909 AMINOSImpurity Peaks on an HPLC Chromatogram: What They Are and What a Certificate Can't Tell You
On an HPLC purity certificate, everything other than the main peak is reported only as "other integrated peak area". A standard certificate doesn't identify what those minor peaks are.
- "99.22% (area %)" means the main peak made up 99.22% of the peak area the method detected and integrated.
- The remaining 0.78% is other integrated peak area; the certificate doesn't identify it.
- Laboratories can calculate area % on different bases, so figures from different laboratories aren't directly interchangeable.
- An HPLC purity figure doesn't cover co-eluting or non-absorbing substances, the amount in the vial, or tests that weren't performed.
1Reading the chromatogram
Retention time: when each component left the column.
Detector response at the method's wavelength.
Normally the target peptide. On the certificate for lot TESA-2609A, the main peak elutes at 9.249 minutes.
Before or after it: other components that absorb at that wavelength.
Example, lot TESA-2609A (KMD certificate 9AL-20260921-C7PG-01):
2What minor peaks can be
This is general background, not an identification of the peaks on any 909 certificate.
In synthetic peptides, minor peaks are often peptide-related substances, molecules very close to the target. Common types include:
| Type | What it is |
|---|---|
| Deletion sequence | One amino acid missing from the chain |
| Truncated sequence | Synthesis stopped early, leaving a shorter chain |
| Incompletely deprotected | A protecting group from synthesis still attached |
| Oxidized form | Typically at methionine, tryptophan or cysteine |
| Deamidated form | Asparagine or glutamine converted to their acid forms |
| Diastereomer | One amino acid in the opposite (D/L) configuration |
A standard purity certificate reports the minor peaks only as a share of total area. It doesn't name them, and it doesn't establish which of these types, if any, are present. Identifying individual peaks takes additional analysis, usually mass spectrometry.
3Two laboratories, two definitions
Purity by area % depends on what goes into the total:
| KMD Analytical (lot TESA-2609A) | ILS Laboratories (lot KLOW-2610A) | |
|---|---|---|
| Result | 99.22% | 99.89% |
| Stated basis | Percentage of total integrated peak area | Target peptide(s) relative to all peptide-related peaks; non-peptide process-related impurities, if detected, excluded |
| Detection | HPLC-DAD | RP-HPLC at 214 nm |
Divides by the total integrated peak area.
Divides by peptide-related peaks only.
Figures calculated on different bases aren't directly interchangeable, so read the stated basis before comparing purity results from different laboratories.
4What an HPLC purity figure can't tell you
A substance that leaves the column at the same time as the main peak is counted as part of it.
Area is detector response, not mass, so a component that absorbs weakly is under-represented and one that absorbs strongly is over-represented.
Water, many salts and some solvents give little or no signal at typical wavelengths.
Purity is a proportion. The quantity of peptide is a separate content result. See Purity vs. Measured Content.
KMD's certificates state that, unless expressly noted, no assessment was made of microbial, endotoxin, heavy-metal or residual-solvent content. A certificate covers only the tests it lists.
5Checklist
- Is the basis of the purity figure stated (all peaks, or peptide-related peaks only)?
- Are the method and wavelength named?
- Is the chromatogram included, so the peaks can be seen?
- Is content reported separately from purity?
- Which tests does the certificate say were not performed?
?Frequently asked questions
Does 99% purity mean 99% of the powder is the peptide?
No. It means the main peak was 99% of the detected peak area. How much of the powder is peptide is a content question, affected by counter-ions and water.
Can a certificate tell me what each small peak is?
Not a standard HPLC purity certificate. It reports minor peaks only as a share of area; identifying them needs further analysis such as mass spectrometry.
Why do purity figures from different laboratories differ?
Laboratories can use different columns, wavelengths and definitions of the total. Compare figures only when the basis is the same.
Sources
- KMD Analytical certificate 9AL-20260921-C7PG-01 (lot TESA-2609A), original PDF
- ILS Laboratories certificate COA-2026-FVFEQK (lot KLOW-2610A), original PDF
Sources are cited for reference. Their publishers have not reviewed or endorsed this guide.
For laboratory research use only. Not for human or veterinary use. Not FDA approved.