Utility of ultrasound assisted-cloud point extraction and spectophotometry as a preconcentration and determination tool for the sensitive quantification of mercury species in fish samples

Abstract

The current study reports, for the first time, the development of a new analytical method employing ultrasound assisted-cloud point extraction (UA-CPE) for the extraction of CH₃Hg⁺ and Hg²⁺ species from fish samples. Detection and quantification of mercury species were performed at 550nm by spectrophotometry. The analytical variables affecting complex formation and extraction efficiency were extensively evaluated and optimized by univariate method. Due to behave 14-fold more sensitive and selective of thiophene 2,5-dicarboxylic acid (H₂TDC) to Hg²⁺ ions than CH₃Hg⁺ in presence of mixed surfactant, Tween 20 and SDS at pH5.0, the amounts of free Hg²⁺ and total Hg were spectrophotometrically established at 550nm by monitoring Hg²⁺ in the pretreated- and extracted-fish samples in ultrasonic bath to speed up extraction using diluted acid mixture (1:1:1, v/v, 4molL^-1 HNO₃, 4molL^-1 HCl, and 0.5molL^-1 H₂O₂), before and after pre-oxidation with permanganate in acidic media. The amount of CH₃Hg⁺ was calculated from difference between total Hg and Hg²⁺ amounts. The UA-CPE method showed to be suitable for the extraction and determination of mercury species in certified reference materials. The results were in a good agreement (with Student's t-test at 95% confidence limit) with the certified values, and the relative standard deviation was lower than 3.2%. The limits of detection have been 0.27 and 1.20μgL^-1, for Hg²⁺ from aqueous calibration solutions and matrix-matched calibration solutions spiked before digestion, respectively, while it is 2.43μgL^-1 for CH₃Hg⁺ from matrix-matched calibration solutions. A significant matrix effect was not observed from comparison of slopes of both calibration curves, so as to represent the sample matrix. The method was applied to fish samples for speciation analysis of Hg²⁺ and CH₃Hg⁺. In terms of speciation, while total Hg is detected in range of 2.42-32.08μgkg^-1, the distribution of mercury in fish were in range of 0.7-11.06μgkg^-1 for CH₃Hg⁺ and in range of 1.72-24.56μgkg^-1 for Hg²⁺.

Keywords: Fish samples; Mercury species; Spectrophotometry; Ultrasound assisted extraction.