Peptide purity explained: what 98% and 99% mean
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A purity figure such as 98% or 99% on a peptide certificate usually describes HPLC purity: the proportion of the detected signal that comes from the main peak. It does not say what fraction of the vial contents is the peptide itself. This guide separates the two ideas and explains the terms around them.
Purity at a glance
| HPLC purity | Share of detected signal from the main peak |
|---|---|
| Net peptide content | Share of the vial mass that is the peptide itself |
| Counter-ion | Ion paired with the peptide, such as acetate |
| Residual water | Water remaining after freeze-drying |
| Identity | Confirmed separately, by mass spectrometry |
What does HPLC purity measure?
Liquid chromatography (HPLC) separates the components of a sample as they pass through a column, and a detector records each as a peak. Purity is the area of the main peak divided by the total area of all peaks. A 99% result means that 99% of the detected signal belongs to the target compound and 1% to other detected components. It is a comparison of peaks, not a weighing of the vial.
What is net peptide content?
A lyophilised vial contains more than the peptide. It also holds counter-ions from the salt form, and a small amount of residual water. Net peptide content states how much of the total mass is the peptide. A batch can have 99% HPLC purity and a net peptide content well below 99%, because the two figures answer different questions. Vial strength on a listing, such as 10mg, refers to the supplied material as described on that listing.
What about counter-ions and water?
Synthetic peptides are commonly isolated as salts, for example as an acetate. The counter-ion adds mass but is not a contaminant peak in the usual sense. Residual water is measured separately. See what does lyophilised mean for why some water remains.
What should the documentation show?
- An HPLC purity figure with the method stated.
- A mass result consistent with the expected compound.
- The salt form, where relevant.
- Net peptide content or water content, if the supplier reports them.
- The batch number matching the vial.
Batch documents are on the COAs page. For a walk-through see how to read a peptide certificate of analysis.
Frequently asked questions
Is 99% purity always better than 98%?
Not automatically. The two figures should be from the same method, and the purity percentage says nothing about water, counter-ions or identity.
Does 98% purity mean the vial is 98% peptide?
No. It means 98% of the detected HPLC signal comes from the main peak. The share of the vial mass that is peptide is the net peptide content.
What is a counter-ion?
An ion paired with the peptide in its salt form. Acetate is a common example.
How is identity confirmed?
Usually by mass spectrometry, which measures the molecular mass for comparison with the value calculated from the sequence.
Where should the figures be found?
On the batch certificate of analysis, which should carry the same batch number as the vial.
All Ascend Research products are sold strictly for laboratory research use only. They are not for human or animal use, and nothing in this article is medical advice.
The Ascend Research team