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 were developed to determinate the content of Th, U and K in ultrapure Li2CO3 samples by inductively coupled plasma mass spectrometry (ICP-MS). The research showed the signals of Th, U and K were significantly inhibited with the increasing of lithium concentration. When the lithium concentration reached 8000 ug g-1, the recovery 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 correction method was simpler to operate, while the TEVA - UTEVA combined resin method was lower in detection limit. For measurement of K, the plasma gas (Ar) in the ICP - MS also induced significant interference. Compared with the standard measurement mode of ICP - MS, the cold flame mode with reduced radio frequency (RF) power can effectively reduce the interference from Ar, greatly improving the detection sensitivity of K. Meanwhile, the detection limit of K in Li2CO3 samples can be as low as 7.53 ng g-1, correcting the inhibitory effect of the lithium matrix by the recovery correction method. A neutron activation analysis (NAA) measurement was also performed for the comparison, yielding results consistent with ICP-MS.