Characterization of neutral fragments in tandem mass spectrometry: a unique route to mechanistic and structural information.

The neutral species eliminated upon fragmentation of fast-moving mass-selected ions can be directly identified by collisional ionization and detection in neutral fragment reionization (Nf R) mass spectra. Establishment of the identity of neutral fragments yields valuable insight into the decomposition mechanism of a precursor ion, as demonstrated for fullerene and alkali metal iodide cluster ions as well as metal ion adducts of amino acids. In addition, neutral fragment reionization also provides structural information that may not be available from the complementary ionic fragments alone; this is illustrated in the differentiation of isomeric mononucleotides. The parameters influencing the appearance of Nf R spectra are discussed and the scope and general applicability of the method are briefly evaluated.