Pressure-Tunable GC Columns with Electronic Pressure Control

Dynamic Control of Gas Chromatographic Selectivity during the Analysis of Organic Bases

A novel method for controlling selectivity during the gas chromatographic (GC) analysis of organic bases is presented. The technique employs tandem stainless steel capillary columns, each coated with a pH adjusted water stationary phase. The first is a 0.5 m trap column coated with a pH 2.2 phase, while the second is an 11 m analytical column coated with a pH 11.4 phase. The first column traps basic analytes from injected samples, while the remaining components continue to elute and separate. Then, upon injection of a volatile aqueous ammonia solution, the basic analytes are released as desired to the analytical column where they are separated and analyzed. Separations are quite reproducible and demonstrate an average RSD of 1.2% for analyte retention times in consecutive trials. Using this approach, the retention of such analytes can be readily controlled and they can be held in the system for periods of up to 1 h without significant erosion of peak shape. As such, it can provide considerable control over analyte selectivity and resolution compared to conventional separations. Further, by employing a third conventional GC column to the series, both traditional hydrocarbon and enhanced organic base separations can be performed. The method is applied to the analysis of complex mixtures, such as gasoline, and much less matrix interference is observed as a result. The findings indicate that this approach could be a useful alternative for analyzing such samples.

Single fiber-in-capillary annular column for gas chromatographic separation

A method for preparation of a stationary phase-adjustable column with in-column stationary phase-coated fused-silica fiber annular column was successfully developed. The surface of a 0.12 mm o.d. bare optical fiber was first coated with a stationary phase and then inserted into a fused-silica capillary (non-coated or coated) as an annular column for gas chromatographic study. The optical fiber and capillary were coated with polydimethylsiloxane (SE-30) and polyethylene glycol 20 M (PEG-20 M) as nonpolar and polar stationary phases, respectively. Among the investigated annular and open tubular columns, the PEG-20 M-coated fiber-in-PEG-20 M-coated capillary annular column showed the highest column efficiency with a minimum plate height of 0.35 mm and an optimum gas velocity of 25 cm/s. When a SE-30/PEG-20 M-coated fiber-in-uncoated capillary annular column was applied to separate a 9-component complex mixture, the total analysis time was 5.3 min and the column length was 12 m. By contrast, when a SE-30-coated fiber-in-PEG-20 M-coated capillary annular column was used to separate the same 9-component mixture, the analysis time was reduced to 3.5 min and the column length was shortened by half to 6m. Our results show that the stationary phase-coated fiber-in-stationary phase-coated capillary annular column is a better choice for gas chromatographic separation as it is more efficient and flexible. In addition, the proposed annular column design provides flexibility in using two or even more types of stationary phases to achieve optimal analytical separation.

Characterization and Quantitative Analysis with GC/TOFMS Comparing Enhanced Separation with Tandem-Column Stop-Flow GC and Spectral Deconvolution of Overlapping Peaks

Time-of-flight mass spectrometry is unique in that ion abundance ratios are constant over the chromatographic peak profile provided that the peak contains only one component. This provides the means for the automated finding and spectral deconvolution of overlapping chromatographic peaks from completely unknown mixtures if the mass spectra for the overlapping components are sufficiently unique. This can greatly reduce the chromatographic resolution requirements, which allows for very rapid quantitative analysis as well as for high-speed mixture characterization. High-speed GC with stop-flow operation of a series-coupled column ensemble can be used to completely separate some component pairs that coelute from the column ensemble, thus eliminating the need for spectral deconvolution of those mixture components. This provides two options for high-speed qualitative and quantitative analysis, using either the mass spectra from deconvoluted overlapping peaks or the mass spectra from the completely separated peaks obtained with stop-flow operation of the tandem column ensemble. These options are compared with respect to the similarity for spectral matching with a library and to peak area linearity with concentration, calibration plot correlation coefficients, and shot-to-shot reproducibility.

Fast gas chromatography and its use in trace analysis

There is revived interest in the development and implementation of methods of faster GC. The paper summarises the advantages of faster GC analysis, general approaches to faster GC method development and practical aspects of fast gas chromatography with the utilisation of open tubular capillary columns with the stress on trace analysis. There are a number of ways to take the advantage of the improved speed of analysis by faster GC. Numerous options exist for pushing the speed of capillary gas chromatography (CGC) analysis. The scope of this paper is also to give an overview of the present state of faster GC instrumentation which is already available for trace analysis. The practicality of fast CGC is a function of sample preparation and the matrix interferences and how they affect the resultant resolution that may be achieved. Researchers have demonstrated the applicability of fast GC to trace and ultratrace analysis of volatile and semivolatile compounds also with narrow bore columns and difficult sample matrices (such as food, and soil extract). The main development of faster GC methods has been observed in the field of environmental analysis. Practical applications are presented. Both optimised sample preparation and experimental conditions for faster GC are the future perspective of trace analysis.

