How to read a peptide COA: every test explained, red flags and how to verify it

A certificate of analysis (COA) is the only evidence most researchers ever see about the peptide in their vial. A good one lets you check identity, purity, how much peptide you are actually weighing out, and whether the material is suitable for an endotoxin-sensitive assay. A poor one gives a single purity figure on a template that could belong to any batch.

This guide walks through a research peptide COA field by field, explains what each result means and what it does not, lists the red flags, and sets out how to verify a certificate with the laboratory that issued it. For an active pharmaceutical ingredient bought for regulated manufacturing, the expectations are stricter; see our semaglutide API COA guide for that case.

What is a peptide certificate of analysis?

The WHO model certificate of analysis describes a COA as a document that “lists tests performed on a particular sample with the results obtained and the acceptance criteria applied, followed by an indication of whether or not the sample complies with the specification” [2]. It is “usually prepared for each batch” [2].

For APIs, ICH Q7 is more explicit. Section 11.4 says “authentic Certificates of Analysis should be issued for each batch of intermediate or API on request”, and that the certificate “should list each test performed in accordance with compendial or customer requirements, including the acceptance limits, and the numerical results obtained (if test results are numerical)”. It should be “dated and signed by authorised personnel of the quality unit(s)” and show the name, address and telephone number of the original manufacturer [1].

Research-use peptides are not made under ICH Q7, but these principles are the right benchmark: one batch, one certificate, numerical results against stated limits, and an identifiable laboratory standing behind it.

Batch-specific vs generic COA: why the lot number matters

A batch-specific COA reports results for the lot you received. A generic, “representative” or “typical” COA reports results for some other lot, or values that were never measured on a single lot. Only the first tells you anything about your material.

A peptide’s impurity profile is a product of how that batch was synthesised, purified and stored. D’Hondt and colleagues describe SPPS-related impurities (deletions, insertions, diastereomers, protecting-group adducts, unwanted counter-ions such as trifluoroacetate) and degradation products such as diketopiperazine, pyroglutamate and succinimide [17]. Results from one lot cannot be assumed for another.

Check that the batch number on the certificate matches the vial label and the shipping documents, and that the certificate states a date of analysis after the date of manufacture.

What should a peptide COA include?

The WHO model lists the information a COA should carry [2]. Adapted to a research peptide:

Field	What to look for	Ref
Issuing laboratory	Name and address of the laboratory that ran the tests	[2]
Certificate number and pagination	A unique certificate ID, with page X of Y on every page	[2]
Product identity	Name, and for peptides the sequence, any modifications and the salt form	[2], [7]
Batch number	Matching your vial and paperwork	[1], [2]
Dates	Manufacture (if available), sample receipt, analysis, and expiry or retest date	[1], [2]
Original manufacturer	Name and address; plus repacker or trader if different	[1], [2]
Tests	Each test with method reference and acceptance criterion	[1], [2]
Results	Numerical where applicable; subcontracted results identified as such	[2]
Conclusion	Whether the sample meets the specification	[2]
Approval	Date and signature of the head of laboratory or another authorised person	[2]
Accreditation	Reference to ISO/IEC 17025 accreditation, if related to the specific analysis	[2]

EMA’s synthetic peptide guideline gives the analytical content of a full specification: appearance, identification, purity (total and individual impurities), high molecular weight impurities, assay or content, counter-ion identity and content, residual TFA, water content, residual solvents, elemental impurities where metal catalysts are used, bacterial endotoxins and microbiological purity [7]. A research-grade COA will rarely carry all of these, but the further it falls short, the less it tells you.

