Peptide Purity Explained: What 98% by HPLC Means
"98% pure" is the number most peptide listings lead with, and it is easy to read more into it than it says. This guide explains what an HPLC purity figure measures, what it leaves out, and which other tests on a certificate of analysis (COA) cover the gaps.
Research use only. Not for human consumption. Not evaluated by Health Canada. This guide covers specifications, storage and documentation. It gives no dosing or usage guidance.
In short.
- HPLC purity is a ratio of peak areas. It is the main peak's share of everything the detector sees at one UV wavelength, usually 210–220 nm.
- It does not tell you how much of the powder is peptide, or which peptide it is. Counter-ions and water are not counted, and identity needs mass spectrometry.
- A COA documents a lot's analysis. It does not by itself establish that a product is safe, authorized or suitable for any use.
What HPLC purity measures.
HPLC stands for high-performance liquid chromatography. For peptides, the standard method is reversed-phase HPLC: the sample is pushed through a column packed with C18-modified silica, and different molecules come off the column at different times. A UV detector at the column outlet records a trace, called a chromatogram, with one peak for each separated component.
Peptide bonds absorb UV light at around 210–220 nm, so that is where the detector is usually set. The purity figure is then worked out from the trace: the area of the main peak, taken as a share of the total area of all peaks, gives the peptide purity.
So a result of 98% means this: of all the material that absorbed UV light at the test wavelength, the target peptide's peak made up 98% of the total peak area. The remaining 2% of area belongs to other peaks, which are usually related peptides left over from synthesis.
Two details follow from that definition:
- It is a relative number. It compares peaks with each other. It does not weigh anything.
- It only sees what absorbs at that wavelength. Material that gives no UV signal at 210–220 nm does not show up in the ratio.
What HPLC purity does not show.
How much of the powder is peptide.
A vial of lyophilized peptide is not 100% peptide by weight, even at high purity. Synthetic peptides are usually isolated as salts, with counter-ions such as trifluoroacetate (TFA) or acetate bound to the basic amino acids and the N-terminus, plus some residual water.
The share of the powder that is peptide is called net peptide content: the fraction of peptidic material relative to counter-ions and residual water. Peptides delivered as TFA salts can carry a substantial share of their weight as TFA. Water content is measured separately, by Karl Fischer titration.
That is why "purity" and "peptide content" are different numbers. A sample can be 98% pure by HPLC and still have a net peptide content well below 98%.
Which peptide it is.
HPLC shows that one main component dominates. It does not say what that component is. A wrong sequence that happened to elute cleanly could still give a high purity figure. Identity is confirmed separately by mass spectrometry, which measures the molecular weight and checks it against the expected value.
Non-peptide contaminants.
Residual solvents and other reagents from synthesis need their own tests. They are not what a peptide purity result is designed to measure.
Common impurities in synthetic peptides.
Most research peptides are made by solid-phase peptide synthesis (SPPS), which builds the chain one amino acid at a time on a resin. Steps that do not run to completion, and side reactions along the way, leave related sequences in the crude product alongside the target, as a 2014 review of related impurities in peptide medicines describes. Purification removes most of them. What is left shows up as the minor peaks on the chromatogram.
| Impurity type | What it is | Where it comes from |
|---|---|---|
| Deletion sequences | Target sequence missing one or more amino acids | An incomplete coupling or deprotection step |
| Truncated sequences | Chains that stopped growing before the full length | Chain growth ending early during synthesis |
| Incompletely deprotected peptides | Target sequence still carrying a protecting group | Protecting groups not fully removed |
| Oxidized peptides | Side chains (for example methionine) oxidized | Exposure during synthesis, handling or storage |
| Diastereomers | An amino acid flipped to its mirror-image form | Racemization during synthesis |
| Counter-ions and residual reagents | TFA or acetate; leftover reagents and scavengers | Cleavage and HPLC purification steps |
Related peptide impurities matter for research because they can affect early experiments and lead to wrong conclusions, as the same review points out.
How COA tests map to these gaps.
Every Vitaliti batch is made by solid-phase synthesis, tied to its own lot code, and tested by an independent third-party lab.
Many COAs include the first two tests below; some labs also report the others. The COA for your lot lists which tests were run on it.
| Test | Question it answers | What it covers |
|---|---|---|
| Purity (HPLC) | How much of the UV-absorbing material is the target peptide? | Deletion, truncated, oxidized and other related-peptide impurities, as peak area |
| Identity (mass spectrometry) | Is the main component the right molecule? | Confirms molecular weight matches the expected sequence |
| Quantity (amino acid analysis) | How much peptide is actually present? | Measures peptide quantity directly, which HPLC purity does not |
| Water content (Karl Fischer) | How much of the powder is water? | Residual moisture, which HPLC purity does not count |
| Residual solvents | Are synthesis solvents left behind? | Non-peptide contaminants outside the scope of HPLC purity |
Amino acid analysis works by breaking the peptide down with strong acid and measuring the individual amino acids that come out. That gives a quantity, which is the missing piece next to a purity ratio. Water is measured by Karl Fischer titration, and counter-ion content, where reported, shows how much of the weight is TFA or acetate.
Blends such as GLOW and KLOW contain more than one peptide, and independent labs report them in different ways. Some give identity and measured amount for each component; some add a purity figure per component; some give a single blend-average purity and total content alongside measured vs labelled amounts per component. Some labs measure identity and amount only for blends, because the peaks of several peptides can overlap and obscure each other's impurities. The COA for your lot shows how its lab reported each component.
Reading a COA.
A few checks make a COA more useful:
- Lot code. It should match the lot code printed on your vial.
- Tests and results. Look for each test listed, the method used and the result.
- Chromatogram and spectrum. Some COAs include the HPLC chromatogram and the mass spectrum. Where they do, you can see the peaks behind the numbers.
- Dates. Note when the analysis was run alongside the expiration date on the vial.
The COA for your exact lot is sent by email on request at contact@vitalitipeptide.com.
What a COA does not establish.
A COA is a record of what was measured in one lot at one point in time. It does not by itself establish that a product is safe, that it is authorized for sale as a drug, or that it is suitable for any particular use.
Labelling a product "research use only" does not by itself make it legal or exempt it from regulatory requirements. Purity data answers a chemistry question. It does not answer a regulatory or safety one.
Peptide quality also depends on what happens after testing. Storage and handling after delivery affect a peptide's condition, which is why storage guidance sits alongside the COA rather than being replaced by it.