Method
HPLC and UHPLC purity
Reversed-phase HPLC separates components along a column and detects them, often by ultraviolet absorbance. UHPLC is the same idea at higher pressure and usually smaller particles. The purity figure is only as clear as the method behind it.
On a peptide certificate, purity is commonly the area of the peak assigned to the main component divided by the total area the method includes, times 100. Wavelength matters. Peptide bonds are often observed near 214 or 220 nm. A percent at one wavelength is not automatically the percent at another, and it is not automatically the mass fraction in the vial.
The chromatogram is the evidence for that percent. Look for the baseline, the main peak, and whether neighbors are separated or hiding in a shoulder. A report that prints “99%” and omits the trace has skipped the part a second scientist would want to see. Impurity profiling claims, including claims below 0.1%, should name the method. A claim without a method is a slogan.
Retention time can also be used for identity: the sample peak lines up with a reference standard run under the same conditions. That is a comparison of behavior, not a measurement of mass. Pair it with the mass spectrometry page when identity is the question you care about.
Content, sometimes printed as milligrams per vial, needs a quantitative approach: a weighed standard, a response factor, or another assay the laboratory describes. Do not subtract impurities from the label claim in your head and call the result an assay. Ask for the content line.
Nothing in an HPLC purity result authorizes a use in people or animals. It describes a chromatogram.