For oil samples of types A and B in Fig. 24, the simplest pretreatment procedure is to remove any visible oil layer by pipette and to filter the water through a lipophobic filter. Transfer a measured volume of water, for instance 1000 mL, to a Marinelli beaker and count with a NaI(Tl) detector, as shown in Fig. 24. A regular cylindrical beaker may also be used, but the counting efficiency is somewhat lower. Owing to complex background radiation, an HPGe detector may be used if desired.

Upconcentration of f50Co(CN)^3- before gamma spectrometry

• After pretreatment to remove oil, the water is percolated through an anion exchange resin in order to concentrate the tracer molecule in a small volume in the resin.

• The column can be mounted directly onto a gamma detector (NaI(Tl) or HPGe) for gamma spectroscopy measurement.

• To prepare a sample for liquid scintillation counting the [Co(CN)6]3- on the column may be eluted with a suitable elution liquid into a small volume (a few millilitres) and mixed with a liquid scintillation cocktail. Thus, the counting efficiency is considerably improved. However, it requires that other beta emitters are not present in the sample.

A gamma spectra of 60Co accumulated with an HPGe detector with high energy resolution is compared with a gamma spectrum accumulated with a NaI(Tl) scintillation detector in Fig. 25.

The [60Co(CN)6]3- ion may also be detected with a liquid scintillation counter of high efficiency. It is the beta radiation and the Compton electrons which are registered. Liquid scintillation count detection is sensitive to quenching effects, as illustrated in Fig. 26 where a sample of 60Co activity is counted in a quench-free scintillation mixture and in a mixture where 8 mL of water is added. Water acts as a quenching agent in this case, resulting in a decrease in counting efficiency.

More details on the analysis of [60Co(CN)6]3- tracer is given in the analysis protocol in Appendix III.

(iv) Analysis of 14C or 35S labelled SCN-

Both 14C and 35S are beta emitters with similar beta energies (156 and 168 keV, respectively). Thus, they cannot be analysed simultaneously in the same counting sample. Therefore, use of these two compounds simultaneously in the same reservoir section should be avoided. If, however, both tracers are needed, it is, in principle, possible to analyse them in the same sample by special sample treatment and separation technique. The SCN- ion may be broken down to leave S in one type of molecule (e. g. SO4- by oxidation) and C in another (e. g. CN — or CO2). These may be isolated separately and counting samples prepared separately for each of them. Although the process given deals exclusively with the separation, enrichment and analysis of only one of them, the basic procedure for the two is identical.

Analysis of radiolabelled SCN — in produced waters from oilfields has been described previously in the literature. Two methods are outlined, one based on liquid-liquid extraction and the other on anion exchange separation of the tracer

ions from produced water samples. A detection limit <0.01 Bq/L was obtained by using low background liquid scintillation counting equipment (Quantulus 1220). A third method proposed by the China Institute of Atomic Energy is given in detail. It is based on solvent extraction with tributyl phosphate of a metallic thiocyanate complex after the purification process of oil removal followed by filtration as described previously.

Details of the procedure are given in the analysis protocol of radiolabelled SCN — in Appendix III, but the general steps include:

(1) Removal of particles and oil droplets by filtration through 0.45 pm filter paper;

(2) Addition of ZnCl2, KSCN carrier and HCl to form Zn(SCN)2 in a clear solution;

(3) Extraction of the electrically neutral Zn(SCN)2 into tributylphosphate;

(4) Conduct of phase separation by gravity segregation or centrifugation;

(5) Removal of sample from an aliquot of the tributylphosphate phase and mix with liquid scintillation cocktail in a liquid scintillation counting vial;

(6) Detection of the activity by liquid scintillation counting.

(v) Analysis [60Co(CN)6]3 in presence of radiolabelled SCN

The general procedure is as follows. Purify the water by removing oil as described previously. Isolate and enrich the two tracers from the bulk water volume, either sequentially or simultaneously. Two possible methods are solvent extraction and ion exchange. After the first step involving extraction/stripping or feed/elution operations, prepare samples for radioactivity detection. Ion exchange is less labour intensive than solvent extraction.

A separation procedure has recently been reported that is described in the analytical protocol for [60Co(CN)3]2- in presence of 35SCN- or S14CN — in Appendix III, but the general steps include:

• Removal of particles and oil droplets by filtration through 0.45 pm filter paper.

• Preparation of an anion exchange column of the resin Dowex 2 x 8.

• Percolation of the purified sample solution through the column. The [60Co(CN)3]2- will bind to the resin while 35SCN- or S14CN — will not.

• Removal of any remaining radiolabelled SCN — on the column by elution with a small volume of low concentration NaClO4 solution, which is added to the raffinate.

• Concentration and purification of 35SCN — or S14CN — in the raffinate by the procedure described for radiolabelled SCN — in Appendix III.

• Evaluation of column directly by gamma spectroscopy. In the cases where liquid scintillation counting is desirable, radiolabelled [Co(CN)3]2- may be stripped from the column by, for instance, a small volume of NH4NO3 solution at medium-high concentration and mixed directly with a scintillation cocktail.

• Detection of activity by liquid scintillation counting.

The necessity for performing a separation operation is underlined in Fig. 27, where a spectrum of 60Co is compared with a spectrum of 14C. The spectra overlap substantially. The liquid scintillation counting equipment may be run in so-called dual label mode, but the sensitivity becomes lower and the uncertainty in the results greater when spectra overlap to this extent.