How to read each test on a peptide COA

COA line	Typical method	What a good entry looks like	Red flag
Appearance	Visual	”White to off-white lyophilized powder” against a stated criterion	No criterion, or appearance as the only test
Identity	ESI or MALDI mass spectrometry	Observed mass next to theoretical mass, spectrum attached	”Conforms” with no number or spectrum
Purity	RP-HPLC, UV 210–220 nm	Area % with the chromatogram, method conditions stated	Purity without chromatogram; a perfectly flat baseline
Peptide content	Amino acid analysis, nitrogen analysis, or assay vs reference standard	Mass fraction of peptide in the powder	Missing, or confused with purity
Water	Karl Fischer titration	% w/w with method	Missing for a lyophilized powder
Counter-ion	Ion chromatography, 19F NMR (TFA)	Salt form named and % content given	Salt form not stated
Residual solvents	GC, against ICH Q3C limits	Each process solvent with ppm result	Generic “complies”
Endotoxin	LAL or rFC (USP <85>, Ph. Eur. 2.6.14)	Numerical EU/mg result with limit	”Pass” with no value or detection limit
Sterility, bioburden	Compendial microbiology	Only if supplied as sterile or if specified	Sterility claimed for a non-sterile bulk powder
Solubility	Stated solvent and concentration	Informative only	Presented as a quality result

Identity: mass spectrometry

Identity is usually confirmed by electrospray ionisation (ESI) or MALDI mass spectrometry. ESI spectra of large molecules show “coherent sequences of peaks whose component ions are multiply charged” [14], so the certificate should report the deconvoluted molecular mass, not just one peak from the spectrum.

The comparison must use the right theoretical value. Using semaglutide as a worked example: FDA’s prescribing information gives the molecular formula C187H291N45O59 and a molecular weight of 4113.58 g/mol [12], while PubChem gives a monoisotopic mass of 4111.1154 Da for the same formula [13]. The difference of about 2.5 Da is simply average versus monoisotopic mass. A COA should say which it reports, and the observed value should agree with the matching theoretical value within the instrument’s stated accuracy.

Mass alone does not confirm sequence. A peptide with the same residues in a different order has the same mass, and a racemised residue (a diastereomer) does not change mass at all. EMA recommends “at least two orthogonal methods” for identity, such as mass, relative retention time, LC-MS, peptide mapping, amino acid analysis or NMR [7].

HPLC purity and the chromatogram

HPLC purity is the main peak as a percentage of total integrated peak area, usually by reversed-phase HPLC with UV detection at 210–220 nm [15]. It is a relative figure, and it depends on method choices. Our guide to peptide purity testing covers how those methods work.

The chromatogram is what makes the number checkable. Look for:

• The method conditions: column, mobile phases, gradient, wavelength and run time. A short, steep gradient can merge impurities into the main peak.
• Baseline resolution around the main peak, and integration of the small peaks before and after it.
• A peak table with retention times and areas that sum to the reported purity.
• Peak shape. The harmonised chromatography chapter added a general peak symmetry requirement of 0.8–1.8 [16]; USP has since proposed limiting it to assays and related substances tests [21].
• A batch and sample identifier on the chromatogram itself, not only on the certificate.

Integration is not neutral. FDA’s data integrity guidance notes that “a dynamic chromatographic record may allow the user to change the baseline and reprocess chromatographic data so that the resulting peaks may appear smaller or larger”, and expects the audit trail for an HPLC run to record the user, date and time, integration parameters and any reprocessing [4].

Net peptide content

Net peptide content is the share of the powder’s mass that is the peptide. It is the number that determines how much peptide you get when you weigh out powder, and it is often well below the purity figure. EMA lists amino acid analysis, nitrogen analysis by Kjeldahl, elemental analysis, qNMR and LC against a reference standard as ways to determine assay or content [7].

The gap is not academic. In a 2024 study, researchers bought three lyophilized semaglutide vials from illegal online pharmacies whose documentation claimed “a purity of at least 99%”. Analysis by LC-MS found semaglutide made up only 7.7% to 14.37% of the powder mass in those vials [20]. The study was small and the products were falsified, but it shows exactly the confusion a “99% purity” line can hide.

Water content (Karl Fischer)

Lyophilized peptides retain water, and EMA expects water content to be part of stability protocols for hygroscopic powders [7]. Water is normally measured by Karl Fischer titration, which is based on the oxidation of sulphur dioxide by iodine with consumption of water [11]. A COA for a lyophilized peptide without a water result leaves a gap in the mass balance.

