Some aspects of additives effects on retention in supercritical fluid chromatography studied by linear free energy relationships method.

This work is dedicated to the investigation of additives effects on retention mechanisms in supercritical fluid chromatography. Additives are compounds which are added to the mobile phase in small quantities and greatly affect retention factors, peak shape, separation selectivity and other chromatographic parameters. Linear free energy relationship (LFER) method with an expanded set of descriptors including the ones taking ionic interactions into account was used to probe the effect of four types of additive: trifluoroacetic acid, diethylamine, ammonium acetate and water - on retention on four polar stationary phase bearing different functional groups: bare silica, cyano, 2-ethylpyridine and zwitter-ionic sulfobetaine. Effects of all additives were shown to be complex, involving different intermolecular interactions and not uniform. The direction and magnitude of retention change depends not only on additive concentration, but on all other component of a chromatography system: type of a stationary phase, mobile phase composition and the nature of the solute. Various subtle effects were registered, the most peculiar among them being the fact that diethylamine and ammonium acetate provide identical changes of LFER constants. We hypothesize that this is caused by the convergence of diethylamine into a methylcarbamate via reaction with carbon dioxide and methanol, which then behaves as a salt additive.