Accurate and precise Th/Th isotope ratio measurement by multi-collector inductively coupled plasma mass spectrometry using a pre-cell mass filter for collision/reaction cell (MC-ICP-MS/MS).

In this paper we present the high-precision and accurate Th/Th isotope ratio measurement by a novel multi-collector inductively coupled plasma mass spectrometer with a pre-cell mass filter and a MS/MS collision/reaction cell (Neoma™ MC-ICP-MS/MS). No tailing correction is needed for the Th signal, as the tailing of an adjacent high intensity peak (abundance sensitivity) is effectively reduced due to high extent of Ar-based ion removal before the electrostatic analyser/magnetic sector. The obtained abundance sensitivity at m-2 (i.e. at m/z = 230) is less than 0.9 ppb, the measured signal in the mass region of interest is comparable to the background. The repeated Th/Th measurements of IRMM-035 and IRMM-036 Th isotope ratio reference materials gave 11.3459 ± 0.0073 ppm (relative uncertainty of 0.064 %, k = 2) and 3.0564 ± 0.0052 ppm (relative uncertainty of 0.17 %, k = 2), respectively. The obtained results agree well with the recommended, consensus values (11.380 ± 0.096 ppm and 3.047 ± 0.024 ppm, respectively) and literature reported ratios, but with a much-improved uncertainty due to the elimination of the need for tailing correction and a high signal-to-noise ratio on the reduced background. The findings offer a new possibility in thorium isotope measurements as well as other high-precision isotope analysis in several geological, environmental, safeguards and nuclear forensic applications.