Long-term evaluation of a reusable radon-in-water proficiency test

Abstract

A proficiency test is an integral part of any analytical procedure; however, there is no known proficiency test in place for radon-in-water analysis. This led us to conduct a long-term study. Successful preparation of a reusable radon (222Rn)-in-water standard containing a ‘radium (226Ra)-loaded filter paper (the source)’ sandwiched between polyethylene sheeting has been reported. As the source ‘226Ra-loaded filter paper’ is sandwiched between polyethylene sheets, the surrounding water (which is sampled and analyzed) in the bottle remains free of 226Ra. With this type of standards, a previous study reported that at full ingrowth (>30 days), 86% of the 222Rn produced by the source was emanated into the water and remained stable thereafter, and the remaining 14% was retarded by the polyethylene sheeting. We periodically measured radon-in-water in two such standard samples allowing a 40- to 50-day ingrowth interval for more than 6 years (2016–2022). In each measurement, we prepared in duplicate the cocktails in four different ways (in Mineral-oil vs. Optifluor in combination with two different ways of ‘pipetting or sample drawing’ and dispensing into the scintillation vials) and measured the radon-in-water using two different Liquid Scintillation Counting (LSC) assays: full-spectrum (0–2,000 keV) versus Region of Interest (ROI) for radon (ROI, 130–700 keV). A substantial number of repeated results unequivocally show that the reusable standards maintained its characteristics satisfactorily for a 6-year long period. Duplicate measurements were precise in almost all cases. We consistently observed significant differences in measured radon concentration between the two different LSC assays and between the two different scintillation fluids, but not between the two sample drawing methods. With full-spectrum assay (0–2,000 keV), both Mineral-oil and Optifluor grossly underestimated the actual radon concentration, and with ROI assay (130–700 keV), Mineral-oil overestimated the radon concentration; therefore, these should be avoided. Preparing the cocktails with Optifluor and measuring by ROI assay (130–700 keV) was the only method that consistently produced results within the acceptance window (±25% of the known), suggesting that a certain way of preparing and measuring the water samples could yield more accurate results for radon. Thus, our findings demonstrate that a proficiency test for radon-in-water using these reusable 226Ra-free radon-in-water standards is a valid and valuable option, and it should be a part of radon-in-water analysis by the laboratories.