Mass spectral fragmentation of perfluoroacyl derivatives of half nitrogen mustards for their detection by gas chromatography/mass spectrometry.

Analytical methods for the detection and identification of half nitrogen mustards (halfNMs), i.e., partially hydrolyzed products of nitrogen mustards (pHpNMs), using silyl derivatives are often associated with low sensitivity and selectivity. In order to overcome these limitations, the derivatization of halfNMs was performed using perfluoroacylation. Two efficient derivatization techniques using trifluoroacetyl (TFA) and heptafluorobutyryl (HFB) groups were developed for the unambiguous identification of halfNMs. A mass spectral database was generated by performing gas chromatography/electron ionization mass spectrometry (GC/EI-MS) and gas chromatography/positive chemical ionization mass spectrometry (GC/PCI-MS). The fragmentation pathways were studied by tandem mass spectrometry (MS/MS) in both EI and PCI mode. The EI-MS spectra of the TFA and HFB derivatives of halfNMs contain intense molecular ions and fragment ions, thus making perfluoroacylation preferable to silylation. In addition, the background-free chromatogram obtained using these derivatives provides unambiguous identification of these compounds in blind samples. The structures of the fragment ions were postulated, and the sources of significant ions were traced by performing MS/MS precursor ion scans. In the PCI-MS spectra, along with the protonated molecule, significant peaks due to neutral losses of HF, HCl, CH3 Cl and CF3 COOH were observed. This is the first report of the elucidation of the fragmentation pathways of perfluoroacyl derivatives of halfNMs. The complementary GC/PCI-MS and GC/PCI-MS/MS data will be helpful in the identification of unknown metabolites in a fast and reliable fashion.