Capillary electrophoresis in the analysis and characterisation of lubricant additives.

Capillary electrophoresis (CE) has proved to be a very efficient separation method in aqueous media. This work reports developments in nonaqueous capillary electrophoresis (NACE) for samples from the lubricants industry.Two commonly used additives in lubricants, zinc dialkyldithiophosphate and the metallic detergent alkylsalicylate, were analysed. Method development included selection of solvent, pH, and electrolyte type and concentration.Improvements in the separation were obtained by altering the injection conditions and the electric field strength. Hexadimethrine bromide (HDB) was shown to be essential in controlling the electroosmotic flow (EOF), and its concentration was optimised. Best separation was seen when analyte dilution prior to injection was carried out with methanol, resulting in sample stacking.Analysis of ZDDPs showed that separation occurred through migration of free dithiophosphate ions. Calcium alkylsalicylate showed similar results where the free alkylsalicylate ions migrated. Identification was achieved through peak patterns and relative migration times.Adequate separation conditions were identified and then coupling of NACE to mass spectrometry was attempted. Direct infusion MS was first utilised with great success in identification of the two additives. Using a solution of methanol and ammonium acetate it was possible to identify free dithiophosphate ions from the ZDDP and free alkylsalicylate ions from magnesium alkylsalicylates. Only the molecular ions were obtained in all instances making identification easier.NACE-MS was carried out successfully; however, sample dilution and adequate interfacing were problematic.Analysis of formulated lubricating oil was performed by NACE, NACE-MS and MS. Degradation products were identified for ZDDPs, where sulphur atoms were replaced with oxygen leading to the formation of phosphates.NACE has been shown to be a technique of potential for the analysis of lubricant additives, and in combination with mass spectrometry it could be very powerful.