Practical considerations for the rapid screening for pesticides using ambient pressure desorption ionisation with high-resolution mass spectrometry

Abstract

A rapid screening method for pesticides has been developed to streamline the processing of produce entering the United States. Foam swabs were used to recover multi-class mixtures of 240, 140, 132 and 60 pesticides from the surfaces of apples, kiwis, peaches and tomatoes. The mixtures were selected to span a large range of chemical classes, polarities, solubilities and sizes to provide a broad look at how this technique will perform for a variety of analytes. The swabs were analysed using direct analysis in real-time (DART) ionisation coupled with a high-resolution Exactive Orbitrap™ mass spectrometer. This study expands the types of commodities analysed using this method and explores the feasibility of compositing multiple units of produce per batch to analyse a representative sample. It was established that whilst smooth-skinned produce, such as apples, maintained a high detection rate for the pesticide mixtures even when ten apples are swabbed with one foam disk, commodities with rougher surfaces, such as peaches, suffered a decrease in detection rate when ten peaches are swabbed with one foam disk. In order to maintain some consistency across the sample preparation process, a composite size of three units was selected. The varying topography of the commodities necessitated minor modifications to the method; for example, analysis of kiwi required that the hair on the surface be shaved prior to swabbing to achieve good recovery. Additionally, the effect of storage conditions on detection rate was analysed by spiking the surface of tomatoes at levels of 5 and 10 ng g⁻¹ for each pesticide, storing them under refrigeration and ambient conditions for 3 and 8 days, and then analysing the surface using this method. After 8 days of storage under both conditions more than 80% of the pesticides in the mixture were detected. Also, analysis of the multi-class mixtures was performed in both positive- and negative-ion mode and many classes were detected in both modes. Some classes, such thiocarbamates, phenylamides and organochlorine pesticides, were only observed in the positive-ion mode, whilst benzoylphenylurea pesticides were only detected in the negative-ion mode.

Keywords:

• LC/MS
• pesticide residues

Acknowledgement

The authors would like to thank Chris Sack (USFDA, Lenexa, KS) for helpful discussions, Jon Wong (USFDA, College Park, MD) for providing pesticide mixtures, Detlef Schumann (ThermoFisher Scientific, Cincinnati, OH) for helpful discussions, and David Meyer (Centre College, Danville, KY) for assistance in sample preparation.