In this article from Issue 11 of the Analytix Reporter, produced by Merck, a matrix compatible solid-phase microextraction (SPME) fiber or cover coated (OC) SPME fiber, was used to determine pesticides in soy milk via direct immersion SPME/GC-MS.
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Abstract
HA matrix-compatible direct-immersion solid-phase microextraction (SPME) fiber, named PDMS/DVB/PDMS or SPME-OC Fiber, was used for the determination of pesticides in soy milk via direct immersion. Combined with gas chromatography-mass spectrometry, it eliminated the need for extensive sample pre-treatment procedures.
Introduction
Soy-based products are a category of nutraceuticals extensively used worldwide for their health benefits and also as a more sustainable alternative to dairy products. Raw soy grains are the starting material for all soy-based products and are often exposed to agrochemicals from agricultural and post-harvesting practices. It is important to monitor the level of pesticide residues in soy derivatives to ensure their compliance with tolerance limits set by various regulatory agencies across the world. Soy milk, being a stable emulsion of oils, water, and proteins, is a challenging sample to treat for the extraction of pesticides residues at ultra-trace levels. To propose an automated and sensitive method, solid phase microextraction (SPME) was considered as an extraction technique in this work.
Experimental
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Results and Discussion
The SPME procedure necessitated the optimization of fiber washing after the extraction (rinsing) and desorption (washing), in order to prolong its lifetime. Previous studies demonstrated that this optimization needed to be performed based on the type of food matrix analyzed and the targeted analytes. Several rinsing and washing solutions were tested.
Method Validation
A matrix-matched calibration approach was used by spiking pesticide-free soy-milk samples with all analytes in a concentration range of 1-1000 µg/kg; with the exception of phosalone which was spiked at 2.5-1000 µg/kg. Calculations were performed using linear regression for each of the targeted analytes, except phosalone, which required a 1/x2 weight. The accuracy and precision of the method were assessed at three concentration levels of 15, 75, and 200 µg/kg in quadruplicate measurements over three days.
Conclusions
A new method for the analysis of pesticides in soy milk was optimized and validated using a matrix-compatible SPME fiber. This DI-SPME-GC-MS method was able to quantitatively monitor the presence of pesticides with LOQs of 1-2.5 µg/kg, with a completely automated workflow including rinsing and washing of the SPME fiber. The excellent robustness of the SPME matrix compatible fiber enabled its use of up to 120 extraction/desorption cycles.
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