Reading a Certificate of Analysis

The headline figure on most peptide COAs is reverse-phase HPLC purity, expressed as a percentage of the integrated chromatogram attributable to the target peak. A reference-standard co-injection on the same column under identical gradient conditions is the only way to assert that the dominant peak is, in fact, the intended sequence — not a deletion product with similar retention behavior.

Impurity peaks below the target retention time typically correspond to truncated sequences and deletion peptides arising from incomplete coupling cycles. Peaks eluting after the target often represent oxidation products at methionine or tryptophan residues, or sequence variants with altered hydrophobicity. A purity number presented without a chromatogram image attached is incomplete analytical evidence; the chromatogram lets the analyst see the shape of the impurity profile, not just its integrated total.

Gradient conditions, column chemistry, detection wavelength, and flow rate should all be disclosed. A purity figure recorded under a shallow gradient on a C18 column at 214 nm is not interchangeable with a steeper gradient at 220 nm on a different stationary phase; co-eluting impurities can hide under the main peak depending on method choice.

An accurate-mass measurement within a few ppm of the calculated monoisotopic mass confirms molecular identity of the synthesized peptide. Charge-state envelopes from electrospray ionization should deconvolute cleanly to the expected molecular weight; a noisy envelope or unexpected adduct pattern can indicate sequence ambiguity that HPLC alone will not surface.

For longer analytical reagents — 30 residues and above — MALDI-TOF is often the more diagnostic modality, since it preserves the intact molecule and resolves close-mass impurities that ESI deconvolution may merge. Either way, the COA should report observed mass, calculated mass, and the mass-accuracy delta in ppm rather than a bare "identity confirmed" statement.

Solid-phase peptide synthesis cycles use DMF, DCM, TFA, acetonitrile, and several other solvents during coupling, deprotection, cleavage, and purification. ICH Q3C guidance defines acceptable residual limits for each. A COA that omits the residual solvent panel leaves an analyst unable to rule out solvent-driven cytotoxicity in cell-based assays — a particularly common confounder in viability and apoptotic readouts.

Trifluoroacetic acid (TFA) residue is the most frequently flagged item, both because it is intrinsic to the cleavage and purification workflow and because TFA counter-ion content materially affects measured peptide mass and apparent potency. A residual TFA figure of 5–15% by mass is typical for HPLC-purified peptides not subjected to a counter-ion exchange step.

When listed, endotoxin (LAL) results indicate suitability for cell-based assay work. LPS contamination at even low EU/mg levels can confound any readout that touches innate immune signaling, including cytokine release, NF-κB reporter assays, and macrophage activation panels. Endotoxin testing is not universal for research-grade peptides, but its absence should be treated as a known unknown rather than as evidence of cleanliness.

Every COA should carry a lot identifier, the synthesis date, the analyzing laboratory, and a signing analyst. Without those four data points, the document describes a chemical class rather than a specific physical lot, and chain of custody cannot be reconstructed if a downstream analytical question arises six months later. The lot identifier on the COA must also match the identifier printed on the vial collar — a mismatch is grounds to quarantine the lot until provenance is resolved.