Abstract. A small percentage (0.3 % w/w) of formic acid was added to supercritical carbon dioxide to increase mobile phase polarity and improve separation of polar solutes on three different commercially available capillary columns. Formic acid reduced retention and altered selectivity for several polar compounds. Thermodynamic studies revealed that the enthalpy of solvation was as much as 27 kcal mol-' more negative when formic acid was added due to increased solute-solvent interactions. Entropy measurements show that formic acid modified mobile phase is more ordered than a pure carbon dioxide mobile phase, apparently as a result of modifier clustering around polar solutes. Van't Hoff plots for some very polar solutes showed a region of retrograde behavior at low temperatures and pressures, indicating an increase in modifier clustering. Comparisons between pure and modified mobile phases were also made by examining practical separations obtained with each, revealing differences in retention, selectivity and peak shape.
A modified simplex algorithm has been used to optimize supercritical fluid chromatography (SFC) separations of samples containing nonhomologo us, nonollgomerlc components. Short capillary columns are employed In the initial separations, with a transfer to a longer column for greater efficiency as needed.Two chromatographic response functions, based on peakvalley ratio or threshold resolution criteria, were found to be suitable for SFC. Two-and three-variable slmplexes utilizing (I) density gradient rate and temperature, (II) simultaneous pressure and temperature gradient rates, or (ill) Initial density, density gradient rate, and temperature provided good results for the samples in this study. Convergence to the global optimum was shown for case I by restarting the simplex In another part of the parameter space. A synthetic mixture of three difficult-to-separate sesquiterpene lactones was separated by optimization on a short column using the three-parameter simplex and then transferring the method to a longer column.
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