Understanding partitioning in two-phase systems with induced miscibility gaps—comparing SWIEET and QuEChERS

Our salt-free SWIEET extraction method, recently developed, uses a mixture of acetonitrile and isopropanol as extraction solvents for extractions from aqueous solutions. Glucose was added to induce a phase separation between an aqueous and an organic phase, both of which contain water, acetonitrile, and isopropanol, but in different fractions. During method development, we observed that the amounts of isopropanol or glucose added greatly influenced analyte recoveries. This study aims to better understand the effects underlying especially the enhanced extraction of very polar and charged analytes. We varied the type and concentration of additives in SWIEET double extractions and investigated their effects on the recoveries of a broad range of analytes. Furthermore, the organic and aqueous phases resulting from the extractions were characterized with NMR, solvatochromic dyes, and Karl Fischer titration, to gain a deeper understanding of the effects of phase composition, the polarity and water content of the phases, and the phase ratio. Significant differences between QuEChERS and SWIEET were observed regarding the water content of the organic phase. In contrast, differences in phase composition between SWIEET double extraction steps were minor, and we discussed the high phase ratio of the organic to aqueous phase to be a major reason for the enhanced recoveries of polar analytes in the second extraction step.