Rapid and quantitative analysis of pesticides in fruits by QuEChERS pretreatment and low-temperature plasma desorption/ionization orbitrap mass spectrometry†
Ambient desorption/ionization high-resolution mass spectrometry (ADI-HR-MS) is a powerful method for the analysis of complex samples. Recently, direct analysis in real time (DART) MS and low-temperature plasma probe (LTP) MS demonstrated potential in direct qualitative pesticide residue screening and quantitative analysis of pesticides in liquid extracts. In the present study, a LTP-HR-MS method for quantitative pesticide residue analysis in fruit extracts was developed and evaluated with respect to the European Union (EU) legislation on pesticides. In particular, this study focused on pesticides in different fruit matrices that were reported to often exceed legal maximum residue levels (MRLs) in Germany in the past (namely acetamiprid, cyprodinil, fenhexamid, and fludioxonil; see report on of the German Federal Office of Consumer Protection and Food Safety in 2009). After method optimization, pesticides in spiked and unspiked fruit QuEChERS extracts were identified successfully by LTP-Orbitrap-MS via accurate mass measurements (<4 ppm). The method is considered useful for MRL verification. Matrix-matched calibration was applied for quantification because it was found that the fruit matrix (still present during extract analysis) has a significant effect on analyte ion abundance. Linear working ranges greater than four orders of magnitude were achieved. Limits of quantification ranged from 0.001 mg kg−1 to 0.07 mg kg−1 (which is significantly below permitted MRLs). Measurement precision was below 15% and method precision was typically close to 14% relative standard deviation. Finally, the validated LTP-HR-MS method was tested with unspiked fruit samples bought from a local grocery store. Pesticide residues of cyprodinil and fludioxonil (0.003–0.03 mg kg−1) were readily detected. These results were directly compared to a standard liquid chromatography electrospray HR-MS method and found to be in good agreement.