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.