Analysis of active sites and heterogeneity in commercial reversed-phase octadecylsilanated silica with numerically calculated sorption distributions.

Sorption isotherms spanning six orders of magnitude of pyridine concentration in a 60:40 methanol:water mobile phase adjusted to pH 5 were obtained with the frontal analysis method on three ODS stationary phases: Zorbax Pro-10/150, Vydac 218TPB10, and YMC 120AS10. The data was fit to a heterogeneous Langmuir model in which the association constant, K, is continuously distributed over a finite range of values. The results indicate a small degree of secondary adsorption for all three phases as a separate peak in K-space at higher values of K than the primary hydrophobic partitioning, and additional adsorption at even higher K values for the Zorbax and to a much smaller degree the YMC phase. Integration of the distributions yields the amount of sorption at each of the modeled sites. The results correlate with information known about the synthesis of these phases and the degree of band tailing in elution experiments at these conditions.