Distribution coefficients of n-alkanes measured on wall-coated capillary columns.

Distribution coefficients K of n-alkanes were determined in wide ranges of temperature and carbon numbers from gas chromatographic retention data measured on wall-coated poly(dimethylsiloxane) commercial capillary columns. A discussion is centered on how to mitigate the difficulties for an accurate determination of K when using weakly retentive columns, as those bearing very high phase ratios or short lengths. Particularly, the errors associated with the estimation of the gas hold-up and the phase ratio of the column are considered. The chromatographic importance for determining K of n-alkanes relies on the fact that these are the most commonly applied references for reporting relative thermodynamic parameters such as the Kovats Index and the relative retention. A great amount of information has been compiled in this form. If K of the reference is known, absolute values of distribution coefficients for a myriad of substances are readily obtainable. The knowledge of K(T) functions of solutes in wide ranges of temperature is a primary necessity in temperature-programmed gas chromatography. This knowledge is needed for the prediction of absolute retention times and for computing separation optimizations of mixtures containing several critical pairs of analytes.