Evaluation of Data for the Gas-Liquid Chromatography
For the determination of the following informations are required: the net retention time of the solute, the temperature, column inlet and outlet pressure, the carrier gas flow rate, and the amount of the stationary phase on the solid support. With the help of these experimentally determined parameters the specific net retention volume (Vg0) corrected to 0 °C is obtained. The activity coefficient at infinite dilution is then given by
where R is the general gas constant, ML the molar mass of the solvent, and PiS the saturation vapor pressure of the solute calculated using Antoine constants taken from the Dortmund Data Bank (DDB). It is also possible to take other vapor pressure equations (Wagner, DIPPR, ...) if they show smaller deviations to the experimental vapor pressure data stored in the Dortmund Data Bank. The saturation fugacity coefficient of the solute φiS is calculated with the help of the Soave-Redlich-Kwong equation of state following Gmehling and Kolbe (1992). The required critical data and acentric factors are taken from the DDB. The net retention time is equal to the difference between the retention time of the solute (solute peak) and the dead time (peak caused by air which is injected together with the solute; see also Fig. 2). The net retention time is a measure of the partition of the solute between the gas (mobile) phase and the stationary phase (solvent). This partitioning is a result of phase equilibrium behavior. With the help of the obtained net retention times it is also possible to determine the separation factor at infinite dilution α 12∞ (Fig. 2), which is of great importance for the solvent selection in extraction or extractive distillation processes.
In order to examine the presence of adsorption effects, it is necessary to vary the relative amount of stationary phase (liquid loading) or the sample volume of the injected solutes. The reliability of the obtained by GLC has already been demonstrated by different authors (Gmehling et al., 1994). Some typical values are shown in Fig. 3. More information about GLC and further results can be found by Gruber et al. (1997), Gruber et al. (1998a) and Gruber et al. (1998b).