When a vendor advertises a peptide as 99% pure, that figure almost always comes from HPLC (high-performance liquid chromatography) and describes purity by peak area. A second, less-advertised figure - net peptide content - answers a different question: of the total mass in the vial, how much is actually peptide, as opposed to water, salts and counter-ions. These two numbers routinely disagree, and understanding why is a core piece of certificate-of-analysis (CoA) literacy. This guide explains what each measures, why they diverge, and why a genuinely transparent CoA may report both. These are materials for research use only.
What does HPLC purity actually measure?
HPLC purity is a relative measurement. The instrument separates a dissolved sample into peaks based on how the components move through a column, then reports the target peak as a percentage of total peak area - for example, 99.0%. It is telling you how much of the detected material is the target peak versus other peaks (related substances, truncated sequences, degradation products).
Crucially, purity by area says nothing about what is not peptide at all. Solvent, trapped water and salts either elute where the method ignores them or are simply not part of the area calculation. So a 99% figure means "99% of the chromatographic peak area is the target," not "99% of the vial's weight is the target." The real evidence is the chromatogram - the plotted trace behind the number. A bare percentage with no chromatogram attached is a weak signal, a point we expand in our guide on how to read a peptide CoA.
What is net peptide content, and how is it measured?
Net peptide content (sometimes called peptide content or assay) is an absolute mass measurement. It answers: within the total dry weight in the vial, what fraction is genuinely peptide backbone? Synthetic peptides are rarely pure dry compound. Freeze-dried powder carries water, residual salts and counter-ions - commonly trifluoroacetate (TFA) or acetate left over from synthesis and purification.
Two laboratory approaches are standard. Amino-acid analysis (AAA) hydrolyses the peptide into its constituent amino acids and quantifies them, giving a direct measure of how much peptide is present. Nitrogen determination measures total nitrogen and back-calculates peptide mass. Both correct for the non-peptide mass that HPLC purity ignores. As a result, net peptide content is typically a lower number than HPLC purity - not because the peptide is contaminated with other peptides, but because part of the weighed powder was never peptide to begin with.
Why do the two numbers differ?
The gap comes down to what each method counts. HPLC purity divides the target peak by the sum of the peptide-related peaks it detects; it is blind to water and salts. Net peptide content divides peptide mass by total mass, including everything inert. So the same batch can honestly be described as, say, high purity by HPLC and noticeably lower by peptide content.
- Water: lyophilised powders are hygroscopic and hold residual moisture that adds weight but not peptide.
- Counter-ions: TFA or acetate salts can account for a meaningful share of the dry mass, especially for sequences with many charged residues.
- Salts and buffer residues: leftovers from purification contribute weight without contributing peptide.
This is why the two figures are not competing claims about the same thing. They are complementary. A vendor reporting only the higher HPLC number is not necessarily wrong, but a CoA that reports both gives you a fuller picture of what is in the vial. Identity and purity answer different questions, and content answers a third one about mass.
Does high purity guarantee correct identity?
No, and this is a common misreading. HPLC purity tells you a sample is chromatographically clean; it does not tell you the clean peak is the molecule claimed. A sample can be 99% pure by area and still be the wrong compound - one clean unknown rather than the target peptide.
Confirming identity is the job of mass spectrometry (MS / LC-MS), which measures molecular mass to verify the compound matches the expected peptide. Purity, identity and net content are three separate axes:
- Purity (HPLC): how clean is the sample, by peak area?
- Identity (MS / LC-MS): is it the molecule claimed?
- Net peptide content (AAA / nitrogen): how much of the mass is actually peptide?
A strong CoA addresses more than one axis. We compare the first two in detail in our overview of purity versus identity testing methods. When you read a certificate, notice which questions it answers and which it silently leaves open.
How should a comparison look on a transparent CoA?
A transparent CoA does not force you to guess which number a claim refers to. It labels each figure, states the method, and shows the underlying evidence. The table below summarises how the two measurements line up.
| Aspect | HPLC purity | Net peptide content |
|---|---|---|
| Method | HPLC (chromatography) | Amino-acid analysis or nitrogen determination |
| Type of measure | Relative (% of peak area) | Absolute (% of total mass) |
| What it accounts for | Peptide-related peaks only | Water, salts, counter-ions |
| Typical relationship | Higher number | Usually lower number |
| Question answered | How clean is the peptide? | How much peptide is in the vial? |
When both figures appear with their methods named, a batch or lot number, and a date, you can reconcile them yourself instead of taking a single marketing headline at face value. That is the difference between a document you can interrogate and a number you must trust blindly.
What should a buyer check when both figures matter?
Treat the two numbers as a set, not a single score. A useful reading checklist:
- Is the method stated for each figure? Purity should cite HPLC; content should cite AAA or nitrogen. An unlabelled percentage is ambiguous.
- Is a chromatogram attached? Purity claimed without the trace is a red flag, not proof.
- Does a batch or lot number match the vial? Results describe one batch at one point in time.
- Is the testing lab named and independent? A named, independent lab such as an accredited third-party testing laboratory is a stronger signal than "tested by a US lab" with no name.
- Can you verify at the source? A code or QR that resolves on the lab's own domain beats a PDF hosted only by the vendor.
None of these figures speak to sterility or endotoxin levels; a purity and content CoA describes composition only. To see how these checks translate into a transparency score, read our scoring methodology, which weights documentary evidence rather than marketing claims.
Why does a transparent vendor report both numbers?
Reporting both purity and net peptide content is a documentary choice, and documentary transparency is a property you can verify rather than a claim you are asked to believe. A vendor showing only the flattering HPLC figure is technically accurate but incomplete; one that adds net peptide content, names the method, and attaches the chromatogram is handing you the evidence to judge for yourself.
The pattern PeptideTrust looks for is consistent: batch-level results from a named, independent lab, a lot number that matches the vial, a date of analysis, and a verification mechanism that resolves at the source. Two well-labelled numbers with methods stated is a chromatogram-and-content pair you can reason about; one bare percentage is not. When you compare vendors, apply this lens on the ranked transparency scoreboard and ask not "how high is the number" but "how much of the evidence can I check."