Determination of 234Th in seawater by direct beta counting.
The following procedure is based on the direct beta counting of 234Th. The particulate phase is collected on a filter, whereas the dissolved phase is coprecipitated with MnO2 which is collected on a second filter. Both filters are counted with a low-background beta counter. The original procedure was developed for 20-L samples and published by Rutgers van der Loeff and Moore (1999). This procedure has subsequently been adapted to smaller sample volumes (2-5L), as described in Buesseler et al. (2001) and Benitez-Nelson et al., (2001). Methods for 234Th analysis have recently been reviewed by Rutgers van der Loeff et al. (2006). This review includes an automated procedure for surface waters.
the particulate phase
234Th can be counted by non-destructive beta counting. In the open ocean, 234Th activity usually overwhelms other isotopes that might contribute to the beta signal from suspended particles, like K-40, Ra-226 or Pb-210. This should be checked occasionally by following the decay over several months. If the activity does not decline with the half-life of 234Th (24.1 day) this is an indication for contributions from other nuclides.
234Th itself decays with relatively weak betas (maximum beta energy 0.20 MeV) to 234mPa, which is a high-energy beta emitter (maximum beta energy 2.29 MeV, half-life 1.17 min.). Thus, if a filter is dried and carefully folded (possible with polycarbonate (e.g. nucleporeTM) filters, not with membrane filters, which will break) to fit the beta detector holder and covered with a thin foil, the activity measured is due mainly to 234mPa, with some contribution of 234Th.
Self-absorption- Beta particles have a continuous energy spectrum extending from zero energy up to a maximum energy Emax. If a flat beta source is measured with a thin absorber of thickness L between source and detector, the count rate I is very closely represented by
I(L) = I(0) e-aL (eq.1)
where I(0) is the count rate measured in the absence of the absorber and a is an attenuation factor which can be derived from Emax and the density of the absorber (e.g. Tsoulfanidis, 1995). If the same activity is homogeneously distributed in the thin absorber, a good approximation in the case of a multiply-folded filter, the count rate measured with a detector on top of the absorber can be described just as in case of the self-absorption of gamma radiation (Cutshall et al., 1983)
I(L) = I(0) . (1-e-aL)/aL = I(0) . Et (eq.2)
where we call Et the transmission efficiency.
If filters have to be measured with a suspended load that is at least comparable with the filter weight , Et has to be calculated with equation (2). The parameter aL has to be measured for each individual sample from measurements of a (strong) beta source with and without the filter in-between, using equation (1). As the equations (1) and (2) only hold for the energy distribution of a single beta decay, all beta measurements have to be made with a cover removing the weak betas of 234Th. A cover of 30 mg/cm2, approximately equivalent to two overhead sheats, transmits 82% of 234mPa but only 4% of 234Th betas.
However, if the absorber has reproducible geometry and thickness, as in case of a carefully folded filter with particle load << filter weight, or in case of a filter with reproducible load of MnO2 precipitate, Et will have a constant value, which can be determined with standard filters. Measurements can be made with a thin cover, improving counting statistics.
Procedure for particulate 234Th
- 20-50L is filtered over 142mm polycarbonate filters with 1µm pore size. Filters are drained by suction, folded twice in two, air dried, and carefully folded 4 more times to produce a 18x18mm, 64-sheet thick package which is wrapped in thin (e.g. 0.01 mm) plastic (polyester or polyethylene) foil, and counted directly in a beta counter (count rate IF).
Blanks- Polycarbonate (NucleporeTM) filters contain some 137Cs, which contributes to the blank. The count rate Ibl of blank filters (including instrument background) is typically around 0.5 cpm, but can vary widely between batches and between pore sizes and definitely has to be checked in advance.
Calibration- Standard filters can be prepared by making a Fe(OH)3 precipitate of a U standard solution. In a small teflon beaker add 50 µl of 50mg/mL Fe(III) solution to a weighed Uranium spike containing an accurately known activity AU of approximately 100 dpm 238U (135 µg U), mix, add dilute (1M) NH3 while mixing until precipitate remains (pH is now 8-8.5). Put filter on graph paper and pipette spike slurry dropwise in regular pattern on the filter. Dry in fume hood, fold as other filters, and count (count rate IS). This standard filter is sufficient for calibration of filters from subsurface ocean water, when suspended load << filter weight.
For filters with suspended load comparable to or larger than the filter weight, aL is determined from the measurement of a strong source (e.g. a planchet prepared from a natural Uranium spike) with (count rate I2) and without (count rate I1) the filter in-between, using a thick (30mg/cm2) cover. Analogous to equation (1) we have
I2 = IF + (I1-Ibg) e-aL (eq. 3)
where Ibg is the background count rate, giving
aL = -ln((I2-IF)/(I1-Ibg )),
which is used to determine the transmission efficiency of the filter EF. The transmission efficiency of a blank filter (1-µm pore size polycarbonate), Ebl, is typically around 80%.
