Peptide Testing and Purity: How to Read a Certificate of Analysis

Research Use Only. This article is written for laboratory and research professionals. The
compounds discussed are supplied strictly for in-vitro laboratory research. They are not drugs,
dietary supplements, or foods. They are not for human or veterinary use, and not for diagnostic
or therapeutic use. Nothing here is medical, legal, or regulatory advice.

The short answer. Peptide testing means checking a specific lot against analytical data: purity by HPLC, identity by mass spectrometry, and net peptide content. A certificate of analysis reports those results for one batch. Check that the lot number matches your vial. Read the chromatogram rather than the headline percentage, and treat any undocumented purity claim as unverified.

Why this problem exists at all

Research chemicals sold this way pass through no quality gate. No premarket approval, no mandatory product registration, no compendial monograph anyone must meet, no required lot-release testing. Nobody opens a vial before it ships. That is not unique to peptides; it is what "research reagent" means as a commercial category. Few buyers follow that to its conclusion: you are the quality control function.

The research-use-only designation says almost nothing about quality. It describes how a material is supplied. It is not a grade, and two vials with identical RUO labelling can differ by twenty percentage points of actual peptide. We cover the regulatory side in our guide to RUO status and peptide legality.

Compare an approved drug substance. FDA's guidance on analytical procedures and methods validation spells out what it takes to document identity, strength, quality and purity: validated methods, defined acceptance criteria, specificity you can demonstrate. Nothing compels a research reagent to meet that bar. That is exactly why it makes the right yardstick for supplier paperwork.

Plenty of sellers here synthesise nothing at all. They buy bulk, repackage, and forward whatever document arrived with the drum.

What a certificate of analysis actually is

A COA is a document from the manufacturer or a testing laboratory reporting analytical results for one specific lot. Not the product line. That batch, tested on a stated date by a named analyst.

Synthesis runs stepwise, and every step yields less than 100%, so impurity profiles differ between sequences and between two runs of the same sequence. A batch is the only unit at which the numbers mean anything. So a COA with no lot number, or one that does not match your vial, describes material you do not have.

Reading a peptide COA field by field

Field	Complete entry	Weak signal
Product name	Full chemical or common name	Trade name only
Sequence	One-letter and three-letter, with modifications	Absent
CAS number	Registry number, where one exists	Missing
Lot / batch number	Alphanumeric, matching the vial	Missing or "N/A"
Molecular formula	e.g. C50H73N15O11	Absent
Molecular weight	Average and monoisotopic, both labelled	One unlabelled value
Appearance	"White lyophilized powder"	Absent
Purity by HPLC	% plus column, gradient, wavelength, chromatogram	A bare percentage
Identity by MS	Method, theoretical and observed mass, spectrum	"Confirmed", no numbers
Net peptide content	% of mass that is peptide, plus method	Absent or generic
Water content	% by Karl Fischer	Absent
Counterion content	TFA or acetate, % by IC or 19F NMR	Absent
Amino acid analysis	Found vs theoretical residue ratios	Rarely provided
Solubility	Solvent used for the analysis	Vague
Storage	Temperature, light, desiccation	Absent
Date of analysis	Specific date, after manufacture	Implausible
Analyst	Named person, signature	Unsigned

Name, sequence, and CAS number

The sequence should appear in both notations. One-letter (RPPGFSPFR) is compact; three-letter (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) is harder to typo. Terminal modifications belong here too, since an amidated C-terminus changes the mass.

Lot or batch number

Must match the number printed on the vial. Not the catalogue code. If you check one thing, check this.

Molecular formula and molecular weight

Two molecular weights exist. A good COA prints both. Average molecular weight uses each element's average atomic mass across natural isotopic abundance; it is the number you use to weigh material and calculate concentration. Monoisotopic mass uses the most abundant isotope of each element, carbon-12 at exactly 12.000000, and is what a high-resolution instrument reports for the first isotope peak.

