DETERMINATION OF RA-228 IN DRINKING-WATER

A procedure for the analysis of Ra-228 in drinking water has been developed. The procedure involves separation of radium by an initial coprecipitation with lead sulfate. The isolated Pb(Ra)SO4 is then dissolved in sodium diethylenetriamine pentaacetate (DTPA). Radium-228 is co-precipitated from this solution with barium sulfate while the DTPA supernate which contains pre-existing Ac-228 is discarded. The purified Ba(Ra)SO4 precipitate is then allowed to ingrow, generating Ac-228, which is then dissolved in DTPA, isolating both Ra-226 and Ra-228 in the precipitate while Ac-228 remains in the aqueous supernate. The supernate is partitioned against di-(2-ethylhexyl phosphoric acid), HDEHP, dissolved in n-heptane, which retains the Ac-228. Actinium-228 is then stripped from the organic phase by partitioning against 1M HNO3. Finally, the Ac-228 is coprecipitated onto cerium oxalate. The precipitate is collected on a filter and counted in a low-background beta counter. Radium-228 standards with concentrations ranging from 0.044 to 1.6 Bq were used to establish the detector counting efficiency for Ac-228 in cerium oxalate samples, as well as monitoring the chemical yield and absorption factors. The resultant average value of 30.3 +/- 2.1 cpm/Bq (uncertainty given at 95% level of confidence) was obtained. Various Ra-228 cross checks from U. S. Environmental Protection Agency (EPA) with concentrations of 0.063-0.5 2 Bq/l were analyzed in order to assess the performance of the procedure. The minimum detectable concentration (MDC) of Ra-228 in water with this procedure is 0.015 Bq/l. This is based on a one liter aliquot of sample, a 100 min counting period, and a 3 hour decay interval between the end of Ac-228 ingrowth and midpoint of counting. Decontamination factor studies were performed to determine the extent of the carry-over of U-238, Ra-226, Po-210, and Sr-90 into the final fraction.