Calculation - The activity AF on a filter with count rate IF is given by
AF = (IF-Ibl)/(IS-Ibl) AU Ebl/EF elThDt
where lTh = ln(2)/ t1/2 = 0.0287 d-1
and Dt is the time lapse between filtration and counting. For low filter loadings the self-absorption correction term Ebl/EF is unity.
To a weighed 20-Liter aliquot of filtered seawater add 6 drops of concentrated ammonia (25 weight % NH3) and 250µL of concentrated KMnO4 solution, followed after mixing by 100µL of a concentrated MnCl2 solution (use the Winkler I reagent, 400g MnCl2.4H2O/L, as used for oxygen determination). After mixing the purple colour disappears rapidly and a suspension of MnO2 is formed. After eight hours, to allow for the MnO2 particles to grow, the suspension is filtered over a 1-µm polycarbonate filter. Use vacuum filtration and rinse the container vigorously to bring remaining adhering MnO2 particles in suspension and on the filter. The filter is rinsed with distilled water, drained by suction and folded while wet in the same geometry as used for the filters containing suspended matter from seawater. The folded filter is held together with plastic paperclip and allowed to dry before it is wrapped in plastic foil. This filter is counted directly in the beta counter. As it is not necessary to stop the weak betas of 234Th, all beta countings can be done with a thin cover.
Calibration: Prepare MnO2-filter blanks by producing an MnO2 precipitate in milli-Q water. Count rates should be only slightly above the filter blanks. Extraction efficiency is better than 99% (as determined from repeated extractions of the same sample), but some precipitate may stick on the walls, tubing or filter holder. The loss can be estimated by rinsing all equipment with a solution of 10 mL H2O2/L in 1mol/L HCl and measuring Mn in the leach with atomic absorption, using the spike solutions (250µL KMnO4 + 100µL MnCl2+ 1 mL dilute H2O2/HCl) made up with distilled water to 500mL as calibration.
Self-absorption and overall counting efficiency of a MnO2-coated filter can be determined as in for the filters with suspended material. The transmission efficiency Et is typically around 78%. The overall efficiency of the entire procedure can be checked by determining 234Th in a sample from mid-depth in the open ocean and comparing the result with the value expected from equilibrium with 238U (Chen et al., 1986).
As in the case of particles, 234Th activity usually overwhelms other isotopes that might contribute to the beta signal from the MnO2 precipitate. However, radium is partially coprecipitated with MnO2 and the contribution from 226Ra daughters (214Bi with maximum beta energy 3.27 MeV) can be significant and has to be checked by following the decay over several months. If the activity does not decline with the half-life of 234Th (24.1 day) this is an indication for contributions from other nuclides. In a typical precipitate from 20 L seawater containing 2.5 dpm/L 234Th and 0.15 dpm/L 226Ra, the contribution from radium daughters, and of traces of Uranium which may have coprecipitated and contribute to supported 234Th, is approximately 4% of the gross beta count rate measured within a week after sampling. The uncertainty in this contribution depends primarily on the steady state between production and outgassing of 222Rn during the two measurements of the sample: before and after 234Th decay.
Accuracy and precision The procedure is reproducible to within 2%, but accuracy is approximately 5% as a result of uncertainties related to the contribution from other isotopes.
- Buesseler K. O., Benitez-Nelson C., Rutgers van der Loeff M., Andrews J., Ball L., Crossin G., and Charette M. A. (2001) An intercomparison of small- and large-volume techniques for thorium-234 in seawater. Marine Chemistry 74(1), 15-28. doi.10.1016/S0304-4203(00)00092-X
- Benitez-Nelson C., Buesseler K. O., Rutgers van der Loeff M. M., Andrews J., Ball L., Crossin G., and Charette M. A. (2001) Testing a new small-volume technique for determining thorium-234 in seawater. Journal of Radioanalytical and Nuclear Chemistry 248(3), doi: 795-799.10.1023/A:1010621618652.
- Rutgers van der Loeff M. M. and Moore W. S. (1999) Determination of natural radioactive tracers. Chapter 13. In Methods of Seawater Analysis, third Edition (ed. K. Grasshoff, M. Ehrhardt, and K. Kremling), pp. 365-397. Verlag Chemie.
- Rutgers van der Loeff M., Sarin M. M., Baskaran M., Benitez-Nelson C., Buesseler K. O., Charette M., Dai M., Gustafsson O., Masque P., and Morris P. J. (2006) A review of present techniques and methodological advances in analyzing 234Th in aquatic systems. Marine Chemistry 100 (3-4) 190-212. doi: 10.1016/j.marchem.2005.10.012