“Purity” is one of the first specs buyers look for, but it can be misunderstood. Purity is typically a measure of how much of a sample corresponds to the desired peptide relative to detectable impurities under a defined analytical method. The method, detection wavelength, and sample preparation all matter.
At a glance
- Purity is method-dependent: results are tied to the specific HPLC method used.
- Identity ≠ purity: MS can confirm mass while HPLC estimates relative composition.
- Higher purity reduces ambiguity in quantitative and sensitive experiments.
How peptide purity is commonly reported
Many suppliers report purity as HPLC area%: the area under the main chromatographic peak divided by total peak area (within the method’s detection limits). This is useful for comparing lots and assessing gross impurity levels, but it is not the same as absolute weight percent.
HPLC vs MS: complementary tools
- HPLC separates components by chromatographic behavior and supports relative quantification (area%).
- Mass spectrometry (MS) helps confirm that the dominant component matches the expected molecular mass.
Used together, HPLC and MS provide a stronger picture: “Is it the right molecule, and how clean is the mixture under this method?”
What impurities can look like
- Deletion sequences: missing one or more residues from incomplete coupling.
- Side products: protected fragments, oxidation variants, or modified residues.
- Salts and residuals: counterions and trace synthesis/purification residues (method-dependent detectability).
Choosing a purity level for your experiment
The “right” purity depends on how sensitive your system is to trace components and how quantitative your readout is.
- Method development / screening: moderate-to-high purity may be sufficient depending on tolerance for minor peaks.
- Quantitative assays: higher purity is generally preferred to reduce off-target signal and improve reproducibility.
- Reference standards: higher purity and strong documentation are typically important.
What to look for on a CoA
- Sequence and modifications (including terminal chemistry).
- Observed mass and expected mass (and ionization mode if listed).
- Purity method details (column type, gradient, detection).
- Lot number and date, so you can trace results to a specific batch.
Trusted Peptides focuses on documentation that’s easy to interpret so you can qualify material quickly and confidently.
FAQ
Is 98% always “better” than 95%?
Not automatically. It depends on the application, the method used to report purity, and what the remaining impurities are. For highly quantitative or interference-prone work, higher purity is often helpful.
Can purity differ between methods?
Yes. A sample may appear “cleaner” under one HPLC method and show more minor peaks under another due to different separation selectivity.
