Study of different HILIC, mixed-mode, and other aqueous normal-phase approaches for the liquid chromatography/mass spectrometry-based determination of challenging polar pesticides.

The aim of the study was to evaluate the performance of different chromatographic approaches for the liquid chromatography/mass spectrometry (LC-MS(/MS)) determination of 24 highly polar pesticides. The studied compounds, which are in most cases unsuitable for conventional LC-MS(/MS) multiresidue methods were tested with nine different chromatographic conditions, including two different hydrophilic interaction liquid chromatography (HILIC) columns, two zwitterionic-type mixed-mode columns, three normal-phase columns operated in HILIC-mode (bare silica and two silica-based chemically bonded columns (cyano and amino)), and two standard reversed-phase C18 columns. Different sets of chromatographic parameters in positive (for 17 analytes) and negative ionization modes (for nine analytes) were examined. In order to compare the different approaches, a semi-quantitative classification was proposed, calculated as the percentage of an empirical performance value, which consisted of three main features: (i) capacity factor (k) to characterize analyte separation from the void, (ii) relative response factor, and (iii) peak shape based on analytes' peak width. While no single method was able to provide appropriate detection of all the 24 studied species in a single run, the best suited approach for the compounds ionized in positive mode was based on a UHPLC HILIC column with 1.8 µm particle size, providing appropriate results for 22 out of the 24 species tested. In contrast, the detection of glyphosate and aminomethylphosphonic acid could only be achieved with a zwitterionic-type mixed-mode column, which proved to be suitable only for the pesticides detected in negative ion mode. Finally, the selected approach (UHPLC HILIC) was found to be useful for the determination of multiple pesticides in oranges using HILIC-ESI-MS/MS, with limits of quantitation in the low microgram per kilogram in most cases. Graphical Abstract HILIC improves separation of multiclass polar pesticides.

Evaluation of nanoflow liquid chromatography high resolution mass spectrometry for pesticide residue analysis in food.

This article reports on the evaluation of nanoflow liquid chromatography-mass spectrometry (LC-MS) for pesticide residue analysis in food. The approach is based on the use of reversed-phase C18nano columns with an integrated emitter, so that separation, ionization and detection are performed minimizing dead volumes. The use of nanoflow not only increases ionization efficiency and minimizes ionization suppression but also boost sensitivity compared to analytical-scale LC-MS methods. The nanoflow LC system was combined with full-scan high resolution mass spectrometry using a Q-Exactive Orbitrap instrument. The analytical performance was assessed for over 60 representative pesticides in five representative commodities (tomato, baby food, orange, fruit-based jam and olive oil). The sensitivity achieved with this configuration enables the implementation of high dilution factors (eg. 1:20, 1:50 or beyond) in pesticide residue workflows without compromising sensitivity, featuring limits of quantitation in the low ng kg-1 range. Using this dilution factors, signal suppression was found negligible in most cases (<10% in most cases, especially with 1:50 dilution), so that matrix-matched standards may be skipped, thus simplifying laboratory workflows. The robustness of the nanoflow LC system and its capability to withstand long analytical runs was also evaluated. Appropriate precision in terms of peak area and retention time was obtained at different concentration levels for over 125 injections without any instrument servicing. The main benefits of the nanoflow liquid chromatography approach are the high sensitivity gain and the outstanding reduction in matrix effects thanks to the high sample dilution factors that can be implemented, along with the substantial reduction in solvent usage.

A feasibility study of UHPLC-HRMS accurate-mass screening methods for multiclass testing of organic contaminants in food.

The feasibility of accurate-mass multi-residue screening methods using liquid chromatography high-resolution mass spectrometry (UHPLC-HRMS) using time-of-flight mass spectrometry has been evaluated, including over 625 multiclass food contaminants as case study. Aspects such as the selectivity and confirmation capability provided by HRMS with different acquisition modes (full-scan or full-scan combined with collision induced dissociation (CID) with no precursor ion isolation), and chromatographic separation along with main limitations such as sensitivity or automated data processing have been examined. Compound identification was accomplished with retention time matching and accurate mass measurements of the targeted ions for each analyte (mainly (de)protonated molecules). Compounds with the same nominal mass (isobaric species) were very frequent due to the large number of compounds included. Although 76% of database compounds were involved in isobaric groups, they were resolved in most cases (99% of these isobaric species were distinguished by retention time, resolving power, isotopic profile or fragment ions). Only three pairs could not be resolved with these tools. In-source CID fragmentation was evaluated in depth, although the results obtained in terms of information provided were not as thorough as those obtained using fragmentation experiments without precursor ion isolation (all ion mode). The latter acquisition mode was found to be the best suited for this type of large-scale screening method instead of classic product ion scan, as provided excellent fragmentation information for confirmatory purposes for an unlimited number of compounds. Leaving aside the sample treatment limitations, the main weaknesses noticed are basically the relatively low sensitivity for compounds which does not map well against electrospray ionization and also quantitation issues such as those produced by signal suppression due to either matrix effects from coeluting matrix or from coeluting analytes present in the standards solutions which often occur as they contain hundreds of the analytes included in the screening methods.

Generic sample treatment method for simultaneous determination of multiclass pesticides and mycotoxins in wines by liquid chromatography-mass spectrometry.

In this work, a generic sample treatment method for simultaneous determination of multiclass pesticides and mycotoxins in wines is presented. The proposed method is based on solid-phase extraction (SPE) using polymeric-type SPE cartridges. To evaluate the proposed sample treatment, a liquid chromatography electrospray time-of-flight mass spectrometry method was used for testing 60 selected representative multiclass pesticides and 9 mycotoxins. Two different polymeric sorbents were evaluated, with hydrophilic-lipophilic-balanced (HLB) polymer cartridges being selected (Oasis HLB) as the most suitable for the present study. The identification and confirmation of the compounds was based on retention time and accurate mass measurements of the protonated molecules ([M+H](+)). Limits of detection were below 1 µg L(-1) for the 87% of the studied compounds. With the selected 4:1 preconcentration factor, 70% of the target compounds showed relatively low matrix effects, corresponding to signal suppressions lower than 30%. Recovery studies (n=10) were carried out at two concentration levels, 2.5 µg L(-1) and 25 µg L(-1), obtaining mean recovery rates between 70 and 120% for the 90% of studied analytes. The relative standard deviation (RSD%) values of the entire procedure were below 15% in most cases (97% of the studied analytes). The proposed method was successfully applied to 24 red wine samples produced in different regions of Spain. The concentration levels of the target compounds found in the studied samples were in compliance with the current regulations. Aflatoxin B(2) and metalaxyl were the most detected compounds (75% and 50% of the studied samples, respectively).