Characterization of a range of alkyl-bonded silica HPLC stationary phases: Chromatographic behavior of neutral, acidic, and basic test solutes

Abstract

The effect of the chain length (C1-C18) of n-alkyl bonded-phase silica packing materials on retention behavior is investigated using three series of substituted test solutes: biphenyls (neutral), barbiturates (weakly acidic), and pyridines (weakly basic). An excellent correlation between the capacity factor (k) and alkyl chain length is observed in the majority of cases. However, deviations from this relationship occur when the alkyl chain length is increased beyond C8 for certain groups of solutes or when there is significant solute interaction at secondary sites on the packing material. Log k plots showed that the neutral and barbiturate solutes have similar retention mechanisms at all the alkyl chain lengths. However, the basic solutes show significant evidence of secondary interactions, which are confirmed by the peak asymmetry data and the lack of correlation between the log10 octanol-water partition coefficient (log P) values of the solutes and retention. There is a significant trend towards improved peak symmetry for the basic solutes when the packing materials having the highest alkyl surface density (C1, C2, and C3 alkyl chain lengths) are used; this suggests that residual silanols are more effectively masked by the shorter alkyl chain lengths. The neutral biphenyl solutes show a progressive increase in efficiency as the chain length increases, but consistent trends are not observed for the acidic and basic solutes. The results show that the length of the bonded alkyl chain is of primary, though not sole, importance in governing the retention of acidic, basic, and neutral compounds on a series of monofunctional n-alkyl-bonded reversed-phase packing materials. Retention mechanisms that might explain the observed separation behavior are discussed.