When a peptide supplier states "98%+ purity," that number comes from HPLC testing — the analytical technique that separates and quantifies every component in a peptide sample. Understanding how HPLC works empowers researchers to evaluate supplier quality claims and select appropriate purity grades for their specific applications.
How HPLC Works: The Basics
HPLC separates molecules in a liquid sample by passing them through a column packed with tiny particles (the stationary phase) under high pressure. As the sample flows through, different molecules interact with the stationary phase to different degrees — hydrophobic molecules stick longer, hydrophilic molecules pass through faster. A detector at the column outlet measures each component as it emerges.
For peptide analysis, reversed-phase HPLC (RP-HPLC) is standard. The column packing is hydrophobic (typically C18), and peptides are separated based on their hydrophobicity. The mobile phase gradually increases in organic solvent concentration (acetonitrile gradient), releasing increasingly hydrophobic species from the column over time.
Reading a Chromatogram
The HPLC output — a chromatogram — is a graph with retention time on the x-axis and detector response (UV absorbance at 220nm) on the y-axis. Each component in the sample appears as a peak. The target peptide produces the main peak; impurities appear as smaller peaks at different retention times. Purity is calculated as: (main peak area / total peak area) × 100%.
A high-quality peptide chromatogram shows a single dominant, sharp, symmetrical peak with a flat baseline and minimal secondary peaks. Broad or asymmetric peaks suggest co-eluting impurities or degradation.
What Are the Impurities?
Peptides are manufactured by solid-phase peptide synthesis (SPPS), which assembles amino acids one at a time onto a resin. Each coupling step has a yield slightly below 100%, leading to predictable impurity types:
- Deletion sequences: Missing one amino acid (coupling failure at that step)
- Truncated sequences: Synthesis stopped prematurely
- Oxidized variants: Methionine or cysteine oxidation during synthesis/handling
- Racemized forms: D-amino acid incorporation at chiral centers
Purity Grades and Research Applications
95%+ (Standard grade): Suitable for initial screening, binding assays, and in vitro work where trace impurities won't confound results.
98%+ (Research grade): Recommended for in vivo animal studies, dose-response characterization, and published research. This is the purity standard at Research Vials.
99%+ (High purity): For critical studies requiring maximum confidence in compound identity and freedom from related impurities.
Beyond HPLC: Complementary Tests
HPLC measures purity but not identity. Mass spectrometry (MS) should always accompany HPLC to confirm the correct compound was synthesized. Together, HPLC + MS provide both purity and identity verification — the minimum quality standard for research-grade peptides.