How to Read a Peptide HPLC Certificate of Analysis

Almost every peptide supplier advertises a purity figure. Far fewer publish the document behind it, and fewer still explain how to read one. A certificate of analysis is the only evidence you have that the vial contains what the label says — but only if you know which numbers matter and which are decorative.

This guide walks through what a COA actually reports, how to read an HPLC trace, the one figure people consistently misunderstand, and the warning signs that a certificate is not worth the paper it is printed on.

What a COA is, and what it is not

A certificate of analysis reports the results of tests performed on one specific batch of material. That last part is the whole point. A COA is not a general statement about a compound, or about a supplier’s typical quality — it describes a particular lot, made on a particular date, tested on particular instruments.

This is why the lot number is the most important thing on the page. A COA that cannot be matched to the number printed on your vial is telling you nothing about your vial.

What should be on the page

A certificate missing the lot number, the analysis date, or the method behind the purity figure is incomplete regardless of how good the number looks.

Reading the HPLC trace

High-performance liquid chromatography separates the components of a mixture by pushing it through a column; different molecules travel at different speeds and emerge at different times. The detector records what comes off, and the result is a chromatogram — a mostly flat line with peaks where something eluted.

Three things to look at:

The main peak. One large, well-shaped peak dominating the trace is what you want. Your target compound is that peak.

Retention time. The horizontal position of the peak, in minutes. Consistent retention time across lots of the same compound, on the same method, is a sign of a consistent process.

Everything else. Smaller peaks are impurities — typically truncated sequences, deletion products, or degradation. A few tiny peaks are normal in synthetic peptide production. Several substantial ones are not.

Peak shape matters as well as size. A sharp, symmetrical peak indicates clean separation; a broad or badly tailing peak can hide co-eluting material that the percentage figure will not reveal.

What “≥99% purity” actually measures

This is the figure everyone quotes and the one most often misread.

HPLC purity is area percentage: the area under the main peak divided by the total area of all peaks, at a particular detection wavelength. Usually 214 nm, which detects the peptide bond itself, or 220 nm.

Two consequences follow, and both matter.

First, it is relative, not absolute. It tells you what proportion of the detected material is your compound. Anything the detector cannot see at that wavelength does not appear in the calculation at all.

Second, the wavelength is part of the claim. A purity figure quoted without the detection wavelength and method is an incomplete statement. Reputable certificates state both.

The figure most people miss: net peptide content

Here is the distinction that catches out even careful buyers.

Purity describes the proportion of peptide material that is the right peptide. Net peptide content describes how much of the powder in the vial is peptide at all.

Lyophilized peptides are not pure peptide by weight. The powder also contains counter-ions — commonly trifluoroacetate (TFA) left over from purification — along with residual water and sometimes salts. Depending on the sequence and the process, peptide can be somewhere around 70–90% of the total mass, with the balance made up of everything else.

So a vial can be legitimately 99% pure by HPLC and still contain noticeably less actual peptide than the label mass suggests, because purity and content are answering different questions. A certificate reporting both is being more transparent than one reporting only the headline purity number.

If net peptide content is not stated and it matters to your work, ask for it. A supplier who has the data will send it.

Mass spectrometry: the identity check

HPLC tells you how pure the material is. It does not, on its own, tell you that the pure thing is the right thing.

Mass spectrometry closes that gap by measuring molecular mass directly. The observed mass on the certificate should match the theoretical mass calculated from the sequence, within the tolerance of the instrument.

A COA with a strong purity figure and no mass confirmation has answered only half the question. Both belong on the page.

Warning signs

• No lot number, or one that does not match the vial. The document cannot be tied to your material.
• No date of analysis.
• No chromatogram — only a typed purity figure with no trace behind it.
• The same certificate reused across different lots. Compare lot numbers and dates if you order repeatedly; identical traces on supposedly different batches mean the testing is not per-lot.
• A purity figure with no method or wavelength.
• Suspiciously perfect numbers. Real synthetic peptide production yields figures like 98.7% or 99.2%, not a flat 100%.
• No testing laboratory named, in-house or third-party.

What to do with it

Three habits make a COA genuinely useful rather than decorative:

1. Match the lot number on the certificate to the number on the vial, every time. This takes five seconds and is the whole basis of the document’s value.
2. Keep them. File the COA with the lot. If material ever behaves unexpectedly, the certificate is your starting point.
3. Compare across orders. Consistent retention times and purity figures across lots tell you a supplier’s process is under control — which is more informative than any single certificate.

Every lot we ship has a certificate of analysis behind it. You can see them on our certificates of analysis page, and the lot number on your vial will match the one on the document.

If something on a certificate does not make sense to you — ours or anyone else’s — that is a reasonable thing to ask a supplier about. How readily they answer tells you something too.

Storing Research Peptides: Temperature, Light and the Freeze-Thaw Problem

A peptide is only as good as the way it has been kept. Purity verified at the point of manufacture tells you what left the lab; it says nothing about what is in the vial after three months in a warm cupboard. Storage is the part of handling most often done casually and most often responsible for material that quietly stops behaving as expected.

