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Apparent recovery compares a calibrated analytical result with a reference value. For measured estimate $\hat{q}$ and nonzero reference amount $q_{\mathrm{ref}}$, the ratio is $R_{\mathrm{app}}=\hat{q}/q_{\mathrm{ref}}$. IUPAC distinguishes this result-level comparison from extraction recovery, which describes yield of a preparation stage.^[ref]

An original example shows why the distinction matters. Begin with $100\mathrm{nmol}$ of analyte, retain 70% during preparation, and suppose the final matrix gives 1.5 times the neat-reference response per retained amount. A neat-reference calibration then reports $100(0.70)(1.5)=105\mathrm{nmol}$, so apparent recovery is 105%, while physical extraction recovery is 70%. No extra analyte need have been created. Loss and response enhancement can partially cancel or conceal each other.

The result also depends on calibration design. If preparation loss has already been incorporated into a valid process-matched calibration, applying the same recovery correction again can overcorrect the reported amount. A result near its reference does not independently establish perfect preparation yield or absence of an analytical matrix effect.

For a spike-recovery estimate, a common incremental comparison is $({\hat{q}}_{\mathrm{spiked}}-{\hat{q}}_{\mathrm{unspiked}})/q_{\mathrm{added}}$ after accounting for volume changes. This tests the observed increment, rather than automatically proving recovery of analyte originally embedded in a sample. An endogenous contribution, unequal matrix response or calibration nonlinearity can change its interpretation.

State the reference quantity, calibration, preparation stage and whether a correction has already been applied. An apparent-recovery percentage is an experimentally defined comparison, not a universal material-conservation fraction.

Spike equilibration is the process by which added analyte comes to share the chemical forms or physical environments relevant to recovery of the native analyte. Mixing a solution uniformly is not sufficient if native material is bound, trapped or otherwise inaccessible to the added material. The spike-recovery estimate is representative only to the extent that the two undergo comparable processing.

IUPAC's recovery guidance makes this equivalence a condition for using an added surrogate to correct native recovery. It notes that even an isotopically modified analyte can fail to equilibrate with native material bound to the sample matrix.^[ref]

An original two-environment model describes a fast-extractable fraction $a(t)$ approaching $a_{\mathrm{eq}}$:Let the fresh spike start entirely accessible, $a(0)=1$, while native material has accessible fraction $a_{\mathrm{eq}}=0.20$. With stipulated spike equilibration time $\tau =5\min$, a 15-minute wait leaves $a\approx 0.240$, not 0.20. If extraction measures only the accessible material, correcting a native recovered amount 20 by the spike's apparent recovery 0.240 gives approximately 83.4, below the true 100 amount units. The mismatch persists despite a high initial spike recovery.

A useful test varies the waiting time before extraction while keeping preparation and added analyte amount fixed. Compare the behavior of the spike with independent native-material or reference material evidence where available. A stable time series supports constancy over that window, but cannot prove that an inaccessible native compartment has been sampled.

More vigorous processing may change chemical forms or destroy the analyte; longer waiting alone does not guarantee equivalence. State addition timing, equilibration conditions and the extraction stage to which the measured extraction recovery applies. These choices are part of the measurement model, rather than details that can be recovered from the final percentage alone.

A spike-recovery estimate compares the measured increment after adding a known analyte amount to a sample with the amount added. It tests the response to the added material under the stated preparation and measurement calibration; it does not automatically measure recovery of analyte originally bound or embedded in the sample matrix.

If matched aliquots give inferred amounts ${\hat{q}}_s$ and ${\hat{q}}_0$ after accounting for dilution, and $q_a$ was added, the incremental apparent recovery isAn original dilution example starts with a $2.00\mathrm{mL}$ aliquot whose unspiked result is $3.00\mathrm{nmol}\mathrm{m}{\mathrm{L}}^{-1}$. Add $0.100\mathrm{mL}$ containing $20.0\mathrm{nmol}\mathrm{m}{\mathrm{L}}^{-1}$ of analyte; the added amount is $2.00\mathrm{nmol}$. If the spiked $2.10\mathrm{mL}$ solution measures $3.60\mathrm{nmol}\mathrm{m}{\mathrm{L}}^{-1}$, thenSubtracting the two concentrations without accounting for the changed volume would give a different answer. Alternatively, bring the spiked and unspiked aliquots to the same final volume before comparing them.

The spike's point of addition defines what is tested. Added before extraction, it experiences preparation loss and matrix-dependent response; added afterward, it cannot establish the earlier extraction recovery. Spike equilibration also matters: freshly added dissolved analyte may not access the same binding sites or compartments as native material. IUPAC's recovery guidance distinguishes recovery of native and added surrogate material and their possible differences.^[ref]

Use multiple spike levels and matched unspiked aliquots to look for concentration-dependent response. An incremental result near 100% can coexist with a constant interference in both aliquots, because subtraction removes that shared offset. The result therefore supports a particular recovery comparison, rather than proving every source of analytical bias absent.