Quality control 07
Testing for Purity and Presence — Two Questions, Not One
A high purity result does not prove that the dominant species is the labeled peptide or that the vial contains the stated peptide content.
'Is it pure?' and 'Is the labeled peptide actually in there?' are different questions, and a vial can pass one while failing the other.
Purity by HPLC or UPLC indicates what fraction of detected material appears as the dominant species under a defined method. It is silent on identity. A vial can be 99% one compound and still contain the wrong compound. High purity of the wrong molecule is still the wrong molecule.
Identity and quantitation answer the questions purity cannot. Mass spectrometry tests whether the measured mass is consistent with the intended peptide. Quantitation asks how much peptide is present, rather than assuming the entire mass of a lyophilized cake is active peptide. Powder mass can also include counter-ions, moisture, excipients, and residual salts.
The useful combination is complementary evidence: mass analysis for identity, chromatography for purity, and a suitable quantitative method for content. Any one alone leaves a gap that can remain invisible until a dose-response shifts without an obvious explanation.
Sources and evidence record
Research-use quality education only. This material does not establish suitability for human or veterinary use and is not dosing guidance.