For bradykinin, a nine-residue mass spectrometry reference peptide, C50H73N15O11 gives an average mass of 1060.21 Da and a monoisotopic mass of 1059.56 Da. The 0.65 Da gap scales with size, reaching roughly 2 Da by 4 kDa. That is enough to make an observed mass look wrong when it is checked against the wrong theoretical value.

Purity by HPLC

Almost always reversed-phase: a C18 column, a water/acetonitrile gradient, typically 0.1% trifluoroacetic acid as ion-pairing modifier.

Detection is at 214 nm because that is where the peptide bond absorbs. The amide carbonyl absorbs strongly between roughly 205 and 220 nm, so every residue contributes and response scales with the number of peptide bonds. At 280 nm only tryptophan, tyrosine and weakly phenylalanine absorb, leaving a non-aromatic peptide nearly invisible.

The percentage comes from area normalisation: main peak area divided by total integrated area. That is also its limit. The number is relative and ignores anything that fails to absorb at 214 nm or never elutes, so a sample can be 99% pure by area and a fifth salt by mass.

Gradient conditions decide what the figure is worth. A shallow gradient over thirty minutes resolves deletion sequences, where one residue failed to couple, and diastereomers from racemisation. A steep 5 to 95% ramp over five minutes buries them and returns a flattering number from identical material. A COA should state column, mobile phases, gradient, flow rate, temperature and wavelength.

Identity by mass spectrometry

Purity says the sample is one thing. Mass spectrometry says it is the right thing.

ESI-MS sprays the peptide from solution and produces multiply charged ions. Software then deconvolutes that charge envelope to a single mass. The method couples directly to the HPLC, with low parts-per-million accuracy on high-resolution instruments. MALDI-TOF desorbs the sample from a crystalline matrix with a laser, producing mostly singly charged ions. It tolerates salts better, but linear-mode accuracy is coarser, near 0.1%.

What matters is observed versus theoretical, and a COA has to print both. Agreement within about ±1 Da is normal on a unit-resolution instrument for a 1 to 5 kDa peptide; high-resolution work should land within tens of parts per million. Some deviations are diagnostic: +16 Da suggests oxidation, often at methionine, +18 Da hydrolysis, −18 Da dehydration, +22 Da a sodium adduct. A deficit matching a residue mass, −57 Da for glycine or −71 Da for alanine, points to a deletion sequence. An observed mass matching theoretical to four decimals is not reassuring, because real measurements have error.

Net peptide content, the field almost everyone misreads

The most valuable line on the page, and the one most often missing. Purity is relative. Peptide content is absolute. Purity asks what share of the peptide-related material is your target sequence; content asks what share of the powder is peptide at all.

Lyophilized synthetic peptides are salts. Purification with TFA leaves a trifluoroacetate counterion on every basic site: arginine, lysine, histidine, the free N-terminus. Freeze-dried powders are also hygroscopic and retain water. Neither appears in an HPLC purity figure, so a vial labelled 10 mg at 98% purity may hold around 8 mg of peptide. Net peptide content for a TFA salt commonly runs 70 to 90%.

This is measurable, not hypothetical. A metrology group publishing in Scientific Reports determined peptide purity by mass balance rather than chromatography alone: water by Karl Fischer, TFA by ion chromatography, inorganic ions by ICP-MS, related impurities by isotope dilution MS. Water came to roughly 5% of the mass and TFA to roughly 10%, leaving about 896 mg of peptide per gram. Chromatographic purity was never the limiting factor. Mass was.

Amino acid analysis measures content most reliably. Nitrogen determination and UV at 280 nm are less rigorous, and the UV route needs tryptophan or tyrosine present. Treat "peptide content: approximately 80%" with no stated method as boilerplate.

Water content and counterion content

Karl Fischer titration gives water content, and unlike loss on drying it responds only to water. Between 1 and 10% is unremarkable for a lyophilized powder.

