DETERMINATION OF ARSENIC AND SELENIUM IN TAP WATER SAMPLES BY TXRF SPECTROMETRY

The quantitative measurements of arsenic and selenium in tap water is a very important task since Brazilian legislation establishes the limit of 0.01 mg L-1 for both elements in this matrix. Besides Atomic Absorption Spectrometry (AAS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES) have been routinely used on this kind analysis, the Total Reflection X-ray Fluorescence spectrometry (TXRF) show to be a very interesting alternative since it provides a fast and easy sample preparation besides low analytical operation and maintenance costs.
This study shows the feasibility of using TXRF to quantify As and Se in tap water. Selectivity and linearity have been evaluated according to CGCRE 008 of Instituto Nacional de Metrologia, Normalização e Qualidade Industrial (INMETRO). All measurements were performed using TXRF spectrometer S2 PICOFOX (Bruker), equipped with an air cooled low power X-ray tube (Mo target). For internal calibration, 1 mL of sample was mixed with 20 uL of a standard solution (Ga; 10 mg L-1). 10 uL of the mixture was placed on a quartz glass carrier and dried in a drying oven.
The F-tests and t-tests were performed considering 95% confidence and 6 degrees of freedom, to evaluate the selectivity. After performing the readings for As and Se, the values of 2.00 and 2.75 respectively for the F-test and 1.789 and 2.161 respectively for the t-test were obtained. These values indicate that the selectivity for both elements was adequate. For the linear correlation coefficient (R2), obtained for reference solutions between 0.005 and 0.02 mg L-1 of As and Se, values of 0.995 and 0.997 respectively were obtained. These values indicate that the method showed good linearity. The accuracy was checked by adding 10 ug L-1 of both elements from 1000 mg L-1 of As (L00431A, Ultra Scientific) and 1000 mg L-1 of Se (L00416, Ultra Scientific) onto tap water sample (10035RMOC, Sabesp). Recovery about 92.1% (9.21+0.82 ug L-1) for As and 94.7% (9.47+0.64 ug L-1) for Se were obtained indicating good accuracy of proposed methods. The limits of detection, calculated by the software Spectra Picofox Version 7.5.3.0, are 0.26 ug L-1 for As and 0.25 ug L-1 for Se and the maximum allowable concentration for both elements is 10 ug L-1. Considering all this, we can conclude that TXRF spectrometry showed to be a very interesting alternative to be used in As and Se measurement on water samples analysis.