A technique for the analysis of protein sequence and structure. A high energy beam of atoms or ions (Cs or Xe) vapourizes, fragments and ionises a protein solution, the mass spectrum of which gives details of the peptide sequence and post-translational modifications such as N-terminal acylation, glycosylation, phosphorylation and disulphide bridging. Flow FAB is a variant that analyses the continuous liquid stream from a high pressure liquid chromatography or capillary electrophoresis separation.