Almost every supplier of research peptides quotes a percentage. But purity isn’t a quality mark; it’s the result of a measurement. And that result only means something if you know how it was measured.
What purity measures
Purity describes what share of the material in the vial consists of the intended peptide. The rest is usually related impurities: chains where an amino acid was skipped during synthesis, partially deprotected variants, and degradation products.
Note that purity says nothing about the total weight in the vial. Salts, residual moisture and counterions don’t count toward an HPLC purity, but they do determine how much peptide you actually have. That’s why our certificates also list the moisture content.
HPLC in brief
Reverse-phase HPLC separates the components in a sample by polarity. Each component leaves the column at its own retention time and shows up as a peak in the chromatogram. Divide the area of the main peak by the total area of all peaks and you have the purity.
So a purity of 99.4% means that 99.4% of the detected peak area belongs to the target peptide. Without a chromatogram that figure can’t be checked, which is why the graph is always part of our certificates.
Mass spectrometry
HPLC shows how pure a sample is, but not what it is. Mass spectrometry determines the molecular mass and so confirms the identity of the peptide. If the measured mass differs from the theoretical mass, that points to a wrong sequence or to modifications.
Where it goes wrong
Three things to check when you read a certificate:
- Is there a batch number, and does it match your vial? Without that number there’s no way to tell whether the report belongs to your material.
- Is the chromatogram included, or only the percentage? A bare figure can’t be verified.
- Does the report belong to this batch, or to a previous one? Every new production batch should get its own analysis.
What a COA should contain
A usable Certificate of Analysis contains at least: product name, batch number, analysis date, the methods used, purity with chromatogram, the mass spectrometry result, a moisture determination and the name of the laboratory that did the analysis.
Our certificates follow that layout and are listed per batch in the COA library.