Prediction of the separation in gas chromatography

Modeling approaches for temperature-programmed gas chromatographic retention times under vacuum outlet conditions

This contribution evaluates the performance of two predictive approaches in calculating temperature-programmed gas chromatographic retention times under vacuum outlet conditions. In the first approach, the predictions are performed according to a thermodynamic-based model, while in the second approach the predictions are conducted by using the temperature-programmed retention time equation. These modeling approaches were evaluated on 47 test compounds belonging to different chemical classes, under different experimental conditions, namely, two modes of gas flow regulation (i.e., constant inlet pressure and constant flow rate), and different temperature programs (i.e., 7 °C/min, 5 °C/min, and 3 °C/min). Both modeling approaches gave satisfactory results and were able to accurately predict the elution profiles of the studied test compounds. The thermodynamic-based model provided more satisfying results under constant flow rate mode, with average modeling errors of 0.43%, 0.33%, and 0.15% across all the studied temperature programs. Nevertheless, under constant inlet pressure mode, lower modeling errors were achieved when using the temperature-programmed retention time equation, with average modeling errors of 0.18%, 0.18%, and 0.31% across the used temperature programs.

Gas chromatography-mass spectroscopy optimization by computer simulation, application to the analysis of 93 volatile organic compounds in workplace ambient air

GC-MS optimization method including both advantages from chromatographic separation and mass spectrometric detection was designed for a set of 93 volatile organic compounds. Only a few experiments were necessary to determine the thermodynamic retention parameters for all compounds on a RTX-VMS column. From these data, computer simulation was used in order to predict the retention times of the compounds in temperature programmed gas chromatography. Then, an automatic selection of ions from the NIST database was performed and compared to the optimum conditions (full separation of VOC). This simulation-selection procedure was used to screen a numerous set of GC and MS conditions in order to quickly design a GC-MS method whatever the set of compounds considered.

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.

Mathematical model for liquid-gas equilibrium in acetic acid fermentations

An experimental study was conducted to propose an adequate mathematical model for liquid-gas equilibrium in acetic acid fermentations. Three operation scales (laboratory, pilot plant, and industrial plant) were employed to obtain the sets of experimental data. The proposed model, based in the UNIFAC method for the estimation of activity coefficients of a solution consisting of several components, takes into account the effect of temperature. However, in the set of equations, it has been necessary to put in the degree of equilibrium (epsilon). This coefficient adequately reflects the physical conditions of fermentation equipment. The experimental and numerical results help to define the fundamental mechanisms for liquid-gas equilibrium in these systems and demonstrate the model validity in the three tested scales. It was also found that in an industrial setting, closed systems are those with lowest evaporation losses. Copyright 1998 John Wiley & Sons, Inc.