Paper spray PS ambient ionization is implemented using a portable mass spectrometer and applied to the detection of alkyl quaternary ammonium salts in a complex oil matrix. These salts are commonly used as active components in the formulation of corrosion inhibitors. They were identified in oil and confirmed by their fragmentation patterns recorded using tandem mass spectrometry MS/MS. The cations of alkyl and benzyl-substituted quaternary ammonium salts showed characteristic neutral losses of C_nH_2n (n carbon number of the longest chain) and C₇H₈, respectively. Individual quaternary ammonium compounds were detected at low concentrations (<1 ng μL ⁻¹) and over a dynamic range of [similar]5 pg μL⁻¹ to 500 pg μL⁻¹ ppb. Direct detection of these compounds in complex oil samples without prior sample preparation or pre-concentration was also demonstrated using a home-built miniature mass spectrometer at levels below 1 ng μL⁻¹.