Identification and Quantitation of Linear Alkanes in Lubricant Base Oils by Using GCxGC/EI TOF Mass Spectrometry

Linear alkanes are a class of compounds known to negatively affect the physical performance of lubricant base oils. The ability to rapidly identify and quantify linear alkanes in lubricant base oils would enable oil companies to more effectively evaluate their refinery methods for converting crude oil to lubricant base oils. While mass spectrometry is a powerful method for elucidation of the structures of compounds in complex mixtures, it is not innately quantitative. An approach is presented here for the identification and quantitation of linear alkanes in base oil samples by utilizing GCxGC/EI TOF MS. Identification of the linear alkanes in base oils was achieved based on their retention times in both GC columns as well as their EI mass spectra. Linear alkane model compound mixtures were used to generate calibration plots for quantitation of the linear alkanes in the base oils. The accuracy of this method was greater than 83.8%, within-day precision lower than 6.2%, between-day precision lower than 16.2%, and total precision lower than 17.2%. All noted figures of merit surpass the acceptable limits for a new validated quantitative method, where accuracy must be better than 80% and precision lower than 20% at the lower limit of quantitation. The n-alkane content in both base oil samples was further validated using a GCxGC/FID method (the gold standard for quantitation), which provided nearly identical results to those obtained using the GCxGC/EI TOF MS method. Therefore, GCxGC/EI TOF MS can be used to both identify and quantitate linear alkanes.