RETENTION IN SFC AT HIGH-DENSITY

The elucidation of the detailed mechanism of retention in supercritica l fluid chromatography (SFC) is a complex problem, especially for high -density supercritical fluid mobile phases, Martire and Boehm develope d a theory of SFC retention based on a statistical thermodynamic treat ment, and introduced a density-squared equation for retention in SFC: In k = a + b/T + c rho + d rho/T + e rho(2)/T, In this paper the syste ms n-alkane-polydimethylsiloxane-CO2 and n-carboxylic acid methyl este r-polydimethylsiloxane-CO2 have been treated using the method introduc ed by Martire and Boehm, In the ranges of reduced temperature from 1.0 79 to 1.293 and reduced SF mobile phase density from 0.705 to 1.632, t wo 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 CO2 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 retenti on values increased with increasing SF mobile phase density, The minim um in SFC retention predicted by Martire and Boehm was not observed, O n the basis of some assumptions and the experimental data, the followi ng retention equation was derived: In k = a + b/T + c rho + d rho/T e rho(2)/T + f rho(3)/T + g rho(4)/T. This equation was consistent wit h the experimental data and can conveniently be used to explain other SFC retention behavior.