Effect of methanol concentration on the speed-resolution properties in adiabatic supercritical fluid chromatography

The influence of the modifier concentration in supercritical mixtures of carbon dioxide and methanol on the speed-resolution properties of columns packed with 1.7μm core-shell particles was investigated from a theoretical viewpoint. Molar fractions of methanol up to 30% were considered. The column was assumed to be operated under strict adiabatic conditions in order to maximize its efficiency. Four inlet temperatures were tested, between 297 and 337K. Four different pressure drops along the column were considered, between 25 and 200bar. The physico-chemical properties (density, viscosity, and heat capacity) of the mixtures of carbon dioxide and methanol were derived from the NIST REFPROP program for temperatures between 287 and 337K and pressures between 150 and 390bar. The axial heterogeneity of the column was taken into account by segmenting it into 500 hundreds slices in each of which all physical and chromatographic properties were assumed to be uniform. The apparent kinetic Poppe plots were built from the apparent column efficiency calculated from the sum of the increments of the retention times and time variances from the column inlet to its outlet. The numerical results showed that the axial heterogeneity of the column due to axial variations of the temperature and the equilibrium constant decreases with increasing molar fraction of methanol in the eluent when the pressure drop is increased from 25 to 200bar. The methanol content decreases the speed-resolution of the column more particularly when the analysis is done at low pressure drops. The results demonstrate also that under adiabatic conditions, for pressure drops larger than 100bar, an increase of methanol concentration does not cause a dramatic loss of speed-resolution. For example, at 310K and with a pressure drop of 25bar, the longest column is expected to deliver 17,000 plates; increasing the molar fractions of methanol from 0 to 30% decreases this efficiency by about 15% and increases the analysis time by +140%. In contrast, under the same experimental conditions except for a pressure drop of 200bar, the maximum efficiency (130,000 plates) would remain unchanged and the analysis time increase by only 40%.