Ultralow-level measurement of organically-bound tritium in bioassay samples

An intercomparison study of urine samples having high levels (5 Bq·L-1) of organically-bound tritium (OBT) was conducted, in conjunction with the oxygen combustion-liquid scintillation counting (LSC) method, to evaluate the suitability and sensitivity of the 3He-ingrowth mass spectrometry (MS) technique for OBT in bioassay samples. The study established that 3He ingrowth-MS has the required sensitivity to measure ultralow levels of OBT-in-urine (∼0.1 Bq·L-1). Cumulative 24 h urine samples from a few members of the general population, living in the vicinity of the heavy-water research reactor facility at Chalk River Laboratories (CRL) at Chalk River, were collected and analyzed for tritiated water (HTO) and OBT. The participants were from Ottawa (200 km east), Deep River (10 km west) and an occasionally occupationally HTO-exposed worker at CRL. HTO-in-urine values were 6.5 Bq·L-1 for the Ottawa resident, 15.8 Bq·L-1 for the Deep River resident, and 1260 Bq·L-1 for the exposed worker. OBT-in-urine levels from these same individuals were 0.06 Bq·L-1 (Ottawa), 0.29 Bq·L-1 (Deep River), and 2.2 Bq·L-1 (exposed worker). With a model developed for calculating OBT dose fraction from the measured ratio of HTO to OBT in urine, we estimated that the dose arising from OBT in the body was about 26% of the total tritium dose for the Ottawa resident and 50% for the Deep River resident. The CRL individual had a 5% dose contribution from OBT, but had higher overall tritium dose due to frequent intakes of HTO. The study indicates that the bulk of the tritium dose to the population is the result of HTO intakes and not due to dietary intake of OBT. (author)