Determination of Th, U and K in ultrapure Li2CO3 by Inductively Coupled Plasma Mass Spectrometry for neutrino-less double beta decay experiment

Abstract: Li₂CO₃,a raw material of Li₂MoO₄ crystal used for searching the neutrino-less double beta decay (0νββ),
must be of high purity and low content of Th, U, and K. In this study, quantitative methods are developed to
determine the contamination of Th, U, and K in ultrapure Li₂CO₃ samples by inductively coupled plasma mass
spectrometry (ICP-MS). Our measurements showed that the signals of Th, U, and K were significantly inhibited
with increasing Li concentration. When the lithium concentration reached 8000 µg · g⁻¹,the recoveries of Th,
U, and K decreased to 10%–30%. For the determination of Th and U, both the recovery correction method and
the TEVA-UTEVA combined resin method can eliminate the influence of the lithium matrix. The recovery cor-
rection method was simpler to operate, while the TEVA-UTEVA combined resin method had more advantages
in terms of the detection limit. In the measurement of K, the plasma gas (Ar) in ICP-MS also induced signifi-
cant interference. Compared to the normal measurement mode of ICP-MS, the cold plasma mode with a lower
radio frequency (RF) power can effectively reduce the interference from Ar, greatly improving the detection
sensitivity of K. The detection limit of K in Li₂CO₃ samples can be as low as 7.53 ng · g⁻¹,correcting for the
inhibitory effect of the lithium matrix using the recovery correction method. Neutron activation analysis (NAA)
measurements were also performed for comparison, yielding results consistent with ICP-MS.