Counterion content identifies the salt form and its share of the mass. Ion chromatography, HPLC or ¹⁹F NMR can quantify TFA. Acetate and hydrochloride forms come from deliberate exchange, usually because TFA interferes with an assay. Without a counterion figure you cannot convert mass to moles.

Amino acid analysis, solubility, and storage

Amino acid analysis hydrolyses the peptide in 6 M HCl near 110 °C, then quantifies the released residues against calibrated standards. It confirms composition independently of MS, and it is what puts a number on net peptide content. Its blind spots are known: tryptophan is destroyed, cysteine needs separate treatment, and asparagine and glutamine convert to aspartate and glutamate.

Solubility should name the solvent used for the analytical sample, so anyone can reproduce it. Storage should specify temperature, light protection and desiccation, ideally with a retest date. That chemistry gets its own treatment in our peptide storage and stability guide.

Date, analyst, and signature

A date of analysis that follows the manufacture date. A named analyst. A named approver or release signature. Unglamorous fields, and the ones a fabricated document tends to fumble.

Apply all of this to every supplier you are evaluating, including this one. Every product in our research catalogue has a certificate of analysis on its product page and on Lab results, so you can run these checks before you order. A supplier who cannot produce one has answered your question.

How to read an HPLC chromatogram

The trace tells you more than the number derived from it.

The main peak should be sharp, symmetric and return cleanly to baseline on both sides. A tailing factor around 0.8 to 1.5 is acceptable. Badly tailing peaks make integration boundaries arbitrary, which makes the percentage arbitrary.

Shoulders matter. A bump on the leading or trailing edge is a closely eluting impurity the integrator may have swept into the main peak. That is where deletion sequences and racemisation products hide. One sharp symmetric peak at 96% is better material than a lumpy peak reported at 99%.

The baseline should be flat and quiet. Drift, noise, or integration marks that start and stop at odd places all make the percentage unreliable. Check the axes too, since autoscaling can make a 0.3% impurity look alarming or bury a real one. The run also has to be long enough to elute hydrophobic material. A trace cropped to the main peak hides the part worth seeing.

Third-party testing versus in-house

In-house means the manufacturer's own QC laboratory ran the analysis. Not worthless, since they hold the method and know the compound. The limitation is structural rather than moral: the party with a commercial interest in the result is reporting it. Third-party testing adds most value when the sample came from the lot you receive and the report names the lab.

Accreditation separates an independent laboratory from one that merely says it is. ISO/IEC 17025:2017 is the international standard covering competence, impartiality and consistent operation of testing and calibration laboratories. A recognised body grants ISO/IEC 17025 laboratory accreditation; nobody self-declares it. It is also scope-specific, so a lab accredited for one class of method is not accredited for everything it can run. Underneath sits traceability. Results are only comparable when instruments are calibrated against characterised reference materials, which is what bodies such as NIST supply through the Standard Reference Materials programme.

Then the caveat most articles omit: a PDF can be fabricated. Logos copy and paste, and a lab's name can be typed onto a report it never saw. An unverifiable "third-party tested" badge is a marketing claim, not evidence.

How to verify a COA is genuine

1. Match the lot number on the document to the vial label, character for character. This one step eliminates most bad documents.
2. Check internal consistency. Does the formula match the sequence, the average molecular weight match the formula, the observed mass sit a plausible distance from theoretical? Fabricated documents rarely survive arithmetic, because whoever produced them did not expect anyone to do any.
3. Check the dates. Analysis after manufacture, neither in the future.
4. Look for a named analyst and approver. Anonymous documents are accountable to nobody.
5. Contact the testing laboratory with the report number. Accredited labs usually confirm whether a report is theirs, and the accreditation body's directory lists each lab and its scope.
6. Ask for the raw traces, chromatogram and spectrum, not the summary page. A lab that ran the work has them; someone who typed the document does not.
7. Distrust round numbers. Exactly 99.9%, or a figure identical across every published lot, is a template rather than a measurement.
8. Ask before you order. A COA produced only after payment is a different document from one produced on request.

