Retention of sulphur and sulphur organics in reversed-phase liquid chromatography

Abstract

In reversed-phase systems with purely or mainly methanolic eluents, sulphur homocycles S n exhibit clearly higher retention than n-alkanes or cycloalkanes that have the same skeleton atom number ( n c + n s). If normalized to equal surface area values, the retention differences is even higher. It has to be concluded that alkanes interact more strongly with the methanol eluent than the (proton-free) sulphur rings. This is reflected by a more negative sorption enthalpy and a more positive sorption entropy of sulphur rings. With more water in the eluent, the retentions of alkanes and sulphur rings approach each other, because alkane—eluent interactions are suppressed by solvophobic forces. The retention behaviour of polysulphanes H 2S n is very similar to that of sulphur rings. Thia substitution in alkanes, yielding thiols, thioethers, or multisulphides, invariably lowers retention, to a degree that depends on the number of nonvicinal substitution sites and their relative locations in the molecule. In a eluent of pure methanol the retention loss caused by thiol formation is ca. 200 index units; the thioether function is somewhat more effective. The retention decrease is attributed to the formation of local polar centres, which are surrounded by “solvation patches”. Among aromatic systems, thia substitution causes some retention increase rather than decrease. In aliphatic polysulphides, both retention loss due to the formation of local polar centres and retention increase due to the presence of undisturbed sulphur atoms, can be observed. The former effect is predominant in short-chain polysulphides, the latter in solutes with long sulphur chains.