= Analytical blank
An <analytical blank> is a material containing none, or as little as possible, of the <analyte> of interest, used to establish a <blank indication>. The material and the measured <blank indication> are different objects: a solution without the intended <analyte> can still produce a nonzero instrument response. A <procedural blank> follows the complete stated procedure, whereas a solvent or instrument blank covers a specified portion of it.<Blank material in the IUPAC Gold Book>[08010, definition and notes3–6]{ref}
An invented additive-response example makes the distinction concrete. Four <analytical blank> measurements give $2,3,1,2$ response units. Their mean is $2$ and their sample <standard deviation> is $\sqrt{2/3}\approx0.816$. A sample response of $8$ then has a blank-subtracted response of $8-2=6$. The <blank correction> removes an estimated offset; it does not erase uncertainty in either measurement.
For independent sample response $X$ and a mean of $n$ independent blank measurements $B_i$, the model gives
$$
\operatorname{Var}(X-\overline B)=\operatorname{Var}(X)+\frac{\operatorname{Var}(B_i)}{n}.
$$
This explains why more blank replicates can improve the estimated background without eliminating sample variability. <Blank correction> also depends on the measurement model: <transmittance> can require division by the blank indication rather than subtraction.<Blank correction in the IUPAC Gold Book>[08008, definition and notes1–2]{ref} An appropriate <analytical blank> must match the part of the procedure whose background contribution is being assessed.
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