Goal: Demonstrate determinations of quaternary cationic pesticides in oat cereal samples by ion chromatography coupled with tandem mass spectrometry.
Pesticide contamination in food as a potential health risk is a growing public concern, resulting in increased interest and attention by health researchers and regulatory agencies.  Due to those concerns, much of the agricultural industry has largely replaced the traditional, toxic, solvent-based pesticides with the widely used, mostly less toxic, ionic pesticides. Collectively grouped into the category of “Ionic and Highly Polar Pesticides” are herbicides, fungicides, defoliants, and desiccants. For traditional solvent-based pesticides, the regulatory test methods are typically gas and liquid chromatography combined with mass spectrometry.
However, polar pesticides, which are ionic and non-volatile, are much more suitable for methods using ion chromatography (IC) separations combined with mass spectrometry (MS) detection. Consequently, determinations of anionic polar pesticide residues, such as glyphosate, glufosinate, and their degradation products, such as AMPA (aminomethyl)phosphonic acid), have been successfully demonstrated in beer, fruit, cereals, vegetables and drinking water by the Food Environmental Research Association (FERA, LTD) [2,3] and others [4-15] using IC with MS (tandem MS, high resolution accurate mass (HRAM) MS, or single quadrupole MS). Compared to the anionic highly polar pesticides, determinations of cationic polar pesticide residues have proven more challenging due to their similar chemical structures. [11,16]
Important cationic pesticides, such as the highly toxic paraquat and diquat, could not be chromatographically resolved on the previously available cation-exchange columns. Because diquat and paraquat have very similar chemical structures and differ in their m/z for molecular and fragment ions by less than 2 a.m.u., they could only be resolved by the HRAM MS.  To chromatographically resolve them, a new stationary phase was developed to assure accurate paraquat and diquat reporting. [18,19] Separate quantitation of paraquat and diquat is driven by the differences in their individual toxicity levels (LD50) and often different legal status (one may be permitted while another may be prohibited) in different jurisdictions.
This application note demonstrates the advantage of the new, designed-for-purpose Thermo Scientific™ Dionex™ IonPac™ CS21- Fast-4μm ion exchange column paired with a triple quadrupole mass detector rather than HRAM MS due to the chromatographic separation of paraquat and diquat.
Earlier, multi-residue extraction methods were developed for polar pesticides. In 2018, the European Union Reference Laboratory for Pesticide Residues in Fruits and Vegetables (EURL-FV) developed the Quick Polar Pesticide Extraction (QuPPE) method. Recently the QuPPE method was modified to improve recoveries for mepiquat and chlormequat by using formic acid with methanol at elevated temperature, 80 °C.20 Hydrochloric acid with methanol at 80 °C was used to improve paraquat and diquat recoveries. 
Four cationic pesticides with similar quaternary amine chemical structures –mepiquat (1,1-dimethylpiperidinium chloride), chlormequat (2-chloroethyl(trimethyl)azanium), paraquat (1-methyl-4-(1-methylpyridin-1-ium-4-yl) pyridin-1-ium), and diquat (1,1’-ethylene-2,2’-dipyridylium) – were selected to evaluate the new cation-exchange column optimized for quaternary amines, the Dionex IonPac CS21-Fast-4µm column. These compounds have similar chemical structures and are registered and reregistered (approved) herbicides in the United States.  Paraquat, a restricted use pesticide, is widely used for commercial applications. To protect personnel, field workers, and adjoining lands, new labeling is required, manual sprayers are banned, aerial spraying is restricted when it may reach adjoining crops, and personnel must wait longer periods before safely entering a sprayed field.  In many countries, including those in the European Union (EU), paraquat and diquat are not registered (approved) pesticides. [24,25] In contrast, the EU has approved plant growth regulators containing chlormequat and mepiquat as the active ingredients, particularly for conventional wheat and oats cultivation. The EU’s Maximum Residue Levels (MRLs) for these pesticides in fruits, vegetables, flowers, herbs, and cereals, range typically from 0.01 to 0.05 mg/kg.26 However, EU MRLs26 (mg/kg) for oat cereals are much higher, due to the expected presence of these pesticides and the challenge of obtaining accurate values from these complex samples. The EU MRL for paraquat was the exception, remaining at 0.02 mg/kg. The MRLs for the US, EU, and Canada are summarized in Table 1.
This IC-MS/MS application demonstrates the advantages of combining the optimum cation-exchange column, suppressed conductivity detection, and tandem mass spectrometry detection. The Dionex IonPac CS21-Fast-4µm cation-exchange column was designed to chromatographically resolve the four quaternary amine pesticides and the matrix ions within 15 min. The electrolytic suppressor neutralizes the eluent for sensitive conductivity detection and for compatibility with MS. Tandem MS detection using selective reaction monitoring (SRM) scans only for the ions of interest, eliminating the sample matrix and increasing sensitivity and selectivity.
In this application note, mepiquat, chlormequat, paraquat, and diquat cationic polar pesticides were determined by cation-exchange chromatography using an electrolytically generated methanesulfonic acid gradient at 0.3 mL/min and 40 °C on a 2 × 150 mm, Dionex IonPac CS21 cation-exchange column.
As the quaternary amines eluted from the column, they were detected by suppressed conductivity, heated to ionized gas by heated electrospray ionization (HESI-II), and detected by tandem mass spectrometry in SRM mode by the Thermo Scientific™ TSQ Altis™ Plus triple quadrupole mass spectrometer. The method was applied to modified QuPPE extractions of ground oat cereal samples and was found to be accurate and sensitive.