Determination of selenium by electrothermal atomic absorption spectrometry (ETAAS) is complicated by the presence of different species of this analyte. The presence of different oxidation states (-II, IV and VI) may result in different sensitivities obtained for each species rendering impossible the use of a single species for calibration. These species also exhibit different behaviours regarding thermal stabilities; the temperature program must be provided to conform to this problem. Chemical modifiers are commonly used for thermal stabilization of selenium species. In this study, experiments were carried out to demonstrate the effect of nitric acid in the presence of chemical modifiers. Nickel and palladium + magnesium were selected as the most commonly used chemical modifiers. Using both aqueous and human serum solutions it has been demonstrated that although chemical modifiers provide thermal stabilization of species so that higher ashing temperatures can be used, equal sensitivities cannot be achieved unless nitric acid is also present. Selenite, selenate, selenomethionine and selenocystine were used in experiments. When equal sensitivities for all these species are achieved, determination of total selenium by ETAAS can be performed by using a single species as the standard; selenite was used in this study. Precision was 5.0% or better using peak height signals. There was no significant difference in detection limits (3s) when Ni or Pd+Mg(NO3)(2) was used as chemical modifier; 37 and 35 pg of selenium were found to be the detection limits for Ni and Pd + Mg(NO3)(2) chemical modifiers, respectively. For chemical modifications, either 5 mu g of Ni or 0.5 mu g of Pd and 5 mu g of Mg(NO3)(2) were used; final solutions contained 2.5% HNO3. In serum analyses, 10 mu g of Ni was used in presence of 2.5% HNO3. (c) 2005 Elsevier B.V. All rights reserved.