S. E. Unger

Detection limits for organic salts in secondary ion mass spectrometry

Abstract Trimethylisopropylammonium iodide is detected at the picogram level by secondary ion mass spectrometry (s.i.m.s.). The intact organic cation can be monitored; the effects of experimental conditions on its abundance are discussed. Mixtures of organic salts can be analyzed without matrix effects at 1:100 weight ratios.The effects of other matrices and the mechanism of sputtering are considered. S.i.m.s. provides greater sensitivity for direct ionization of polyatomic salts than for cationization of molecules.

Negative and positive secondary ion mass spectra of organic acids

Abstract Organic compounds can be ionized by sputtering the solid sample. The resulting negative and positive secondary ions provide mass spectra which characterize both the molecular weights and the structures of the compounds. Ionization occurs either by direct ejection of charged species from the solid into vacuum or by electron or proton transfer. The sputtered secondary ions dissociate unimolecularly to give fragment ions. These reactions are identical to those which occur when the secondary ions are independently generated by chemical ionization, selected by mass and dissociated in a high energy gas phase collision. The negative ion SIMS spectra show molecular ions (M −. ) or (M-H) − ions as the dominant high mass species together with fragments due to decarboxylation, dehydration and losses of other simple molecules. Stronger acids show larger (M-H) − /M −. abundance ratios. The positive ion spectra are complementary and also useful in characterizing molecular structures. Attachment of cations to organic molecules (cationization) occurs much more readily than anion attachment and this makes negative SIMS spectra simpler than these positive ion counterparts.