Electron Ionization vs. Chemical Ionization for Targeted Metabolomics: Method development, Validation, and Application

Abstract

Insect populations have decreased dramatically the past decades, with one reason being unintentional pressure from pesticides. Insecticides such as neonicotinoids can cause sub-lethal effects in insects, such as altered metabolic regulation, which can be studied using targeted metabolomics. Although commonly studied using High-Performance Liquid Chromatography tandem Mass Spectrometry (HPLC-MS/MS), GC-MS/MS can also be used when studying volatile, polar metabolites. Electron ionization (EI) is the premier choice for ionization source in GC due to it producing reproducible mass spectra and general ease of use. Chemical ionization (CI), being underrepresented within targeted metabolomics, could be used to produce more selective transitions. The aim of this project was to in part develop and compare two methods for targeted metabolomics in insect brains using either EI or CI as the ion source, followed by an application studying the effects of acetamiprid on metabolic regulation in Eristalis Tenax brain tissue. The methods, after optimization of Selected Reaction Monitoring (SRM) transitions, were validated following the Internation Council of Harmonization guideline M10 on bioanalytical method validation, evaluating selectivity, sensitivity, precision, and accuracy. Using the better performing method, extracted brain tissue samples from drone flies exposed to an acute, short-term dose of Acetamiprid were compared to control samples, to quantify changes in metabolite concentrations. The selectivity of both methods was found to be good for most analytes. The linearity was generally good for both methods, although the sensitivity was higher for the EI method, with LOD values about a magnitude lower compared to CI. The precision was poor for analytes in low endogenous concentrations in both methods, and the accuracy was found to be bad for most analytes in both methods. Through Principle Component Analysis (PCA) and Analysis of Variance (ANOVA), no statistical variation could be found between treated and control flies.

Popular Abstract

For the past decades, insect populations have decreased in drastic manners. Pollinators, such as honeybees, bumblebees, and flies of various kinds, are among the decreased numbers, and one of the possible causes is an increased use of pesticides. Insecticides come in various flavours, and most of them are designed to target specific insects, meaning field-realistic doses are not enough to kill common pollinators. But studies have shown that low doses can induce so called sub-lethal effects, such as changes in insect metabolism, which can affect the survival rate of entire colonies.
Metabolomics, or the study of metabolomes, can be done through gas chromatography (for separation of the metabolites), coupled to mass spectrometry (for the detection), GC-MS/MS. The mass spectrometer can use several different types of ionization to fragment the molecules before detection. Most used is Electron ionization (EI), where electrons impart molecules with high energy, producing plenty of fragments that often have very reproducible spectra, likened to molecular fingerprints. Chemical ionization (CI) on the other hand is underrepresented in metabolomics. It’s a softer type of ionization which ionizes, but does not fragment molecules as much, which can be useful for the selectivity of the method. Especially in conjunction with Selected Reaction Monitoring (SRM) which act as a sort of filter for ionized molecules and fragments.
The aim of this project was to in part develop, validate, and compare two GC-MS/MS methods using either EI or CI as the ion source, and to use the better performing method for an applied study to see if the insecticide Acetamiprid affects metabolism in drone fly brains after exposure. The validation of the methods was done according to a guideline from the International Council of Harmonization, to assess different qualities such as selectivity, sensitivity, precision, and accuracy. The study found that the EI method was superior to the CI method, but both lacked in accuracy. The application study was done by hand feeding flies either acetamiprid-laced sugar solution or neat sugar solution and analysing their brains to see if any analytes looked at were statistically different. From the short application, there was no statistical difference between the treated flies and the control.

• author: Strandberg, Teddy
• supervisor: Simon Palmer
• organization: Department of Chemistry
• course: KEMR30 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Eristalis, GC-MS/MS, Ionization, Metabolomics, Analytical chemistry
• language: English
• id: 9200589
• date added to LUP: 2025-06-23 11:24:45
• date last changed: 2025-06-23 11:24:45