Peptide Impurities Explained: Common Synthesis Impurities and How HPLC and LC-MS See Them
For in vitro laboratory research use only. This article explains analytical chemistry terms. It makes no claims about the effects of any material.
A peptide purity result such as 98% tells you the main component makes up most of what the method detected. The remaining few percent are impurities: other substances that came through the same process. This guide explains where peptide impurities usually come from, the common types, and how the two methods on a certificate of analysis, HPLC-UV and LC-MS, each see them.
Where peptide impurities come from
Most research peptides are made by solid-phase peptide synthesis. The chain is built one amino acid at a time on a solid support. Each new amino acid carries temporary protecting groups so it only reacts where it should. At the end, the finished chain is cut from the support, the protecting groups are removed, and the product is purified, usually by preparative HPLC, then freeze-dried.
Each coupling step works very well, but not perfectly. Over many steps, small amounts of slightly different molecules build up. Purification removes most of them. What is left is measured as impurities.
Common types of synthesis-related impurities
These are the impurity types most often described in the analytical literature for synthetic peptides. Many of them change the mass in a predictable way, which is why mass spectrometry helps tell them apart.
| Impurity type | What it is | Typical mass change |
|---|---|---|
| Deletion sequence | One amino acid is missing because a coupling step didn’t finish | Lower by the mass of the missing residue (for example, about 57 Da for glycine) |
| Insertion sequence | One amino acid appears twice | Higher by the mass of the extra residue |
| Truncated sequence | The chain stopped growing before the full length | Lower, often by several residues |
| Incomplete deprotection | A protecting group was not fully removed | Higher by the mass of that group (for example, about 56 Da for a tert-butyl group) |
| Oxidation | An oxygen is added, most often at methionine | About +16 Da |
| Deamidation | Asparagine or glutamine converts to aspartic or glutamic acid | About +0.98 Da (close to the 1 Da spacing of isotope peaks, so it can be hard to tell apart) |
| Isomers (diastereomers) | Same sequence and mass, but one amino acid has the opposite 3D form | No change |
Our guide to peptide sequence notation shows how residue masses add up, and our molecular weight and mass spec explainer covers how those masses appear in a spectrum.
What isn’t counted as a peptide impurity
A freeze-dried powder can also contain water and counter-ions such as trifluoroacetate (TFA) or acetate. These add weight but are not peptide impurities, and they usually don’t appear as peaks in a UV purity method. They affect net peptide content, which is a separate result. Our explainer on net peptide content vs purity covers that difference.
How HPLC-UV sees impurities
In an HPLC-UV purity run, impurities usually show up as small peaks before or after the main peak, because a small change in structure changes how long a molecule stays on the column. The purity figure is the main peak’s share of the total integrated peak area. Our guide to reading an HPLC chromatogram walks through that calculation.
HPLC-UV has limits here:
- It doesn’t name the peaks. A small peak shows that something else is there, not what it is.
- Close neighbors can overlap. An impurity that comes off at almost the same time as the main peak can sit inside it or show up only as a shoulder.
- It only sees what absorbs UV at the chosen wavelength. Substances that don’t absorb there won’t be counted.
More on what the percentage does and doesn’t mean is in our HPLC purity explainer.
How LC-MS sees impurities
On a certificate of analysis, identity (LC-MS) is liquid chromatography with mass spectrometry, used to check that the measured mass matches the expected compound. Because many impurity types shift the mass by a known amount, the same kind of analysis can also suggest what a minor peak is, for example a mass about 16 Da higher than expected pointing to oxidation.
LC-MS has limits too. Isomers have the same mass, so mass alone can’t tell them apart; separation by chromatography is needed for that. And the size of a peak in a mass spectrum doesn’t directly give the amount, since different molecules ionize to different extents. That is one reason purity is usually reported from HPLC-UV, while LC-MS is used for identity. Our LC-MS identity explainer covers what that result confirms and what it doesn’t.
What a COA usually reports
A typical certificate reports a single HPLC-UV purity figure and an LC-MS identity result. It usually does not list each impurity by type. A purity of 98%, for example, means about 2% of the detected peak area belongs to other components, without saying which ones. A certificate reports results for the sample tested. It is not a guarantee for other lots. For the terms used here, see our research peptide glossary.
Peptix certificates
The peptide lots with a published certificate on our peptide COA lookup page each have a third-party Certificate of Analysis showing the lot number, HPLC-UV purity and LC-MS identity. For example, you can view the BPC-157 10 mg product with its lot certificate, or browse all research peptides.
For research use only. Not for human or animal use.
