted-ion chromatograms from the complete mixture for a single 0.5 second wide pulse initiated at (a) 30 seconds, (b) 40 seconds, and (c) 52 seconds after injection, respectively. 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 seconds after injection as seen in FIG.  15 ( a ), components  201  and  202  are well-separated, but component groups  203 / 204 / 205  and  206 / 207  show no significant change in the ensemble peak patterns relative to the no-pulse case. For a pulse initiation time of 40 seconds, a complete separation of components  203 ,  204 , and  205  is achieved, but coelutions are observed for component pairs  201 / 202  and  206 / 207 . For a pulse initiation time of 52 seconds, component pair  206 / 207  is well-separated, but the pattern of peaks for component groups  201 / 202  and  203 / 204 / 205  are essentially the same as for the no-pulse case. 
     Therefore, it can be seen that the test mixture used in this example results in three congested regions of the ensemble chromatogram. Accordingly, as seen in FIG. 16, a sequence of three pulses can be used to improve the separation of all three of the groups of mixture components. The broken vertical lines indicate the required pulse initiation times and duration. It is important to note, however, that the required pulse-initiation times are altered by any previously applied pulses, because each previously applied pulse reduces the time required for subsequent component groups to reach column junction  128 . This can be taken into account by using an algorithm to determine the band position vs time plots or, alternatively, by monitoring first column  114  using first detector  118  to determine the appropriate pulse initiation times. 
     FIGS.  17 ( a )-( d ) illustrate a direct comparison of the analytical ion chromatograms (AIC) (FIGS.  17 ( a ) and ( c )) and extracted-ion chromatograms (EIC) (FIGS.  17 ( b ) and ( d )) for the no-pulse case (FIGS.  17 ( a ) and ( b )) and for the case when three sequential 0.5 second (nominal) pulses are used as described in FIG. 16 (FIGS.  17 ( c ) and ( d )). The analytical ion chromatograms are obtained by summing all the extracted ion chromatograms that exceed a user-defined signal-to-noise ratio for the found peaks. The analytical ion chromatograms provide information that is similar to that obtained in a total-ion chromatogram but at substantially increased signal-to-noise ratios. For this work, the analytical ion chromatograms are used to simulate the use of a single-channel detector, such as an flame ionization detector. 
     With particular reference to FIGS.  17 ( a ) and ( b ) where no pulse is applied, the analytical ion chromatogram illustrates only five peaks as a result of the overlap of component pairs  201 / 202  and  206 / 207 . In addition, the peaks of components  203  and  204  show excessive overlap, which prevents accurate peak-area calculations. As seen in FIGS.  17 ( c ) and ( d ), when three pulses are applied by vent valve  122 , the analytical ion chromatogram illustrates that a complete separation is achieved with at least baseline resolution for all adjacent components. 
     As should be appreciated from the foregoing discussion in connection with the second embodiment of the present invention, the use of a low-dead-volume valve  122  and an atmospheric pressure vent  136  can be very effective in enhancing the ensemble separation of specified groups of mixture components. Furthermore, the present invention provides a simple and convenient method of controlling the selectivity in relatively small, specified regions of the ensemble chromatogram. As described in connection with the first embodiment, the second embodiment provides that all of the mixture components that are on the same column during the time that vent valve  122  is open (pulse duration) will be shifted to larger or smaller retention times, but the pattern of peaks for these components from the output second column  116  will be relatively unaffected by the pressure pulse. Lastly, gas chromatography system  110  of the present embodiment provides simpler, less expensive, more flexible method of enhancing the selectivity for series coupled, gas chromatography column ensembles, and further provides a faster pulse response than that provided by the first embodiment of the present invention. 
     Still further, it was seen that for cases in which more than one coeluton 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 first column  114  that are caused by each successive pulse. It should be understood that although the apparatus described herein used isothermal gas chromatography, multiple-pulse techniques should be very effective with temperature-programmed gas chromatography, since 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 vent valve  122  closed, and the column junction-point pressure is the value that would occur without any additional connection to column junction  128 . 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. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.