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.

Evaluating a direct swabbing method for screening pesticides on fruit and vegetable surfaces using direct analysis in real time (DART) coupled to an Exactive benchtop orbitrap mass spectrometer

“…An ideal swab would be made from a sturdy woven material that could withstand temperatures up to 350°C, which is the maximum temperature used in FDA DART methods. Starting at a low temperature and increasing the temperature of the helium gas exiting the DART source over a specific time interval enabled thermal separation of pesticides in very complex mixtures as shown by Edison et al [6,7].…”

“…Generally the DART ionization process in positive ion mode creates protonated molecules that are singly charged yielding spectra that are very easy to interpret if the sample is not in a complex mixture. In the case of complex mixtures, the use of a temperature gradient can facilitate some thermal separation of analytes as shown by Edison et al [6,7].…”

“…Direct analysis in real time ionization coupled with mass spectrometry (DART-MS) has been gaining momentum in the past few years in food quality and food safety, particularly in the application areas of authenticity [1][2][3], adulteration [4,5], contamination [6][7][8][9][10][11], and food packaging [12,13]. A review paper on the challenging applications in food quality and safety accomplished with the DART technique gives a perspective of where this technology has progressed to over the past 4 years [14].…”

“…In recent publications, the United States Food and Drug Administration (US FDA) scientists have developed a DART-based method utilizing high-resolution accurate mass detection to rapid screening of pesticides on the surfaces of fruits and vegetables with the objective of creating a simple protocol for testing that might be deployed at selected border points of entry. Using their experimental protocols pesticide screening per sample was completed in under 3 min per swab [6,7]. The use of such a rapid method for detection of many pesticides at levels at or above tolerance limits, as well as the presence of unapproved pesticides on specific commodities using a combination of accurate mass and thermal desorption characteristics could facilitate more inspections.…”

Future perspectives in Orbitrap™-high-resolution mass spectrometry in food analysis: a review

“…Three fungicides spiked onto the surfaces of the produce were detectable at levels as low as 20 ng/g, and the swabbing took 30-60 sec and the automated DART-HRMS took only another 10-20 sec. Similarly, Edison et al, (2011) used foam swabs to recover multi-class mixtures of 240, 140, 132 and 60 pesticides from the surfaces of apples, kiwis, peaches and tomatoes. Rough and hairy surfaces were found to be more difficult to handle using the swab, but nevertheless impressively low detection limits down to 5 ng/g were reported.…”