Counter-ion and TFA content

Basic residues need a counter-ion. Peptides purified with trifluoroacetic acid “are obtained as trifluoroacetate salts”, while most marketed peptide drugs are acetates [9]. The COA should name the salt form and, ideally, give its content. Ion chromatography is described as “a technique of the first choice” for counter-ion determination, and TFA can also be measured by ¹⁹F NMR [9].

This matters for research. In vitro, TFA at 10⁻⁸ to 10⁻⁷ M reduced osteoblast cell numbers, and TFA salts of amylin and calcitonin consistently gave less proliferation than hydrochloride salts, which the authors said could lead to “wrongly attributing an antiproliferative effect” [10]. EMA also lists “residual ion content (TFA)” as a separate specification test from counter-ion content [7].

Residual solvents (ICH Q3C)

Peptide synthesis and purification use organic solvents, and the COA should list those actually used in the process with results against ICH Q3C limits. Class 1 solvents “should not be employed” because of unacceptable toxicity or environmental effect; Class 2 solvents “should be limited”; Class 3 solvents are of lower risk, and amounts corresponding to 5,000 ppm (0.5%) “would be acceptable without justification” [8]. Option 1 concentration limits for solvents often met in peptide work:

Solvent	ICH Q3C class	Concentration limit (Option 1)
Acetonitrile	2	410 ppm
Dichloromethane	2	600 ppm
N-Methylpyrrolidone (NMP)	2	530 ppm
N,N-Dimethylformamide (DMF)	2	880 ppm
Methanol	2	3,000 ppm
Acetic acid, ethanol, ethyl ether, tert-butyl methyl ether	3	5,000 ppm general limit

Source: FDA, Q3C Tables and List [8]. TFA is not a Q3C solvent, which is why it appears as its own line.

Endotoxin

For in vitro immune work or any in vivo research, the endotoxin result matters as much as purity. It should be reported as a number in EU/mg with the method (LAL gel-clot, turbidimetric, chromogenic or rFC). The harmonised bacterial endotoxins test allows results to be expressed in EU per mL, per mg, per mEq or per Unit [18]. Schwarz and colleagues found that human CD1c+ dendritic cells could be activated in vitro by LPS amounts equivalent to the endotoxin contamination they detected in some commercial recombinant proteins [19]. In the 2024 test-purchase study, the authors reported endotoxin in all three online semaglutide vials, at levels they gave as 2.1645 to 8.9511 EU/mg, two of the three results being upper bounds [20]. See our guide to endotoxin testing for peptides.

Appearance and solubility

Appearance is a basic check, useful for spotting gross problems but not evidence of quality. Solubility is a property, not a release test: EMA lists “solubility in different media” among general properties to describe [7]. A COA that leads with appearance and solubility and gives no identity or purity data is not a meaningful certificate.

Red flags: how do you know if a COA is legit?

Red flag	Why it matters	Ref
No batch number, or a batch number that does not match your vial	The results cannot be linked to your material	[1], [2]
Same COA for different lots, or identical numbers across lots	Indicates a template, not testing	[2]
“Complies” or “Pass” where a number should be	ICH Q7 expects numerical results where tests are numerical	[1]
No acceptance criteria	A result means nothing without a limit	[1], [2]
No issuing laboratory, address or contact	The report cannot be verified	[2]
Unsigned, or signed by sales staff	Approval should come from the quality unit or an authorised person	[1], [2]
Reseller letterhead with no original manufacturer	ICH Q7 and WHO expect the original manufacturer to be named and its certificate attached	[1], [2]
Purity without a chromatogram, or identity without a spectrum	The headline number cannot be checked	[4]
Purity presented as peptide content	Hides water, counter-ion and solvent	[7], [20]
Analysis date before manufacture date, or no dates	Impossible or unverifiable chronology	[2]
Accreditation logo with no certificate number or scope	Accreditation is for a defined scope of tests	[5], [6]
Endotoxin “negative” with no value or method	Detection limit unknown	[18]

