Analysis of metal dialkyldithiophosphate additives in lubricating oil by MALDI mass spectrometry

Abstract

Reaction of zinc dithiophosphate with dimethylsulfoxide was used to analyze zinc dialkyldithophosphates and diaryldithiophosphates lubricating oil additives by matrix assisted laser desorption ionization mass spectrometry (MALDIMS) analysis. The MALDI-MS analysis was carried out by mixing the [Zn(DTP)(DMSO)[subscript 2]][superscript +] with various type of matrix such as 2,5-dihydroxybenzoic acid, dithranol and alpha-cyano-4-hydroxycinnamic acid. The mixture of [Zn(DTP)(DMSO)2]+and matrix was applied to the multiprobe and bombarded with laser to genarate ionic analyte. The following MALDI-MS conditions were found to provided the good MALDI-MS spectrum of [Zn(DTP)(DMSO)[subscript 2]][superscript +] : dithranol as a matrix with tetrahydrofuran as solvent, the ratio of [Zn(DTP)(DMSO)[subscript 2]][superscript +] : matrix was 1 : 75 by volume and laser power was 187 [micro]J. The MALDI-MS spectrum of zinc dialkyldithiophosphates showed the peaks between m/z 517 and 615 and zinc diaryldithiophosphates showed the peaks between m/z 697 and 795. The MALDI-MS analysis of [Zn(DTP)(DMSO)2][superscript +] provided information on the carbon number of the two alkyl chains (R1+R2) which is an individual fingerprint of zinc dithiophosphates for any given commercial lubricating oils. Therefore, the developed MALDI-MS was used to analyze zinc dithiophosphates in commercial lubricating oils.