Studies to improve the extraction yields of incurred pesticide residues from crops using the QuEChERS method.

The influence of various factors on the extraction yields of incurred pesticides from crops using the QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) method was thoroughly studied. These factors included extraction time, extraction temperature, agitation approach, and in the case of dry commodities, sample comminution grade. Extraction yields increased with increasing extraction time, eventually reaching a plateau. Extraction temperature also played an important role in speeding up extraction, whereas the agitation approach had little influence. Based on our results we propose an extension of the first QuEChERS extraction step to 15 min when using deep frozen samples and to 2 min when using samples at ambient temperature. The extension of the second QuEChERS extraction step was shown to be less effective. Mechanical shakers can be used to facilitate extraction. This minor modification of the QuEChERS method does not alter its simple structure nor increase the manual labor and costs involved. It was further shown that the extraction yields of incurred pesticides strongly depend on their physiochemical properties, with lipophilic pesticides typically showing stronger retardation and higher yields when extraction time and/or temperature are increased. The impact of prolonging the first QuEChERS extraction step from 1 to 15 min on the extraction yields of incurred pesticides from frozen samples was studied on 132 real samples containing 85 different pesticides throughout the polarity range and representing 55 different commodity types. Out of the 408 pesticide/commodity combinations studied, 34% showed >25% yield increases when the extraction time was extended to 15 min. Also, more than half of the 132 studied samples contained at least one incurred pesticide for which the extraction yield increased by more than 25%. Similar extraction retardation effects were also observed for spiked pesticides but only if these were spiked on commodities with intact surface, not to homogenates thereof.

Analysis of "Amitraz (sum)" in pears with incurred residues - Comparison of the approach covering the individual metabolites via LC-MS/MS with the approach involving cleavage to 2,4-dimethylaniline.

We developed a method for the hydrolysis of amitraz, and its degradation products 2,4-dimethylphenyl-N'-methylformamidine (DMPF) and 2,4-dimethylformamide (DMF) into 2,4-dimethylaniline (DMA), directly from QuEChERS extracts. The hydrolysis product DMA was analysed using liquid chromatography-electrospray tandem mass spectrometry (LC-ESI-MS/MS) in the positive ion mode with DMA D6 as an internal standard. Validation of DMPF and amitraz via DMA, following hydrolysis directly from the QuEChERS raw extracts, was performed at 0.05 mg kg(-1) and at 0.5 mg kg(-1). Individual recoveries of amitraz and DMPF, determined as DMA, ranged between 100 and 120% and between 96 and 118% respectively and the relative standard deviations (RSDs) below 5.2% and below 9.5% respectively (n=5 at each spiked level). Successful validation for amitraz, and DMPF at 0.01 mg kg(-1), was also conducted following fivefold pre-concentration of QuEChERS extracts prior to hydrolysis. Pear samples with a history of amitraz treatment, containing residues of the amitraz metabolite DMPF but no detectable residues of amitraz or its other metabolites, DMF and DMA, were extracted using the QuEChERS method. DMPF-results obtained using direct LC-MS/MS analysis were found to be comparable with DMA-results obtained when the same extracts were subjected to alkaline hydrolysis, suggesting that DMPF constituted virtually the only source of DMA in the analysed pears and that the determination of amitraz (sum) in pears does not necessarily require a cumbersome method involving cleavage to DMA.

QuEChERS-based method for the multiresidue analysis of pesticides in beeswax by LC-MS/MS and GC×GC-TOF.

The validation of an analytical procedure for the determination of pesticide residues in beeswax, an interesting matrix for environmental pollution monitoring, is presented. Using the QuEChERS template, the impacts of wax particle size, sample amount, and cleanup procedure (water addition, dispersive solid phase extraction, freeze-out, and combinations thereof) on extraction yield and coextractive load were studied. Sample preparation through liquid-liquid partitioning between acetonitrile and melted wax (∼80 °C), followed by freeze-out and primary-secondary amine dispersive cleanup, was performed on incurred and pesticide-free samples for 51 residues. Determinations were made through LC-MS/MS and GC×GC-TOF, and the whole procedure was validated. Matrix effects were evaluated, with recoveries between 70 and 120% and RSDs below 20% in almost all cases. LC-MS/MS LOQs ranged from 0.01 to 0.1 mg/kg for most pesticides, but for GC-amenable pesticides, GC×GC-TOF sensitivity was lower (0.1-0.2 mg/kg). This methodology can be applied for routine analysis of pesticide residues in beeswax.

Extracting fumonisins from maize: efficiency of different extraction solvents in multi-mycotoxin analytics.

The complete extraction of analytes is of utmost importance when analyzing matrix samples for mycotoxins. Mycotoxins consist of substances with widely different physicochemical properties; therefore, the loss of toxins that occurs in multi-mycotoxin methods due to compromises in the extraction solvent is currently a topic under discussion. With regard to fumonisins, several extractants from recently published multi-mycotoxin methods were investigated when analyzing unprocessed and processed maize matrices. All extractants were tested in a validated on-site method and the extraction yields were compared to those of an HPLC-FLD reference method (EN 14352). Most of the compared multi-mycotoxin methods that have been published were only for analyzing fumonisins in maize or maize-meal; we have applied the extractants of these methods to processed, complex maize matrices for the first time. Our results show that, for extractions with aqueous acetonitrile mixtures with the addition of acid, e.g. MeCN/H2O/acetic acid (79/20/1, v/v/v), higher extraction yields are obtained than with MeCN/H2O (80/20, v/v), in both spiked and naturally contaminated maize matrices. But compared to the results of the reference method EN 14352, the two extractants did not show a similar extraction efficiency. Overall, the extractant MeCN/MeOH/H2O (1/1/2, v/v/v) turned out to be the most appropriate extractant applied in all experiments, obtaining the best and most comparable extraction yields and recoveries. Furthermore, our investigations showed that, with some of the tested extraction solvents, e.g. MeCN/H2O (75/25) containing 50 mmol/l formic acid, stark differences occur when analyzing spiked and naturally contaminated matrices. With spiked matrices, recoveries of approximately 80-110% were obtained, but with naturally contaminated matrices no results comparable to the EN method have been achieved. In contrast, a double extraction with MeCN/H2O/formic acid (80/19,9/0,1, v/v/v), followed by a second polar extraction step with MeCN/H2O/formic acid (20/79,9/0,1, v/v/v), led, for most naturally contaminated samples, to comparable results with the EN method. However, for spiked samples, the same extractant led to raised recoveries of between 120 and 140 %. For some processed matrices, like taco-chips, all tested extractants showed a poor extraction efficiency for fumonisins. By extending the extraction time from 1 to 15 min, a result comparable to that of the reference method could also be obtained for the extractant using MeCN/MeOH/H2O (1/1/2, v/v/v). As this extractant has been used in our recently published method (Trebstein et al. Mycotoxin Res 25:201, 2009), this work also presents an update on this method with respect to the extended extraction time.