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Autoxidation lag standard addition estimates an initiation rate or an equivalent background trapping capacity by adding known amounts of a reference radical-trapping antioxidant to a matched sample matrix. It uses a series of autoxidation inhibition periods, rather than treating one delay as a chemical concentration.

Under constant effective initiation rate $R_i$, fast quantitative trapping and unchanged reference stoichiometry $n_s$, let $K_b$ be the initial background capacity and $C_s$ the added reference concentration. Capacity balance givesThus the linear regression intercept and slope have different roles. The concentration $a/b$ is an equivalent amount of the chosen reference; it does not identify the background chemical species.

For an original series, $C_s=0,4,8\mu \mathrm{M}$ gives delays $30,70,110\mathrm{s}$. Then $a=30\mathrm{s}$ and $b=10\mathrm{s}\mu {\mathrm{M}}^{-1}$. With $n_s=2$, the model gives $R_i=0.20\mu \mathrm{M}{\mathrm{s}}^{-1}$ and $K_b=6\mu \mathrm{M}$ of trapping capacity, equivalent to $3\mu \mathrm{M}$ of that reference. Using only the largest addition and dividing $n_s{C}_s$ by its full delay would give about $0.145\mu \mathrm{M}{\mathrm{s}}^{-1}$, because it incorrectly includes the background delay in the reference-only balance.

At first order, if $n_s$ is known, $u(R_i)/R_i\approx u(b)/b$. Inference of $K_b=n_{s}a/b$ also involves the fitted intercept-slope covariance; treating fitted parameters as independent can misstate the measurement uncertainty. Replicates at several concentrations test linearity and estimate the delay variability.

The chemical observation must still be defined. A primary spectrophotometric method monitors a BODIPY-based signal carrier co-autoxidizing with a hydrocarbon and distinguishes trapping kinetics from stoichiometry.^[ref] The regression above is a stated capacity model, not an assertion that this paper used this particular numerical series. Keep solvent addition, initiation, temperature, oxygen supply and breakpoint rule matched. Curvature can reveal altered chemistry, oxygen-limited autoxidation or time-varying initiation; it should not be hidden by forcing a straight line.

An inhibited autoxidation assay measures how an added substance changes the time course of a substrate's autoxidation under specified conditions. A radical-trapping antioxidant competes with radical chain propagation, so both the speed of interception and the amount of trapping capacity can affect the observed curve. A smaller oxidation rate and a longer delay before rapid oxidation are distinct observations.

An experiment compares a defined oxidizable substrate with and without the test substance, using matched solvent, temperature, oxygen availability and radical chain initiation. The readout must track the oxidation process rather than merely the test substance's own optical background. One primary method uses a visible-light spectrophotometer and a BODIPY-based signal carrier that co-autoxidizes with a hydrocarbon substrate.^[ref] The instrument observes a probe response; interpreting it as substrate oxidation requires the method's coupled-reaction model.

An original limiting model explains an autoxidation inhibition period. Let radical-chain initiation supply chains at constant concentration rate $R_i$. Suppose the antioxidant initially has concentration $C_{A0}$ and can intercept $n$ chain carriers per molecule, with fast, nearly quantitative interception until it is exhausted. Capacity balance givesFor chosen values $C_{A0}=10\mu \mathrm{M}$, $n=2$ and $R_i=0.10\mu \mathrm{M}{\mathrm{s}}^{-1}$, the modeled period is $200\mathrm{s}$. Doubling the antioxidant concentration doubles it under these assumptions. But $n=4$ at $5\mu \mathrm{M}$ gives the same capacity, so an inhibition period alone does not separately determine trapping stoichiometry and antioxidant concentration.

Autoxidation initiation-rate calibration is consequently part of the inference. If initiation varies between samples, a longer inhibition period need not mean more antioxidant. Finite trapping rates, side reactions, inhibitor regeneration and changing oxygen availability can invalidate the simple capacity model. Comparing the inhibited slope with the uninhibited curve asks a kinetic question in addition to the capacity question.

A 1983 primary study used inhibited autoxidation to estimate lipid-soluble chain-breaking antioxidant concentrations in human plasma and erythrocyte ghosts, and compared those results with vitamin E measured by high-performance liquid chromatography.^[ref] That independent comparison tests a relationship between activity and composition in the studied preparations; an arbitrary antioxidant-assay signal alone does not establish chemical identity or a health benefit.

Initiator-limited antioxidant exhaustion describes whether a finite radical source can consume all the trapping capacity in an inhibited autoxidation assay. A missing end to an autoxidation inhibition period can result from too little integrated initiation, rather than an unlimited amount of radical-trapping antioxidant.

Let $K_0=n{C}_{A0}$ be the initial radical-trapping capacity, measured in concentration of trapped chain carriers. Assume quantitative trapping, no regeneration or competing loss, and an initiation flux $R_i(t)=R_0{e}^{-kt}$ with $R_0,k>0$. Before exhaustion the remaining capacity isThe source can deliver only $Q_{\infty }=R_0/k$. Consequently, $K_0<Q_{\infty }$ gives finite exhaustion timeIf $K_0=Q_{\infty }$, capacity approaches zero only as $t\to \infty$. If $K_0>Q_{\infty }$, a positive capacity $K_0-Q_{\infty }$ remains even at infinite time. These are different outcomes despite both lacking a finite exhaustion breakpoint.

For an original example, $R_0=0.20\mu \mathrm{M}{\mathrm{s}}^{-1}$ and $k=0.010{\mathrm{s}}^{-1}$ give total supply $20\mu \mathrm{M}$. Initial capacities $10$, $20$ and $25\mu \mathrm{M}$ respectively give exhaustion at $69.3\mathrm{s}$, asymptotic exhaustion, and a residual $5\mu \mathrm{M}$. The initial-rate estimate $K_0/R_0$ would wrongly predict a finite $100\mathrm{s}$ exhaustion time in the middle case.

The logarithm explains a sensitive regime. For $\alpha =K_0/Q_{\infty }\ll 1$,but $t_{\ast }$ diverges as $\alpha \to 1^{-}$. Small capacity or initiation-calibration changes near that threshold can cause large lag changes.

A primary visible-light autoxidation method distinguishes trapping kinetics from stoichiometry through a coupled oxidation readout.^[ref] In the model here, a controlled second initiator addition can test whether capacity remains after the first source declines. A matched blank and independent autoxidation initiation-rate calibration are essential: extra initiation can change the optical reaction, and a persistent background source would eventually exhaust even capacity above $Q_{\infty }$. An absent breakpoint alone does not distinguish these mechanisms.