Chemical ionization mass spectrometric analysis of lubricating oils and the kinetics of some associated ion-molecule reactions

Abstract

Selection of suitable reagent gases and ion-source pressures can lead to formation of mono-ionic plasmas for chemical ionization. If sample ionization occurs by a single mechanism, discrimination between chemically different components in mixtures is possible. This has permitted the analysis of specific components in lubricating base oils. Ammonia chemical ionization of esters, at ammonia pressures, ca.0.6 Torr, leads only to the formation of the cluster ions (M.NH)+. These conditions have been used to analyse aviation lubricant esters. A method for ion-source pressure determination, of ammonia, directly from the abundance of NHg and NH+ plasma ions was developed from data obtained in a study of ion-molecule reactions consequent upon its electron-impact ionization. The collision-stabilized NO+ ion, generated from nitrogen-nitric oxide mixtures was used to ionize synthetic hydrocarbon lubricants; semi-quantitative determination of components were obtained. Aromatic components in mineral oils, with ionization potentials 9.1 eV, were preferentially ionized by the fluorobenzene molecular ion. The rates of reaction of selected aromatic compounds with fluoro benzene molecular-ion have been determined to permit quantitation. Good agreement with calculated collision rates (ADO) suggests a direct charge transfer mechanism. The dependence of ion-abundances of primary ions derived from several fluorinated benzenes have been investigated at ion-source pressures between 0.01 and 0.25 Torr.