Direct-immersion SPME in soy milk for pesticide analysis at trace levels by means of a matrix-compatible coating

Highlights

• A matrix-compatible SPME fiber, PDMS/DVB/PDMS, was used for analysis of pesticides in soy milk.

• Optimized washing conditions minimized the occurrence of matrix fouling on the SPME device.

• Matrix modification optimization for the extraction of hydrophobic pesticides was performed.

• The matrix-compatible SPME device lifetime was evaluated up to 120 consecutive extractions.

Abstract

This study demonstrates a newly developed PDMS/DVB/PDMS fiber's suitability for the determination of pesticides in soy milk via direct-immersion solid-phase microextraction (SPME) combined with gas chromatography-mass spectrometry, eliminating the need for extensive sample pre-treatment procedures. Fouling accumulation on the coating surface was further minimized by implementing rapid and effective pre- and post-desorption cleaning steps. Under optimum conditions, the fiber was used to perform over 120 extractions while maintaining RSD values of less than 24.5% for 10 extracted pesticides. By comparison, the RSD values ranged from 8.4% to 42.8% over 80 extractions using a commercial PDMS/DVB fiber. The optimized conditions were used to fully validate a quantitative method for the targeted analytes by matrix-matched calibration and isotopically labeled internal standard correction. Significantly, the proposed method was able to achieve limits of quantitation (1–2.5 μg/kg) for the targeted analytes that were below the Maximum Residue Levels mandated for soy-based products. Accuracy, intra- and inter-day repeatability were also satisfactory. The proposed PDMS/DVB/PDMS fiber dramatically improved repeatability and suitability for direct-immersion SPME in soy milk, and represents a good alternative to other extraction methods for high-throughput quantitative analysis of pesticide residues in soy-based products.

Introduction

The use of nutraceutical products has rapidly grown due to their many health benefits and their usefulness in the prevention or treatment of diseases [1]. Soy-based products are one of the most popular categories of nutraceuticals among consumers due to their multiple health benefits, including their role in lowering cholesterol levels and preventing cardiovascular diseases and osteoporosis, as well as their effectiveness in treating menopausal symptoms [2]. Soy-based products are available in a wide variety of forms, such as beans, tofu, and milk; however, raw soy grains can be exposed to harmful chemicals from agricultural and post-harvesting practices. These compounds tend to accumulate in the final commercial product, often as a consequence of processing and concentrating the starting raw material. Thus, it is critical to monitor contaminant levels in soy derivatives to ensure that they comply with limits or tolerances set by Regulatory Agencies, as this is a key step in protecting consumers from the well-known health risks associated with pesticide exposure. Moreover, methods of determining trace levels of pesticides in soy-derivatives are extremely useful for monitoring imported products, especially those originating in countries with limited or no regulations regarding acceptable levels of pesticide residues in food. Soy milk, which is a stable emulsion of oil, water, and proteins, is the most commonly used “plant milk” for replacing regular diary milk, largely due to its high nutraceutical value. In terms of sample preparation, soy milk represents a particularly challenging matrix due to the type and number of analytical procedures typically required to isolate compounds of interest in clean extracts, while also avoiding the occurrence of interfering co-extractives.

In the literature, various methods have been reported for analyzing contaminants in soybeans, but only a few studies have focused on the analysis of pesticide residues in soy milk products. In general, QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) [[3], [4], [5]] and solid-phase extraction (SPE)[6] are the two most commonly used sample-preparation techniques for soy milk analysis. Although regulatory agencies, such as the European Union, set maximum residue levels (MRLs) for contaminants in raw food materials (Regulation EC 396/2005), these MRLs are not always followed for the derivatives of these raw materials. However, since the production of nutraceutical products requires large amounts of raw materials—thus increasing the potential concentration of contaminants in the final products—it is likely that MRLs for toxic substances in these products will be regulated in the near future. As such, it would be beneficial to find new analytical methods for determining contaminants in soy milk that are able to achieve adequate sensitivity, while also featuring easy and green sample-preparation protocols that minimize chemical waste and enhance throughput. To this end, solid-phase microextraction (SPME) represents a highly attractive extraction technique, as it not only satisfies the requirements of green analytical chemistry [7], but it also combines sampling, isolation, concentration, and sample introduction into a single step. As a result, SPME is able to provide high-throughput analysis and excellent analytical performance with respect to reproducibility, repeatability, sensitivity, and selectivity, which has led to its widespread acceptance and use in numerous analytical fields [[8], [9], [10]]. In recent years, the implementation of matrix-compatible coatings for the analysis of complex food matrices via gas-chromatography has greatly expanded the application of direct-immersion SPME, as it eliminates many of the typical drawbacks associated with conventional SPME coatings, such as fouling and diminished extraction capability after a limited number of extractions in complex matrices [11].

Matrix-compatible PDMS/DVB/PDMS fibers combine the high extraction efficiency of a PDMS/DVB coating and the antifouling properties of PDMS, deposited on the original SPME coating as a thin outer layer [12]. One of this coating's first applications was for the determination of triazole fungicides in grape and strawberry pulps [13]; significantly, the PDMS/DVB/PDMS coating was able to provide limits of quantitation (LOQs) that were below the maximum residue levels (MRLs) allowed for both matrices, and at least one order of magnitude lower than those achieved by the QuEChERS method [13]. Subsequent studies evaluated the PDMS/DVB/PDMS coating's endurance using raw blended fruits and vegetables, as well as to investigate and model its extraction kinetics for a set of representative analytes [14,15]. Moreover, several quantitative methods were implemented for a variety of matrices, including spaghetti sauce, baby food, Brazilian cachaça, avocado samples, and dried seaweed [14,[16], [17], [18], [19]]. Furthermore, the implementation of additional pre- and post-desorption rinsing steps was found to further extend the coating's lifetime [14,20]. However, the composition of the rinsing solutions needs to be adjusted based on the type of matrix being considered and the analytes under investigation, which necessitates an additional optimization step. For example, the addition of acetone into the pre-desorption rinsing solution for high-fat content matrices, such as avocado, was found to extend the coating's lifetime and prevent instrument contamination, thus enabling adequate reproducibility [14].

This paper presents an investigation of the PDMS/DVB/PDMS matrix-compatible coating's suitability for the extraction of pesticides with different polarities from soy milk. To avoid fouling during extraction, pre- and post-desorption rinsing conditions were optimized. The entire procedure was automated, and optimized conditions were used to evaluate coating lifetime in comparison to a conventional PDMS/DVB coating. Moreover, special conditions for the enhanced extraction of hydrophobic analytes were optimized and applied in order to validate the quantitative method and to screen commercial samples of soy milk.