Portable gas chromatograph with tunable retention and sensor array detection for determination of complex vapor mixtures

A prototype portable gas chromatograph that combines a multiadsorbent preconcentrator/focuser, a tandem-column separation stage with individual column temperature control and junction point pressure modulation, and a detector consisting of an integrated array of polymer-coated surface acoustic wave microsensors is described. Using scheduled first-column stop-flow intervals and independent temperature programming of the two columns, it is possible to adjust the retention of eluting analyte vapors to maximize vapor recognition with the microsensor array and minimize the time of analysis. A retention window approach is combined with Monte Carlo simulations to guide retention tuning requirements and facilitate pattern recognition analyses. The determination of a 30-vapor mixture of common indoor air contaminants in < 10 min is demonstrated using ambient air as the carrier gas. Detection limits of < 10 ppb are achieved for the majority of compounds from a 1-L air sample on the basis of the most sensitive sensor in the array. Performance is assessed in the context of near-real-time indoor air quality monitoring applications.

Band-trajectory model for temperature-programmed series-coupled column ensembles with pressure-tunable selectivity

A model and a spreadsheet algorithm is described for the prediction of solute-band migration trajectories in a series-coupled combination of two capillary GC columns with pressure-tunable and -programmable selectivity and operated under temperature-programmed conditions. The model takes into account the acceleration of carrier gas in the two columns as a result of decompression effects, the deceleration of carrier gas as a result of the increase in viscosity during temperature programming, the decrease in solute retention factors with increasing temperature during the temperature program, the differences in retention factors for the two columns, and programmed changes in the carrier-gas flow rates in the two columns during selectivity programming. In the model, the 20-meter-long column ensemble is divided into 1-cm-long intervals, and the carrier-gas velocity and column temperature are assummed to be constant in any interval. Migration times for all of the mixture solutes are computed for each column interval, and the solute-band positons in the column ensemble are plotted versus the running sum of these migration times to obtain band trajectory plots. The sum of these migration times for all 2,000 intervals gives the ensemble retention times for the solutes. Isothermal retention factors (k) for all of the mixture components at various column temperatures (Tc) are used as imput to the algorithm. Slope and intercept values of In(k) vs 1/Tc plots are used in the algorithm. General features of the model are tested using a mixture of C12-C24 normal alkanes. A mixture of polar and nonpolar compounds is used to test the utility of the model for the predicition of peak separations and retention times with pressure-tunable and -programmable selectivity. Good agreement is observed in all cases.

A tandem column ensemble with an atmospheric pressure junction-point vent for high-speed GC with selective control of peak-pair separation

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.

Pulsed flow modulation for high-speed GC using a pressure-tunable column ensemble

A computer-driven pressure controller is used to deliver pressure pulses to the junction point of two series-coupled columns using different stationary-phase chemistries. The column ensemble consists of a trifluoropropylmethyl polysiloxane column followed by a dimethyl polysiloxane column. Each pressure pulse causes a differential change in the carrier gas velocities in the two columns, which lasts for the duration of the pulse. A pressure pulse is used to selectively increase the separation of a component pair that is separated by the first column but coelutes from the series-coupled ensemble. If both components are on the same column when the pulse is applied, a small change in the ensemble separation occurs. If one component of the pair is on the first column and the other component is on the second column, a pressure pulse can result in a much larger change in the ensemble separation for the component pair. A model with a spreadsheet algorithm is used to predict the effects of a pressure pulse on the trajectories of component bands on the column ensemble. The effect of the initiation time of a pressure pulse is investigated for a two-component mixture that coelutes from the column ensemble. For the case where the entire pressure pulse occurs when one of the components is on the first column and the other component is on the second column, the peak separation from the ensemble increases nearly linearly with the product of the pressure pulse amplitude and the pulse duration. Peak shape artifacts are observed if the pressure pulse occurs when a solute band is migrating across the column junction point.

Programmable control of column selectivity for temperature-programmed GC

A computer-driven pressure controller connected to the junction point of a series-coupled ensemble of two capillary GC columns having different stationary-phase selectivity is used to obtain on-the-fly (programmable) changes in ensemble selectivity. Changes in the junction-point pressure result in differential changes in the local carrier gas velocity in the two columns, and this results in changes in the pattern of peaks eluting from the ensemble. When used with relatively fast temperature programming (30 degrees C/min), the pattern of eluting peaks can be very sensitive to the time at which a selectivity (junction-point pressure) change is implemented. These elution pattern changes are described for a set of six PCB congeners that elute with a small range of retention times. The components are considered as a group, and changes in their elution pattern are described for a single junction-point pressure change, which is implemented at various times after sample injection. If the pressure change is implemented after the components have migrated across the junction point, the final pressure has relatively little impact on the ensemble retention pattern. Pressure changes implemented prior to the components reaching the junction can have a large effect and usually result in a pattern of peaks similar to the pattern obtained when the final pressure is used for the entire separation. For pressure changes made when the group of components is near the junction point, the observed peak pattern may be very sensitive to the time of the pressure change. The time at which the junction-point pressure change occurs is varied in 1.0-s intervals. Artifacts such as peak doubling and peak focusing or broadening are observed if a migrating band is crossing the column junction point at the time of the programmed pressure change.

