Comparative study on retention behaviour and enantioresolution of basic and neutral structurally unrelated compounds...

A comparative study on the retention behaviour and enantioresolution of 54 structurally unrelated neutral and basic compounds using five commercial cellulose-based chiral stationary phases (CSPs) and hydro-organic mobile phases compatible with MS detection is performed. Four phenylcarbamate-type cellulose CSPs (cellulose tris(3,5-dimethylphenylcarbamate), Cell1; cellulose tris(3-chloro-4-methylphenylcarbamate), Cell2; cellulose tris(4-chloro-3-methylphenylcarbamate), Cell4 and cellulose tris(3,5- dichlorophenylcarbamate), Cell5) and one benzoate-type cellulose CSP (cellulose tris(4-methylbenzoate), Cell3) are assayed. Mobile phases consist of binary mixtures of methanol (30-90% MeOH) or acetonitrile (10-98% ACN) with 5 mM ammonium bicarbonate (pH = 8.0). The existence of reversed phase (RPLC) and hydrophilic interaction liquid chromatography (HILIC) retention behaviour domains is explored. In MeOH/H₂O mobile phases, for all compounds and CSPs, the typical RPLC retention behaviour is observed. When using ACN/H₂O mobile phases, for all compounds in all CSPs (even in the non-chlorinated CSPs) a U-shaped retention behaviour depending on the ACN/H₂O content is observed which indicates the coexistence of the RPLC- (< 80% ACN) and HILIC- (∼80-98% ACN) domains. The magnitude of retention changes in both domains is related to the hydrophobicity of the compound as well as to the nature of the CSP. The study of the effect of the nature and concentration of the organic solvent, as well as the nature of the CSP on the enantioresolution reveals that: (i) the use of MeOH/H₂O or ACN/H₂O greatly affects the enantioselectivity and enantioresolution degree of the chromatographic systems, being, in general, better the results obtained with ACN/H₂O mobile phases. (ii) The ACN-RPLC-domain provides much better enantioresolution than HILIC-domain. (iii) Cell2, especially with ACN/H₂O mobile phases, is the CSP that allows baseline enantioresolution for a higher number of compounds. (iv) Phenylcarbamate-type CSPs do not offer clear complementary enantioselectivity to that of Cell2. (v) Cell3 is the only CSP that provides marked complementary enantioselectivity to that of Cell2, almost orthogonal in MeOH/H₂O mobile phases.