Ensuring selectivity and sensitivity by timed- and ultra-selective reaction monitoring during gas chromatography-tandem mass spectrometric determination of pesticides

In multiresidue analysis, as the complexity of matrix increases, matrix co-extractives might co-elute and interfere with the detection of target analytes, and thereby result in false positives and erroneous quantifications. This paper aims to evaluate the combination of enhanced mass resolution and concurrent selectivity and sensitivity in analyzing a complex mixture of 341 pesticides in grape, orange, tomato, okra and spinach by gas chromatography triple quadrupole mass spectrometry (GC-MS/MS). Timed-selective reaction monitoring (t-SRM) was found advantageous over segment SRM in terms of ease of method optimization and sensitivity. The optimized t-SRM method was used to test the applicability of ultra-SRM (USRM) in filtering out co-eluting interfering matrix compounds. False detections of benfluralin, dimethoate, etc. could be avoided when mass resolution was increased from 0.7 to 0.4 and 0.2Da (full width at half maximum). Similar observations were noted for chlorobenzilate, spiromesifen, tebuconazole, etc., in grape; omethoate, bendiocarb, monocrotophos, etc. in orange; omethoate, dimethoate, ethoxyquin, atrazine, etc., in tomato; and dichlobenil, omethoate, propoxur, monocrotophos, etc., in okra. The analysis at higher mass resolution could significantly minimize matrix effects (ME), e.g., 34% at 0.7Da for spiroxamine in grape to 0.6% (0.4Da) and -44% (0.7Da) for dichlorvos in okra to 8.8% (0.1Da). A feature called "quantitation enhanced data-dependent (QED) scan feature" was found effective in targeted screening to confirm the detection of atrazine, azoxystrobin, λ-cyhalothrin, etc. at trace quantities in incurred samples and avoid false detection of chlorpyrifos.