How to read a peptide Certificate of Analysis

A COA is a record of tests performed on a sample drawn from a particular batch (or lot). It is evidence about that batch and nothing else. A COA is not a certificate of quality in the abstract, not a licence, and not a statement that the material is fit for any particular purpose.

Three consequences follow, and they matter more than any single number on the page:

• A COA is batch-specific. A report for lot A tells you very little about lot B from the same supplier.
• A COA describes the sample tested, which is assumed to represent the batch. Sampling practice is rarely stated and almost never audited.
• A COA is only as good as the laboratory behind it. An unattributed report with no instrument, no method and no analyst is a design exercise, not a measurement.

Before any result, check that the document ties itself to something real. A usable COA header carries:

• Product name and sequence — for a peptide, the one-letter or three-letter amino-acid sequence, not just a trade name.
• Batch or lot number — and it should match the number printed on the vial you hold.
• Date of manufacture and date of analysis — these are different dates and both matter.
• Quantity and presentation — nominal fill weight, lyophilised or in solution.
• Testing laboratory — a name, and ideally a report reference you could quote back to them.

If the batch number on the COA does not match the vial, the document is decorative. See why batch numbers matter for what that link is doing.

Identity is normally established by mass spectrometry. The report should state the theoretical molecular weight calculated from the sequence and the observed mass, and those two should agree within the tolerance of the instrument.

This is a different question from purity, and the distinction is the single most common source of confusion when people read these documents. Identity asks is the right compound present? Purity asks how much of what is in the vial is that compound? A sample can pass one and fail the other in either direction. Identity vs purity works through why both are needed.

Peptide purity is almost always reported from reversed-phase HPLC as an area percentage: the area of the main peak divided by the total area of all detected peaks, expressed as a percentage.

Read that definition again, because it constrains the number heavily. It is relative, it applies only to species that the detector can see, and it depends on the method used to separate them. A good report therefore states the column, the gradient, the flow rate and the detection wavelength — usually 214 nm, where the peptide bond absorbs, or 280 nm for sequences containing tryptophan or tyrosine.

What ≥98% HPLC purity actually means covers the limitations of this number in detail. The short version: it is a real and useful measurement, and it is not a statement about how much peptide is in the vial.

Here is the section that separates a thorough COA from a cosmetic one. Purity and peptide content are not the same measurement.

A lyophilised peptide vial contains the peptide plus whatever came with it: bound water, residual salts, and the counterion left over from purification — usually trifluoroacetate (TFA) from the reversed-phase step. A sample can be 99% pure by HPLC and still be only 70–80% peptide by mass, because the non-peptide material does not absorb at 214 nm and therefore never appears on the chromatogram.

Net peptide content is determined separately, by amino acid analysis or nitrogen determination. Where it is reported, weigh-outs based on the label become meaningful. Where it is absent, any calculation from nominal vial weight carries an unstated error that is frequently double digits. This feeds directly into molar mass and solution concentration.

Three supporting measurements round out a serious report:

• Water content, by Karl Fischer titration. Lyophilised peptides are hygroscopic; water both dilutes the nominal mass and accelerates degradation in storage.
• Counterion identity and content — typically TFA, sometimes exchanged to acetate or hydrochloride. Relevant because TFA is biologically active in some in-vitro systems at concentrations that are easy to reach accidentally.
• Residual solvents from synthesis and purification, by GC or headspace GC.

Very few research-market COAs report all three. Their absence is not automatically a red flag — it is normal for this market — but their presence is a strong positive signal about the laboratory.

When a COA lands in front of you, this order gets to a judgement fastest:

1. Does the batch number match the vial? If not, stop.
2. Is there a named laboratory and a date of analysis? If not, treat every number below as unverified.
3. Is there an MS result with theoretical and observed mass? This is your identity check.
4. Is there an actual chromatogram, or only a typed percentage? A trace lets you see the baseline, the peak shape and the shoulders. A number alone hides all three.
5. Is the HPLC method stated? Column, gradient, wavelength.
6. Is net peptide content reported? If yes, this supplier is doing more than the minimum.

Six checks, perhaps ninety seconds. Red flags on a peptide COA covers what failure at each step tends to indicate.