Peptide purity grades: what the percentages mean
Purity is a measurement, not a grade. Two suppliers quoting "98%" can be measuring different things under different conditions.
The common labels
| Label | Typical HPLC area % | Common use |
|---|---|---|
| Crude | 50–80% | Further purification, non-quantitative work |
| Research grade | ≥95% | General in-vitro research |
| High purity | ≥98% | Assays, reference work |
| Ultra purity | ≥99% | Quantitative and sensitive applications |
Why the same number differs
- Wavelength — 214 nm and 280 nm see different impurities.
- Gradient — a shallow gradient resolves more peaks and can lower the reported purity.
- Integration — how small peaks are integrated changes the percentage.
- Sample load — overloading the column distorts the profile.
- Net content — purity excludes water and counter-ions; content does not.
Choosing a grade
Match the grade to the question your experiment asks. Screening work rarely needs 99%; a quantitative assay usually does. Buying ultra-purity for a qualitative check is waste; buying research grade for a reference standard is a false economy.
What to request
- Chromatogram with method details (column, gradient, wavelength)
- MS spectrum for identity
- Water content and, where relevant, net peptide content
- The batch number, so the data matches what you receive
The method behind the number
A purity figure is the output of a method, and the method is a set of choices. Before comparing two certificates, compare four parameters:
- Wavelength: 214 nm and 280 nm detect different classes of impurities; a peptide with no aromatic residues may be nearly invisible at 280 nm.
- Gradient: a shallow gradient resolves more peaks and can lower the reported purity — not because the product is worse, but because the method is more honest.
- Column and load: overloading a column compresses peaks and hides impurities.
- Integration: the threshold for integrating small peaks changes the percentage more than most buyers realise.
If the certificates do not state these parameters, the numbers are not comparable. A "98%" from a method that hides impurities and a "96%" from a method that resolves them may describe the same material.
A buying rule
Choose the lowest grade that satisfies your application, then verify the batch. Buying ultra-purity for qualitative work wastes money; buying research grade for a quantitative assay introduces avoidable error. In both cases, the specification — not the label — is what you should hold the supplier to.
What to write into your specification
A grade label is not a specification. If you need a defined purity, write it as a test with a method and an acceptance criterion:
- "Purity ≥ 98.0% by RP-HPLC, C18 column, stated gradient, detection at 214 nm, impurities integrated above 0.05%."
- "Identity confirmed by ESI-MS, observed mass within ±0.1% of the calculated value for the stated salt form."
- "Water content ≤ 5.0% by Karl Fischer."
- "Net peptide content ≥ 85% by assay."
With that wording, two suppliers are finally comparable — and a third party can verify the result without asking what "98%" was supposed to mean.