## Role of the Stationary Phase

The stationary phase influences retention in SFC, as in all chromatography, through interactions with the solute: dispersive, dipole-dipole, dipole-induced and hydrogen-bonding acidic and basic interactions are all possible and retention can be modelled by means of solubility parameters, respectively ¿d, ¿0, ¿m, ¿a and ¿b. Retention is then described by:

where V; is the solute molar volume and a-f are weighting factors for given stationary and mobile phases at a given temperature.

The coefficients a-f for each combination can be deduced by means of values of k for probe solutes for which solubility parameters and molar volumes are known. These coefficients then allow retention to be predicted for other solutes, solubility parameters of which have been calculated. In fact, the five individual solubility parameters can be calculated from refractive indices, dipole moments and energies of hydrogen bonding, with calibration using compounds for which relevant data are available. With values for the coefficients a-f and the solubility parameters for a given compound, k for a given mobile and stationary phase is then obtained from eqn [9].

Retention factors and solubility parameters for each probe molecule, stationary phase and mobile phase are combined into an expression of the form of eqn [9]. The series of simultaneous equations is then solved, using a matrix, to give values of the coefficients a-f for each stationary/mobile-phase combination. The coefficients vary greatly on moving from pure CO2 to 1% modifier, thereafter showing a more gradual variation with increasing modifier content in the mobile phase; this observation is consistent with the formation of a thin layer of modifier on the stationary phase. The coefficients a-f are a measure of the ability of the stationary phase to take part in the five different types of molecular interaction.

Use of the coefficients a-f, solubility parameter and molar volume substituted into eqn [9] yield predicted k values in fair agreement with experimental data. Correspondingly, solubility parameters for compounds for which they are not otherwise available may be obtained. The model has also been extended to difunctional compounds.

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