Determination of Pesticide Residues in Foods by Acetonitrile Extraction and Partitioning with Magnesium Sulfate: Collaborative Study

A collaborative study was conducted to determine multiple pesticide residues in fruits and vegetables using a quick, simple, inexpensive, and effective sample preparation method followed by concurrent analysis with gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/ tandem mass spectrometry (LC/MS/MS). For short, the method is known as QuEChERS, which stands for quick, easy, cheap, effective, rugged, and safe. Twenty representative pesticides were fortified in 3 matrixes (grapes, lettuces, and oranges) at 3 duplicate levels unknown to the collaborators ranging from 10 to 1000 ng/g. Additionally, 8 incurred pesticide residues were determined. Thirteen laboratories from 7 countries provided results in the study, and a variety of different instruments were used by collaborators. The QuEChERS procedure simply entails 3 main steps: (1) a 15 g homogenized sample is weighed into a 50 mL centrifuge tube to which 15 mL acetonitrile containing 1% HOAc is added along with 6 g MgSO4 and 1.5 g NaOAc, and the tube is shaken and centrifuged; (2) a portion of the extract is mixed with 3 + 1 (w/w) MgSO4-primary secondary amine sorbent (200 mg/mL extract) and centrifuged; and (3) the final extract is analyzed by GC/MS and LC/MS/MS. To detect residues <10 ng/g in GC/MS, large-volume injection of 8 microL is typically needed, or the extract can be concentrated to 4 g/mL in toluene, in which case 2 microL splitless injection is used. In the study, the averaged results for data from 7-13 laboratories (not using internal standardization) for the 18 blind duplicates at the 9 spiking levels in the 3 matrixes are as follows %recovery and reproducibility relative standard deviation (RSD(R), %): atrazine, 92 (18); azoxystrobin, 93 (15); bifenthrin, 90 (16); carbaryl, 96 (20); chlorothalonil, 70 (34); chlorpyrifos, 89 (25); cyprodinil, 89 (19); o,p'-DDD, 89 (18); dichlorvos, 82 (21); endosulfan sulfate, 80 (27); imazalil, 77 (33); imidacloprid, 96 (16); linuron, 89 (19); methamidophos, 87 (17); methomyl, 96 (17); procymidone, 91 (20); pymetrozine, 69 (19); tebuconazole, 89 (15); tolylfluanid (in grapes and oranges), 68 (33); and trifluralin, 85 (20). For incurred pesticides, kresoxim-methyl (9.2 +/- 3.2 ng/g) and cyprodinil (112 +/- 18) were found in the grapes; permethrins (112 +/- 41), lamda-cyhalothrin (58 +/- 11), and imidacloprid (12 +/- 2) were determined in the lettuces; and ethion (198 +/- 36), thiabendazole (53 +/- 8), and imazalil (13 +/- 4) were determined in the oranges. Chlorpyrifosmethyl (200 ng/g) was used as a quality control standard added during sample homogenization and yielded 86% recovery and 19% RSD(R). Intralaboratory repeatabilities for the method averaged 9.8% RSD for all analytes. The results demonstrate that the method is fit-for-purpose to monitor many pesticide residues in fruits and vegetables, and the Study Director recommends that it be adopted Official First Action.