Ammonia laser desorption/chemical ionization with ammonium bromide: Fourier transform ion cyclotron resonance mass spectrometry of aromatic hydrocarbons

Laser desorption/ionization combined with Fourier transform ion cylcotron resonance mass spectrometry (LD/FT/ICR/MS) is a proven technique for the analysis of nonvolatile materials. Unfortunately, LD tends to produce a large excess of neutral species compared to ions. Laser desorption followed by chemical ionization (LD/CI) by use of a reagent gas is a seIective and sensitive means of control in the analysis of nonvolatile compounds. In this article we demonstrate the technique of ammonia LD/CI by addition of a small amount of ammonium bromide (NH₄Br) to an involatile sample, i.e., the ammonium salt is used in place of ammonia reagent gas. For various aromatic hydrocarbons, abundant (M + H)⁺ ions are produced as a result of CI A primary advantage of this method in FT/ICR/MS is that selective LD/CI experiments may be conducted at low pressure as in pulsed valve CI (but without the need for pulsed valve operation), thereby providing the potential for obtaining high-resolution FT/ICR mass spectra.