Hihg-speed GC/MS of gasoline-range hydrocarbon compounds using a pressure-tunable column ensemble and time-of-flight detection

A pressure-tunable series-coupled ensemble of two capillary GC columns is combined with a time-of-flight MS detector for the high-speed characterization of mixtures containing hydrocarbon compounds. The column ensemble consists of a nonpolar 5% phenyl poly(dimethylsiloxane) column and a very polar poly(ethylene glycol) column. The TOFMS instrument uses time-array detection to obtain up to 500 complete electron mass spectra per second. Instrument software allows for automated peak finding and the spectral deconvolution of severely overlapping unknown chromatographic peaks, if their fragmentation patterns are significantly different and if at least two spectra can be recorded between the peak apexes. By adjusting the carrier-gas pressure at the column-junction point, the separations between adjacent peak pairs can be adjusted to enhance the capabilities of the TOFMS detector. The sensitivity of peak-pair separation to changes in junction-point pressure is studied for combinations of alkanes, olefins, and aromatic compounds. When complete separation is required, the use of pressure-tunable column ensembles cannot always provide sufficient control of peak-pair separation for structurally similar compounds. However, complete chromatographic separation typically is not required with the TOFMS detection, and a pressure-tunable column ensemble is very useful for the high-speed characterization of hydrocarbon mixtures.

Pressure-tunable column selectivity for high-speed vacuum-outlet GC

A pressure-tunable ensemble of two series-coupled capillary columns operated at subambient outlet pressure is described. The ensemble consists of a 4.5-m length of nonpolar dimethyl polysiloxane column followed by a 7.5-m length of polar trifluoropropylmethyl polysiloxane column. Air at an inlet pressure of 1.0 atm is used as carrier gas, and a vacuum pump is used to pull the carrier gas and injected samples through the column ensemble. Detection is provided by a photoionization detector operated at a pressure of 0.3 psia. Ensemble selectivity is controlled by means of an electronic pressure controller located at the junction point between the columns. The minimum pressure step size is 0.1 psi, and 50 different set-point pressures can be used, each one producing a different pattern of peaks eluting from the column ensemble. Measured ensemble retention factors for a set of target compounds produce straight lines when plotted versus the ratio of the calculated holdup time of the first column in the ensemble to the total ensemble holdup time. A component band trajectory model is used to describe the effects of ensemble junction-point pressure on the elution patterns generated by the ensemble. Ensemble retention times predicted by the model are in good agreement with values obtained from chromatograms. The use of on-the-fly set-point pressure changes during a separation (selectivity programming) is demonstrated and used to improve the quality of the separation of a 19-component test mixture.

High-speed gas chromatography: an overview of various concepts

An overview is given of existing methods to minimise the analysis time in gas chromatography (GC) being the subject of many publications in the scientific literature. Packed and (multi-) capillary columns are compared with respect to their deployment in fast GC. It is assumed that the contribution of the stationary phase to peak broadening can be neglected (low liquid phase loading and thin film columns, respectively). The treatment is based on the minimisation of the analysis time required on both column types for the resolution of a critical pair of solutes (resolution normalised conditions). Theoretical relationships are given, describing analysis time and the related pressure drop. The equations are expressed in reduced parameters, making a comparison of column types considerably simpler than with the conventional equations. Reduction of the characteristic diameter, being the inside column diameter for open tubular columns and the particle size for packed columns, is the best approach to increase the separation speed in gas chromatography. Extremely fast analysis is only possible when the required number of plates to separate a critical pair of solutes is relatively low. Reducing the analysis time by reduction of the characteristic diameter is accompanied by a proportionally higher required inlet pressure. Due to the high resistance of flow of packed columns this seriously limits the use of packed columns for fast GC. For fast GC hydrogen has to be used as carrier gas and in some situations vacuum-outlet operation of capillary columns allows a further minimisation of the analysis time. For fast GC the columns should be operated near the conditions for minimum plate height. Linear temperature programmed fast GC requires high column temperature programming rates. Reduction of the characteristic diameter affects the sample capacity of the "fast columns". This effect is very pronounced for narrow-bore columns and in principle non-existing in packed columns. Multi-capillary columns (a parallel configuration of some 900 narrow-bore capillaries) take an intermediate position.