Analysis of saxitoxin and analogues by reverse-phase liquid chromatography coupled to mass spectrometry after derivatisation with ethyl chloroformate

Verification of saxitoxin (STX), a Schedule 1 chemical under the Chemical Weapons Convention, requires highly selective analytical methods for diverse sample types. In this study, STX was derivatised with ethyl chloroformate to form less polar derivatives without compromising the structural differences that differentiate it from non-scheduled congeners. Following optimisation of the derivatisation conditions, the method was applied to STX-spiked samples relevant to chemical weapons testing scenarios including authentic sample analysis and chemical forensics. The protocol enabled the rapid and selective identification of STX at 40 ppb in water and beverages in the presence of non-scheduled congeners including neosaxitoxin, decarbamoyl saxitoxin, and decarbamoyl neosaxitoxin by reverse-phase liquid chromatography mass spectrometry (LC-MS). Additionally, samples from the Organisation for the Prohibition of Chemical Weapons First Trial Biotoxin Proficiency Test were derivatised and analysed, facilitating the identification of a blank sample and two samples spiked with STX at the ppb level. Finally, the protocol was applied to contaminated oyster homogenate to demonstrate its utility in a food matrix. The protocol was found to complement existing methods for the analysis of STX in a chemical weapons verification context and rapidly produces analytical data for verification of STX in several types of samples. The ability to distinguish STX from non-scheduled congeners by reverse-phase LC-MS, which is a standard technique in most chemical weapon residue testing laboratories, is a further benefit of this work. • Derivatisation of STX and three congeners with ethyl chloroformate is described. • Separation of the derivatives by reverse-phase liquid chromatography is presented. • Identification of 40 ppb STX in beverages by untargeted and targeted LC-MS(HESI) is demonstrated. • Verification of STX in samples from OPCW's Trial Biotoxin PT was achieved.