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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.^[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 $\sqrt{2/3}\approx 0.816$. 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 givesThis 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.^[ref] An appropriate analytical blank must match the part of the procedure whose background contribution is being assessed.

Blank correction removes the modeled effect of a blank indication from a measured response. The analytical blank supplies evidence about a background contribution; the measurement model specifies how to correct for it. The quantities combined must be of the same kind.^[ref]

For an additive example, a sample response of12 units and mean blank response of2 units give a corrected response of10 units. The correction retains measurement uncertainty. If sample response $X$ is independent of the mean of $n$ independent blank measurements, thenWith sample variance4, single-blank variance9 and $n=9$, the corrected variance is5, not zero. Dependence requires the corresponding covariance term in the propagation.

A different measurement model can require division. Invented transmittance indications0.4 for the sample and0.5 for the blank give corrected transmittance $0.4/0.5=0.8$. The associated absorbance is $-{\log }_{10}(0.8)\approx 0.09691$. Subtracting0.5 from0.4 would represent a different operation.^[ref] These examples show why specifying the blank material alone does not specify a valid blank correction.