Rapid method of arsenic estimation in geological samples by WD-XRF using a novel concept of As-Pb concentration equivalence

The presence of arsenic in ground waters of many countries has been a subject of global concern due to its toxicity. Primary sources of arsenic are geogenic, i.e. weathering and erosion of rocks and soils containing arsenic. This paper presents a rapid method for determination of arsenic in solid geological samples by wavelength dispersive X-ray fluorescence spectrometer. To achieve the best LLD (lower limit of detection), the most intense X-ray fluorescence line Ka(1,2) is preferably used for determination of elemental concentrations because it pertains to the most probable transition. But the greatest challenge in arsenic estimation is the serious line overlap of AsKa(1,2) lines with the equi-energy PbLa1,2 lines. By using the conventional line overlap correction methods, uncertainty and detection limits in arsenic determination are degraded to an unacceptable degree in samples which contains high lead and low arsenic concentrations. The proposed method bypasses the line overlap issue in employing a novel concept of arsenic-lead concentration equivalence factor for the cumulative peak of AsKa(1,2) and PbLa1,2 fluorescence lines. The constancy of this factor for all geological matrices facilitates arsenic determination in samples universally irrespective of matrix elements. For the method validation, 22 international certified reference materials have been analysed and the results proved to be propitious wherein only one value out of 22 determinations showed relative error more than 20% of the certified values. This attests to the high accuracy of the proposed method which can effectively determine arsenic below 5 mg/kg in the presence of high lead concentration up to 1000 mg/kg.