S. Vezzani

Fast and accurate method for the automatic prediction of programmed-temperature retention times

Abstract Sic calculation methods for the prediction of the retention times in programmed-temperature gas chromatography by starting from isothermal data were applied to capillary column analysis. The minimum number of isothermal runs necessary to obtain accurate results was evaluated. The application of the various methods and the accuracy of the prediction are discussed. A quadratic interpolation method which requires only three isothermal retention values as the starting data was found suitable and easily applied to computer programs.

Automatic prediction of retention times in multi-linear programmed temperature analyses

A method for the prediction of retention times of compounds analysed during programmed-temperature gas chromatography with capillary columns is described, which employs an explicit equation for the calculation of the gas hold-up time. The method can be applied to analyses carried out with different temperature profiles: constant linear programming rate, initial and final isothermal tract, different temperature gradients in the same analysis. The effect of the various parameters of the analysis on the accuracy of the predicted values was investigated.

Comparison of different methods for the prediction of retention times in programmed-temperature gas chromatography

Four calculation methods that can be easily applied by simple BASIC programming on personal computers were compared for the prediction of the retention times of various substances during linear temperature programming with and without an initial isothermal period on polar and non-polar capillary columns. The methods are based on curve-fitting techniques or a numeric iterative integration approach (Simpson and trapezoid methods). The comparison with experimental data obtained in various programmed-temperature analyses showed that all the tested methods permit the prediction of the retention times. The computation times and deviations of the results are compared.

Retention models for programmed gas chromatography

The models proposed by many authors for the prediction of retention times and temperatures, peak widths, retention indices and separation numbers in programmed temperature and pressure gas chromatography by starting from preliminary measurements of the retention in isothermal and isobaric conditions are reviewed. Several articles showing the correlation between retention data and thermodynamic parameters and the determination of the optimum programming rate are reported. The columns of different polarity used for the experimental measurement and the main equations, mathematical models and calculation procedures are listed. An empirical approach was used in the early models, followed by the application of thermodynamic considerations, iterative calculation procedures and statistical methods, based on increased computing power now available. Multiple column arrangements, simultaneous temperature and pressure programming, applications of two-dimensional and fast chromatography are summarised.

Theoretical calculation of gas hold-up time in capillary gas chromatography Influence of column, instrument parameters and analysis conditions and comp

The gas hold-up time or dead time, t m , of some capillary columns with different bonded liquid or carbon layer stationary phases was calculated by using equations derived by the classical theory of gas behaviour in narrow tubing, e.g., Poiseuilles law. The parameters of the equations (pressure, temperature, gas viscosity, diameter and length of the column) were measured experimentally and the effect of the approximation of the measure on the final value of t m was evaluated over a wide temperature range. The calculated t m values were compared with those obtained by using the retention time of methane, the extrapolation of homologous series, the elution of the front of the solvent peak, flow-rate measurement with a bubble flow meter and the automatic systems of commercially available instruments.

Comparison of the behaviour of gas-liquid and gas-liquid-solid capillary columns through the determination of thermodynamic characteristics

Interpolation methods, used for the prediction of retention times in programmed temperature capillary gas chromatography, employ equations whose parameters are correlated, through the capacity factor and the phase ratio of the column, with the thermodynamic parameters, that can be calculated by using retention data obtained at various temperatures. Homologous series of organic compounds (linear alkanes and alcohols) were analysed on bonded phase polar, non-polar and carbon layer open tubular columns and the values of some thermodynamic parameters were obtained. Their dependence on the polarity of the liquid phase and of the solutes, film thickness, temperature of the column, was investigated and allowed comparison of the behaviour of the different phases tested.

Automatic prediction of retention times in programmed-pressure isothermal gas chromatography

A method for the prediction of the retention times of the compounds analysed during programmed-pressure gas chromatography with capillary columns is described. The method can be applied to analyses carried out with different pressure profiles: constant linear programming rate, initial and final isobaric tract. Only one preliminary isobaric run is necessary in order to find the input data which permit to obtain accurate results. The effects of the various parameters of the analysis on the accuracy of the predicted values were investigated.

Prediction of the plate height of capillary columns operated at any inlet pressure of the carrier gas by using few retention data measured under isobaric conditions.

Programming inlet pressure in gas chromatography permits to decrease the analysis time without changing the elution order of compounds of different polarity whose relative retention changes with changing temperature. The choice of the best values of the inlet pressure and flow-rate of the carrier gas often requires many preliminary analyses with different parameters to be carried out. A method for the prediction of the separation by starting from few experimental data measured in isothermal and isobaric conditions decreases the time required for the optimisation of the analysis. The efficiency of the separation depends on the change of the theoretical plate height at various pressures and temperatures, due to pressure drop along the column. By calculation of the diffusion coefficients of the analysed compounds into the mobile and stationary phase it is possible to evaluate the column efficiency and predict the number of theoretical plates at any inlet pressure. A procedure for the prediction of the plate height of a capillary column at any inlet pressure of the carrier gas and column temperature by using retention data of polar and non-polar compounds (1-alcohols and linear alkanes) obtained in few isobaric runs is described.

Structure-retention correlations of chlorobenzenes as a method for the comparison of the behaviour of gas-liquid-solid and gas-liquid capillary columns of various polarities

Abstract The structure-retention correlations of chlorobenzenes were investigated by using retention data measured on narrow-bore capillary columns: bonded-phase gas-liquid non-polar polydimethylsiloxane, polar polyglycol, carbon and graphite layer gas-liquid-solid modified by addition of various amount of polar liquid phase (polyglycols and terephthalic acid). The retention values obtained in various isothermal runs (ranging from 60 to 200°C) were correlated with the available physical properties data: boiling point, dipole moment, vapour pressure at the analysis temperature and the effect of the reciprocal position of the chlorine atoms on the aromatic ring was evaluated.

Gas chromatographic separation and automatic identification of complex mixtures of organic solvents in industrial wastes

Abstract The analysis of complex mixtures of industrial solvents in chemical wastes was carried out by the simultaneous use of non-polar and polar wide-bore capillary or packed columns. The co-eluting peaks on one of the two phases were resolved on the other phase, and the identification and quantitative analysis of mixtures containing up to 32 compounds was possible. Some integrators and data collectors were tested to allow the automatic identification of the compounds by comparing the chromatograms obtained from a 50:50 split of the sample into two parallel columns connected to identical flame ionization detectors.