Determination of spirocyclic tetronic/tetramic acid derivatives and neonicotinoid insecticides in fruits and vegetables by liquid chromatography and mass spectrometry after dispersive liquid–liquid microextraction
Introduction
While insecticides permit high production and good quality food, they may have drawbacks, such as contamination of different environmental matrices or toxicity to humans and other species (Chagnon et al., 2015, Ippolito et al., 2015). Spirocyclic tetronic/tetramic acid (ketoenol) derivatives and neonicotinoids are two of the most extensively used insecticides for the control of important agricultural crop pests (Demaeght et al., 2013, Xu et al., 2006). Spirocyclic tetronic/tetramic acid derivatives and neonicotinoids act as lipid biosynthesis inhibitors and antagonists of the nicotinic acetylcholine receptor, affecting the synapses in the insect central nervous system, respectively (Bretschneider et al., 2007, Roberts and Hutson, 1999, Tomlin, 2009). European Union legislation has established stringent maximum residue levels (MRLs) for these substances in fruits and vegetables, and so efficient, selective and sensitive methods are needed for the simultaneous determination of these insecticides in fruits and vegetables.
Due to their thermolability, low volatility and high polarity, neonicotinoid residues in foods have usually been analyzed by high-performance liquid chromatography with different detectors, for example mass spectrometry (HPLC–MS or LC/MS2) (Fenoll et al., 2010, Di Muccio et al., 2006, Obana et al., 2003, Radišić et al., 2009), or diode-array detection (HPLC-DAD) (Obana et al., 2002, Watanabe et al., 2007). However, little has been published on the evaluation and monitoring of trace levels of spirocyclic tetronic/tetramic acid derivatives in fruits and vegetables using chromatographic methods (Tran et al., 2012, Xu et al., 2008, Zhang et al., 2011, Zhang et al., 2013, Zhu et al., 2013).
Extraction and purification are crucial steps for the determination of different pesticides in fruits and vegetables. However, classical methods for sample preparation are often expensive, laborious and time consuming (Fenoll, Hellín, López, González, & Flores, 2007). In recent years, new sample preparation techniques, such as dispersive liquid–liquid microextraction (DLLME), have been developed to overcome some of these limitations. DLLME, which was developed by Rezaee et al. (2006), is based on the generation of fine droplets of the extracts in an aqueous solution when a mixture formed by the solvent (immiscible with water) and another organic solvent (used as dispersant and soluble in both phases) is injected rapidly into the aqueous solution. The turbid mixture is centrifuged and the enriched extract injected into the chromatographic system (Viñas, Campillo, López-García, & Hernández-Córdoba, 2014). Therefore, DLLME has a number of advantages over classical extraction methods, such as its simplicity, speed, low cost and high preconcentration factor (Zhang, Yang, Yin, Wang, & Wang, 2012).
DLLME has been little reported for the analysis of neonicotinoids in foods using chromatographic techniques (Campillo et al., 2013, Jovanov et al., 2013, Wu et al., 2011, Zhang et al., 2012) and no references have been found describing the use of this technique for the determination of spirocyclic tetronic/tetramic acid derivatives in foods. As far as we know, combined DLLME–LC–ESI-MS2 has not previously been used to analyze these insecticides in fruits and vegetables. The present work deals with the quantification of five neonicotinoids (thiamethoxam, clothianidin, imidacloprid, thiacloprid and acetamiprid) and three spirocyclic tetronic/tetramic acid derivatives (spirodiclofen, spiromesifen and spirotetramat) in different fruits and vegetables using DLLME combined with ESI-LC–MS2 in selected reaction monitoring (SRM) mode.
Section snippets
Chemicals and reagents
Acetamiprid ((E)-N1-[(6-chloro 3-pyridil)methyl]-N2-cyano-N1-methylacetamidine) (99.9%), thiamethoxan (3-[(2-Chloro-1,3-thiazol-5-yl)methyl]-5-methyl-N-nitro-1,3,5-oxadiazinan-4-imine) (99.7%), thiacloprid ((Z)-3-(6-chloro-3-pyridylmethyl)-1,3-thiazolidin-2-ylidenecyanamide) (99.9%) and clothianidin ((E)-1-(2-Chloro-1,3-thiazol-5-ylmethyl)-3-methyl-2-nitroguanidine) (99.9%) were purchased from Fluka (Buchs, Switzerland). Imidacloprid
Optimization of HPLC separation and detection conditions
The separation power of two different reversed-phase chromatographic columns for the insecticides studied was checked: a C8 analytical column of 150 mm × 4.6 mm and 5 μm particle size (Zorbax Eclipse XDB-C8) and a Spherisorb ODS2 (Teknokroma) 150 mm × 4 mm, 5 μm. Different mobile phases were applied to each stationary phase. In the case of the C8, the best separation was obtained when the programme consisted of an isocratic step with 90:10 0.1% formic acid:acetonitrile mixture for 5 min and then a linear
Conclusions
Dispersive liquid–liquid microextraction was seen to efficiently preconcentrate the insecticides studied after extraction from the solid matrix using acetonitrile in a simple solvent extraction step. The selectivity of the triple quadrupole mass spectrometer coupled to liquid chromatography provided unequivocal identification of the eight compounds and their quantification with a high degree of sensitivity.
Conflict of interest and author agreement
We wish to confirm that there are no known conflicts of interest associated with this manuscript and there has been no significant financial support for this work that could have influenced its outcome. All named authors have read and approved this version of the manuscript and there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.
Acknowledgements
The authors acknowledge the financial support from the Ramón and Cajal Subprogram and the Comunidad Autónoma de la Región de Murcia (Fundación Séneca, Project 15217/PI/10). We are also grateful to Inmaculada Garrido, Juana Cava and María V. Molina for technical assistance and to FEDER and European Social Funds.
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