A subtraction fitting method for independent determination of enantioselective and nonselective adsorption isotherms based on the single-component isotherms in the framework of the two-site model

Abstract

Based on theory of two-site model, a subtraction fitting method (SFM) is proposed to independently fit enantioselective adsorption sites and the nonselective adsorption sites on the surface of chiral stationary phases (CSP) based on polysaccharide derivatives. Compared with the direct fitting method, the method allows independent fitting the isotherm models of two types of adsorption sites. By the SFM, the adsorption data of methyl mandelate on the CSP based on cellulose tri-(3, 5-dimethylphenylcarbamate) was measured. The adsorption on enantioselective sites was well fitted with Langmuir model, and the adsorption on nonselective sites was well fitted with Tóth model or Langmuir-Freundlich model. The adsorption isotherms of the enantiomers of Corey PG-lactone diol on chiral stationary phase coated with different amounts of amylose tri-(3,5-dimethylphenylcarbamate) (Wchiral selector: Wsilica = 20 : 100, 30: 100 and 40: 100, respectively) were also studied. It was found that the saturated adsorption capacities of both sites increase with the increase of the chiral selector loading. The chiral stationary phase with low chiral selector loading (Wchiral selector: Wsilica = 20 : 100) has a high true separation factor (αtrue = 16.8) and the lowest apparent separation (αapp = 1.70) due to the large nonselective adsorption, which indicates that a chromatographic support with an inert surface is important for this type of CSP.