= Transporting blank corrections between batches
\b[Transporting blank corrections between batches] uses background measurements from one preparation or acquisition group to correct another. The transfer requires a justified relation between their backgrounds; sharing a nominal protocol does not by itself establish equality. In <blank-corrected carbon recovery in polymer tests>, OECD's general principle warns that endogenous activity with a test substance need not match its blank control.<Test No. 301: Ready Biodegradability by OECD>[General principle §4]{ref}
An original additive model writes a target specimen reading as $T_b=\mu_b+\beta_b+\epsilon_b$ and a source-batch blank as $B_a=\beta_a+\eta_a$. Let $\Delta_{ba}=\beta_b-\beta_a$. A transferred correction is
$$
\widehat\mu_b=T_b-B_a-\widehat\Delta_{ba}.
$$
Ignoring the shift biases the result by $\beta_b-\beta_a$. For stipulated specimen reading 18 mg, source background 2 mg and target background 5 mg, direct source-blank subtraction gives 16 mg instead of 13 mg. A precisely measured source blank does not eliminate this three-milligram mismatch.
Transfer uncertainty must also be retained. Under independent errors with <standard uncertainties> $u(T_b)=1$ mg, $u(B_a)=0.5$ mg and $u(\widehat\Delta_{ba})=2$ mg, the corrected result has uncertainty $\sqrt{1+0.25+4}\approx2.29$ mg. Omitting the transfer term gives only 1.12 mg. These chosen values illustrate the measurement model, not observed test performance. The GUM requires significant correction and influence quantities to be included rather than treating the functional relationship as just a numerical formula.<Guide to the expression of uncertainty in measurement by JCGM>[§4.1.2]{ref}
<Shared-reference cancellation in transferred blank corrections> is a complementary warning. If the estimated shift is literally $\widehat\Delta_{ba}=B_b-B_a$, then $T_b-B_a-(B_b-B_a)=T_b-B_b$. The source blank cancels. Treating $B_a$ and this shift as independent would double-count uncertainty; their <covariance> contains the cancellation.<Guide to the expression of uncertainty in measurement by JCGM>[§5.2.2, correlated-input propagation]{ref}
A transfer study can pair blanks across preparation conditions, quantify a reproducible shift and test whether it depends on collection time or material. Such evidence supports a specified transfer model. It does not prove that a blank remains chemically additive in the presence of the specimen.
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