A single vs. double spike approach to improve the accuracy of 234Th measurements in small-volume seawater samples

Abstract 234 Th is a particle-reactive radionuclide widely used to trace biogeochemical oceanic processes occurring over short timescales. During the last few years, small-volume techniques based on the co-precipitation of 234 Th with MnO 2 coupled with beta-counting have been developed as an alternative to large volume gamma-spectrometric techniques. Here a procedure has been developed to enhance quantitative measurement of 234 Th in MnO 2 precipitates. The main objectives were to obtain a purified Th fraction for beta-counting and to determine the chemical recovery of 234 Th using Th spikes and alpha-spectrometry as an alternative to ICP-MS based methods. Two variations of the procedure are presented. In the first “1 spike” method a 230 Th tracer is added to the sample prior to precipitation of MnO 2 , and UTEVA® extraction chromatography is used to obtain a NdF 3 (Th) purified source that can be used for both beta-counting of 234 Th and alpha-spectrometry of 230 Th. In the “2 spike” method a 230 Th spike is added and the MnO 2 (Th) precipitate is directly beta-counted for 234 Th and subsequently spiked with 228 Th or 229 Th prior to UTEVA® purification and alpha-spectrometry. The results confirm the need to process small-volume seawater samples for 234 Th measurement in presence of a yield tracer, and show that both the 1 spike and 2 spike methods allow an accurate and precise determination of 234 Th (relative percent difference, RPD, between expected and mean measured value 234 Th measurements accomplished with both versions of the NdF 3 procedure is promising (∼6% for 2-L samples), the precision of the 234 Th flux estimation will ultimately depend on the degree of disequilibrium between 234 Th and 238 U.