When you receive your Certificate of Analysis (COA) from a peptide testing lab, the purity number is often the first thing you look at. But what does that number actually tell you about your peptide, and how should you use it in your research?
1. What “Purity” Means in HPLC Testing
HPLC purity (typically reported by UV at 214 nm or 220 nm, sometimes with CAD) represents the percentage of the main peak area relative to the total integrated peak area in the chromatogram. A reported purity of 98.5% means that 98.5% of the UV-absorbing material in your sample is the intended peptide, while 1.5% consists of other UV-active compounds (related substances, deletion sequences, oxidation products, etc.).
2. Related Substances vs. Unknown Impurities
Modern HPLC methods for peptides can resolve and report individual related substances. Good labs will list the major impurities with their relative retention times and approximate percentages. This is far more valuable than a single purity number because:
- You can identify common synthesis problems (e.g., deletion sequences, racemization, oxidation).
- You can set scientifically justified acceptance criteria instead of arbitrary “>95%” rules.
- You have data to support batch release or further purification decisions.
3. How to Read Your Report Effectively
When reviewing an HPLC report from Nexus or any other lab, focus on these elements:
Peak Purity / Peak Homogeneity
Look for diode-array or mass spec confirmation that the main peak is spectrally or mass-pure. A single sharp peak on UV does not always mean a single compound.
Related Substance Profile
Pay attention to the number and size of impurity peaks. A 99% pure peptide with one 0.8% impurity is very different from a 99% pure peptide with ten 0.1% impurities from different sources.
Method Details
Always check the HPLC method used (column, gradient, detection wavelength, injection volume). Different methods can give different purity numbers for the same sample. Ask for the raw data files so you can re-integrate if needed.
4. Setting Realistic Acceptance Criteria
For most research applications we recommend the following starting points (adjust based on your specific peptide and downstream use):
| Application | Recommended HPLC Purity | Key Additional Tests |
|---|---|---|
| Basic in vitro screening | ≥ 95% | Identity (MS) |
| Cell-based assays | ≥ 97% | Identity + Endotoxin |
| Animal in vivo studies | ≥ 98% | Identity + Impurities + Metals + Endotoxin |
| Publication-grade research | ≥ 98.5% + full impurity profile | Full orthogonal characterization |
5. Common Questions We Receive
Q: My peptide is only 94% pure — is it still usable?
It depends on the nature of the 6% impurities and your application. Many researchers successfully use 90–95% pure material for initial screening, but you should characterize the impurities and consider further purification or orthogonal testing.
Q: Why did my purity drop after storage?
Peptides can degrade via oxidation, deamidation, or hydrolysis. Always store lyophilized peptides at –20°C or lower, protected from moisture and light. We can help you investigate degradation products.
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If you want help interpreting a recent HPLC report or designing the right testing package for your next peptide batch, our team is here.