Peptide purity: what the number means
"99% purity" is the most common number in the peptide world, and one of the least understood. This article explains what the figure actually measures, what makes up the rest of the sample, how purity is tested, and why the salt form on a certificate of analysis (COA) matters as much as the headline number.
What a purity figure measures
When a COA reports 99% purity, it means that 99% of the material the instrument detected is the target peptide. The figure comes from liquid chromatography, which separates a sample into its components and draws a peak for each one. The classic method is high-performance liquid chromatography (HPLC); many labs now run it coupled to a mass spectrometer (LC-MS or LC-MS/MS). Purity is the area of the main peak as a share of the total area of all peaks.
Two points follow from that:
- Purity is a ratio, not an amount. It tells you how much of what's in the vial is the correct peptide, not how many milligrams of peptide the vial holds.
- Purity only covers what the method detects. Water, salts and counter-ions don't show up as peaks in the same way, so they aren't part of the percentage.
A sample that is 95% pure isn't a vial with 5% less peptide in it. It is a sample in which 5% of the detected material is something other than the target.
What the other few percent is
Peptides are built one amino acid at a time, and each step in that process has a small chance of going wrong. The impurities in a peptide sample are mostly the by-products of those small failures:
- Truncated sequences: chains that stopped growing early because a coupling step failed.
- Deletion sequences: chains missing one amino acid from somewhere in the middle, because a single coupling was incomplete and the next one went ahead anyway.
- Modified residues: for example, des-amino variants that lack the N-terminal amino group, or oxidized methionine, which can form during synthesis or storage.
- Aggregates: peptide molecules that have clumped together.
- Synthesis by-products: leftover protecting groups or reagents that purification didn't fully remove.
These related substances matter because they are similar enough to the target peptide to be mistaken for it, and different enough to behave differently. In laboratory work, impurities can interfere with assay results, which makes experiments harder to interpret and harder to repeat.
How purity and identity are tested
No single test answers every question about a sample. Laboratories combine methods, each of which checks something the others can't:
- Liquid chromatography (HPLC, or LC coupled to mass spectrometry) measures chromatographic purity. Look for one dominant, sharp peak and a clean baseline, with the chromatogram included rather than just a number.
- Mass spectrometry (MS) confirms identity by measuring molecular mass. Chromatography tells you how pure a sample is; MS tells you what it is. A sample can score well on purity and still be the wrong compound. Labs often combine the two as LC-MS or LC-MS/MS, so identity and purity come from a single run.
- Amino acid analysis checks that the composition matches the expected sequence.
- Endotoxin testing, usually by the LAL method, measures bacterial endotoxin, which can interfere with cell-based experiments.
Not every COA includes every test. A good COA is clear about which tests were run on that lot, by what method, and when, with each result shown against its specification. Our guide to reading a certificate of analysis walks through each section.
Salt form and net peptide content
This is the part of peptide quality that gets the least attention, and it explains a common source of confusion.
Peptides are usually purified by HPLC using trifluoroacetic acid (TFA) in the mobile phase. TFA binds to positively charged amino acids such as lysine, arginine and histidine, and it stays with the peptide as a counter-ion. Many manufacturers exchange it for acetate or another salt in an extra step. Others leave it as TFA.
Either way, the powder in a vial is the peptide plus its counter-ion, plus a little residual water. That leads to two different figures:
- Gross weight: the weight of all the powder.
- Net peptide content: the share of that weight that is the peptide itself.
Net peptide content is always below 100%, and how far below depends on the sequence and the salt form. That's normal, and it isn't the same thing as purity. A sample can be 99% pure by HPLC and still have a net peptide content well under that, because the counter-ion and water aren't impurities in the HPLC sense.
The practical point: look for net peptide content on the COA. It reports how much of the powder is the peptide itself, usually in milligrams against the amount on the label, and the salt form is the main reason it sits below the total weight of the powder.
Why lot-specific reports matter
Every synthesis run is a separate event. Coupling efficiency, purification and drying all vary a little from one run to the next, so purity varies too. A supplier's average figure, or a report from an earlier lot, says nothing reliable about the lot in front of you.
That's why a COA should carry a lot number that matches the label on the vial. It is the only way to tie the test results to that specific material.
A short checklist
- Chromatographic purity (HPLC or LC-MS) with the chromatogram shown, not only a number
- An identity test (mass spectrometry), with observed mass close to theoretical
- Net peptide content, not only the total weight of the powder
- A lot number that matches the vial
- A report number and the dates the sample was received and analyzed
- Each result shown against its specification
Can you judge purity by eye?
No. A 95% sample and a 99.5% sample look the same: a white to off-white powder. Visual inspection can catch obvious problems such as discoloration or moisture in the vial, but purity differences can only be measured with analytical instruments.
How we handle it
Every lot we sell is tested by an independent laboratory before it goes on sale, by liquid chromatography–tandem mass spectrometry (LC-MS/MS): identity and chromatographic purity in one run, plus net peptide content and label accuracy. The report for each lot is shown on its product page under a "View COA" button. It carries the lot number printed on the vial, a report number and the received and analyzed dates, and it lists exactly which tests were run, each against its specification, with the method details and chromatogram. The purity figure on the page is the one on that report.
All products are sold for laboratory research use only and are not for human or veterinary use.