In the case of the failure of library identification, the molecular
ion and isotope ratio of compounds can provide the determination
of the molecular formula. For this reason, CI as a soft ionization
technique has been commonly used to enhance the signal intensity
of molecular ions. Ten n-nitrosamines including deuterated
internal standards were analyzed by using GC/MS both in EI and CI
modes. Apparently, the mass spectrum of NDBA in EI was ambiguous
due to extensive fragmentation, and consequently low
abundance of molecular ion. On the other hand, identification of
the molecular ion of NDBA in CI mass spectrum was clearly con-
firmed by detection of additional methane reagent gas adducts
([Mþ29]þ for [MþC2H5]þ and [Mþ41]þ for [MþC3H5]þ) as
shown in Fig. 4. Individual target nitrosamines including deuterium-labeled
three internal standards were clearly identified in GC/
CI–MS chromatogram by means of their protonated molecular ion
and two adduct ions (supplementary material, Fig. S3).