 Blank-corrected carbon recovery in polymer tests

Blank-corrected carbon recovery in polymer tests subtracts measured background carbon from a specimen-containing experiment before comparing recovered carbon with the specimen's initial inventory. It is a carbon-basis closure for polymer degradation calculation with an explicit background model. Uncertainty in that subtraction and differences between blank and specimen conditions both affect the result.

For total measured carbon $T$, mean blank carbon $\overline{B}$ and initial specimen carbon $C_0$, writeAll terms must use the same carbon-mass or carbon-amount basis. For independent $T$ and $\overline{B}$, $u^2(Q)=u^2(T)+u^2(\overline{B})$; a shared correction can instead introduce covariance. Uncertainty in $C_0$ also enters the recovery ratio. The GUM's correlated-input rule supplies the general propagation framework.^[ref]

An original numerical example measures $T=10$ mg of carbon, with standard uncertainty 0.5 mg. Three independent blank runs give 2, 4 and 6 mg. Their mean is 4 mg, sample standard deviation is 2 mg and estimated standard uncertainty of the mean is $2/\sqrt{3}\approx 1.155$ mg. Net carbon is therefore 6 mg with standard uncertaintyWith stipulated exact $C_0=24$ mg, recovery is 25%, with standard uncertainty approximately 5.24 percentage points. A precise total measurement can still leave a relatively uncertain net result after subtraction.

Shared-blank covariance in carbon recovery matters if the same estimated blank corrects several specimens. With independent specimen totals $T_i$, each corrected quantity is $Q_i=T_i-\overline{B}$, soFor four specimen totals each having the 0.5 mg standard uncertainty above, averaging the corrected values gives variance $0.25/4+4/3\approx 1.396\mathrm{m}{\mathrm{g}}^2$. Treating all four corrected values as independent would incorrectly divide the shared blank contribution by four. Taking more specimen readings does not recreate independent blank information.

The OECD 301 B apparatus aerates inoculated medium with carbon-dioxide-free air and measures evolved gas captured in hydroxide traps. Its measured carbon dioxide is corrected for blank inoculum production.^[ref] Use matched collection duration, volume and analytical conversion when comparing blanks and specimen vessels; a gas-mass reading must be converted to carbon mass before mixing it with other carbon fractions.

A blank is not guaranteed to reproduce every background process inside the treated vessel. The guideline explicitly notes that endogenous activity in the presence of the test chemical may differ from that in the blank control.^[ref] Replication estimates scatter around the blank model; it does not prove additivity. Report the blank design, correction, uncertainty and any evidence for altered background activity alongside the final recovery.