Checking foodstuffs for pesticide residues is an essential part of food safety. Mirabelli et al. (2016) show that mass spectrometric-based detection either matches or outperforms current methods for detection and measurement in food matrices. The researchers present a workflow that couples nano-liquid chromatography directly with ambient dielectric barrier discharge ionization and mass spectrometry (nano-LC-DBDI-MS) to enable efficient detection and highly sensitive pesticide quantitation in apple products1.
Using organic apples and organic apple puree baby food as blank matrices, the researchers developed the nano-LC-DBDI-MS workflow by examining analytical-grade commercial pesticide solutions. These commercial stocks include triazines, carbamates and organophosphorus pesticides. They follow an EU QuEChERS multi residue method, CEN 15662 in preparation for mass spectrometric analysis.
Mirabelli et al. prepared stock solutions of each pesticide then combined them to create a reference mixture in acetonitrile. After homogenizing the apples (including peels), they then extracted the analytes according to CEN 15662 but omitting the dSPE (solid phase extraction) cleanup step. The team examined the extracts of food matrices spiked with pesticide solution using an LTQ-Orbitrap mass spectrometer in full scan mode (resolution lowered to 30,000 to increase scan speed and obtain sufficient data points), in addition to direct analysis of the acetonitrile stock using an LCQ Dexa XP mass spectrometer (both Thermo Scientific).
The analytical equipment comprised a lab-built vaporization assembly coupled directly to the nano-LC system. From here, the nano-LC feeds directly into the vaporization chamber and DBDI source inside the mass spectrometer. In this way, ionization takes place directly inside the MS inlet. Mirabelli et al. describe this as “active capillary ionization” or ambient DBDI.
First, the team created calibration curves for the pesticides using dilution of stock solutions. They achieved LODs of 3µg/mL with reproducible results over different experimental runs. They determine an extraction efficiency ranging between 82 and 108% for the QuEChERS method for all pesticides.
Quantifying the analytes using the LTQ-Orbitrap mass spectrometer, the team saw no discernible interference from the food matrices; suppression or enhancement of pesticides was not significant. The team achieved limits of detection (LODs) of 10pg/mL, in addition to limits of quantitation (LOQs) compliant with current European Union (EU) food safety regulations.
In conclusion, Mirabelli et al. are confident that the nano-LC-DBDI-MS workflow is robust and sensitive for food safety analysis and quantitation of pesticide residues in food matrices. The advantage of this method is from both the nano-LC, which requires a nanolitre-per-minute flow rate that reduces analyte dilution and saves on solvent use, and the enhanced sensitivity of mass spectrometric detection. Furthermore, this method enables measurement in both the liquid- and gas phases. Covering a large linear dynamic range, the new workflow matches and outperforms current methods in use for food safety protocols today.
1. Mirabelli, M.F. et al. (2016) “Pesticide analysis at ppt concentration levels: coupling nano-liquid chromatography with dielectric barrier discharge ionization-mass spectrometry“, Analytical and Bioanalytical Chemistry 408 (pp.3425-34)