How to read a peptide certificate of analysis
A certificate of analysis (COA) is a lab's report on one specific batch. Read well, it tells you whether the material is what it claims to be and how pure it is. Read carelessly, a COA can look reassuring while proving very little.
The parts of a COA
| Field | What it tells you | What to check |
|---|---|---|
| Product and sequence | What was tested | Name, and ideally the amino acid sequence or molecular formula |
| Lot / batch number | Which batch the results belong to | It matches the lot number on your vial |
| Test date | When the analysis was run | Recent relative to the batch, not years old |
| Testing lab | Who ran it | A named lab you can look up; note any accreditation such as ISO/IEC 17025 [1] |
| HPLC purity | Share of UV-detectable material that is the target peptide [2][3] | A chromatogram is attached, with the method or detection wavelength |
| Mass spectrometry | Whether the molecule has the expected mass (identity) [4] | Observed mass is reported next to the theoretical mass |
| Net peptide content (if listed) | How much of the powder's weight is peptide [2] | Often below 100% even at high purity, because of water and counterions |
| Appearance | Physical description | Usually "white lyophilized powder" or similar |
Purity is not the same as content
HPLC purity answers "how much of the detectable material is the target peptide?" Net peptide content answers "how much of the weighed powder is peptide at all?" Water and counterions such as TFA are invisible to standard HPLC, so a peptide can be 99% pure while the powder is meaningfully less than 100% peptide by weight [2][3]. For calculations that need exact amounts, the net content matters.
Identity and purity are separate tests
A purity number alone doesn't prove the peak is the right molecule; mass spectrometry confirms the molecular weight [4]. A strong COA reports both. See HPLC purity testing and identity testing by mass spectrometry.
Red flags
- No lot number, or a lot number that doesn't match the vial.
- A purity figure with no chromatogram.
- No mass spectrometry or identity result.
- No testing lab named, or no test date.
- The same report reused across products or batches.
- Signs of editing: mismatched fonts, misaligned numbers, blurry or cropped figures.
What a COA doesn't tell you
A standard purity and identity COA usually doesn't cover sterility, endotoxin, water content or counterion levels unless those tests are listed; they are separate analyses [5]. And a COA describes one lot at the time of testing, not material from a different batch or material that has been stored badly since.
How this works at XS
Every XS vial carries a lot number, and each lot is sent to an outside lab. You can request the report for your lot from any product page or from Your shelf, and it's matched to the lot number you received. See lot numbers and traceability.
Questions
What is a COA for peptides?
A certificate of analysis is a lab's report on one batch: typically HPLC purity, mass spectrometry identity, the lot number and the test date.
What purity should a research peptide COA show?
It depends on the application. More important than the headline number is that the report includes a chromatogram, a mass spectrometry identity result, the lot number and the testing lab.
Why is net peptide content lower than purity?
Purity measures UV-detectable material by HPLC. Net peptide content measures how much of the powder's weight is peptide, which excludes water and counterions such as TFA that HPLC doesn't detect.
References
- ANAB: ISO/IEC 17025 testing laboratory accreditation
- Iris Biotech: Net content and purity, two key parameters in peptide synthesis
- LifeTein: What is peptide purity?
- Bio-Synthesis: Peptide identification and confirmation
- GenScript: Examining the components of your peptide sample (QC methods)
References describe general laboratory practice from the cited sources; stability and handling vary by compound. Follow the documentation for the specific peptide.