Development and comparative evaluation of chemical ionization and electron ionization for GC-MS targeted metabolomics applied to the brain of a pollinating insect.

Metabolomics has been suggested as an informative tool to assess effects of chemical pollution, including pesticide exposure in insects. However, the small size of insects is challenging and has often precluded tissue-specific analyses in single animals. In the present study, a gas chromatography-chemical ionization/mass spectrometry (GCCI/MS) method employing single reaction monitoring (SRM) was developed, including optimization of ion source temperature, reagent gas pressure and ionization energy, for targeted profiling of 51 metabolites in central carbon metabolism. The method was compared to an independently optimized GC-electron ionization (EI)/MS SRM method using standard validation parameters. Finally, the best-performing method was used to assess the immediate (1 min) metabolic effect of acetamiprid, a commonly used insecticide, in the brain of the hoverfly Eristalis tenax. A high reagent gas pressure benefited detection of the vast majority of metabolites, with effects of ion source temperature and ionization energy being more ambiguous. Both methods demonstrated adequate precision, poor trueness, and high selectivity, with EI outperforming CI with regards to sensitivity and detection limits. While trueness was poor, EI provided for adequate relative quantification of a majority of the targeted metabolites in a single E. tenax brain. Profiling of brain metabolites in E. tenax exposed to acetamiprid revealed a large variation in the responses to acetamiprid exposure. In conclusion, EI was found to perform better than CI in profiling of metabolites in insect brains and shows promise to further develop our understanding of pesticide effects in pollinating insects.