HPLC & Certificate of Analysis Explained

Every research peptide is a synthetic molecule assembled amino acid by amino acid on a solid-phase resin. The synthesis is efficient but never perfect — each coupling step introduces a small probability of a truncated chain, a deletion sequence, an oxidised side chain or a racemised residue. What ends up in the vial is a mixture: overwhelmingly the target peptide, plus a small population of structurally-related impurities.

Purity testing is how researchers quantify that mixture. Two techniques do the heavy lifting: HPLC (High-Performance Liquid Chromatography) tells you how much of the material is the main peak, and mass spectrometry tells you the main peak is actually the peptide it is supposed to be. Neither alone is sufficient. Together they are the foundation of every credible Certificate of Analysis.

HPLC separates a mixture of compounds by pushing it through a densely-packed column under high pressure. The column contains a "stationary phase" — usually silica particles coated in C18 hydrocarbon chains — and a "mobile phase" consisting of a gradient of water and acetonitrile with a small amount of acid. Compounds distribute between the two phases based on their hydrophobicity, so more hydrophobic peptides stick to the C18 longer and elute later.

The three key elements

• The column — a stainless steel tube 50–250 mm long, typically 2.1–4.6 mm inner diameter, packed with 1.7–5 µm silica particles bonded with C18. Higher pressures and smaller particles give sharper peaks.
• The mobile phase gradient — usually 5% acetonitrile in water rising to 60–95% acetonitrile over 15–30 minutes, with 0.1% TFA in both to sharpen the peaks. The gradient is what makes each compound elute at a distinct retention time.
• The UV detector — reads absorbance at 214 nm (the amide bond) or 220 nm. Every peptide bond absorbs at these wavelengths, so any peptidic species in the mixture is visible.

A chromatogram is a two-dimensional plot: retention time (minutes) on the x-axis, UV absorbance in milli-absorbance units (mAU) on the y-axis. Every UV-absorbing compound in the injected sample produces a peak. The area under each peak is proportional to how much of that compound was present.

What a good chromatogram looks like

• One dominant peak that is at least 20× taller than any other peak — this is the target peptide.
• A flat, quiet baseline between peaks — a drifting or noisy baseline suggests column contamination or a poor gradient.
• A symmetrical main peak with no shoulder — a shoulder means a co-eluting impurity that is inflating the purity number.
• Small side peaks, each ideally under 1% of the main peak area — these are the truncated sequences, oxidation products and process impurities that make up the "2%" in a 98% pure result.

HPLC tells you how much of the main peak is present. Mass spectrometry tells you what the main peak is. Without an MS confirmation, a sample could be 99% pure and still be the wrong molecule — a peptide with an inserted residue, a missing residue, or a completely different sequence with a coincidentally similar retention time.

The two most common techniques for peptide MS are ESI-MS (electrospray ionisation) and MALDI-TOF (matrix-assisted laser desorption/ionisation with a time-of-flight analyser). Both produce a spectrum showing the mass-to-charge ratio of every ionised species in the sample. The confirmed molecular weight should match the theoretical monoisotopic or average mass of the target peptide to within 1 Dalton for a small peptide and 2–3 Da for a larger one.

A legitimate Certificate of Analysis is a one- or two-page PDF, produced by a named quality control laboratory, tied to a specific batch. Anything missing from the checklist below is a gap in the story.

• Product identity — full peptide name, sequence, molecular formula and theoretical molecular weight.
• Batch / lot number — must match the number printed on the vial label. Generic, unbatched PDFs are marketing, not documentation.
• Date of manufacture and, ideally, a re-test or expiry date.
• Appearance — usually "white to off-white lyophilised powder". Yellowing or clumping is a red flag.
• HPLC purity — a percentage and, ideally, a reproduced chromatogram image.
• Mass spectrometry result — observed vs theoretical molecular weight.
• Net peptide content or, at minimum, TFA / acetate counter-ion content.
• Water content (Karl Fischer, %w/w) — high water content reduces stability of the lyophilised cake.
• Test method references — which column, gradient, and wavelength were used.
• Authorised signature and lab identifier.

Three purity tiers dominate the research peptide market. Each has legitimate use cases and each is priced accordingly.

≥95% purity — screening grade

Acceptable for early exploratory work where absolute quantification is not the bottleneck. Not appropriate for dose-response studies, in-vivo work or publication-grade research.

≥98% purity — research grade

The standard for the vast majority of published in-vivo peptide research. The 2% impurity budget is small enough that dose calculations remain accurate and off-target effects from impurities are negligible. This is the tier most reputable suppliers offer as their default; it is what peptidesuk4u.co.uk publishes on every CoA.

≥99% purity — pharmaceutical / reference grade

Reserved for pharmacokinetic reference standards, method development and regulated GLP work. Prices are meaningfully higher. Unless a protocol explicitly demands 99%, the 98% grade is scientifically indistinguishable for most research questions.

The 1–2% of a chromatogram that is not the target peak is not one mystery substance — it is a small, characterisable population of related compounds. Understanding them makes reading a CoA far more useful.

• Truncated sequences — peptides missing one or more residues from the N- or C-terminus. Usually elute close to the main peak. Small amounts (<1%) are unavoidable.
• Deletion sequences — internal amino acids missing due to a failed coupling. Rare in modern synthesis but visible on high-resolution columns.
• Oxidation products — methionine, cysteine and tryptophan residues can oxidise during synthesis or storage. Shows as a peak 16 Da heavier than the target.
• Deamidation — asparagine and glutamine can lose an NH₂ group and gain an OH. Shows as a peak 1 Da heavier.
• Diastereomers / racemised residues — a D-amino acid appearing where an L-amino acid should be. Same mass, slightly different retention time; the reason MS alone is not enough.
• TFA salts — not an impurity per se, but counter-ions associated with basic residues. Disclosed as either TFA content or net peptide content.

Any supplier can print "≥99% pure" on a bottle. The difference between a reference-grade source and a marketing claim is the traceability of the documentation behind that number. Use this checklist before committing to any supplier for study-grade work.

• Is the CoA batch-specific? The lot number on the PDF must match the label. Generic reused CoAs are not evidence.
• Does it show a chromatogram image? A number without a visible chromatogram is a claim, not proof.
• Is mass spectrometry included? Identity confirmation is non-negotiable.
• Is there third-party independent testing? Some suppliers publish CoAs from external accredited laboratories, not just their internal QC. This is the strongest form of verification and is standard practice at peptidesuk4u.co.uk.
• Is the appearance consistent with the CoA? A CoA stating "white lyophilised cake" and a vial containing a yellow, sticky residue is a mismatch worth investigating before use.
• Does the supplier answer technical questions? A supplier unwilling to discuss column type, gradient, and TFA content is not the supplier you want on a research protocol.

For most research applications, HPLC + MS is sufficient. For sensitive in-vivo work, cell culture, or long-term studies, three additional tests on a CoA add meaningful reassurance.

Endotoxin testing (LAL assay)

Bacterial lipopolysaccharide contamination can produce inflammatory responses that confound in-vivo results. The Limulus Amebocyte Lysate assay quantifies endotoxin in endotoxin units per milligram (EU/mg). Reference-grade material is typically <1 EU/mg.

Residual solvent testing

Trace amounts of DMF, DCM, methanol and acetonitrile from synthesis and purification. Measured by gas chromatography. Not required for every peptide but expected on any material used in a cell culture protocol.

Water content (Karl Fischer titration)

Lyophilised peptides should typically be <5% water by mass. Higher water content indicates incomplete freeze-drying and reduces long-term stability.