Red flags checklist

• [ ] No COA offered, or offered only after purchase
• [ ] A generic COA not tied to a specific lot
• [ ] Document lot number does not match the vial
• [ ] Purity claimed with no method, column, gradient or wavelength
• [ ] A purity percentage with no chromatogram
• [ ] No mass spectrometry data, or "identity confirmed" with no observed mass
• [ ] No net peptide content, or a generic figure with no method
• [ ] No water content and no counterion figure
• [ ] Molecular weight given as a single unlabelled number
• [ ] No analyst name, no signature, no date of analysis
• [ ] "Third-party tested" with no laboratory named
• [ ] No lot number printed on the vial itself
• [ ] The supplier makes human-use claims, an easy compliance point to get right, which invites the question of what else gets handled loosely
• [ ] No physical address, company name, or way to reach a person
• [ ] Pricing far below plausible synthesis cost

None of these alone proves anything. Three or four together is a pattern.

Now apply the list, starting with us. Every product we list has a certificate of analysis from Horizon Analytical, a laboratory in the USA. Each one names the lot tested, the analysis date and the chemist, reports identity, measured amount against the label and purity by UPLC/MS, and includes the chromatogram and mass spectrum. Read them on each product page or on Lab results before you order. What they do not cover, such as sterility, residual solvents and heavy metals, is listed on our Testing method page. Then put the questions below to whoever you are considering buying from, here or anywhere else. The point is not where you shop. It is that you stop accepting undocumented claims.

Questions to ask a supplier

1. Can I see the COA for the exact lot I would receive, before ordering?
2. Who performed the analysis, and is that lab independent of the manufacturer?
3. Is that lab accredited to ISO/IEC 17025, and for which methods?
4. What were the full HPLC conditions, and can I see the chromatogram?
5. What is the net peptide content, and how was it determined?
6. Which counterion is present, and at what level?
7. What is the water content, and was it measured by Karl Fischer?
8. Which MS method was used, and what were the theoretical and observed masses?
9. What are the lot's manufacture, analysis and retest dates?
10. Will the vial's lot number match the certificate?
11. Do you hold stability data under your stated storage conditions?
12. What happens if the material does not match its documentation?

Compliance notice. Nothing in this article should be read as guidance for administering any
compound to a human or animal subject. "Research Use Only" describes the conditions under which
a material is supplied; it is not a determination of legal status and confers no exemption from
the Federal Food, Drug, and Cosmetic Act. Researchers are responsible for compliance with all
applicable federal, state, and institutional requirements, and for obtaining any approvals their
work requires.

Frequently asked questions

What does peptide testing actually measure?
Purity by reversed-phase HPLC at 214 nm, identity by mass spectrometry, and net peptide content by amino acid analysis. Water content, counterion content and appearance support those three.

What is the difference between peptide purity and peptide content?
Purity is relative: the main peak's share of total HPLC peak area. Content is absolute: the share of the vial's mass that is peptide. Material can be 98% pure and 80% peptide by mass, because water and counterion are invisible to HPLC.

What purity level should research grade peptides have?
The experiment sets the requirement, not the vendor. Above 95% is a common baseline; above 98% is typical where quantitation demands it. What matters more than the figure is the method and the chromatogram behind it.

Can a certificate of analysis be faked?
Yes. It is a PDF, and anyone with a word processor can produce one. That is why the checks matter: match the lot number, confirm formula, molecular weight and observed mass are consistent, and contact the named laboratory.

What does "third-party tested" actually mean?
In principle, that a laboratory with no commercial interest in the outcome analysed a sample from the lot. In practice the phrase turns up with no laboratory named and no report attached, which leaves nothing to check.

Does a certificate of analysis tell me whether a peptide is legal to buy?
No. A COA is a chemistry document. It reports what is in the vial and says nothing about regulatory status, permitted use or import restrictions, which are governed separately.