Formation of protonated phosphonates in the ion‐trap mass spectrometer under electron impact conditions

The gas‐phase behaviour of organophosphorus compounds such as diethylmethylphosphonate towards electron impact (EI) ionization occurring in a quadrupole ion‐trap instrument is investigated. If all ions observed in conventional EI mass spectra are detected, ion/molecule reactions are seen to occur leading to formation of protonated molecules and adduct ions such as 2M +· and [2M + H] + . The keto‐to‐enol‐like isomerization of the molecular M +· ion seems to favour proton transfer to neutral M species and a “self chemical ionization” ion is observed with corresponding fragment ions.