A high accuracy reference method for the determination of minor elements in steel by ICP-OES

An ICP-OES method is described for the high accuracy analysis of minor elements in steel (Mn, Mo, Cr and Ni) with full uncertainty evaluation of ∼1% at the 95% (k = 2) confidence interval. The technique is ideally suited to the production of reference materials and standards. Samples are analysed by the exact matching technique in which a single elemental standard is prepared by closely matching the concentrations of sample and standard so that the emission intensity ratio of sample to standard is ∼1. Sample and standard are doped with an internal standard solution of gold to correct for instrumental drift. Critically, the sample and standard are also matched with respect to acid strength and Fe, the dominant matrix element. The Fe matrix is matched to within 3%. Without Fe addition, significant biases in ICP performance are observed. The single point calibration technique is readily amenable to an estimation of a complete uncertainty budget, taking into account all of the uncertainty contributions that derive from the protocol. The combined uncertainty is dominated by repeatability of the method, linked to sample preparation, rather than instrumental effects.