State-of-the-art and progress in precise and accurate isotope ratio measurements by ICP-MS and LA-ICP-MS Plenary Lecture

Abstract

The capability to determine isotope abundances is a main feature of mass spectrometry. The precise and accurate determination of isotope ratios is required for different application fields, such as: isotope ratio measurements of stable isotopes in nature, especially for the investigation of isotope variation in nature or age dating; determining isotope ratios of radiogenic elements in the nuclear industry; quality assurance of fuel material for reprocessing plants, nuclear material accounting and radioactive waste control; and tracer experiments using highly enriched stable isotopes or long-lived radionuclides in biological or medical studies. Inductively coupled plasma mass spectrometry and laser ablation ICP-MS (LA-ICP-MS) provides excellent sensitivity, precision and good accuracy for isotope ratio measurements with practically no restriction with respect to the ionization potential of the element investigated. Therefore both ICP-MS and LA-ICP-MS are increasingly replacing thermal ionization mass spectrometry (TIMS), which has been used as the dominant analytical technique for precise isotope ratio measurements for many decades. In the last few years instrumental progress for improving figures of merit in isotope ratio measurements in ICP-MS and LA-ICP-MS with a single ion detector has been achieved by the introduction of the collision cell interface, in order to dissociate disturbing argon-based molecular ions, to reduce the kinetic energy of ions and neutralize the disturbing argon ions of the plasma gas (Ar⁺). The application of the collision cell in ICP-MS results in higher ion transmission, improved sensitivity and better precision of isotope ratio measurements compared to ICP-MS without the collision cell. The most important instrumental improvement for isotope analysis by sector field ICP-MS was the application of a multiple ion collector device (MC-ICP-MS) (developed about 10 years ago) in order to obtain better precision of isotope ratio measurements of up to 0.002%, RSD.