Effect of temperature and organic modifier on the isocratic retention characteristics of nitrated polycyclic aromatic hydrocarbons on a reversed-phase octadecylsilane column

Abstract

Nitrated polycyclic aromatic hydrocarbon (nitro-PAH) retention behavior was evaluated on a reversed-phase polymeric octadecylsilane column under isocratic conditions as a function of organic modifier and temperature. Several conclusions can be drawn from the retention and organic modifier study ( viz., methanol and acetonitrile—water). First, the logarithm of the capacity factor (log k′) was linearly dependent on organic—water volume fraction (ϕ). Second, nitro-PAH k′ values increased with decrease in organic modifier concentration. Third, the slopes of log k′ vs. ϕ plots were solvent-dependent. Larger slope values were found for nitro-PAHs in methanol—water than in acetonitrile—water mixtures for a particular compound. Fourth, slope values were dependent on the molecular structure of individual nitro-PAHs. Nitro-PAH retention times decreased with increase in column temperature. A linear dependence of log k′ on the reciprocal of the absolute column temperature, the Van `t Hoff plott, was observed for both organic modifiers over the temperature range studied. The standard enthalpic range (Δ H 0) for nitro-PAH transfer from the mobile to the stationary pahse was determined. Δ H 0 was dependent on organic modifier tupe and composition as well as solute structure. The enthalpy—entropy compensation effect was evaluated by plotting log k′ vs. — Δ H 0 and used to interpret nitro-PAH retention mechanisms. A compensation effect was found for some mononitrated PAHs for both organic modifiers. The compensation temperatures were within the accepted range described for reversed-phase high-performance liquid chromatography systems.