Next, a range of eutectic solvents were considered. (Deep) eutectic solvents are prepared by mixing two (or more) solid components, one HBD and one HBA, that form intermolecular bonds which yield a lower melting point than the individual components. Eutectic solvents are interesting due to the possibilities for fine-tuning solvent properties by mixing various components in different ratios. In EME, monoterpenes, such as menthol and thymol, have been explored as HBD components in eutectic solvents, and applied for peptides and some basic and acidic drugs , Menthol and thymol were paired with three HBA components with complementary properties, camphor (log P = 2.5), dodecyl methyl sulfoxide (DDMS, log P = 3.6), and 6methylcoumarin (log P = 2.3) in 2:1, 1:1, and 1:2 ratios. Camphor and DDMS are non-aromatic and offer HBA and dipole interaction, while the highly aromatic 6-methylcoumarin offers additional πtype interaction (π-π stacking, anion-π). Menthol-based solvents were all found unsuitable due to either poor solvent stability (did not remain liquid at room temperature), poor extraction efficiency, or due to dissolution into the acceptor solution. Mixtures with DDMS/6methylcoumarin and thymol in 2:1 ratio were also not liquid at room temperature. Figure shows the extraction performance of the best ratios for each combination. As seen, DDMS:thymol (1:2) and 6-methylcoumarin:thymol (1:2) were both slightly more efficient than camphor:thymol (1:1), and were therefore considered superior. With DDMS:thymol (1:2), recoveries were also fairly insensitive to a plasma matrix. The Kamlet-Taft values were α = 1.11, β = 0.30, π = 0.97, which indicated strong HBD and dipole interactions, and weak-to-moderate HBA interactions. Changing the ratio to 1:1 caused the acceptor solution to become cloudy, which was attributed to the very low solubility of DDMS and consequent formation of microdroplets. For 6methylcoumarin:thymol, the extraction efficiency was similar for both 1:1 and 1:2 ratios (2:1 was not liquid at room temperature), but the current was somewhat lower for the 1:2 ratio. The Kamlet-Taft values for this ratio were determined to be α = 1.01, β < 0.55 (exact value could not be determined due to high background absorbance), π = 1.09. Based on these observations, DDMS:thymol (1:2) and 6-methylcoumarin:thymol (1:2) were selected as the best candidates. As an additional remark, Hong et al. recently suggested that aromatic liquid membranes may be more susceptible to fouling by matrix components when extracting bases, causing a decrease in extraction recovery . The current data may indicate that this phenomenon is less prominent for acidic analytes, as no major decrease from neat standard to plasma was observed with 6methylcoumarin:thymol (1:2). We suggest this could be related to the lack of cation-π interactions for the reversed voltage polarity.