Influence of sample acidification in the presence of humic substances in determination of pesticide by LC-MS/MS

The strength of the interaction between a pesticide and the soil organic matter is a key parameter to assess the risk of it reaching to groundwater with potentially harmful effects to human health. The humic substances (HS) play important role in the adsorption of pesticides. The mechanism of these reactions includes cation exchange, chelation, coordination, hydrogen bonding, charge transfer, hydrofobic bonding, π bonding, and van der Waals bonding. Many factors could affect the pesticide signals in LC-MS/MS analysis.  The chemical and physical properties of the analytes and the composition of the sample matrix are amongst them. Acidification of the solution can also affect the ionization of analytes (pesticides) in the ion source.

In this work, a approach that allows measuring such pesticide interactions  in acidic solutions (formic acid - FA, acetic acid - AA). These were representatives of pesticides phenoxycarboxylic, chloroacetanilide, urea, organophosphate, triazine, triazole and representatives of other pesticides. The measurements showed that different groups of pesticides exhibit different behaviors in the presence of humic acids. There were differences both between different groups of pesticides and between individual representatives within certain groups.

The resulting measured concentration values differed most from each other for organophosphate pesticides. At the same time, the structures of the molecules of these pesticides vary greatly. Dimethoat had a more pronounced signal drop than diazinon at lower pesticide concentrations (25 ng/l). The effect of the addition of FA and AA on the resulting concentration values also differed between the two pollutants studied. In diazinon, the addition of AA signal decreased in dimethoat, on the contrary, increased. The addition of FA in diazinon further dampened the signal, while dimethoat experienced a similar increase in analyte values as with the addition of AA. From the results of the measurements, it can be deduced that representatives of organophosphate pesticides show different behavior in the presence of HA.