Purity
Purity, on a peptide certificate, usually means the proportion of the detected material that was the intended molecule.
It is most often measured by chromatography and reported as the area of the main peak as a percentage of all peaks the detector saw. That phrasing matters: it is a percentage of what that instrument detected, under one set of conditions.
Anything the method does not detect is not in the denominator. A high purity figure is therefore compatible with the presence of substances the test was never looking for.
For example
A result of 99.1 percent means the main peak accounted for 99.1 percent of the detected peak area. It does not mean the container is 99.1 percent the intended compound by mass.
What it is not
Purity is not a safety measure and not a complete picture of contents. A purity figure is silent on sterility, on bacterial endotoxin, on residual solvents, and on how much of the mass is salt rather than peptide.