Thallium in spruce needles: a comparison of the analytical capabilities of spectrochemical methods

Abstract

Four spectrochemical methods are employed for the determination of Tl in spruce needles: ICP-MS (inductively coupled plasma mass spectrometry), ETV-ICP-MS (electrothermal vaporisation coupled to ICP-MS) analysis of decomposed and slurry samples (SlS), and SS-HR-CS-GFAAS (solid sampling high-resolution continuum source graphite furnace atomic absorption spectrometry). The methods are compared considering practical laboratory aspects (sample pretreatment, optimisation, validation, time-consumption), as well as their analytical performance. For this specific case of Tl in spruce needles, all the methods had similar requirements regarding the time required for sample pretreatment (1–2 hours, ETV-ICP-MS, SS-HR-CS-GFAAS < ICP-MS < SlS-ETV-ICP-MS). The method optimisation can be considered as routine for ICP-MS and more complicated for slurry and solid sampling, due to the differences in the matrix of standards and samples. ICP-MS optimisation required a several dozen minutes and dozens of hours for the ETV methods, which increases operational costs. ETV-ICP-MS requires a well experienced operator, skilled in both techniques (graphite furnace and ICP-MS). The order of complexity can be classified as follows: ICP-MS < SS-HR-CS-GFAAS < ETV-ICP-MS, SlS-ETV-ICP-MS. Requirements for validation in terms of the chemicals, calibration standards, and reference and control materials are similar. Major differences were evident in time demands. The procedural LODs were found typically below 1 μg kg⁻¹ of Tl (in μg kg⁻¹): 0.18 ICP-MS, 0.010 ETV-ICP-MS, 0.024 SlS-ETV-ICP-MS and 1.2 SS-HR-CS-GFAAS, which are appropriate for the Tl levels found in environmental samples. The range of recoveries did not differ significantly for the different methods used (in %): ICP-MS 92–107, ETV-ICP-MS 90–114, SlS-ETV-ICP-MS 94.5–117 and SS-HR-CS-GFAAS 90–115. ICP-MS revealed better repeatabilities (1.9–6.2%) than the others (ETV-ICP-MS 5.2–19%, SlS-ETV-ICP-MS 6.3–17% and SS-HR-CS-GFAAS 0.17–15%), as expected. In practice, ICP-MS shows significant advantages for analysis of Tl in spruce needles. However, it is necessary to consider all of the factors discussed. ETV techniques may have a justified place in special routine applications, in which the difficult development and validation of methods is much more convenient than the daily preparation of highly resistant or otherwise complicated samples.