Using ICP/MS or ICP/OES Method for Detection of Dimethyl Thallium in Water Samples

The high toxicity of thallium (Tl) means this element is of significant environmental relevance. Industry, waste handling and mining are a few examples of the various processes that result in TI emission. In environmental studies, it is usually the inorganic species of thallium that is considered.

Research into marine ecosystems is currently being conducted to establish the occurrence of organic Tl species such as the dimethyl thallium cation. Only very limited information is available on the environmental relevance of this organometallic compound, mainly because analytical procedures for determining dimethyl thallium at low concentration levels are lacking.

Detection of dimethyl thallium in water samples at low concentration levels

To date, the occurrence of dimethyl thallium cation Me₂Tl – an organic species of thallium – in marine ecosystems has only been revealed by a few investigations. Schedlbauer and Heumann were the first to present evidence of its occurrence, in 1999 (Schedlbauer, O.F., 2000).

This research is focused on the development of suitable analytical procedures for determining trace levels of dimethyl thallium. Two methods were explored, one being ion chromatography coupled with inductively coupled plasma mass spectrometry (IC-ICP/MS) and the other being a liquid chromatography/mass spectrometry (LC/ MS)-based method. (Schedlbauer, O. F., 2000)

Results

The detection and quantification of Me₂Tl in standard solutions and natural samples was achievable using the IC-ICP/MS method. This is the most suitable technique available, since it ensures any effects of  sample pre-treatment on the stability of the Me₂Tl complex are kept to a minimum, as shown in Figure 1.

Figure 1. Full IC-ICP/MS run of a 1 μg/L 23-element standard solution from Merck after addition of 1 μg/L of Tl, Rb, and Me₂Tl, respectively. The first Tl peak is formed by Me₂Tl⁺ and the second one by Tl⁺. The low concentration was chosen to be able to evaluate the suitability of the method for the low concentrations of Me₂Tl expected in environmental samples. Standard peak area could be used for internal standard calibration of these kinds of samples. Column: Metrosep C 4 -250/4.0; eluent: 1.7 mmol/L HNO₃; flow rate: 0.9 L/min.

Indirect anodic stripping voltammetric determination of Tl(I) and Tl(III) in the Baltic seawater samples

Plant tissues and water have been found to contain the oxidized forms of thallium and Me₂Tl has been detected in Atlantic seawater. Here, the proposed analytical method for studying Tl speciation in water samples involves chromatographic separation with subsequent detection by inductively coupled plasma mass spectrometry (ICP/MS) or by inductively coupled plasma optical emission spectrometry (ICP/OES).

To ensure that monovalent thallium Tl(I) was retained in the presence of EDTA, a microcolumn containing immobilized oxine on surfactant-coated alumina was used and Tl(I) was thereby isolated from other species of thallium. Evaluation of Tl(III) concentration was carried out on the basis with or without the addition of DTPA, which serves as a stabilizer and prevents the reduction of Tl(III).

Results

The above method was used to examine the Baltic seawater sample enriched with thallium. The chromatograms indicated that Tl(III) was reduced by ascorbic acid, a reduction that could be prevented by stabilizing Tl(III) with DTPA, as illustrated in Figure 2. Chromatograms of two prepared samples that contained different amounts of Tl(III) are shown in Figure 3.

The peak of Tl(III)-DTPA was recorded at a retention time of 350 seconds. Chromatographic analysis of both thallium species revealed a Tl(III) concentrations of 24 ± 4 ng/mL and a Tl(I) concentration of 284 ± 6 ng/mL.

Figure 2. IC chromatograms of seawater spiked with standards of 280 μg/L Tl(I) and 25 μg/L Tl(III) and conserved with ascorbic acid or DTPA solution (diluted 1:10). The signals were recorded with online ICP/MS detection of ²⁰⁵Tl. Column:  Hamilton PRP-X100 (250/4.1); eluent: 100 mmol/L ammonium acetate, 5 mmol/L DTPA (pH 6.2); flow rate: 1.5 mL/min.

Figure 3. IC chromatograms of seawater spiked with standards of 280 μg/L Tl(I) and 25 μg/L Tl(III) (a) and of the same sample after addition of 500 mg/L Tl(III)-DTPA (b). Both samples were diluted 1:10. The signals were recorded with online ICP/MS detection of ²⁰⁵Tl.

Thallium – Further Applications with IC-ICP/MS

Tl-speciation of aqueous samples – a review of methods and application of IC-ICP/MS/LC-MS procedures for the detection of (CH₃)₂Tl⁺ and Tl⁺ in river water. Sindern, S.; Schwarzbauer, J.; Gronen, L.; Görtz, A.; Heister, S.; Bruchmann, M. (2015) Int. J. Environ. Anal. Chem. 95(9), 790–807

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