A blank is the full analytical procedure carried out on a sample containing none of the material being measured.
A blank is the full analytical procedure carried out on a sample containing none of the material being measured. Anything the instrument reports is coming from the procedure itself — the solvents, the glassware, the column, the air — rather than from the sample.
Whatever a blank shows is subtracted, conceptually, from every real measurement. A method whose blank is noisy cannot report small quantities reliably, which is how blanks and detection limits are connected.
Residue from a previous, more concentrated sample can appear in the next run. A blank between samples detects this, and its absence is how a strong result from one sample can contaminate the appearance of the next.
A solvent blank tests the reagents. A method blank carries an empty sample through every preparation step. A field blank travels with the samples. Each catches contamination entering at a different point, and a report naming which was run is telling a reader where it looked.
Without them, a low-level finding cannot be distinguished from laboratory background. The result may be entirely correct; there is simply no evidence within the report that it is, and that is the distinction that matters for a trace figure.
Certificates issued commercially rarely include blank results, because the audience is assumed to trust the laboratory's internal quality control. That assumption is normally reasonable and it does mean the published document is thinner evidence than the work behind it.
Accredited laboratories run blanks as a matter of procedure and record them internally, whether or not they appear on a certificate. This is part of what an accreditation credential is actually certifying.
For a headline purity figure, blanks matter little. For any claim that something is absent or present in trace amounts, they are most of the evidence, and a report making such a claim without mentioning them is asserting rather than demonstrating.
The claims on this page about the test with nothing in it, and what its absence allows can be checked against sources outside this site: