Evaluation of different quantification modes for a simple and reliable determination of Pb, Zn and Cd in soil suspensions by total reflection X-ray fluorescence spectrometry

In view of the large number of soil analyses in environmental studies dealing with metal contamination, the use of simple, fast, sustainable and cost-effective methodologies is desired. In this sense, the role of total reflection X-ray fluorescence spectrometry (TXRF) has to be highlighted due to the possibility of direct analysis of solid suspensions without the need for a digestion process. Internal standardization is successfully used as a quantification approach in multielement TXRF analysis of liquid samples, including soil digests. However, its application for the analysis of soil suspensions is not always possible. In the present manuscript we found that although acceptable results were obtained for TXRF quantification by internal standardization when considering soil certified reference materials (recoveries: 80–120%), a systematic underestimation of the concentration values was found when using the same approach for real soil sample analysis (recoveries <70%). In view of these problems, we explored the possibilities of different quantification approaches, including external calibration, which can be used in combination with soil suspension preparation and TXRF analysis to obtain reliable results for metal determination in soils. Data obtained showed that the best strategy was the use of internal standardization with correction of the TXRF results using a calibration curve obtained in the analysis of a set of soil samples with a matrix similar to the unknown ones by using a reference technique (digestion and ICP-OES analysis). With this analytical approach quantitative results were obtained for all the studied metals with recovery values around 95%. As a study case, the proposed TXRF method has been applied for the specific determination of Pb, Zn and Cd concentrations in soils from an abandoned metal mining area but the developed methodology could be extended to other environmental studies dealing with the determination of other metals and soil types. Taking into account the selectivity and sensitivity limitations when determining high Z elements such as Cd using TXRF systems equipped with Mo X-ray tubes, we compare the analytical performance of TXRF systems with different sources (Mo and W) as well.