Negative ions

Abstract

During the last few years, improvements in ionization techniques have revived interest in negative ion mass spectrometry and its applications. Ion sources that operate at pressures of up to 1 Torr (133 Pa) produce high concentrations of low energy electrons which may react directly with suitable sample molecules to form negative molecular and fragment ions. Alternatively, a reagent gas may be used to generate negative chemical ionization mass spectra which, in general, exhibit less fragmentation than many positive chemical ionization mass spectra. The sensitivity of the electron attachment processes may be very high for compounds containing electronegative elements and it may be enhanced for other compounds by making use of huorinated derivatives. Under chemical ionization conditions, reagent ions which react as Bransted bases frequently give [M —H] - ions in great abundance. Use of selected ion monitoring leads to sub-picogram detection levels in favourable cases. The gas phase ion chemistries of such ions as O -. and O 2 -. exhibit several unusual types of reaction which reveal further structural and mechanistic information, examples of which are discussed.