234U and 238U in mineral water: reference value and uncertainty evaluation in the frame of an interlaboratory comparison
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- Cite this article as:
- Spasova, Y., Benedik, L., Vasile, M. et al. J Radioanal Nucl Chem (2009) 281: 113. doi:10.1007/s10967-009-0069-8
In 2007/2008 the Institute for Reference Materials and Measurements (IRMM) organised an interlaboratory comparison (ILC) on the determination of 226Ra, 228Ra, 234U and 238U activity concentrations in mineral water. This paper describes the determination of the reference values for the activity concentrations of 234U and 238U by radiochemical separation and α-particle spectrometry performed at two independent laboratories. The experimental uncertainty of the reference values is discussed in detail.
KeywordsNatural radioactivityUraniumMineral waterReference valueUncertainty
Radioactivity levels in environmental samples and foodstuff are of concern for international organisations, national authorities and the general public with respect to health aspects of the population. As a support to the European Union policies the Institute for Reference Materials and Measurements (IRMM) organises on regular basis interlaboratory comparisons (ILCs) among those national laboratories monitoring the radioactivity levels in the environment of their countries.
Determination of the reference value is a particularly important phase in conducting and evaluating an interlaboratory comparison. IRMM provides a reference value and corresponding uncertainty for the measurand, independently from the results of the participants in the ILC round, according to paragraph 5.5 of ISO 13528 .
The recent intercomparison exercise organised by the Institute for Reference Materials and Measurements was designed to evaluate the performance of the participating laboratories in measuring the 226Ra, 228Ra, 234U and 238U activity concentrations in mineral water. The intercomparison material was bought from three different companies in Hungary. All water samples were bottled in 1.5 L PET (polyethylene terephthalate) bottles.
In this paper the procedure to assess the reference values of 234U and 238U is presented. The most influential parameters contributing to the uncertainty of the reference value are investigated. In particular, the uncertainty contributions due to the alpha spectrometry measurements, the variation between different samples of one batch (homogeneity study) as well as the stability of the samples with respect to adsorption on the walls of the bottles is discussed.
In order to establish the reference values of the activity concentrations of the radionuclides studied measurements were done by two independent laboratories—Institute for Reference Materials and Measurements (IRMM, Belgium) and Bundesamt für Strahlenschutz (BfS, Germany). Whereas three batches were analysed, only one of them (Water 3—W3) will be discussed in detail.
Measurements at BfS
At the Bundesamt für Strahlenschutz (BfS, Germany) the analysis of uranium isotopes was performed with proven methods of radiochemical separation and activity measurement . After addition of 232U tracer the 1 L water sample was evaporated to dryness and the residue was wet-ashed with HNO3 and H2SO4. Subsequently, the remaining residue was dissolved in diluted HNO3. The radiochemical separation of uranium ions was performed by extraction chromatography using a UTEVA column , and the measurement source was prepared from the uranium containing eluate by electrodeposition. The radionuclides were deposited within 4 h in cathodic electrochemical reactions at 300 mA on a stainless steel disc. The sources were measured by α-particle spectrometry applying Ortec alpha spectrometer chambers (SOLOIST) and Passivated Implanted Planar Silicon (PIPS) detectors with an active area of 450 mm2. Measuring times between 100,000 and 300,000 s and Ortec MAESTRO (Vers. 6.04) Alpha Analysis Software for the data evaluation were used.
Measurements at IRMM
Also at IRMM the activity concentrations of 234U and 238U in the water were determined by α-particle spectrometry after extensive chemical separation in order to remove interferences from other alpha emitters. After adding 232U as tracer, uranium was preconcentrated from water samples by coprecipitation with iron (III) hydroxide at pH 9–10 using an ammonia solution . The radiochemical separation was performed using a UTEVA column and the source for alpha counting was prepared by micro-coprecipitation with CeF3 . 232U as a tracer for the determination of the chemical yield of the radiochemical procedure was measured together with the two other isotopes by α-particle spectrometry. Special care was taken to ensure traceability to the SI Units by using a calibrated balance and 232U tracer standardised at IRMM within 0.6% by defined solid angle counting .
The experimental set-up used is a common alpha spectrometry system for environmental samples which consists of Canberra alpha spectrometer chambers (Model 7401 VR) and PIPS detectors with 450 mm2 sensitive area. The measured source is deposited on a flat substrate that is placed in a parallel plane, centred at the symmetry axis of the detector. The data acquisition and analysis was done using the Genie-2000 (Vers. 3.1) Alpha Analysis Software.
Results and discussion
Uncertainty of uranium determination
Uncertainty budget for 234U and 238U in mineral water, showing the typical propagated uncertainty contributions for a single measurement at the 1 s level
Counting statistics measurand (incl. background)
Counting statistics tracer (incl. background)
Choice of region of interest (ROI)
Volume of sample
Weighing of tracer
Activity of tracer
Combined uncertainty (quadratic sum)
With respect to sample preparation, the estimated uncertainty on the volume of the sample is 0.1%. The uncertainty on the weight and the activity per mass of the tracer solution is 0.5% and 0.6%, respectively. The propagated uncertainty due to the decay correction factor of the different isotopes (i.e. half-life) is negligible.
The uncertainty on the chemical yield cannot be easily determined. The uranium isotopes and 232U tracer should have the same chemical behaviour and therefore the same yield. Nevertheless, one cannot completely exclude that the uranium in the mineral water could behave differently from the tracer because of matrix effects, for example if it were encapsulated in silica grains which are not easily dissolved. At this stage no uncertainty component is added for such possible effects.
ANOVA test results for 234U and 238U in mineral water (%)
Sample volume (L)
In spite of the good repeatability, the ANOVA test also reveals higher variability from one source to another (sbb) (Table 2). This can in principle be due to inhomogeneity from bottle to bottle, instability of the material or an unforeseen change in the chemical yield ratio.
As stability check, the adsorption of uranium on the walls of the bottles was tested, considering that no special treatment of the water was carried out during bottling. The check was done by filling some of the emptied bottles with distilled water and a few millilitre of concentrated nitric acid. The bottles were stored for a period of at least 1 month. Then solid sources were prepared following the same radiochemical procedure as the one above. The sources were counted for 4 days. The measured activity concentrations were below the detection limit (<0.2 mBq L−1). Therefore, no uncertainty due to adsorption contributes to the combined uncertainty of the reference value.
Reference value and uncertainty calculation
Activity concentration results for 234U and 238U in mineral water (mBq L−1)
Reference value ± combined standard uncertainty uC
Mean ± s
Mean ± s
14.4 ± 1.3
15.8 ± 1.1
15.1 ± 1.1
10.3 ± 0.9
12.2 ± 1.1
11.3 ± 0.8
3.9 ± 0.3
4.2 ± 0.7
4.0 ± 0.3
0.91 ± 0.14
0.92 ± 0.15
0.91 ± 0.15
44.7 ± 1.5
42.9 ± 2.3
43.8 ± 1.6
22.1 ± 0.9
21.3 ± 0.9
21.7 ± 1.0
The reference values of 234U and 238U in mineral water determined in the frame of an interlaboratory comparison (ILC) were obtained independently from the participants results of the ILC round. They were calculated as a mean of the results obtained at two independent expert laboratories, using partially different methods of determination. The mean values of both laboratories were in very good agreement. The main uncertainty contributions to the combined uncertainty of the reference value came from the uranium determination with α-particle spectrometry (2%) and the homogeneity (3% and 4% for 234U and 238U, respectively). The estimated 3.6% and 4.6% combined uncertainty was considered to be adequate for the purpose of the interlaboratory comparison.