Distinction of α- and β-aspartyl and α- and γ-glutamyl peptides by fast atom bombardment/tandem mass spectrometry

A fast atom bombardment/collisional activation/linked‐scan at constant B/E (tandern mass spectrometric) method is described which can distinguish between α‐ and β‐aspartyl and α‐ and μ‐glutamyl underivatized peptides. The method is based upon differences in loss of CO from aspartyl or glutamyl B‐fragment ions (IB) in these isomers which are rationalized from the stability of the resultant A‐fragment ions (IA). It was observed that the ratio of IB:IA which was used in this determination was dependent upon the collision cell pressure. The higher the collision cell pressure, the larger the difference between the IB: IA ratios for these linkages.