Development of a sample preparation strategy for the determination of tungsten in soil samples by inductively coupled plasma mass spectrometry using a response surface methodology


Abstract: An alternative rapid digestion method has been developed for the determination of tungsten (W) in soil samples using fractionation studies and response surface methodology. The digestion method using the Kjeldahl instrument was applied to samples, and soluble tungsten was determined via inductively coupled plasma mass spectrometry. Digestion parameters were selected depending on the results of fractionation studies and optimized using a four-factor and five-level central composite design. The contributions of the phosphoric acid concentration (X$_{1})$, digestion temperature (X$_{2})$, digestion time (X$_{3})$, and hydrochloric acid concentration (X$_{4})$ were evaluated for the determination of W from samples in which its levels were maximum. Optimum conditions for factors were found to be X$_{1}$ = 9.00 M, X$_{2}$ = 124 $^{\circ}$C, X$_{3}$ = 45 min, and X$_{4}$ = 1.52 M using a predicted W level of 86.82 $\mu $g L$^{-1}$. This yielded an expected level of 1736.40 mg kg$^{-1}$ under the optimized conditions. Experimental W levels were in good agreement with this predicted level and found to be 1682.69 mg kg$^{-1}$. Thus, measured W levels showed the versatility of the central composite design in such a complex soil matrix for method development.

Keywords: Tungsten, tungsten mine, soil, environmental analysis, response surface methodology, central composite design, ICP-MS

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