Positive and negative ion chemical ionization mass spectra of amino acid carboxy-n-butyl EsterN-Pentafluoropropionate Derivatives

Abstract

The positive and negative chemical ionization mass spectra of 29 amino acid carboxy-n-butyl ester N-(O,S)-Pentafluoropropionate derivatives hae been determined. The positive ion (PI) mass spectra conform to the expected fragmentation patterns for this amino acid derivative but their methane negative ion chemical ionization (NICI) mass spectra displayed marked differences between various amino acid classes and bvetween positional isomers within the same class. Isomeric butyric acid (α, β and γ) and tyrosine (o, m amd p-hydroxylation) derivatives all had remarkably difterent NICI mass spectra. Aliphatic amino acid CO2-n-butyl-N-PFP derivatives (except glycine) can be readily recognized by a characteristic fragmentation process corresponding to [M–H–C4CO2H]−. Deuterium labellung studies were used to delineate some of the major NICI fragmentation processes. While the PICI mass spectra all gave comparable detection responses differences were observed with various amino acids when operating in the NICI mode. The order of sensitivity of detection for six representative amino acid-carboxy-n-butyl-N-PFP derivatives were: hydroxyproline > lysine > cysteine > glycine=leucine=methionine in the ratio of 100:20:7:1. In PICI mass spectrometry derivatized leucine yielded a response 25 times less than its NICI mass spectrometric value.