A CAVITY-THERMAL IONIZATION MASS SPECTROMETER DESIGNED FOR HIGH-PRECISION 142ND ISOTOPE ANALYSIS

Thermal ionization mass spectrometry (TIMS) is a mature technique used extensively in the geosciences for making precise isotope ratio measurements. Despite overlap with multi-collector inductively coupled plasma mass spectrometers, TIMS remains an important tool for the geosciences. There is room for improvement in both instruments in detecting a higher percentage of the sample atoms available for analysis. Various groups have attempted different approaches to improve the ionization efficiency of TIMS instruments, for instance a ‘cavity’ where the sample is loaded into the back of a relatively long (few cm), narrow (mm) tube heated to ionizing temperatures. This ‘cavity TIMS’ (C-TIMS) technique has long been used in the nuclear community to analyze picogram quantities of elements such as U and Pu. However, a C-TIMS instrument has never achieved routine use in the geosciences despite its ability to provide higher ionization efficiency (up to a factor of 10-40) than traditional methods. Flat-filament TIMS sources also suffer decreasing ionization efficiency with increasing sample size, making it difficult to maintain the large (>10^-10 amp) signals needed to push isotope ratio precisions into sub-ppm range. These high precisions are important for the exploitation of short-lived radioactive systems used for understanding early terrestrial and planetary processes. In particular, the short-lived ¹⁴⁶Sm-¹⁴²Nd system, where the parent isotope, ¹⁴⁶Sm, has a half-life of ~103 Ma, has proven valuable for investigating the timing of processes operating on silicate bodies. Though current techniques can make isotope-ratio measurements (¹⁴²Nd/¹⁴⁴Nd) at the 2-3 ppm-precision level, going beyond this limit will require larger signals integrated for longer times, and thus higher ionization efficiencies. Here we present a new C-TIMS source design with results for ionization of Nd, as well as documentation of mass fractionation behavior within the cavity. Cavity ionization appears to follow the same exponential law fractionation behavior and is therefore correctable using commonly utilized equations. Additionally, cavity ionizers do not appear to suffer from domain mixing more than flat filament sources, and the higher ion beams make domain mixing more readily identified and filtered during an analysis.