How to read a certificate of analysis for a research peptide

Written 23 September 2026, last changed 27 October 2026 by the trade desk. About 11 minutes to read.

Read a peptide COA in this order: the batch number against the one on your vial, then identity (the measured mass against the calculated one), then purity by RP-HPLC against the release limit, then net content against the label. The certificate is the batch’s test record, one per batch.

What ours carries

The batch number, the product and strength, the date of analysis, and three results, each with its method: purity by RP-HPLC, net content, and identity by mass spectrometry, with the chromatogram and the spectrum. The specification for the line is printed on its compound page and the retest date is on the batch record; the desk gives both for any batch number.

The three results, and the rest of the specification

The certificate reports three results: purity, net content and identity. Water, acetate, related substances and residual solvents are the specification the batch is made to; they are not reported batch by batch, and the desk answers for a particular batch on request. Endotoxin, sterility and heavy metals are not in a peptide line’s specification and are not on its certificate; the protein, glycoprotein and preparation lines report endotoxin on theirs, and the quality page lists them, each with its origin.

Purity by RP-HPLC. The share of the peak area belonging to the target peptide, measured by reverse-phase liquid chromatography at 220 nm. Our release limit is not less than 98.0 percent; the specification also caps any single related substance, such as a truncated or deletion sequence or an oxidised form, at 1.0 percent.

Identity by mass spectrometry. The measured mass matched to the calculated one; the tolerance is in the mass line section below, and the HPLC and mass results, one by one, are in HPLC and mass spectrometry on a peptide certificate.

Water content (the specification). Lyophilised peptides hold water. The specification sets not more than 8 percent by Karl Fischer titration; it is not one of the certificate’s three results. It matters for weighing: 10 mg of powder at 6 percent water is 9.4 mg of solids.

Net content. The fill weight per vial, the labelled amount within plus or minus 5 percent. This is the weight of material, not of peptide; peptide by weight (the assay, or net peptide content) is a separate figure, not one of the certificate’s three results; the desk answers for a particular batch on request.

Acetate and residual solvents (the specification). How much counter-ion and purification solvent remain; the specification’s limits are below.

The mass line: measured against calculated

Identity is confirmed by electrospray mass spectrometry. The certificate prints the average mass calculated from the sequence beside the mass measured once the charge states have been worked out. A measured figure within about one unit of the calculated average mass conforms; a change of under one unit, a deamidation (+0.984) or PT-141 in place of MT-2, sits inside that tolerance; telling those apart takes a high-resolution mass, or HPLC run beside a reference of the other, and the certificate carries neither.

What common changes do to a chain’s mass:

• An oxidised methionine or tryptophan: 16 more.
• A tryptophan oxidised to kynurenine: 4 more.
• A deamidated asparagine or glutamine, or a C-terminal amide hydrolysed to the free acid: 0.984 more. A shift this small sits inside the tolerance of the average-mass line; the mass line section above says what telling the two apart takes.
• A lactam ring that has not closed, as in MT-2: 18 more. A disulfide ring that has not formed, as in oxytocin: 2 more. An aspartimide or another dehydrated form: 18 less.
• An N-terminal glutamine that has closed to pyroglutamate: 17 less.
• Hydrolysis of the backbone itself splits the chain into two smaller pieces rather than adding 18.
• A sodium or potassium adduct: about 22 or 38 more, and not a different peptide.

The certificate reports the conclusion, and a batch whose mass line does not conform is not released.

Acetate, TFA and the counter-ion

A peptide with basic residues carries positive charges in solution, and each pairs with an anion. Preparative HPLC runs with trifluoroacetic acid in the mobile phase, so the peptide leaves the column as its trifluoroacetate salt. An exchange step then swaps trifluoroacetate for acetate, the salt form we supply, and a trace of TFA survives.

The specification bounds them, and the desk answers for a particular batch: acetate by ion chromatography, not more than 15.0 percent; residual TFA, not more than 1.0 percent; acetonitrile, the other purification solvent, not more than 410 ppm by gas chromatography. A sequence with many lysine or arginine residues sits nearer the acetate limit than one with few.

Both anions are part of the weighed fill. A 10 mg vial holds 10 mg of acetate salt with its water, and the peptide by weight is less than that. This is why net peptide content is a separate figure, answered by the desk for a particular batch, and not something read off the label.

Reading a percentage honestly

Purity is an area percentage of a chromatogram, not a weight. A vial labelled 10 mg at 99 percent purity does not contain 9.9 mg of peptide. It contains 10 mg of material by weight; 99 percent is the share of the chromatogram's peak area that belongs to the peptide. So purity says nothing about the water and the acetate in that 10 mg; the specification bounds those separately, water at not more than 8.0 percent and acetate at not more than 15.0 percent. If you need peptide by weight, ask for net peptide content, which is a separate figure. How 98 and 99 percent compare: what 98 or 99 percent purity means.

Real or decorative: six checks

A certificate earns trust only through what can be checked against something else, and on ours six things can be; a protein or preparation line’s certificate reports by the methods its own page names. The same kind of check, run on a whole supplier rather than one certificate: how to choose a wholesale peptide supplier.

1. The batch number is the number on your vial, character for character, and it can be looked up on the register the certificate was issued from, not only in a PDF: for ours, the certificate page looks it up there.
2. The dates agree: the date of analysis is no earlier than the month of manufacture in the batch number (2607 in L2607-RT30-0002 is July 2026) and before the goods left.
3. Every result has its method beside it: RP-HPLC with its wavelength for purity, mass spectrometry for identity, the fill weight against the label for net content.
4. The measured mass matches the calculated average mass on the compound page within about one unit; what adds 16, 18 or 2 is listed above.
5. The product and strength match the vial, and another strength of the same compound carries another batch number.
6. No buyer's brand on it: ours carries its issuer's name and the batch number, and we do not issue one in a buyer's name.

A certificate that passes all six is consistent; the register it was issued from and the retained sample are what make it verifiable. One forwarded three times is worth checking against the source before it is relied on.

How long a certificate stays valid

A certificate stands until the retest date on the batch record. How the date is set from the date of manufacture and the 24-month shelf life, and what a re-analysis changes, is in Retest dates and shelf life.

When a batch is re-analysed, the certificate is re-issued. The batch number does not change; the analysis date, the results and the retest date do, and the certificate says it records a retest. The earlier version is superseded, and the certificate page returns the current one.

The certificate belongs to the batch, not to the order. Two orders drawn from L2607-RT30-0002 share one certificate, and an order with three lines from three batches has three. Keep each with the stock it describes, so a query a year on can be answered from your own shelf.

Look-alikes: peptides of the same or nearly the same mass

Most pairs on this list sit hundreds of mass units apart. A few sit close, and the table lists them.

Every pair in the table sits more than one unit apart except MT-2 and PT-141, which sit within one unit; telling those two apart takes a high-resolution mass, or HPLC run beside a reference of the other, and the certificate carries neither. Where a comparison exists, its page sets the two side by side, with the ions each gives in electrospray.

Applies to

This guide was written with Retatrutide and BPC-157 in mind, and it holds for the other lines of their kind; the full list is on the compounds page.

Questions about reading a certificate

Read next