This is a practical guide to keeping lyophilized and reconstituted peptides intact: what actually degrades them, what temperature buys you, why freeze-thaw cycles cost more than people think, and where published stability figures stop being useful.

What actually degrades a peptide

Four things do most of the damage, and they compound.

Water. Hydrolysis breaks peptide bonds, and it needs water to happen. This is the entire reason lyophilization exists — take the water out and the dominant degradation pathway largely stops. It is also why a vial that has been opened and exposed to humid air is in worse shape than the seal suggests.

Heat. Reaction rates rise with temperature. Every degradation pathway available to the molecule runs faster warm than cold, which is why the same peptide has a very different useful life at −20 °C than at room temperature.

Oxygen. Methionine, cysteine and tryptophan residues oxidise readily. A sequence containing them is inherently more fragile than one that does not, which is part of why blanket stability figures are unreliable — the answer depends on what the sequence contains.

Light. UV in particular drives photo-degradation, again concentrated on aromatic and sulphur-containing residues. Amber glass and a closed box solve this cheaply.

Lyophilized versus in solution

The single largest factor in how long a peptide lasts is whether it is dry.

Lyophilized powder kept cold, dark and sealed is in its most stable form by a wide margin. The same peptide in solution is exposed to hydrolysis continuously, and the clock starts the moment you add diluent. This is why reconstituting the whole vial when you only need part of it is usually a mistake: you have converted stable material into unstable material for no reason.

If the material is only needed occasionally, keeping the vial dry until you need it costs nothing and buys a great deal.

Temperature, in order of preference

The single most useful habit here is the least technical: put it away immediately. Material left on a bench over a weekend has spent a meaningful fraction of its useful life for no benefit.

The freeze-thaw problem

Freezing a peptide in solution is not free. Each freeze-thaw cycle concentrates solutes at the ice boundary, shifts local pH, and creates ice-water interfaces where the molecule can unfold. The damage is cumulative, and it is a per-cycle cost rather than a per-day one — five thaws over a month is worse than one thaw and a month refrigerated.

The fix is aliquoting. If reconstituted material must be frozen, split it into single-use portions first, in sealed sterile vials, so each portion is thawed exactly once. It takes ten minutes and it removes the problem entirely.

When thawing, let the vial come up slowly in the fridge rather than under warm water. And once thawed, do not refreeze — use it or discard it.

Light and containers

Keep vials in their box. That is most of the answer. Amber glass helps, but an opaque container in a dark fridge is just as effective and does not depend on the vial you happened to receive.

Avoid decanting into unknown plastics for storage. Some peptides adsorb onto polypropylene surfaces, which quietly reduces the concentration of what you actually draw — a particular risk with dilute solutions, where the proportion lost to the container wall is largest.

When a shipment arrives

Lyophilized peptides tolerate transit at ambient temperature well — this is exactly what the dry form is for, and a package that arrives warm is not automatically compromised. What matters is what happens next.

1. Unpack promptly and get the vials into cold storage rather than leaving the box on a desk.
2. Inspect the cake. It should be a dry, intact solid. A cake that has collapsed into a film or shifted to one side suggests the vial got warm enough to matter in transit.
3. Check the vial seal is intact and the stopper has not been disturbed.
4. File the certificate of analysis against the lot number on the vial. You will want it later, and matching it up after the fact is tedious.

Labelling

Write the reconstitution date on the vial. Not on a note, not in a spreadsheet you will not open — on the vial.

In-solution stability is measured in weeks, and the single most common way people end up using degraded material is simply losing track of when they reconstituted it. A pen solves this.

Worth recording: compound, lot number, reconstitution date, diluent volume and resulting concentration. The last two save you re-deriving the maths every time you draw. Our reconstitution calculator gives you the concentration figure to write down.

Where published figures stop being useful

You will find stability tables everywhere, including a general one in our own shelf-life estimator. Treat all of them as orientation, not specification.

Real stability depends on the specific sequence, its length, whether it contains oxidation-prone residues, the excipients present, the residual moisture left after lyophilization, and how the material was handled before it reached you. Two peptides in identical vials in the same fridge can behave quite differently.

The document that actually describes the material in front of you is the certificate of analysis for that lot. Where a published range and a lot-specific figure disagree, the lot-specific figure wins.

The short version

• Keep it dry as long as possible; reconstitute only what you need.
• Cold and dark, always. Put it away immediately.
• Aliquot before freezing so nothing thaws twice.
• Write the reconstitution date on the vial.
• Keep the COA with the lot, and trust it over any general table.

How to Reconstitute Lyophilized Peptides: A Step-by-Step Guide

Lyophilized peptides arrive as a dry cake or powder at the bottom of a sealed vial. Before they can be used in any liquid-phase work they have to be brought back into solution — reconstituted. The chemistry is simple, but the arithmetic trips people up constantly, and a vial reconstituted at the wrong volume is either unusable or wasteful.

