Unimolecular decomposition of deprotonated glyphosate: Discerning cyclic versus linear dehydration products using ion mobility mass spectrometry

Abstract

As observed previously by multistage electrospray ionisation mass spectrometry, collision induced dissociation (CID) of deprotonated glyphosate results in several fragmentation pathways. These include decarboxylation and dehydration, with the formation of characteristic fragments and isomeric product ions depending on the fragmentation mechanism. Herein, the potential of ion mobility – mass spectrometry (IM-MS) to separate the isomers of dehydrated glyphosate was investigated. Separation of the two isomers of dehydrated glyphosate was not observed with single-pass cyclic ion mobility experiments, nor by multi-pass experiments. However, using characteristic fragments the cyclic isomer appeared to be more stable the linear isomer. The CH₄O₃P⁻ fragment (m/z 95) of dehydrated glyphosate was observed for the first time here. This is suspected to be the product of an ion molecule reaction between the dehydrated glyphosate anion and background water molecules within the travelling wave ion mobility (TWIMS) cell. Hydrolysis of the dehydrated glyphosate ion resulted in methyl phosphonate anion and neutral glycine imine formation. A concerted mechanism is supported by ¹³C labelling and H/D exchange experiments.