How to verify a peptide COA

1. Match the lot. Compare batch number, product name and sequence on the vial, the packing list and the COA.
2. Read the dates. Analysis should follow manufacture, and retest or expiry dates should be stated with storage conditions.
3. Identify the laboratory. The WHO model expects the issuing laboratory’s name and address, and results from subcontractors to be identified as such [2].
4. Check accreditation and scope. ISO/IEC 17025:2017 “sets out requirements for the competence, impartiality, and consistent operation of laboratories” [5]. Accreditation bodies that sign the ILAC Mutual Recognition Arrangement accredit testing laboratories against ISO/IEC 17025 and accept each other’s accredited results [6]. Look up the accreditation body’s directory and confirm that the scope covers the methods on your certificate. WHO says an accredited laboratory “should include in the CoA a reference to the accreditation, if related to the specific analysis” [2].
5. Contact the laboratory directly. Use contact details you find yourself, not those printed on the PDF. Ask the laboratory to confirm the certificate number, sample identity, date and key results.
6. Ask for the raw data. The chromatogram, peak table and mass spectrum for the same sample ID. In GMP settings, US regulations require laboratory records to include “a complete record of all data secured in the course of each test, including all graphs, charts, and spectra”, with the signature of the analyst and of a second reviewer [3].
7. Check the chain of custody. If a trader or repacker issued the COA, ICH Q7 says it should name the laboratory that performed the analysis, refer to the original manufacturer and attach the original batch certificate [1]. WHO says the same for repackers and traders [2].
8. Retest if it matters. For critical work, send a sample from your vial to a laboratory you choose, and ask for identity, HPLC purity and, if relevant, endotoxin.

In-house vs third-party testing: which COA can you trust?

“Third-party tested” is a popular claim, but who ran the test matters less than whether the result is traceable and the method competent.

	In-house (manufacturer’s QC)	Third-party laboratory
Strength	Knows the process and impurity profile; full data and batch records available; under GMP, overseen by an independent quality unit	Independent of the seller; useful as a check on claims
Weakness	Conflict of interest if there is no functioning quality system	Tests only the sample it received; has no view of the batch records
What to ask for	Raw data, method and the quality system behind the result	Proof that the sample came from your lot; accreditation scope; the full report
Framework	ICH Q7 section 11 for APIs	ISO/IEC 17025 accreditation for the tests performed
Ref	[1]	[2], [5]

WHO notes that its model certificate is “essentially designed for QCLs not related to manufacturers”, because manufacturers’ own QC laboratories hold some of the information in other quality-system documents [2]. Both routes can produce reliable results. What makes either untrustworthy is a result that cannot be traced to a sample, a method and a named laboratory.

Two limits of third-party testing are often missed. First, a third-party report covers the sample tested. A report on a sample the seller chose and sent says nothing about the lot you received unless the lot numbers match. Second, independent testing is not a substitute for manufacturing controls: it checks a result, it does not establish how the batch was made.

What changes for a peptide API COA?

If the peptide is an API for regulated manufacturing, the COA is part of a GMP supply chain. ICH Q7 section 11.4 applies in full, with each test listed “in accordance with compendial or customer requirements” [1]. EMA’s guideline expects LC assay limits to be “expressed in terms of the counter-ion free, anhydrous substance, unless otherwise justified” [7]. Our semaglutide API COA guide walks through a full API certificate, and how to qualify a peptide API supplier covers the audit side.

How Winhycare Pharma supplies research peptides

Winhycare Pharma supplies research peptides from Hyderabad, India, as research-use-only material at purity of ≥99%, as stated in our product brochure, and offers custom peptide synthesis. Semaglutide, tirzepatide, retatrutide, cagrilintide and BPC-157 are also supplied as API to licensed manufacturers; see peptide APIs and semaglutide. Ask for the batch Certificate of Analysis for the lot you are quoted, and tell us which tests your work needs: request research peptides or enquire about peptide APIs.

For laboratory research use only. Not for human or veterinary use.