How to use nucfilm’s Ra adsorbing sheets

Warning : The method described is recommended only for untreated waters having drinking water quality. For waste waters, for acid leachates and also for some intercomparison samples (barium spiked) the method may be used, but requires special precaution.

Nucfilm’s Ra adsorbing sheets are polyamide sheets covered on both sides with a thin MnO ₂ film.

Sampling

There is no need to acidify waters having drinking water quality. Simply avoid glass containers. Use polyethylene bottles and fill them completely. If you are scared about precipitation, you may add up to 1g Na ₂ -EDTA(Titriplex III) / liter.

Exposition to the water sample

Fill 100 ml of your untreated water sample into a 100 ml FEP beaker. Don't use glass beakers. FEP (Fluor-Ethylene-Propylene) showed negligible adsorption. If your sample has been acidified rise pH with NaOH to to 5 to 7. Below pH 4.5 adsorption efficiency sharply decreases.
To highly mineralised (g/liter) or sparkling samples add some 30 mg Na ₂ -EDTA (Titriplex III) / 80 ml to avoid precipitation during exposition. Up to 100 mg Na ₂ -EDTA / 80 ml showed no noticeable effect on adsorption efficiency. For waters having a very low mineralization (conductivity below 100 micro-Siemens/cm) it is recommended to add NaHCO ₃ to complex the uranyl ion (300 mg HCO ₃ /l). If not you risk to have uncomplexed uranyl cations adsorbing on the MnO ₂ films.
Rinse the sheet with distilled or deionized water and carefully wipe it dry with a soft paper (e.g. Cleenex). Fix the sheet with a stainless steel clamp in your sample as shown in the figure . Stir at approx. 200 rpm using a teflon covered stirring rod. After 6 h remove the sheet from your sample, rinse it with distilled or deionized water and dry it with not too hot air (hair drier).

For some samples containing high CO ₂ concentrations gas bubbles may form on the film surface. In this case just lift the sheet shortly out of the sample and put it back. This removes the bubbles. This prodcedure may be necessary several times during the first hour of exposition.

It is not known if these bubbles have an effect on adsorption. Any data are welcome .

Measurement

It is recommended to measure both sides of the sheet. In general the side that has been exposed towards the flow ("upstream") holds some 20% more activity than the "downstream" side.
With the sheet at a distance of about 10 mm from the surface of a 900 mm ² alpha detector you have a detection efficiency around 20% .
Summing up the counts from both sides and assuming a 100% adsorption efficiency
1 count/1000s thus corresponds to around 65 mBq/l in your sample.

Adsorption efficiency and calibration

Barium at mg/l concentrations, Sr at some 10 mg/l and U at Bq/l levels are known to reduce adsorption efficiency for radium considerably. Apart from U in some mineral waters these levels are not found in drinking waters. For drinking waters you thus can expect to have adsorption efficiencies > 80% for 20 mm x 20 mm sheets exposed as given above. To verify adsorption efficiency you can expose successively several sheets in the same 80 ml sample. Assuming that every sheet takes up the same fraction of the radium that remained in the sample, adsorption efficiency will be 1- (Ra activity on sheet n+1)/(Ra activity on sheet n).
Radium standard solutions have to be essentially free from barium (<< 1mg/l). NPL (GB) sells a radium standard having very low barium. A 1 Bq/l solution prepared from the original 200 Bq/ml acid solution by simply diluting it (in 3 steps) with deionized water showed a 93% to 95% adsorption efficiency.

I nterferences from ²³⁴ U

MnO ₂ generally adsorbs radium considerably stronger than uranium. In drinking waters having a pH above 6 uranium adsorption showed to be at least an order of magnitude lower than radium adsorption. However in some initially strongly acid uranium ore leachates (with the pH risen by NaOH to around 4.5 for the measurement) radium adsorption may be very low, even below 10%, with a uranium adsorption of the same order of magnitude.
In most cases energy resolution will not be sufficient to distinguish between ²²⁶ Ra
lines and ²³⁴ U lines. This may lead to an overestimate of the ²²⁶ Ra concentration. To solve this problem one can assume as a first approximation that ²³⁸ U and ²³⁴ U are in equilibrium. The amount of ²³⁸ U adsorbed can be determined without interference problems and then be used to correct for the ²³⁴ U contribution. However ²³⁸ U and ²³⁴ U are rarely in perfect equilibrium in natural water samples. Although ²³⁴ U/ ²³⁸ U ratios mostly are in the range of 0.8 to 1.3 values up to 3 have been reported. In some cases it thus may be necessary to determine the ^{2 34} U/ ²³⁸ U ratio by a different method, e.g. by liquid scintillation alpha spectrometry (PERALS) or by adsorption on U-selective thin films.


Bad samples

Unfortunately some organisers of intercomparison exercises only feel good if their samples are acidified down to pH 1 and they like to use barium rich radium standards.These samples are hard to bring to a pH of 5 without precipitation. Adding some g/l Na ₂ EDTA may help, but radium adsorption efficiency is generally bad, even below 10%. In this case it does not help much to expose several sheets. The only solution is to add some barium-free radium tracer after the first exposition and then to expose a second sheet. From the counts difference between the first and the second sheet and the known added activity one then can calculate the efficiency.


October 15, 2004 Heinz Surbeck