This guide covers the whole procedure: what you need, how to pick a diluent volume, the maths worked through with real numbers, correct technique, and how to store the result. If you would rather skip the arithmetic, our reconstitution calculator does it for you.

What reconstitution actually is

Lyophilization — freeze-drying — removes water from a peptide solution under vacuum, leaving a solid that is far more stable than the liquid it came from. A properly lyophilized peptide stored cold and dry can remain intact for a long time. In solution, the same peptide may degrade in weeks.

Reconstitution reverses that. You add a measured volume of sterile diluent, the solid dissolves, and you are left with a solution of known concentration. Everything downstream depends on that concentration being right, which is why the volume you choose matters more than it first appears.

What you need

• Bacteriostatic water. Sterile water containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol suppresses microbial growth, which is what allows a reconstituted vial to be drawn from more than once. Bacteriostatic water is the standard diluent for multi-draw work.
• Preservative-free sterile water is an alternative where no benzyl alcohol is wanted — but it offers no protection against contamination, so it suits single-use preparations only.
• A graduated syringe fine enough to measure your intended volume accurately. For volumes below about 0.1 mL a microlitre syringe is worth the bench space, since reading error dominates at that scale.
• Alcohol wipes for both vial stoppers.

The three numbers that decide everything

Every reconstitution calculation comes down to three values:

From the first two you get concentration. From concentration and the third you get the volume to draw.

The maths, worked

Concentration is mass divided by volume:

concentration (mg/mL) = peptide mass (mg) ÷ diluent volume (mL)

Take a 10 mg vial reconstituted with 2 mL of bacteriostatic water:

10 mg ÷ 2 mL = 5 mg/mL

Now suppose your assay calls for 500 mcg per aliquot. Convert to matching units first — 500 mcg is 0.5 mg — then divide:

0.5 mg ÷ 5 mg/mL = 0.1 mL

And the vial yields:

10 mg ÷ 0.5 mg = 20 aliquots

That is the entire calculation. The single most common error is a units mismatch — milligrams against micrograms — which produces answers wrong by a factor of 1,000. Convert before you divide, every time.

Choosing your diluent volume

Nothing forces a particular volume, but the choice has consequences in both directions.

Less diluent gives a more concentrated solution and smaller draw volumes. Past a point the volumes become too small to measure accurately — below roughly 0.05 mL, small errors in reading become large errors in quantity.

More diluent gives larger, easier-to-measure draws, but fills the vial faster and may exceed its capacity. It also means more total liquid sitting refrigerated for the life of the vial.

A practical rule: pick the volume that puts your typical draw somewhere between 0.1 mL and 0.5 mL. That range is comfortable to read on ordinary graduations and tolerant of small measurement error. The calculator works out the draw volume for any combination, so you can try a few before committing.

Technique

1. Let the vial reach room temperature. Adding liquid to cold glass encourages condensation and slows dissolution.
2. Wipe both stoppers with alcohol and let them dry.
3. Draw your measured diluent volume into the syringe.
4. Angle the needle against the inside wall of the peptide vial and let the water run down the glass. Do not fire the stream directly into the peptide cake — shear from a fast stream can damage the peptide.
5. Let it stand. Most peptides dissolve on their own within a minute or two.
6. Swirl gently if anything remains. Never shake. Agitation denatures peptides and the foam it produces makes accurate drawing difficult.

A correctly reconstituted vial should be clear and free of visible particulate. Cloudiness, floating material, or a solid that will not dissolve after several minutes of gentle swirling all warrant stopping and checking the compound and the diluent before going further.

Storage after reconstitution

The stability clock starts the moment the peptide goes into solution. Lyophilized material kept cold, dark and sealed lasts far longer than the same peptide in solution, and solution kept at room temperature is the shortest-lived state of all.

These are general patterns, not guarantees. Stability varies considerably between compounds — sequence, length, and the presence of oxidation-prone residues all matter — so treat any published range as a starting point and check the certificate of analysis for the lot in front of you. Our shelf-life estimator gives typical ranges by state and storage condition.

Keep reconstituted vials refrigerated and out of light. Avoid repeated freeze-thaw cycles: each one costs you a little material. If a peptide must be frozen in solution, aliquot it first so that each portion is thawed only once.

Common errors

• Unit mismatches. Milligrams and micrograms differ by a factor of 1,000. Convert first.
• Shaking instead of swirling. Fast destruction of exactly what you paid for.
• Firing the stream into the cake. Run it down the vial wall instead.
• Over-dilution. Producing draws so large the vial empties in a handful of uses.
• Under-dilution. Producing draws too small to measure reliably.
• Losing track of the reconstitution date. Write it on the vial. In-solution stability is measured in weeks, and memory is not a storage condition.

Quick reference

For any combination not listed, the reconstitution calculator will work it out, including the volume to draw and the number of aliquots per vial.