Purity testing is routinely quoted as a single number. In practice it is the output of at least two separate analyses that answer different questions.
Mass spectrometry answers "what is it"
MS measures mass-to-charge ratio, giving the molecular weight of the compound present. For a peptide of known sequence the expected mass is calculable, so a match is strong evidence of identity. A mismatch means the material is not the labelled compound, whatever its purity.
HPLC answers "how much of it"
High-performance liquid chromatography separates a sample by how strongly its components interact with a column. The detector output is a set of peaks; integrating their areas gives the proportion of the sample represented by the main compound.
Why both are needed
HPLC alone can report a clean single peak for the wrong molecule. MS alone confirms a target compound is present without describing how much of the sample is something else. Together they establish that the labelled compound is present and is the dominant component.
What neither test covers
Purity and identity are not the same as sterility, endotoxin load or residual solvent content. Those require their own methods, and a COA that does not report them is not making a claim about them.
For laboratory and research use only. Not intended for human or animal consumption, or for any medical, diagnostic or therapeutic use. These statements have not been evaluated by the FDA.