Removal of isobaric interferences in isotope dilution analysis of vanadium in silicon nitride fine ceramics powder by DRC-ICP-MS

Abstract

The National Metrology Institute of Japan (NMIJ) is developing certified reference materials (CRMs) of non-oxide fine ceramics such as silicon carbide and silicon nitride, and isotope-dilution mass spectrometry (IDMS) has been used as one of the key analytical methods for the determination of trace metals in CRMs. IDMS essentially requires two isotopes free from spectral interference; however, it is quite difficult to find such isotopes, especially in the determination of first transition metals. Therefore, specific measurement techniques are necessary to exclude the effect of the spectral interference. Spectral interferences due to the matrix and/or solvent can be removed by ICP-SFMS under a medium resolution. However, it is impossible to remove isobaric interference even with ICP-SFMS. Particularly in the isotope-dilution (ID) analysis of V in fine ceramics using ICP-MS, isobaric interference causes severe problem because the mass spectra for one of the two V isotopes overlaps with that for ⁵⁰Ti and ⁵⁰Cr. The authors proposed a new selective method for determination of V in the presence of Ti and Cr using DRC-ICP-MS with a CH₃F + NH₃ mixed reaction gas. We found formation efficiencies of [MF₂(NH₃)₄]⁺ (M = V, Cr, Ti) significantly differed with elements, and the optimum condition for selective formation of [VF₂(NH₃)₄]⁺ was found by controlling the gas-flow rates of CH₃F and NH₃. The IDMS analytical result of V in fine ceramics with this improved system agreed well with that obtained by a matrix matching calibration method within the expanded uncertainty. In conclusion the reduction of isobaric interference in DRC-ICP-MS expands the application fields of IDMS.