Determination of zinc O,O'-dialkyl dithiophosphate lubricant additives via the corresponding methyl and p-nitrobenzylic ester derivatives using GC-MS, GC-NPD and HPLC-MS

A quantitative fingerprinting of automotive lubricants with respect to zinc dialkyl dithiophosphates (ZDP), major anti-wear/antioxidant additives, is presented. ZDPs in lubricant solutions are converted into the corresponding methyl and p-nitrobenzylic esters, respectively. After removal of the lubricant matrix the methyl esters are submitted to gas chromatography (GC). Mass spectrometric detection (MS) and comparison with reference methyl esters enable the characterisation of practical ZDP mixtures with respect to alkyl chain length and isomery of the single components. Overall recovery rates are higher than 90% and phosphorus-selective detection (NPD) allows a quantitative determination down to 0.1 pg/mu l. The p-nitrobenzylic esters may be analysed by HPLC. Identification and quantification is performed by on-line HPLC-UV and HPLC-MS (APCI) with a determination limit of 20 pg/mu l. The ZDP quantification via the methyl esters is applied to seven lubricants from the German market. The method is applicable to used oils allowing the monitoring of ZDP consumption during engine operation.