APCI-MS/MS: A powerful tool for the analysis of bound residues resulting from the interaction of pesticides with DOM and humic substances

Mass spectra (MS) in positive and negative ionization mode of different humic substances (HS) and dissolved organic matter (DOM) were measured using electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) as transfer ionization techniques. With APCI the average molecular masses were reduced by a factor of 4 to 6 compared to the softer electrospray ionization. This might be the result of breaking charge-transfer bonds, hydrogen bonds or weak covalent bonds between DOM molecules. The calculated average molar masses of HS were in a range as reported by other authors. For bound anilazine residues ESI and APCI were used combined with selective reaction monitoring (SRM), a MS/MS technique for the quantification of specific molecules in a complex mixture. Applying APCI we found a high release of the main anilazine metabolite dihydroxyanilazine. These amounts were similar to those obtained by silylation, a technique reported to release sequestered pesticides. Furthermore, the release corresponding to molecular weight of bound residues was analyzed. Anilazine is bound to all molecular weight fractions as was revealed by size exclusion chromatography(SEC) with radioactivity and APCI-SRM detection. In addition, we tested the matrix effects of different HS with APCI revealing a deviation of the correlation of measured MS signal intensity in function of ingredient concentration that depended on the type and concentration of HS.