3.1. TRACER STABILITY AND INTEGRITY

The quality of a new tracer candidate is examined by subjecting it to a test sequence where stability and associated properties are examined. Another sequence where detectability is examined and developed will only be represented here by an example where a method for analysis of radiolabelled [Co(CN)6]3- and SCN — in the same sample is employed.

3.1.1. Thermal degradation

The thermal stability of tracers is typically tested in batch experiments where solution aliquots of the actual tracer candidate are heat sealed in individual glass cylinders and exposed to different temperatures for different time periods. HTO is always added in a known quantity to act as a standard reference tracer. The experiments may be carried out under aerobic or anaerobic conditions. Water of different quality may be used, ranging from distilled deoxygenized water to seawater and various formation waters.

Samples are analysed with respect to the remaining original tracer concentration as a function of time at the different temperatures. The analysis is carried out by liquid scintillation counting or gamma spectrometry if the radionuclide permits.

If RTo is the volume specific count rate (cps/mL) of the tracer in the original vials before the start of the experiment (t = 0) at temperature T, and RTt is the volume specific count rate of the tracer after time t, the surviving fraction, Y, is found by the simple expression:

One example of a Y-plot is shown in Fig. 34. For an ideal water tracer under test conditions, Y should stay at ~100%. In the example given, a measurable degradation of S14CN — at 120°C in seawater occurs over time.

" MATRIX: Seawater

110 и——————- .——————- .————

Contact time at 120°C (days)

11 Seawater