Use of dielectric barrier discharge ionization to minimize matrix effects and expand coverage in pesticide residue analysis by liquid chromatography-mass spectrometry

Abstract

Although electrospray ionization (ESI) remains the gold standard ionization source for LC-MS, it exhibits two main limitations: the occurrence of matrix effects and the limited ionization coverage towards nonpolar compounds. Dielectric barrier discharge ionization (DBDI) has gained attraction in recent years as a versatile ionization method in different applications and formats. Here, we report a thorough evaluation of DBDI as ionization interface for LC-MS, which reveals attractive advantages over ESI and atmospheric pressure chemical ionization (APCI) provided its singular ionization mechanism versatility. A suite of 80 pesticides across a wide range of physicochemical properties were selected and the results were compared with both ESI and APCI sources. Using a helium plasma operated in homogeneous regime with square-wave AC waveform and relatively low voltages (2.5 kV), not only DBDI was able to ionize compounds only amenable so far by GC-MS (eg. organochlorine species), but also offered a competitive performance in terms of sensitivity when contrasted with the commercial electrospray ionization source under equivalent conditions. Unlike ESI, DBDI mechanism occurs in the gas-phase, so the method is less affected by liquid-phase surface phenomena that yield ion suppression in ESI. Data collected in the positive ion mode revealed negligible matrix effect values (<10% suppression) for most of the studied compounds in different complex matrix extracts such as wastewater, orange or olive oil. This is also consistent with the absence of adduct formation whereas with ESI source, Na adduct formation is quite common with these species. In general, both sensitivity and average limits of quantitation for DBDI were similar to those obtained by ESI and better than APCI. Results showed that analyte coverage with DBDI is enhanced with respect to ESI and APCI sources being able to effectively analyze organochlorine compounds.