Method validation and uncertainty for the determination of rare earth elements, yttrium, thorium and phosphorus in monazite samples by ICP-OES

A method is described for the inductively coupled plasma optical emission spectrometric (ICP-OES) determination of rare earth elements (REE), yttrium (Y), thorium (Th) and phosphorus (P) in monazite samples. Sample preparation was carried out by fuming with sulphuric acid followed by fluoride fusion of the remaining residue. The method was validated using the single laboratory approach by assessment of analytical performance characteristics like specificity, linearity, range, accuracy and precision. Spectral interferences were observed in the case of some heavy REE (Ho,Er,Tm) by light REE (Nd) and correction factors were deduced and applied. The limit of quantification, instrument linearity and the method range were evaluated. Relative standard deviation (RSD) values ranging from 2.6 to 10.2 % were obtained for repeatability studies and RSD values ranging from 1.7 to 11.1% for intra-lab reproducibility studies. Accuracy was established by application to a monazite certified reference material (CRM) and also through comparison of results obtained by present method with those obtained by an alternate method. The validation results were compliant with the acceptance criteria for the various parameters assessed. A simple procedure has been described for the estimation of associated measurement uncertainty using the GUM “bottom-up” modelling approach and results presented in this paper. The validated method was applied to the determination of REE, Y, Th and P in some monazite samples from India.