Spike-recovery estimate

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.