Precise measurement of 228Ra/226Ra for 226Ra determination employing total integration and simultaneous 228Th correction by multicollector ICP-MS using multiple ion counters

A precise method of measuring Ra-228/Ra-226 for Ra-226 determination used in U-series disequilibrium studies has been developed using the total integration method with simultaneous 228Th isobaric interference correction by multicollector ICP- MS with multiple ion counters (MIC). A sample solution of 0.2 ml was separately taken into a test tube, and all Ra-226(+) and Ra-228(+) signals as well as Th-229(+) and (ThO+)-Th-228-O-16 from beginning to end were integrated simultaneously using four channeltrons, IC1, IC3, IC4 and IC5, respectively. Gains between IC1 and IC3 and the mass discrimination factor for Ra-228/ (226) Ra determination were canceled out by measurement of a Ra spike solution with an accurately known Ra isotope ratio determined by TIMS, with which samples were bracketed. The isobaric interferences of Th-228(+) were simultaneously corrected from the signal of (ThO+)-Th-228-O-16 and the oxide forming ratio of Th-228, which was separately determined by simultaneous measurements of Th-229(+) and 2(29)Th(16)O(+) by IC3 and IC5. The gains between IC3 and IC5 for the Th correction were also canceled out. The ion yield of Ra in MIC- ICP- MS was similar to 1.5%. The intermediate precision using 6.6, 3.3, 1.7, 0.83 and 0.41 fg of Ra-226 with Ra-228/Ra-226 similar to 5 was 0.33, 0.93, 0.76, 1.6 and 1.5%, respectively. The Ra-228/Ra-226 ratio showed no systematic change dependent on Th-228/Ra-228 up to 0.25, indicating the validity of the Th-228 correction. At similar amounts of Ra, our method gives intermediate precision values similar to or better than previous MC-ICP-MS and TIMS studies. The precision of the method was verified using another Ra spike with Ra-228/ Ra-226 = similar to 1 determined by TIMS. The analytical performance of the method was further investigated using the silicate samples JB-2 and JB-3 basalts issued by GSJ.