Determination of iodine, strontium and neodymium isotope ratios using a thermionic quadrupole mass spectrometer with an ion counting detector

Abstract

Isotope dilution mass spectrometry (IDMS) as a definitive method is a sensitive analytical technique for accurate trace analysis down to the sub-ppb level. The application of IDMS requires precise and accurate isotope ratio measurements. It has been demonstrated that these measurements can be carried out with a small, cost-efficient and easy-to-use quadrupole mass spectrometer equipped with both a Faraday detector and a secondary electron multiplier (SEM). Here we describe a new detector system for this instrument which allows pulse counting measurements of both positive and negative ions. The performance of the new detector system is shown for I, Sr and Nd isotope ratio measurements. A synthetic mixture of127I and129I was measured using negative thermal ionisation. The127I/129I isotope ratio was determined with a relative external precision of 0.6% using I loadings of 0.08 and 0.8 nmol. For Sr and Nd isotope ratios a relative external precision of 0.1%–0.2% and an accuracy of 0.06% were obtained for loadings of 0.01 to 0.05 nmol.