Naturally occurring 210Pb2+ can be found in altered water samples with originally high Radon concentrations. The maximum concentration in drinking water is 70 mBq/L in order not to exceed 0.1 mSv/a for small children assuming regular water consumption.
A simple tool to estimate the 210Pb concentration in water is LS measurement after enrichment and separation on Radium RAD Disk filters. The method makes use of the fact that Lead is quantitatively extracted together with Radium into crown-ether 21 and is thus retained on the filter. Lead may be quantitatively eluted with small amounts of DHC, while Radium isotopes still remain on the filter to more than 90 %. The method in detail is described in chapter 126.96.36.199. After filtration and washing, 210Pb is eluted dropwise with 5 mL 0.2 M DHC (fig. 21). The measurement is done after homogenization with 16 mL OptiPhase HiSafe III in a low energetic β-channel. The relatively high detection limit value of 50 mBq/L for the single PM Triathler device is due to the high luminescence background in the low energetic β-area but could be improved with a multi PM low level counter, especially when using TDCR.
210Pb can also be determined through the measurement of the high energetic β-emitting daughter 210Bi by Cerenkov counting, when equilibrium can be assumed.
Materials and Equipment
- 0.5 M HNO3
- 0.2 M DHC
- 0.25 M EDTA alkaline
- RAD Disk filter
- OptiPhase HiSafe III
- Gelating cocktail
(modified method with Radium RAD Disk in presence of Pb-isotopes, see fiure 21)
- 3 L of the 2 M HNO3 acidified water sample are sucked through a preconditioned Radium RAD Disk filter.
- After washing the filter with 10 mL 0.5 M HNO3 and 10 mL H2O, 210Pb is eluted dropwise with 5 mL 0.2 M DHC.
- The filter is washed with further 10 mL H2 226Ra-isotopes are then eluted from the filter twice with 5 mL 0.25 M EDTA.
- Each fraction is mixed with 16 ml OptiPhase HiSafe III and measured with TDCR after 3 hours storage.
Quench correction using β-standards for 210Pb may be neglected if the TDCR technique is applied.
The activity concentration of 210Pb is calculated according to
RN = Net rate (cps)
ε = Measuring efficiency (95 % for 226Ra and 60 % for 210Pb)
h = Elution yield (95 %)
V = Sample volume (3 L)
Detection Limit (MDA): 226Ra 5 mBq/L, 210Pb 70 mBq/L (Triathler) and 10 mBq/L (LL LS counter)
Total analysis uncertainty: for 226Ra 8%, and for 210Pb 15% (due to the calibration uncertainty)
As alternative to the direct measurement, 228Ra may be quantified by ingrowth of 228Ac after a storage time of 24 hours (T1/2 (228Ra) = 6.13 hours), or after 15 days when 224Ra with its complex daughter nuclides has been fully decayed (fig. 21 left side). This modification is more time consuming and unsuitable for in-situ analysis, however, allows a considerably lower limit of detection and a higher counting efficiency compared to direct 228Ra measurement because of the high β-energy of 228Ac.
Figure 20: Elution behavior of 226Ra, 210Pb and 210Po on Radium RAD Disk filter
(a) With 0.25 M EDTA alkaline
(b) With 0.2 M DHC
Figure 21: Modified Radium RAD Disk filter method for rapid determination of 226Ra, 228Ra and 210Pb