When a vendor publishes a certificate of analysis showing "98% purity," that number almost always comes from high-performance liquid chromatography — a workhorse analytical technique used across pharmaceutical manufacturing, academic research, and third-party peptide testing labs. Understanding what HPLC measures, and what it cannot see, helps researchers interpret those certificates more critically.
How HPLC Works
High-performance liquid chromatography separates the components of a mixture by pushing a dissolved sample through a column packed with fine particles, under high pressure, using a liquid mobile phase. Different molecules travel through the column at different speeds depending on how strongly they interact with the packing material. As each component exits the column, a detector — most commonly a UV/Vis detector set around 214–220 nm — records a signal proportional to absorbance.
The result is a chromatogram: a graph of detector signal over time. Each peak corresponds to a distinct chemical species. The purity percentage is typically calculated as:
> Target peak area ÷ total peak area × 100
So "98% purity" means the peptide of interest accounts for 98% of the UV-absorbing material detected in that run. It is an *area-percentage*, not a mass fraction.
What the Percentage Means — and Its Limits
A high HPLC purity figure is meaningful, but it carries important caveats:
| What HPLC detects well | What HPLC may miss |
|---|---|
| Truncated sequences and deletion peptides | Impurities that do not absorb UV at the chosen wavelength |
| Oxidised or deamidated variants | Water, residual solvents, counter-ions (e.g. TFA or acetate) |
| Related peptide by-products | Inactive aggregates that co-elute with the main peak |
| Solvent-related carry-over | Endotoxins or microbial contamination |
Because the calculation is area-based, substances that absorb weakly at 214 nm — including some counter-ions and many non-peptide process impurities — contribute little to the denominator. A sample could theoretically show 99% HPLC purity while still containing meaningful levels of materials the detector simply cannot see. This is why pharmaceutical-grade testing typically pairs HPLC with complementary methods such as mass spectrometry (MS) to confirm molecular identity, Karl Fischer titration for water content, and limulus amebocyte lysate (LAL) assays for endotoxins.
The column chemistry matters too. Reversed-phase HPLC on a C18 column — the most common setup for peptides — separates primarily by hydrophobicity. Two structurally similar impurities that differ only in charge might co-elute with the target peak and inflate the purity reading. Some labs run orthogonal methods (e.g., ion-exchange or size-exclusion chromatography) to cross-check.
Reading a Third-Party Certificate of Analysis
Independent labs such as Janoshik and Peptigrity publish HPLC results alongside their raw chromatograms. When evaluating a CoA, researchers commonly look for:
- Retention time — confirms the compound elutes where expected for that peptide.
- Peak shape — a sharp, symmetrical peak suggests a homogeneous compound; tailing or shoulders may indicate impurities co-eluting.
- Wavelength used — 214 nm is standard for peptide backbone absorbance; 254 nm is common for aromatic residues.
- Instrument and method disclosure — a credible CoA names the column, gradient, and flow rate used.
Finnrick.com aggregates third-party test results from multiple labs and displays the raw chromatogram images alongside numeric purity values, which allows a degree of cross-lab comparison. Bachem and PolyPeptide Group, two contract manufacturers supplying pharmaceutical-grade peptides, publish method validation data showing how HPLC is used alongside MS and amino-acid analysis in regulated production.
Why It Matters for Research Peptides
Peptides sold for research use — not approved for human consumption — are typically characterised primarily by HPLC. A published purity figure is a starting point, not a complete quality picture. Researchers working in laboratory settings are better positioned when they understand that HPLC purity reflects UV-detected chemical homogeneity under one set of conditions, and that complementary analytical data (MS confirmation of molecular weight, endotoxin testing) fills gaps the chromatogram alone cannot address.