Chinese scientists have optimized a method for the determination of Tl(I) and Tl(III) in water samples.
Background: Thallium (Tl) is used in various industrial applications such as semiconductors, pharmaceutical products, catalysts, glass production, pigments, thereby increasing the risk of occupational poisoning and environmental pollution. It is also a by-product of heavy-metal sulphide ore mining, and is present in flue dust from coal-fired power plants and cement kilns and thus is being distributed in the environment as anthropogenic pollution. Soluble Tl salts are toxic, and they were historically used as rat poisons and insecticides, but later banned because of their non-selective toxicity and their popularity as a murder weapon. Thallium is one of the 13 priority metal pollutants listed by the US EPA and for mammals is known to be more toxic than Hg(II), Pb(II) or Cd(II).
In the aquatic environment, Tl may be present in two oxidation states, Tl(I) and Tl(III), with Tl(I) being the more stable species and therefore often the dominating species. Tl(III), the more reactive species, is also the much more toxic species. For a meaningful risk assessment and understanding of the environmental behaviour, speciation analysis differentiating the two species is mandatory.
The new study: The group of researchers from China focused their study on the optimization of the chromatographic separation of the two thallium species by using ion exchange methodology under isocratic conditions. The key mechanism used for separation is complex formation of Tl(III) with DTPA whereby the charge of the Tl-DTPA complex depends on pH. While at pH<2, the complex is protonated and therefore positively charged, at pH>6 the complex is negatively charged. Anyhow, in principle, Tl(I) and Tl(III)-DTPA can be separated using IEC.
The authors compared anion and cation exchange chromatography and optimized pH, mobile phase (DTPA and ammonium acetate concentration) injection volume and flow rate. Best detection power was obtained by AEC. Optimum values for the mobile phase used for anion exchange column were pH=4.2, 3 mmol/L DTPA and 200 mmol/L ammonium acetate. In order to obtain high detection power, the injection volume was maximized to 100 µL and the flow was set to 1 mL/min because of pressure limitations. Under these conditions LODs of 3 and 6 ng/L were obtained for Tl(1) and Tl(III) within a chromatographic run of 10 min.
When a cation-exchange column was used, the LODs were a factor of 2-3 worse, however separation was achieved within less than 6 min. The mobile phase for cation exchange chromatography was 15 mmol/L HNO3 and 3 mmol/L DTPA.
The authors mentioned that real natural water samplers could contain redox active species such as iron, leading to the oxidation of Tl. For such reason, samples should be analysed as soon as possible.
The original study:
Yuexin Zhao, Fang Cheng, Bin Men, Yi He, Hui Xu, Xiaofang Yang, Dongsheng Wang,
Simultaneous separation and determination of thallium in water samples by high-performance liquid chromatography with inductively coupled plasma mass spectrometry, J. Sep. Sci., 42/21 (2019) 3311-3318.
DOI: 10.1002/jssc.201900593
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Simultaneous separation and determination of Tl(I) and Tl(III) by IC-ICP-OES and IC-ICP-MS. Water. SA., 29/1 (2003) 17–22. DOI:
10.4314/wsa.v29i1.4940
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last time modified: September 22, 2024
