Compressibility effects in packed and open tubular gas and supercritical fluid chromatography

The influence of the pressure drop on the efficiency and speed of anal. in packed and open tubular supercrit. fluid chromatog. (SFC) is described: methods previously developed to describe the effects of mobile phase compressibility on the performance of open tubular columns in SFC have been extended to packed columns. The Horvath and Lin equation has been used to elucidate the influence of variations in velocity, diffusivity, and capacity factor along the column on the overall efficiency of packed column SFC. In packed columns, in contrast with the situation in open tubular columns, because the diffusion coeffs., the increase in both linear velocity and capacity factor which result from a significant pressure drop cause the plate height to increase along the column. The effect of liq. decompression along the length of the column on the speed of anal. in SFC has been studied and numerical expressions derived which enable calcn. of compressibility correction factors for the plate height. Both the f1 and f2 correction factors remain very close to unity for acceptable pressure drops, which means that the pressure drop has virtually no effect on the no. of plates generated per unit time for an unretained component. For retained species, the decompression of the mobile phase across the column causes the capacity factor to increase and hence leads to increased anal. time

Electron capture detection in high-speed narrow-bore capillary gas chromatography: fast and sensitive analysis of PCBs and pesticides

In this work the combination of high-speed narrow-bore capillary GC with electron capture detection is evaluated. The make-up gas flow rate is a key parameter in the successful coupling of narrowbore columns and ECD detection. The make-up flow has to be as high as possible to eliminate peak tailing caused by the large detection cell volume. The sensitivities at these elevated make-up flow rates (400 to 1000 ml/min), measured for some pesticides like HCB and dieldrin, were very good. Detection limits for these compounds of 0.1 pg were obtained, resulting in minimum detectable concentrations of approximately 0.2 ppb. The performance of the system is illustrated by several high-speed analyses of environmentally relevant samples of PCBs and pesticides

High-speed GC/MS using narrow-bore columns and ion trap detection

Fast gas chromatog. sepns. can be achieved by vacuum-outlet operation and by applying narrow-bore columns. The combination of 50 mm internal diam. columns with ion trap mass spectrometric detection is evaluated. Detection limits in the electron ionization mode, and in the chem. ionization mode using CH4 as the reaction gas, are 1 pg and 5 pg, resp. Owing to the high sensitivity of the ion trap mass spectrometer, a significant improvement in the working range in comparison with other detection systems is obtained. The small column flows cause no loss of mass spectral resoln. and sensitivity. The performance of the system is demonstrated by the anal. of some real-world sample

Optimization of temperature-programmed gas chromatographic separations, II: Off-line Simplex optimization and column selection

In this work a method is described which allows off-line optimization of temperature-programmed GC separations. The method is based on a new numerical method that allows the off-line prediction of retention times and peak widths of a mixture containing components with known identities in capillary GC. In the present work we apply this method for off-line optimization of single- and multi-ramp temperature-programmed GC separations. First, it will be shown how the numerical methods are incorporated in a Simplex optimization method. Next, it is described how the method can be used to determine the optimal temperature program for the separation of a mixture containing components of different functionalities. Finally, it is shown that the optimization strategy followed here allows selection of the capillary column most suited for the separation problem under study from a given set of capillary columns containing the same stationary phase and varying inner diameters and film thicknesses. The process can be performed without any experimental effort. The results indicate that fully off-line simulation and optimization of single- and multi-ramp temperature-programmed GC separations as well as column selection is possible

Optimization of temperature-programmed GC separations. II. Off-line simplex optimization and column selection

In this work a method is described which allows off-line optimization of temperature programmed GC separations. Recently, we described a new numerical method to predict off-line retention times and peak widths of a mixture containing components with known identities in capillary GC. In the present work we apply this method for off-line optimization of temperature programmed GC separations. First, it will be shown how the numerical methods are incorporated in a Simplex optimization method. Next, it is described how the method can be used to determine the optimal temperature program for the separation of a mixture containing components of different functionalities. Finally, it is shown that the optimization strategy followed here allows selection of the capillary column most suited for the separation problem under study, from a given set of capillary columns containing the same stationary phase and varying inner diameters and film thicknesses. The process can be performed without any experimental effort. The results indicate that complete off-line simulation, optimization of GC separations and column selection is possible

Optimization of temperature-programmed gas chromatographic separations, 1: Prediction of retention times and peak widths from retention indices

A numerical method is described to predict retention times and peak widths of a mixture containing components with known identities in capillary gas chromatography. The procedure is based on extracting thermodynamic values (enthalpy and entropy terms) from Kováts retention indices. Next, a numerical procedure is developed that uses these data to calculate retention times and peak widths on any capillary column containing the same stationary phase but with a different phase ratio. The estimations are based on a sound theoretical basis. The predictions can be performed either in the isothermal or temperature-programmed (single- or multi-ramp) mode. In the temperature programs, which cover a broad temperature range, isothermal plateaus are allowed. Errors in the predictions of retention times are generally less than 4%. Prediction of peak widths under the same conditions can be performed with errors of about 10%. An attractive feature of the approach is, that once the thermodynamic values of the solutes of interest are known, future optimizations can be performed without the need to perform experimental input runs. This indicates that the concept can be used for complete off-line simulations and/or optimizations of gas chromatographic separations

Optimization of temperature-programmed gas chromatographic separations, II: Off-line Simplex optimization and column selection

In this work a method is described which allows off-line optimization of temperature-programmed GC separations. The method is based on a new numerical method that allows the off-line prediction of retention times and peak widths of a mixture containing components with known identities in capillary GC. In the present work we apply this method for off-line optimization of single- and multi-ramp temperature-programmed GC separations. First, it will be shown how the numerical methods are incorporated in a Simplex optimization method. Next, it is described how the method can be used to determine the optimal temperature program for the separation of a mixture containing components of different functionalities. Finally, it is shown that the optimization strategy followed here allows selection of the capillary column most suited for the separation problem under study from a given set of capillary columns containing the same stationary phase and varying inner diameters and film thicknesses. The process can be performed without any experimental effort. The results indicate that fully off-line simulation and optimization of single- and multi-ramp temperature-programmed GC separations as well as column selection is possible