Control of spectral and non-spectral interferences in the determination of thallium in river and marine sediments using solid sampling electrothermal atomic absorption spectrometry

Abstract

The direct analysis of solid samples using electrothermal (graphite furnace) atomic absorption spectrometry (ETAAS) has been investigated for the determination of thallium in river and marine sediment reference materials, because complete digestion of sediment samples requires the use of hydrofluoric acid and/or an alkaline fusion, and the extraction with aqua regia might be incomplete and cause interferences in the determination of thallium. The determination of thallium in river sediments using direct solid sampling ETAAS was straight forward, and could be carried out with good accuracy even without the use of a chemical modifier or calibration against aqueous standards. The analysis of marine sediments, in contrast, proved to be extremely difficult due to severe spectral and non-spectral interferences. The latter ones were caused by the relatively high chloride content of marine sediments, compared to river sediments, and could eventually be controlled by the addition of ammonium nitrate as a chemical modifier together with ruthenium as a permanent modifier. The spectral interference could only be overcome with Zeeman-effect background correction, and was most likely caused by sulfate. After optimization of the procedure, thallium in marine sediment reference materials could be determined by calibration against a certified river sediment reference material. According to the experience gained with river sediments, it might be assumed that aqueous standards could equally be used for calibration; this approach, however, was not further investigated. The results were in good agreement with non-certified ‘information’ values for thallium and with results obtained by ICP-MS using electrothermal vaporization and isotope dilution calibration. A characteristic mass of 13 pg was obtained, and the limit of detection of the proposed method, based on the zero-mass response (three times the standard deviation of 10 atomization cycles with empty platforms), was around 0.02 µg g−1 Tl.