Total reflection X-ray fluorescence spectrometric determination of ultra-trace uranium in natural water samples using a dispersive liquid–liquid micro-extraction method

Abstract

A total reflection X-ray fluorescence (TXRF) method in conjugation with dispersive liquid–liquid micro-extraction (DLLME) was developed for the determination of uranium present at ultra-trace levels in water samples. Calibration standard solutions, having uranium concentrations in the range of 5 ng mL⁻¹ to 50 ng mL⁻¹ (in 4 M HNO₃), were prepared by diluting a MERCK single element standard of uranium. The uranium present at ultra-trace levels was selectively extracted using 60 µL of the extractant, tributylphosphate (TBP) in dodecane. The uranium, extracted in the organic TBP phase, was back extracted by equilibrating it with Milli-Q water. Yttrium was added as an internal standard. The TBP from which U was stripped can be again reused for the same purpose and this leads to generation of negligible organic analytical waste. The detection limit of uranium was found to be 0.39 ng mL⁻¹. The developed methodology was further extended to analyze two tap water samples, spiked with different uranium concentrations with good agreement between the TXRF determined and spiked amount of uranium. This method can find applications for uranium determination in water samples of high purity, drinking water, etc. and to monitor the contamination of uranium in different water bodies.