DETERMINATION OF N-NITROSODIMETHYLAMINE IN COMPLEX ENVIRONMENTAL MATRICES BY QUADRUPOLE ION STORAGE TANDEM MASS-SPECTROMETRY ENHANCED BY UNIDIRECTIONAL ION EJECTION

A gas chromatography (GC)/tandem mass spectrometry method using a quad rupole ion storage mass spectrometer (QISMS, trademark of Varian Assoc iates Inc.) operated in chemical ionization (CI) mode has been develop ed for the determination of N-nitrosodimethylamine (NDMA) in complex e nvironmental matrices. Using a customized scan function, the method al lows for the simultaneous storage of [M + H](+) ions of both NDMA and NDMA-d(6). Collisionally activated dissociation of both native and deu terated [M + H](+) ions, carried out by consecutive resonant excitatio n of each isolated ion species, permits daughter ion quantitation base d on an internal standard. The method is capable of subpicogram detect ion limits when unidirectional ejection of stored daughter ions from t he ion trap toward the electron multiplier is effected by the superimp osition of a dipole held applied across the end-cap electrodes. Enhanc ed sensitivity was obtained by operating the electron source at a high er than recommended filament emission current, and by setting the elec tron multiplier to a voltage greater than that required for a gain of 10(5). Background interferences were eliminated via the implementation of a low-mass radio frequency (rf) sweep and positive direct current (dc) amplitude application during [M + H](+) isolation. In CI mode wit h automatic reaction control (ARC) disabled, linear calibration plots were obtained over a concentration range of 0.5-128 pg. In contrast, w hen ARC is enabled, calibration plots over a concentration range 0.5-2 000 pg are characterized by polynomial curve-fitting equations. Concen trations of NDMA in aqueous extracts were found to be comparable to th ose obtained by high-resolution mass spectrometry. Interferences such as chlorobenzene, ethylbenzene, and the o-, m-, and p-xylenes, reporte d when NDMA concentrations are determined by gas chromatographic separ ation followed by low-resolution mass spectrometry, were not detected using the GC/QISMS protocol.