Separation of zinc dialkyldithiophosphates in lubricating oil additives by normal-phase high-performance liquid chromatography

Eleven zinc dialkyldithiophosphates have been synthesised and their mixtures separated by normal-phase HPLC on a 5-μm silica column. Dichloromethane- or heptane-based eluents containing alkylamine (⩽0.4%) and acetic acid in a volume ratio of 2:1 and 1% methanol were suitable for separation of mixtures of these metal complexes in commercial lubricating oil additives. In all cases tested, the zinc complexes eluted from the column without decomposition and there was no evidence for ligand-exchange reactions having taken place during synthesis or on the column.