Abstract
The elucidation of the detailed mechanism of retention in supercritical fluid chromatography (SFC) is a complex problem, especially for high‐density supercritical fluid mobile phases. Martire and Boehm developed a theory of SFC retention based on a statistical thermodynamic treatment, and introduced a density‐squared equation for retention in SFC: In k = a + b/T + cρ + dρ/T + eρ^2^/T. In this paper the systems n‐alkane‐polydimethylsiloxane‐CO~2~ and n‐carboxylic acid methyl ester‐polydimethylsiloxane‐CO~2~ have been treated using the method introduced by Martire and Boehm. In the ranges of reduced temperature from 1.079 to 1.293 and reduced SF mobile phase density from 0.705 to 1.632, two problems were discovered in the treatment of SFC retention behavior by the method of Martire and Boehm: (1) the experimental value of the site of CO~2~ in the lattice field, r~a~, of the two systems was ca 0.9, which is obviously different from the theoretical values of 3.6 or 5.4; and (2) the difference between experimental and calculated retention values increased with increasing SF mobile phase density. The minimum in SFC retention predicted by Martire and Boehm was not observed. On the basis of some assumptions and the experimental data, the following retention equation was derived: In k = a + b/T + cρ + dρ/T + eρ^2^/T + fρ^3^/T + gρ^4^/T. This equation was consistent with the experimental data and can conveniently be used to explain other SFC retention behavior.