Abstract

The capability of in situ trapping of selenium and arsenic hydrides within a bare and modified (Pd, Pt, Ir and Re ) tungsten tube atomiser (WETA) was investigated by electrothermal atomic absorption spectrometry and by a radiotracer technique using ⁷⁵Se radionuclide. Low efficiency of trapping (below 5%) was found within the bare tungsten tube. On the other hand, modification with 100 µg of Pt, Ir and Re, respectively, enhanced significantly the efficiency of trapping. Platinum-treated tubes were very efficient at collecting hydrides in the low temperature range of 100–200 °C, reaching efficiency close to 100%. Iridium- and rhenium-treated tubes were capable of trapping hydrides in the high temperature range of 700–900 °C, approaching an overall efficiency of 10%. Palladium modifier caused fatal deterioration of the tungsten tube. Low- and high-temperature mechanisms of trapping on the inner tube surface were seen, which were probably related to the low temperature dissociative chemisorption of hydrides and to the thermal decomposition of hydrides with simultaneous adsorption of products on the surface, respectively. Radiography experiments proved that the major part of selenium was collected at low temperature on a relatively small area in the central part of the platinum-treated tube opposite the injection hole. On the other hand, selenium collected at higher temperatures was found in a more diffuse area when iridium-treated and bare tubes were used. The following analytical figures of merit were found when sequestrating hydrides of selenium and arsenic within a platinum-treated tube: characteristic masses of 74 and 31 pg Se, 27 and 14 pg As pertaining to integrated absorbance and absorbance height of the signal, respectively; and limits of detection, based on 3 s-criterion, of 150 pg As and 270 pg Se were achieved. They related to a sample volume of one millilitre and fluctuation of overall blanks. The useful lifetime of platinum modified tubes reached 250 firings under optimum conditions.