Natural deep eutectic solvent-based ultrasound-assisted-microextraction for extraction, pre-concentration and analysis of methylmercury and total mercury in fish and environmental waters by spectrophotometry.

In this study, a simple, pH-controlled, and natural deep eutectic solvent-based microextraction (NADES-ME) procedure is proposed before the spectrophotometric analysis of methylmercury (MeHg) and total mercury in fish and environmental waters. The method is based on charge-transfer sensitive ion-pair formation between MeHg and 3-(dimethylamino)-7-(methylamino)phenothiazin-5-ium chloride (Azure B - AzB) in the presence of chelating citrate-phosphate buffer at pH 6.0, and then extraction of the formed ion-pair complex with the NADES. The important variables affecting extraction efficiency were evaluated and optimised. At optimal conditions (300 µL of 1.0 × 10-3 mol L-1 AzB, 600 µL NADES (betaine-sorbitol, 1:3) containing 10% water (v/v) as extractant, 375 µL acetonitrile as aprotic solvent for sonication of 7 min at power of 300 W/40 kHz, and centrifugation at 4000 rpm for 3 min, respectively), the figures of merit for MeHg include a good linearity in the concentration range of 0.7-340 µg L-1, limits of detection and quantification of 0.25 and 0.83 µg L-1, and pre-concentration and sensitivity enhancement factors of 120 and 95, respectively. Figures of merit for total Hg (iHg, where Hg22+ ions are predominantly present in natural oxygen-rich waters such as freshwaters and seawater as well as elemental Hg and Hg2+ ions at very low amounts) after pre-oxidation at pH 5.0 include linearity range of 3-270 µg L-1 with limits of detection and quantification of 0.92 and 3.10 µg L-1, and pre-concentration and sensitivity enhancement factors of 120 and 35, respectively. The reliability (with recovery of 92-98.7% and RSD of 1.9-5.5%) was statistically validated by analysis of two standard reference materials (SRMs) with and without spiking. The method was successfully applied to the speciation analysis of total Hg, iHg and MeHg in fish and environmental waters.