Optimisation of CO₂ absorption and liquid scintillation counting method for carbon-14 specific activity measurement...

A method for the determination of ¹⁴C activity in the ambient air was optimised with the development of a simple setup for the regeneration of CO₂ from carbonate sample and saturating the absorber in <45 min for direct determination of activity by liquid scintillation counting (LSC). Atmospheric CO₂ was trapped in NaOH solution and precipitated as BaCO₃ by adding BaCl₂. The carbonate sample was taken in a newly designed regeneration system, subjected to acid hydrolysis, and the absorber (CarboSorb-E) was saturated with the CO₂ regenerated from carbonate sample. This allowed optimisation of CO₂ absorption by the absorber (up to ~ 2.3941 g of CO₂/10 mL with an average of 2.1688 g) and a minimum detectable activity value of 14 Bq kg^-1C for a counting time of 300 min (8 Bq kg^-1C for 1000 min) was achieved with Quantulus - 1220 LSC system. The necessity of (i) the measurement of the total volume of air sampled, (ii) the determination of trapping efficiency for CO₂ in the NaOH, recovery of ¹⁴C in chemical processing of BaCO₃, and subsequent regeneration and absorption processes, and (iii) independent determination of carbon content in the air for expressing the results in terms of ¹⁴C specific activity (Bq kg^-1C), are avoided in this method. The method is capable of yielding accurate results, in a considerably shorter time when compared to previously reported methods, with a deviation of <2.2% from the target value (with a relative standard deviation of 1.1%, and a relative error of 0.53%) when ambient air samples from clean air region (region not affected by local anthropogenic sources of ¹⁴C) are analysed. Validation of the method was performed by (i) analysing BaCO₃ sample derived from ambient air by accelerator mass spectrometry, and (ii) analysing the CO₂ produced from the combustion of IAEA C3 reference material. Upon validation, the suitability of the method for determining small excess ¹⁴C specific activity in the vicinity of a nuclear power plant was demonstrated.