An analytical procedure was developed allowing the simultaneous determination of acidic pesticides and their conjugates by addition of an alkaline hydrolysis step into the European Union (EU) version of the QuEChERS method. The procedure resulted additionally in hydrolysis of most esters of phenoxy acids. On the basis of information from metabolism studies and the hydrolytic conditions employed in supervised field trials as well as results on the influence of physical and chemical parameters (temperature, time, type of solvent, type of matrix), alkaline hydrolysis for 30 min at 40 °C was deemed a good compromise for the determination of residues of 2,4-D, dichlorprop, fluazifop, haloxyfop, MCPA, and MCPB. The applicability of the proposed method was tested by analyzing food samples with incurred residues in six German laboratories not involved in method development. Up to 6 times higher residues are measured by using the QuEChERS extraction procedure with the newly developed alkaline hydrolysis step.

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.

Matrix effects (MEs) can adversely affect quantification in pesticide residue analysis using GC. Analyte protectants (APs) can effectively interact with and mask active sites in the GC system, and are added individually or in combination to sample extracts and calibration solutions to minimize errors related to MEs. Unfortunately, APs cannot sufficiently compensate for MEs in all cases. Plant extracts, containing a broad range of natural compounds with AP properties, can also be used for this purpose. In this study, the applicability of cucumber extract as a natural AP mixture was investigated both alone and in combination with traditional APs. Extracts of two selected difficult matrices (onion and garlic) were prepared according to the citrate-buffered QuEChERS (quick, easy, cheap, effective, rugged, and safe) procedure. ME values of 40 representative GC-amenable pesticides were compared when calibrating against standards in pure solvent and in cucumber extract, with and without the addition of APs. Using a GC system with a contaminated inlet liner, the use of a cucumber-based calibration solution decreased MEs remarkably. The combination of APs with cucumber raw extract further decreased MEs, resulting in more than 85% of the tested pesticides showing ≤ 10% ME in onion and ≤ 20% ME in garlic. These results demonstrate that the preparation of calibration standards based on cucumber extracts (with or without the addition of APs) is a very useful and practical approach to compensate for MEs in pesticide residue analysis using QuEChERS and GC-MS/MS. The use of various internal standards is furthermore critically discussed.