An Automated Method for Chemical Composition Analysis of Lubricant Base Oils by Using Atmospheric Pressure Chemical Ionization Mass Spectrometry.

Since its invention in the 1950s, field ionization mass spectrometry (FI MS) has been, and currently is, the go-to technique employed by the petrochemical industry for the identification of the different types of nonvolatile compounds in their products. Unfortunately, FI MS has several inherent drawbacks, such as poor reproducibility. The performance of positive-ion mode atmospheric pressure chemical ionization mass spectrometry (APCI MS) with O2 gas as the sheath/auxiliary gas and a saturated hydrocarbon solvent/reagent was recently compared with that of FI MS and found to show promise as an alternative, highly reproducible method for lubricant base oil analysis. We report here on the automation of the APCI/O2/saturated hydrocarbon MS method. Isooctane was chosen as the optimal APCI solvent/reagent for base oil ionization due to the low level of fragmentation it provided for model compound mixtures. Three minutes was determined to be the shortest possible cleaning time between samples, regardless of the base oil viscosity. The total analysis time for each sample was 5 min. The reproducibility of the method was assessed by determining within-day and between-day precisions and total precision for hydrocarbon class distributions measured for three different base oils. All total precision values were found to be better than 6.2%, suggesting that the automated (+)APCI/O2/isooctane method is reproducible and robust.