Arsenic is a toxic element, acting as a nerve poison and proven carcinogen. Its main source f or humans is drinking water where the most important are arsenic (V) compounds. With respect to the Slovak standard, in force since 1998, the maximum tolerable value is defined as the important characteristic of drinking water. If the value 0.01 mg.l –1 of arsenic is exceeded, water is not drinkable. The mentioned standard defines criteria fo r precision of the arsenic determination and the corresponding limit of detection – being 10 % of the maximum tolerable value. Arsenic content in the drinking water samples was measured by the AAS method. Internal Quality Control for analysis by the ETA AAS method was based on the harmonised guidelines recommended by IUPAC, ISO and AOAC. The following metrological characteristics were calculated: precision, accuracy, and the yield of the arsenic standard addition to the studied sample – matrix spike and analytical spike. In addition to it, a thorough study of the limit of detection, LOD, and the limit of quantification (determination), LOQ , was performed. The LOD and LOQ values were calculated in the traditional way as well as by the ULA technique, newly recommended by IUPAC. In the latter case, the calculated LOD value was close to the demanded 10 % limit of the maximum tolerable value, which is 1 mg.l –1 As. The repeatability and reproducibility, found at two concentration levels 56,02±0,73 mg.l –1 and 6,0±0,9 mgl –1 , were 3.2 % and 3.9 %, respectively, and 7.1 % and 9.8 %, respectively. Trueness of the applied AAS method was proven satisfactory on the reference material SRM 1643 Trace Elements in Water.

        Key words: arsenic determination, AAS, drinking water, limit of detection, quality assurance, metrological characteristics