Division 2.1.2 Residual activity

When the ¹⁴C-labelled part of an organic substance is biodegraded, the major part of the ¹⁴C is converted to ¹⁴CO₂, while another part is used for growth of biomass and/or synthesis of extra-cellular metabolites. Therefore, complete ‘ultimate’ biodegradation of a substance does not result in a 100 % conversion of its carbon into ¹⁴CO₂. The ¹⁴C built into products formed by biosynthesis is subsequently released slowly as ¹⁴CO₂ due to ‘secondary mineralisation’. For these reasons plots of residual organic ¹⁴C activity (measured after stripping off CO₂) or of ¹⁴CO₂ produced versus time will show a ‘tailing’ after degradation has been completed. This complicates a kinetic interpretation of the data and for this purpose, only the initial part of the curve (after the lag phase has ended and before approx. 50 % degradation is reached) should normally be used for the estimation of a degradation rate constant. If the test substance is degraded, the total residual organic ¹⁴C activity is always higher than the ¹⁴C activity associated with the remaining intact test substance. If the test substance is degraded by a first order reaction and a constant fraction α is mineralised into CO₂, the initial slope of the ¹⁴C disappearance curve (total organic ¹⁴C versus time) will be α times the slope of the corresponding curve for the concentration of test substance (or, to be precise, the part of the test substance labelled with ¹⁴C). Using measurements of the total organic ¹⁴C activity uncorrected, the calculated degradation rate constant will therefore be conservative. Procedures for estimating the concentrations of the test substance from the measured radiochemical activities based on various simplifying assumptions have been described in the literature (2)(9)(10)(11). Such procedures are most easily applied for rapidly degradable substances.