Accurate-Mass Databases for Comprehensive Screening of Pesticide Residues in Food by Fast Liquid Chromatography Time-of-Flight Mass Spectrometry

Because of the international trade of fruits and vegetables and the lack of harmonized regulations on the use of pesticides worldwide, the development of comprehensive screening methods for analyzing hundreds of pesticides and other banned chemicals is very convenient. This work reports the development and evaluation of a rapid automated screening method for determining pesticide residues in food using liquid chromatography electrospray time-of-flight mass spectrometry (LC-TOFMS) based on the use of an accurate-mass database. The database created includes data not only on the accurate masses of the target ions but also the characteristic in-source fragment ions (over 400 fragments included) and retention time data. This customized database was associated to commercially available software which extracted all the potential compounds of interest from the LC-TOFMS raw data of each sample and matched them against the database to search for targeted compounds in the sample. This automatic screening method requires a careful optimization of the accurate-mass window and retention time tolerances, which play a determinant role on the selectivity, accuracy, and throughput of the whole procedure. Values of 10 mDa for preliminary screening and 1 mDa/5 ppm for confirmation along with a +/- 0.15 min retention time window were found to be optimum for the compounds and samples tested. The optimized methods enable the automated screening of ca. 300 compounds in less than 20 min including the LC-MS run and data processing. The proposed method was applied to 60 real samples, and the results of the positive findings compared well with those obtained using a liquid chromatography tandem mass spectrometry (LC-MS/MS) method (triple quadrupole). The rates obtained on the identification of compounds in spiked and real samples in an automated fashion at different concentration levels were over 95% of the compounds, thus revealing as a convenient tool for the large-scale screening of pesticides in foodstuffs.