Effect of Temperature on HPLC Separations Using C₁, C₄, C₈, and C₁₈ Alkyl Chain Bonded Silica Columns

Abstract

The effect of temperature on retention In reversed phase high performance liquid chromatography was studied using a C₁, C₄, C₈ and C₁₈ alkyl chain bonded columns and four groups of compounds: (1) anthraquinone, methyl anthraquinone and ethyl anthraquinone; (2) naphthalene and biphenyl; (3) dimethyl phthalate and diethylphthalate, and (4) p-nitroaniline and caffiene. The results indicated that the mechanism of separation using the four groups of solutes and the four columns at temperature ranges of 0°C to 77°C is the same and that a linear relationship was obtained for a plot of lnk' vs 1/T. Enthalpy values were observed in the following orders C₁₈<C₈ = C₄<C₁ depending on the solutes studied. The results also showed that a for a pair of adjacently eluting solutes decreased by approximately 10% with increase in temeperature from 30°C to 70°C. The greatest decrease in α (25%) was observed between 0°C and 27°C. It was also observed that in order to achieve the same retention time on all the columns for a certain compound, the volume of organic modifier should be different for each column, C₁₈<C₈<C₄<C₁. Finally, the best separation was obtained at lower rather than higher temperature.