Analysis of TOF-SIMS spectra using quaternary ammonium ions for mass scale calibration

A novel method with quaternary ammonium salts as internal additives has been applied to the mass scale calibration of time-of-flight secondary ion mass spectrometry (TOF-SIMS). Five kinds of quaternary ammonium salts, octyltrimethylammonium bromide (C8TMA), tetradecyltrimethylammonium chloride (C14TMA), octadecytrimethylammonium chloride (C18TMA), cetylpyridinium chloride (CPC) and benzyldimethyltetradecylammonium chloride (Bzl) were diluted with distilled water and conc-NH3 and were mixed in equal quantities. The solution was added to Tinuvin 770 coated on a Si wafer. Then, the sample was measured by TOF-SIMS. The relation between the relative mass accuracy of molecular ions of Tinuvin 770 and the combinations of ions selected for the mass scale calibration was investigated. To improve the mass accuracy of the molecular ion of Tinuvin 770, ions should be selected for the mass scale calibration in the following two ways: one way is for ions to consist only of molecular ions of C8TMA, C14TMA and C18TMA, and the other way is for ions to include the molecular ion of C18TMA, along with the CXHY fragment ions. These results support the proposition of ISO 13084. The novel method with internal additives is effective to improve the mass accuracy of high-mass ions. Quaternary ammonium salts are potential candidates of internal additives.