Retention properties of the fluorinated bonded phase on liquid chromatography of aromatic hydrocarbons

The unique characteristics of a bonded-fluoroalkylsilane column, Fluofix, are described by comparison with those of a bonded octylsilane column, C8, in the reversed-phase chromatography of aromatic hydrocarbons. The selectivity of homologous aromatics on the fluorinated column varied with the methanol concentration in the eluent. At a larger proportion of methanol than 80:20 (v/v) the larger aromatics eluted faster than the smaller ones. However, with less methanol, the aromatics were eluted in order of their molecular size, as usually reported for a conventional hydrocarbonaceous column. A dependence of the retention mechanism on the composition of the eluent was suggested by the decrease in the entropy-enthalpy compensation temperature, beta, with increase of methanol concentration in the eluent. In order to explain the concentration dependence of the retention, the molecular interaction energy in the retention process was calculated by computer simulation. The interaction energy between aromatics and the stationary ligand on the Fluofix column was smaller than that on the C8 column and comparable to the interaction energy between the aromatics and methanol. At higher methanol concentrations, solute-fluorinated ligand complex formation was obstructed by the methanol molecules solvating the solute, reducing the retention of the larger aromatics.