Validation of analytical methodologies for the determination of rare earths in environmental samples by ICP-MS

The group called Rare Earth Elements (REE) are highlighted in the strategic, economic and social spheres, due to their association with products of greater technological value. Indeed, the significant increase in the exploitation of REE makes their availability to the environment increasingly evident. In this context, practical and reliable methods to monitor REE in environmental samples becomes a fundamental prerogative, since the effects of their exposure are not yet well established. Furthermore, the group's coherent behavior makes REE excellent markers of geochemical and hydrological processes. In this work, the results of validation of two analytical methods for the determination of Rare Earths by inductively coupled plasma mass spectrometry (ICP-MS) are reported, in order to be applied to surface water and sediment samples. The sample preparation procedures were in accordance to US EPA (United States Environmental Protection Agency) standard methods 3015A for surface water samples and 3051A for sediment samples. The performance was evaluated on the following figures of merit: working range and linearity, detection and quantification limits, precision in terms of repeatability and intermediate precision, and accuracy in terms of trend and analytical recovery. The use of elements commonly used as internal standards (IS) in ICP-MS was evaluated, revealing double charge spectral interference, caused by both Rare Earth isotopes and other elements commonly found in environmental samples, such as Ba. Both the surface water method and the sediment method showed a linear response in the investigated work ranges, with determination coefficients greater than 0.9980, significance of linear regression by the least squares method and good fit to the applied model. The validated working range for the surface water method was 5.5 – 116.6 µg L-1. For the sediment method, the working range for most elements was 0.05 – 2.5 mg kg-1. The exceptions were La, Ce and Nd, with a range of 0.05 – 125 mg kg-1. The detection limits of the methods were obtained at ultra-trace levels, with values less than 0.375 µg L-1for the surface water method and less than 0.015 mg kg-1 for the sediment method. The theoretical limits of quantification were estimated and confirmed for their precision and accuracy at analytical concentration levels of 2.0 µg L-1 for surface water and 0.1 µg µg L-1 for sediments. Regarding the method, the LQ values were 5.5 µg L-1 for surface water and 0.05 mg kg-1 for sediments. The methods showed adequate precision along the entire length of the validated linear range, with coefficient of variation (CV) values lower than 4.0%. The exception was the lowest quantification level of the sediment method, 0.1 µg L- 1, with the highest CV value reaching 19.8%. Accuracy was satisfactory for all isotopes evaluated, with analytical recoveries remaining within the range of 80 – 110% for both the surface water and sediment methods. Validated methods meet established performance criteria and are suitable for their intended use.