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 mg g 21 Tl.

High-resolution continuum-source atomic absorption spectrometry: what can we expect?

“…Accurate results could be obtained only after a careful optimization of the temperature program and the modifiers, using a combination of ammonium nitrate to remove the bulk of the chloride and ruthenium to stabilize the analyte (Table 2). 39 When the same experiments were repeated using CS AAS, essentially the same results were obtained using no modifier and a low pyrolysis temperature of 300 °C, and with the modifier combination that had to be used for LS AAS with Zeemaneffect background correction. 37 The result of ≈ 0.2 µg/g Tl for SRM 1646a has been confirmed by several independent techniques in our laboratory in the meantime, and is not in contradiction to the recommended value of < 0.5 µg/g Tl.…”

“…The three-dimensional plot in Figure 5b shows the absorbance spectrum over time for Tl and for a matrix component after correction for continuous background absorption. As a strong correlation between the signal suppression for Tl and the sulfur content of the sediment had already been found with conventional GF AAS, 39 it was suspected that the molecular absorption spectrum in Figure 5b was part of the electron excitation spectrum of SO 2 . Hence, a reference spectrum was measured by atomizing 0.01 mg of KHSO 4 under the same conditions, which is shown in Figure 8a, and which obviously has great similarity with that in Figure 5b.…”

“…39 Even Zeeman-effect background correction could apparently not cope with the complex matrix of these samples. Accurate results could be obtained only after a careful optimization of the temperature program and the modifiers, using a combination of ammonium nitrate to remove the bulk of the chloride and ruthenium to stabilize the analyte (Table 2).…”

“…The authors are also grateful to Table 2. Determination of thallium in marine sediment reference materials using solid sampling LS-GF AAS with Zeeman-effect background correction (Z AAS), 39 and slurry sampling CS-GF AAS (CS AAS), 36 respectively, without a chemical modifier using a pyrolysis temperature of 300 °C (No Modifier), and using a combination of 0.4 mg Ru as a permanent modifier and 5% w/v NH 4 NO 3 added in solution (Ru+NH 4 NO 3 ). All values in µg/g; results are averages with standard deviation of at least n = 3 measurements…”

Increasing the working calibration range by means of artificial neural networks for the determination of cadmium by graphite furnace atomic absorption spectrometry

“…This parameter takes into account the variation in the degrees of freedom experienced by the residuals with the inclusion of further terms in a polynomial expression. Therefore, it provides a statistically more accurate reflection of the polynomial fitness than the coefficient of determination employed otherwise [35].…”