Thorium isotope ratio measurements at high abundance sensitivity using a VG54-30®, an energy-filtered thermal ionization mass spectrometer

Th isotope ratios have been measured on 3 standards using a VG54-30® mass spectrometer in static mode with a Faraday cup and ion counting Daly. The use of a 30-cm energy filter improves abundance sensitivity by an order of magnitude over a conventional VG Sector54 mass spectrometer. An abundance sensittivity of1·10⁻⁷ at mass 237 with respect to mass 238 and5·10⁻⁸ at mass 230 with respect to mass 232 can be achieved. Analysis of one standard on 4 different instruments over 18 months gives an accurate result compared with the alpha-counted value with a total external precision of±0.85% (1 standard deviation). Thus the instrument is capable of routine Th isotope ratio measurements.