The model of retention in HPLC with binary mobile phase

The retention model in HPLC is proposed, which is based on quasichemical equilibria of the components of the chromatographic system. This model provides a better undestanding of the retention mechanism in HPLC and it can be applied for quantitative description of the dependence of retention on the nature and composition of the mobile phase. H i g h performance liquid chromatography (HPLC) is known to be one of the most effective methods for analysis the substance mixtures of complex composition. A great number of various organic substances of different nature and properties required various technical approach and instrumentations to separate complex mixtures, identify and determined their components. The choice of the appropriate chromatographic techniques required their optimization based on understanding the retention mechanisms and molecular interactions of the chromatographic system components. In HPLC, along with the sorbate-sorbent molecular interactions, of great importance are also the sorbate-mobile phase, sorbent-mobile phase interactions, in particular, the interactions between the components of binary, ternary and other mobile phases. At present, it is often impossible to compute quantitatively the thermodynamic characteristics of retention in these complicated systems. Further development of the HPLC theory is largely contributed by the determination of correlation dependences between the retention parameters and various properties of sorbates (S) (ref. I + ) , mobile (ref. 3-8) and stationary phases (ref. 9-12). To describe the retention mechanism of substances in most often used variants of HPLC, namely, normal-phase (NP HPLC) and reverse-phase (RP HPLC), it must be used the common approach based on the distribution of substances (sorbates) in dynamic equilibrium between the mobile and stationary phases. This approach provides a common standing for studying these variants of HPLC. Now consider the system most frequently used in HPLC: the stationary phase (SP), sorbate (S) and the two-component mobile phase (Mp) consisting of the solvent (L) as the basic component and the modifying additive (M). As a rule, RP HPLC includes: L water, M organic solvent (methanol, acetonitrile, tetrahydrofuran and other) and NP HPLC includes: L alkane, M weak polar or polar organic solvent (methylenechloride, chloroform, alcohols, etc.). In this case, sorption (distributiyn) from the three-component solution (L,M,S) is exhibited, and L < M >>S.