 Autoxidation inhibition period

An autoxidation inhibition period is a defined interval during which oxidation is suppressed relative to a matched uninhibited reaction. In an inhibited autoxidation assay, it can report how long a radical-trapping antioxidant retains effective capacity, but the operational start and end of the interval must be specified.

For a limiting model with antioxidant concentration $C_{A0}$, trapping stoichiometry $n$ and constant effective chain-initiation rate $R_i$, let $K_0=n{C}_{A0}$. If trapping is rapid and quantitative until exhaustion,At chosen values $C_{A0}=10\mu \mathrm{M}$, $n=2$ and $R_i=0.10\mu \mathrm{M}{\mathrm{s}}^{-1}$, the model gives $200\mathrm{s}$. Equal periods can arise from different combinations of concentration, stoichiometry and initiation rate, so an independent autoxidation initiation-rate calibration is needed to infer a capacity.

For changing initiation rate, capacity is consumed according to the integrated flux:An original decaying-flux example takes $R_i(t)=R_0{e}^{-kt}$, with $k>0$. Thenwhen $0<k{K}_0/R_0<1$. For $R_0=0.10\mu \mathrm{M}{\mathrm{s}}^{-1}$, $k=0.002{\mathrm{s}}^{-1}$ and $K_0=20\mu \mathrm{M}$, this gives about $255.4\mathrm{s}$, rather than the initial-rate approximation $200\mathrm{s}$.

Initiator-limited antioxidant exhaustion arises because this model supplies only total flux $R_0/k$. Here that limit is $50\mu \mathrm{M}$: a capacity $60\mu \mathrm{M}$ would never be exhausted by this modeled initiation source. Absence of a breakpoint would not demonstrate unlimited antioxidant capacity.

Experimentally, a delay can be defined by a stated curve-fitting or threshold rule, while the suppressed region can retain a nonzero oxidation slope. Primary spectrophotometric co-autoxidation work treats kinetics and stoichiometry as distinct properties of the radical-trapping reaction.^[ref] A longer apparent period can also reflect changed initiation, oxygen depletion or a readout effect. Comparing a matched blank, a reference antioxidant and a concentration series helps test which interpretation is supported.