Use of chromatographic retention measurements to obtain solubilities in a liquid or supercritical fluid mobile phase
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A thermodynamic analysis is given of the relationship between the capacity factor in high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) and the solubility of the solute in the mobile phase. The analysis shows that in certain circumstances, where particular assumptions are valid, there is a simple inverse relationship between the two quantities. In these circumstances the proportionality constant has a fixed value for a particular column, temperature and solute, and is independent of the mobile phase. Furthermore, the logarithm of the constant is approximately linearly related to the inverse of the absolute temperature over a limited temperature range. Some experimental studies are then described which show that practical conditions exist where these relationships are valid. It is shown that the same equation-of-state, with the same parameters, can be used to predict quantitatively both supercritical fluid solubilities and the temperature dependence of retention in SFC. Experimental results illustrate the inverse relationship between solubility and retention where the same column was used in both SFC and HPLC. Results are also given for the variation of the proportionality constant with temperature. The various ways in which these relationships could be used to measure solubilities are discussed and examples of experimental measurements are given for some methods.