In short
- HPLC purity is the main peak's share of the counted peak area, measured by one method at one wavelength.
- Going from 98% to 99% purity halves the measured impurity, but small differences between methods are normal.
- Counter-ions such as TFA, and water, add weight to the powder but don't appear in the purity figure.
- Net peptide content, often somewhere around 60 to 90%, is what links the powder's weight to the amount of peptide.
- Purity measures how clean a sample is, not how much peptide the vial holds.
Purity is the number suppliers put first: 98%, 99%, sometimes 99.5% or higher. It is a real measurement, but it is often read as something it isn't. A purity figure doesn't say the vial is 99% peptide by weight, and it doesn't say how many milligrams of peptide you have.
This guide explains where the percentage comes from, what the gap between 98% and 99% means, why the same sample can give different results, and how purity relates to net peptide content, counter-ions and water.
Where the percentage comes from
For peptides, purity almost always means HPLC purity by area percent. The sample runs through an HPLC column, a UV detector set at around 210 to 220 nm records each compound as it comes off, and software measures the area under each peak.
Purity is the main peak's area divided by the total area of all counted peaks. Peaks from the solvent and the instrument itself are excluded, and labs usually ignore very small peaks below a set reporting threshold. So a result of 99.2% means that, of everything this method detected and counted at this wavelength, 99.2% of the signal came from the main peak.
The example chromatogram below shows the features that feed into that calculation.
- Main peak. The compound you ordered. Purity is this peak's area as a share of all peak areas.
- Shoulder or close neighbour. Often a related impurity, such as a truncated or oxidised sequence.
- Early small peaks. Other impurities. A clean trace has few, and each is small.
- Baseline. Should be flat and quiet. A wandering baseline makes area figures less reliable.
98% vs 99%: look at the impurities
It is often more useful to read a purity figure from the other side. At 98%, about 2% of the counted area belongs to other peaks. At 99%, about 1% does. The step from 98 to 99 looks small, but it halves the measured impurity. The step from 99.0% to 99.5% halves it again.
Those impurities are mostly related peptides from synthesis: sequences missing an amino acid, chains that stopped short, and oxidized or otherwise modified forms. Which impurities are present matters as well as their total, which is why a peak table tells you more than a single number.
Two more details are worth checking:
- Specification vs result. “≥98%” is a limit the batch had to meet. “99.2%” is what it measured. A product described only as “98%+” may be quoting the limit rather than a result.
- Small differences. A few tenths of a percent between batches or labs can come from the method and the integration rather than the material.
Why the same sample can give different numbers
An HPLC purity figure belongs to the method that produced it. The column, the solvent gradient, the run time, the detection wavelength and the rules for drawing peak boundaries all affect the result.
The biggest source of difference is co-elution. An impurity that leaves the column at the same time as the main peak is counted as part of the main peak. A method with sharper separation can pull it out as a visible shoulder, and the purity figure drops even though the sample hasn't changed. Detector response also varies somewhat from compound to compound, so area percent is a close estimate of composition rather than an exact one.
That is why a purity result means most when it comes with the method details and the chromatogram, and why comparing figures from different methods is rough at best.
What purity doesn't count: counter-ions and water
A freeze-dried peptide is not pure peptide by weight. The powder in a vial typically contains:
- The target peptide, the molecule you ordered.
- Peptide-related impurities, the small peaks on the chromatogram.
- Counter-ions, small charged molecules bound to the peptide's basic sites. Trifluoroacetate (TFA), left over from the acid used in purification, is the most common. Acetate and chloride appear when the salt form has been exchanged. Counter-ions often make up roughly 10 to 30% of the weight, and more for peptides with many basic amino acids.
- Water, held by the dried powder. It is often under 10%, but it can be higher for peptides that attract moisture.
Counter-ions and water don't show up in the HPLC purity calculation. A sample can be 99% pure by HPLC while a fifth or more of its weight is salt and water.
Net peptide content: the number that relates to quantity
Net peptide content is the share of the powder's total weight that is peptide. It is usually measured by amino acid analysis, which breaks the peptide down into its amino acids and quantifies them, or by nitrogen analysis, since water and common counter-ions such as TFA and acetate contain no nitrogen.
Net peptide content commonly falls somewhere between about 60 and 90%, and it can be lower. It is a separate test from purity, and many research COAs don't report it.
The two numbers combine. Take a vial holding 10 mg of powder, with a net peptide content of 80% and an HPLC purity of 99%:
- 10 mg × 80% = 8.0 mg of peptide material.
- 8.0 mg × 99% ≈ 7.9 mg of the target peptide.
The purity figure on its own would never reveal that gap of about 2 mg.
What the amount on the label means
A label reading “10 mg” can mean different things. It may be the gross weight of powder filled into the vial, the net amount of peptide, or a target that individual vials meet within a tolerance. Suppliers don't always say which.
Some COAs report a measured amount, such as “10.3 mg per vial”. When you see one, check how it was measured, whether against a reference standard or by weighing, and whether it refers to peptide or to powder. A content result also comes from the vials the lab tested. Fill weights vary from vial to vial, so it is an estimate for the batch, not a guarantee for every vial.
If the basis isn't stated, it is reasonable to ask the supplier.
Putting the numbers together
When you look at a peptide's documentation, read the figures as a set:
- Purity: an HPLC area percent, with the wavelength, the method and a chromatogram.
- Identity: a mass spectrometry result showing observed and expected mass. Purity alone can't confirm that the main peak is the right molecule.
- Content: net peptide content or a measured amount, with its basis stated.
- Counter-ion and water: the salt form, and the water content if it was tested.
- Batch: all of it tied to the batch number on your vial.
A high purity figure is a good sign. It is one part of the picture, and it measures how clean the sample is, not how much peptide is there.
Questions
Is 99% purity always better than 98%?
On the same method, 99% means roughly half the measured impurity of 98%. Across different labs or methods the comparison is less reliable, because each method separates and counts impurities differently.
Why isn't net peptide content on my COA?
It is a separate test, often amino acid analysis, and many research suppliers don't run it routinely. If quantity matters for your work, ask whether content was measured for that batch.
What is TFA on a peptide certificate?
TFA stands for trifluoroacetate, a counter-ion left over from the acid used in purification. It binds to the peptide and adds weight without counting against HPLC purity. Some suppliers exchange it for acetate or chloride and state the salt form on the COA.
Does high purity mean the peptide is the right one?
No. Purity only says how dominant the main peak is. Identity needs a separate test, usually mass spectrometry, that compares the observed mass with the expected one.
This guide is about laboratory documentation and handling. It contains no dosing or medical advice, and nothing here describes use in people or animals.