A cavity ion source for high-ionization efficiency neodymium isotope-ratio analyses in the geosciences

Abstract

The principles governing ionization techniques used in thermal ionization mass spectrometers are relatively well understood and have remained largely unchanged for many decades. Though significant advances have been made in ion signal quantification for isotope ratio measurements, particularly for analyses of small samples by using multiple detector systems and low-noise amplifiers, the fundamental approach to sample ionization has received little focus. Modern TIMS techniques attempting to achieve parts-per-million level isotope ratios precisions are realizing limits imposed by the physics of the ionization source. A type of high-ionization efficiency thermal source employed in nuclear physics communities for decades is the so-called cavity thermal ionization source. Here, we provide a proof-of-concept study that shows cavity sources may provide a path forward to achieve a new level of precision in isotope ratio measurements from solid samples. We document our new, simple, cavity ion source design, show preliminary results from Nd isotope measurements, and discuss these new data in the context of current precision limits imposed during traditional thermal ionization methods. We show that, within the limits of our testbed mass spectrometer, mass fractionation within the cavity ion source appears similar to that from filament ion sources. We also demonstrate that oxide-versus-metal ion production plays a significant role in cavity ionization processes for Nd. Cavity ion sources may provide a viable path forward to achieving isotope ratios precisions at the sub-ppm precision level.