Selective Determination of Dimethoate Using Ion Mobility Spectrometry with Single and Mixed Alternate Reagent Ions

Dimethoate, one of the organophosphorus pesticides, was selectively determined by ion mobility spectrometry (IMS) along with the single and mixed alternate reagent ions. By doping ammonium, acetone, ethanol, ether, and methanol into the carrier gas, the best selective detection was obtained by the addition of ammonium having the highest proton affinity among these single alternate reagent ions. Three mixed alternate reagents including ammonium−acetone, ammonium−ether, and acetone−ether, which were also used to improve the selectivity, showed better results than that single-solvent dopants gained. The best resolution and the highest abundance of the dimethoate peak were observed in its IMS spectrum after the addition of the mixed ammonium−acetone among all the above-mentioned dopants. The relationship curves for the concentration of ammonium and mixed ammonium−acetone to the response of dimethoate were studied. Ammonium at ∼1.0 μg/L levels or 1.0 μg/L ammonium at plus acetone at ∼1.0 μg/L levels exhibited the strongest response of dimethoate. In addition, the direct determination of dimethoate residues was compared to its selective detection with mixed ammonium−acetone as the alternate reagent ions in a field test. A model for the selective analysis of an analyte having middle proton affinity in mixed compounds is proposed.