Pesticide residue fast screening using Thermal Desorption MION MS with selective chemical ionization

A new methodology for pesticide screening bypassing sample pretreatment was proposed and tested with bromide, acetonylacetone and water as reagent ions.

Pesticides are substances used to prevent, control, or destroy pests, diseases, and weeds in plants, as well as to protect humans from vector-borne diseases. Their usage has steadily increased over recent years, reaching approximately 4.1 million tons annually and is projected to increase with the need to feed growing population. However, pesticides can be also toxic and pose both acute and chronic health risks, depending on exposure levels and methods.

Some of the most common techniques to monitor pesticide levels in food are gas or liquid chromatography with tandem mass spectrometry, GC-MS/MS and LC-MS/MS, respectively. Both these methods require substantial sample preparation.

Instrument set up and methods

A novel thermal desorption unit was mounted in front of the Multi-scheme chemical IONization inlet (MION2), that was coupled to Orbitrap mass spectrometer (Fig. 1). Filters used for measurements were 37 mm in diameter and made of metal mesh. Sample preparation and chromatographic separation were not needed thanks to unique features of MION2 capable of fast switching between reagent ions.

Bromide (Br-), protonated acetonylacetone (C6H10O2H+), oxygen (O2-) and protonated water (H3O+) were used as reagent ions.

The pesticide standard mixtures, fruit extracts and sliced fruits were provided by the Finnish customs. The standard mixtures were divided into three groups according to the methods regularly used for the analysis: 'LC' (73 pesticides), 'GC1' (65 pesticides) and 'GC2' (70 pesticides). Solutions with 1 ng of mixtures were injected onto the filters and then desorbed, liquid fruit extracts were directly injected into the system, and sliced fruit (pineapple) was swabbed with a filter.

Molecular quantum chemical calculations were performed with Spartan’20 to obtain Br chemical ionization characteristics and to confirm its sensitivity towards a variety of pesticides.    

The thermal desorption unit, Ion source MION2 and mass spectrometer Orbitrap
Figure 1: Schematic of the thermal desorption unit, MION and Orbitrap.

Highlighted results

In total, 115 of 206 pesticides were detected from the standard mixtures using different reagent ions and bypassing sample preparation. In the 'LC' mixture, 92% of all pesticides were identified. 45% and 35% of pesticides were detected in mixtures 'GC1' and 'GC2', respectively. This result is slightly surprising as the newly proposed technique is more similar to traditional gas chromatography than liquid chromatography.

The swabbing method was proposed and tested. This method allows skipping an extensive extraction procedure and sample preparation, thereby, saving time. The core of the method is rather quick swabbing of the actual piece of a fruit or vegetable with a filter for 30 s. Less pesticide species but in similar amount were detected compare to the liquid injection of the fruit sample from the same pineapple (Fig. 2).

Mass spectra of bromine adducts obtained from pesticides screening.
Figure 2: Spectra of fluidioxonil adducts with bromide obtained through a)standard mixture injection b)pineapple juice injection and c)swabbing the fruit surface

No difference were observed between injecting 'LC' and 'GC' mixtures simultaneously or individually, which suggests that at these concentrations chemical ionization of individual pesticides is not affected.

The bromide (Br) chemical ionization characteristics were further inspected using quantum chemical computations: Br- is known for forming strong bonds with hydroxyl groups (-OH) and amino groups(-NH), however, also found to bind even into weakly hydrogen bonding sites.

Reference: Partovi, Fariba, et al. "Pesticide Residue Fast Screening Using Thermal Desorption Multi-Scheme Chemical Ionization Mass Spectrometry (TD-MION MS) with Selective Chemical Ionization." ACS omega 8.29 (2023): 25749-25757. This publication is distributed under CC-BY 4.0.

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Fast pesticides screening with TD MION MS

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