Scope
The short answer
HPLC purity asks how much of the detected signal belongs to the main compound. Peptide content asks how much of the weighed powder is peptide. A report can be strong on one and silent on the other.
This guide describes laboratory analytical reports for research materials. It is documentation education, not guidance for human or veterinary use, and it makes no claims about effects.
What an HPLC purity number measures
In reversed-phase HPLC, a dissolved sample is separated on a column and detected, for peptides typically by UV absorbance at 210 to 220 nm. The purity result is generally the area of the main peak relative to the total area of all detected peaks [1][2].
That makes it a relative measure under stated conditions. Water and salts are not detected by this method [1], so the percentage says nothing about them.
What net peptide content measures
Net peptide content (NPC) is the percentage of peptide relative to non-peptidic material in the powder, mostly counter-ions and moisture [2]. After purification, peptides are usually delivered as salts, most often with trifluoroacetic acid (TFA) or acetic acid bound to the N-terminus and to basic side chains such as Arg, Lys and His [1][2]. Peptides can also be hygroscopic and hold considerable water [1].
Two details are easy to miss. First, NPC counts peptide-related impurities as peptide, so it is not a purity figure either [1][2]. Second, even a 100% pure peptide can have an NPC below 100%, especially when it has many basic amino acids [1]. NPC is typically lower for peptides rich in basic residues and for hydrophilic peptides [2].
Laboratories commonly determine content by amino acid analysis or by elemental analysis (nitrogen content) [1][2], and water by Karl Fischer titration [2]. Some reports state a quantity per vial instead of a percentage. If one does, check how it was determined.
An illustrative example
Imagine a report that lists HPLC purity of 99.1% and net peptide content of 87.4%. Both can be true together. The first says the main peak is 99.1% of detected peak area. The second says 87.4% of the powder by weight is peptide, with the remainder mostly water and counter-ions. This is the same teaching example used in the Interactive COA Decoder, and it is not a real product report.
Where mass spectrometry fits
LC-MS reports the mass of what elutes, which supports identity. It does not by itself establish purity or content. The HPLC vs. LC-MS guide covers that distinction in more depth. A fuller picture usually pairs identity (MS), purity (HPLC) and, where it is tested, content.
Checklist when you read a report
- Which method produced the purity number, and is it labeled as area percent?
- Is there a content or assay result, and how was it determined?
- Does the report name the laboratory, a report number and a date?
- Is there a lot or batch reference that matches the material under review?
- Does the labeled amount match the listing?
If the content result sits a little above or below the label, why a vial can test above its label claim explains what is normal and what to ask about.
The How to Read a COA guide and the COA library show how these fields appear on real documents.
Limitations and risks of over-reading a COA
A COA covers one tested sample. It does not guarantee other lots, and it does not by itself establish sterility, endotoxin levels, long-term stability or suitability for any particular experiment. Purity also varies between batches [2]. Reports can be altered or misrepresented, so confirming the issuing laboratory directly is worth the effort. Methods differ between laboratories, so two purity numbers are not strictly comparable without knowing the conditions.
References
- Iris Biotech, "Net content and purity, two key parameters in peptide synthesis."
- Bachem Knowledge Center, "Quality control of amino acids and peptides: a guide."
Both are manufacturer technical pages, not peer-reviewed articles. Figures in the illustrative example are teaching numbers only.