A new simple, highly sensitive and selective spectrofluorimetric method for the speciation of thallium at pico-trace levels in various complex matrices using N-(pyridin-2-yl)-quinoline-2-carbothioamide

Abstract

A very simple and non-extractive new spectrofluorimetric method for the determination of Tl^I and Tl^III individually and for mixtures of both analytes at pico-trace levels using N-(pyridin-2-yl)-quinoline-2-carbothioamide (PQCTA) has been developed. PQCTA reacts in a slightly acidic (0.0025–0.05 M H₂SO₄) solution with Tl^III in 20% ethanol and is oxidized itself to produce highly fluorescent oxidized product in aqueous solution, which has the excitation and emission wavelengths of λ_ex = 324 nm and λ_em = 379 nm, respectively. The determination of Tl^I is based on the rapid oxidation of this ion by bromine water heating for 15 min with the concomitant formation of fluorescent Tl^III-PQCTA. Fluorescence due to the sum of Tl^I and Tl^III is measured and Tl^I is determined from the difference in fluorescence values. Constant and maximum fluorescence intensities were observed for the period between 2 min and 24 h. Linear calibration graphs were obtained for 0.001–600 μg L⁻¹ of Tl, providing a detection limit of 0.16 ng L⁻¹. The quantification limit of the reaction system was 1.6 ng L⁻¹ and the RSD was 0–2%. A large excess of over 80 cations, anions, and complexing agents (such as chloride, phosphate, azide, tartrate, oxalate, and SCN⁻) do not interfere in the determination. The developed method was successfully used in the determination of thallium in several Standard Reference Materials (SRM) as well as in some environmental waters, biological fluids, soil and food samples, solutions containing both Tl^I and Tl^III, and complex synthetic mixtures. The results of the proposed method for biological samples, vegetables, food, soil, and water analyses were found to be in excellent agreement with those obtained by ICP-OES, AAS, and ICP-MS.