Development of rapid analysis methods for radium-226 in environmental water samples

Abstract

Rapid, routine chemical analysis methods are available for most radionuclides. However, for Ra-226 methods are available, but these are far less routine and are therefore generally cost and time intensive. This is particularly true for the analysis of Ra-226 in environmental waters, as the activity concentrations of this sample type is comparatively low. To address this, accurate and traceable analytical methods for measuring Ra-226 in environmental water samples (1 L), in < 24 h, have been developed and optimised. Initial investigations were focused on gamma and alpha spectrometry with a view of them being implemented in a mobile facility. On site analysis, using a mobile facility, has the potential to increase the time and cost efficiency of analysis through negating the need for sample dispatch and transport. Investigations showed that non-destructive gamma spectrometry, on a portable ORTEC HPGe detector provided a minimum detectable activity (MDA) of 3.6 ± 0.5 Bq/L. These measurements were subject to U-235 interferences, corrections for which are discussed within the text. Investigation were also carried out into using co-precipitation with barium sulphate prior to measurement by gamma spectrometry. Co precipitation removed the U-235 interference by providing separation of Ra-226 from U-235 prior to measurement. Co-precipitation of Ra as Ba(Ra)SO4 gave chemical recoveries > 90% and increased the counting efficiency, through improving the sample-detector geometry. This combination allowed for an MDA of 31 ± 4.5 mBq to be achieved. Alpha spectrometry was also investigated as a portable technique. The developed method pre-concentrated samples using manganese dioxide resin and separated Ra from the sample matrix using barium sulphate micro co-precipitation. The optimised method provided MDAs 75%. The procedural time for non destructive gamma spectrometry was less than that for alpha spectrometry, 1.5 h and 8 h respectively. Gamma spectrometry with co precipitation required operator times of < 30 min as compared to 4 h for alpha spectrometry. However, the higher background count rate and lower counting efficiencies for gamma spectrometry, meant that the count times required were longer and the MDAs achievable much higher than alpha spectrometry. The count times for radiometric techniques is what limits sample throughput. Multiple detectors for alpha spectrometry is an economically viable option, with each counting chamber requiring very little space. The potential of multiple detectors for gamma spectrometry is primarily limited by cost. However, if more detectors were purchased there is the added complication of interferences between simultaneous measurements in a confined space. Direct measurement by inductively coupled mass spectrometry (ICP-MS), using an Agilent 8800 ICP-QQQ-MS (Agilent Technologies, UK), was considered. Although it is not field deployable, it provided a means of eliminating the count times associated with radiometric techniques which proved to be the limiting factor in sample throughput. The instrument’s collision reaction cell was optimised to allow for online separation of interferences reducing the background to 0 at a mass to charge ratio of 226 (m/z 226) resulting in a limit of detection of 0.01 Bq L 1 based on the signal to noise ratio. Handling of complex matrices was achieved through use of the instrument’s high matrix introduction system. Direct analysis of Ra-226 in samples of 10 and 100 Bq/L provided measurement uncertainties of < 30 and < 10% respectively. Direct measurement of samples by ICP-MS removes the count times associated with radiometric techniques, with measurement times of < 5 min per samples, allowing for a much higher sample throughput for higher activity samples. Combining ICP-MS with a suitable pre-concentration technique was also investigated. To do this, the extraction chromatography and ion-exchange resins (TK100 and AG® 50W X8) were both characterised for Ra-226, with a focus of loading high volume (1 L) samples. The Kd values for Ra on TK100 resin were 2-3 orders of magnitude lower than on AG®50W at acid concentrations of 0.01-0.1 M, with peak values of 7 × 102 and 6 × 104, respectively. However, TK100 was more selective than AG® 50W-X8, showing no affinity at pH 2 for Ca, a major element in most environmental waters. TK100 was used to pre-concentrate samples prior to ICP-MS measurement. This allowed for a pre-concentration factor of 100, with minimal signal suppression suggesting larger pre-concentration factors are feasible. A pre-concentration factor of 200 would allow for the measurement of Ra-226 at concentrations in the order of 0.5 Bq L 1 with measurement uncertainties < 10%. These findings proved that ICP-MS could be a viable alternative to alpha spectrometry as a means of assessing drinking water quality, with respect to its Ra-226 content, providing a similar sample throughput with the added benefit of being a cost effective way to measure multiple stable elements and other long-lived radionuclides.