Ammonium salting out extraction with analyte preconcentration for sub-part per billion quantitative analysis in surface, ground and drinking water by flow injection tandem mass spectrometry†
A recently-reported extraction method that merged principles of QuEChERS and salting out liquid–liquid extraction (SALLE) by using ammonium salts (chloride, acetate, formate) has been improved for high-throughput quantitation of bioactive chemicals in water by flow injection tandem mass spectrometry (FI/MS/MS). The new method for water analysis uses the ratio of acetonitrile/water adjusted to yield extract preconcentration in one step: 7.0 mL water aliquot, 3.5 mL of acetonitrile extraction solvent and 3.0 g of NH4Cl (s) resulted in post-extraction acetonitrile phase volume of 1.4 mL (extract preconcentration factor = 5). This preconcentration factor can be adjusted by changing the acetonitrile/water aliquot ratio to achieve the desired sensitivity while optimizing method performance. The acetonitrile/water partition coefficients (Kaw) of analytes in the acetonitrile/water/NH4Cl ternary systems were measured during method development. Kaw values were used to predict analyte recoveries when the acetonitrile/water ratios were varied in the extraction procedure. The effect of aqueous system pH was evaluated and the results were used for extraction optimization. A validation study was successfully conducted in creek, pond, ground, and drinking water for the following pesticides: aminocyclopyrachlor methyl, azimsulfuron, chlorantraniliprole, chlorimuron ethyl, chlorsulfuron, cyantraniliprole, diuron, flupyrsulfuron methyl, hexazinone, oxamyl, methomyl, sulfometuron methyl, triflusulfuron methyl. The method met the stringent 0.1 μg L−1 (ppb) limit of quantitation (LOQ) specified by the European Union for 10 out of 13 pesticides tested. The LOQ for the 3 least responsive analytes, i.e. chlorsulfuron, oxamyl and methomyl, was 0.3 μg L−1. Limits of detection (LODs) were between 10 and 100 ng L−1 (ppt). FI/MS/MS acquisition time was 30 seconds/injection. The correlation between analyte recoveries and publicly-available physicochemical properties, such as octanol/water partition coefficients (Kow) and aqueous solubility, was also assessed, allowing the prediction of method applicability to other chemicals, such as pharmaceuticals and other pesticides not tested in the study.