You ordered peptides. The vendor sent a Certificate of Analysis. It shows 98.5% purity by HPLC. Looks good — so you move forward.

But that single number is doing a lot of heavy lifting. It tells you something about chemical purity. It tells you nothing about whether the compound is actually the molecule you ordered, whether it carries biological contaminants, what percentage of the vial is active peptide versus salt, or whether the report even corresponds to the batch you received.

Most COAs in the research peptide market report one test: reversed-phase HPLC. That’s the minimum. It’s also where most vendors stop. Below are five gaps that standard COAs routinely leave open — and what each one means for your research.

Gap 1: No Identity Confirmation

HPLC measures purity — the proportion of a single peak relative to total UV-absorbing species in the chromatogram. What it cannot determine is what that peak actually is.

Reversed-phase HPLC separates compounds by hydrophobicity. Two peptides with similar amino acid compositions and chain lengths can produce nearly identical retention times and peak shapes. A truncated synthesis product missing a single amino acid, a diastereomer with an inverted chiral center, or an entirely different peptide of similar hydrophobic character can all appear as a clean, sharp peak at >99% area — and HPLC alone cannot distinguish between them.

This is why mass spectrometry (MS) exists as a complementary method. LC-MS or LC-MS/MS measures the mass-to-charge ratio of the eluting compound, providing molecular weight confirmation that matches or excludes the target sequence. Sharma et al. reviewed the analytical characterization of synthetic peptides and emphasized that HPLC-UV alone is insufficient for definitive identification — mass spectrometric confirmation is required to verify molecular identity [1].

If your COA shows purity without mass spec data, you know the compound is pure. You don’t know it’s correct.

Gap 2: No Endotoxin Data

A peptide can pass HPLC at 99% purity and still carry enough bacterial endotoxin to invalidate every downstream experiment. Endotoxins — lipopolysaccharide (LPS) fragments from Gram-negative bacteria — are invisible to HPLC, invisible to mass spec, and biologically active at picogram concentrations [2].

LPS activates Toll-like receptor 4 (TLR4), triggering NF-κB signaling and inflammatory cytokine release including TNF-α, IL-1β, and IL-6 [3]. In any research model involving inflammation, immune response, cell viability, or tissue repair pathways, endotoxin contamination produces a baseline signal indistinguishable from the peptide’s actual pharmacology. The result: confounded data that doesn’t replicate.

Detection requires a dedicated assay — the Limulus Amebocyte Lysate (LAL) test, first described by Levin and Bang using the clotting cascade of horseshoe crab blood [4]. The kinetic chromogenic variant quantifies endotoxin concentration down to 0.005 EU/mL and is recognized by USP <85>, EP 2.6.14, and JP 4.01 as the compendial standard [5].

If endotoxin results don’t appear on the COA, the test wasn’t performed. No amount of HPLC purity compensates for an untested biological contaminant. For a deeper dive into endotoxin methodology, see our Endotoxin Testing Deep Dive.

Gap 3: HPLC Purity ≠ Net Peptide Content

This is the gap that costs researchers money without them realizing it.

When a COA reports “98% purity by HPLC,” it means 98% of the UV-absorbing material in the chromatogram is the target peptide. But the chromatogram doesn’t see everything in the vial. Specifically, it misses:

• Counterion salts — Most synthetic peptides are manufactured using solid-phase synthesis with trifluoroacetic acid (TFA) cleavage and HPLC purification with TFA-containing mobile phases. The resulting product retains TFA as a counterion. Depending on the peptide’s charge state, TFA can constitute 10–30% of total vial weight [6].
• Residual moisture — Lyophilized peptides typically contain 2–8% residual water content even after freeze-drying.
• Residual solvents — Trace amounts of acetonitrile, TFA, and other process solvents can remain bound to the peptide matrix.

The practical result: a vial labeled “10 mg” with 98% HPLC purity may contain only 7–8 mg of actual active peptide. The rest is salt, water, and solvent. This difference between HPLC purity and net peptide content (NPC) is well-established in pharmaceutical peptide characterization — McCarthy et al. specifically noted the need for reference standards that account for counterion and moisture burden in synthetic peptide quality assessment [7].

Some COAs report net peptide content. Most don’t. If yours doesn’t, your dosing calculations are based on total material weight, not active peptide — and your effective concentration is lower than you think.