What a Peptide Certificate of Analysis Proves, and What It Doesn’t

For laboratory research use only. Not for human or veterinary use. This article covers analytical documentation and laboratory testing standards.

A certificate of analysis is a document, not a guarantee. It reports what one laboratory measured, on one sample, on one day, using methods it chose. That can be exactly what you need, or it can be close to meaningless, and the difference isn’t always visible at a glance.

Two questions decide which one you’re holding. Does the data cover the attributes that matter for your work? And can you verify the data is what it claims to be? This guide covers both, along with the pharmacopeial framework that defines the attributes in the first place.

Purity and Content Are Two Different Numbers

Start here, because it’s the most common misreading of a peptide COA.

HPLC purity is a relative measure. Reversed-phase HPLC with UV detection separates the components of a sample, and purity is reported as the area of the main peak against the total area of all detected peaks. It tells you how homogeneous the detected material is.

Net peptide content is an absolute measure. It’s the mass of peptide as a fraction of the total mass of the powder. A lyophilized solid also contains counterions left from purification, residual water, and sometimes residual solvents. Bachem puts typical peptide content at 70 to 90% of the solid [1].

So a lot can be 99% pure by HPLC and around 80% peptide by weight at the same time. Both numbers are correct. They answer different questions, and only the second one tells you how much peptide you’re weighing out.

The rigorous version of this is the mass balance approach. In a 2023 paper in Pharmaceutical Research, McCarthy and colleagues describe assigning content by measuring every detectable component and subtracting from 100%, listing “peptide related impurities, counter ion, water, residual solvents, non-combustible residues” [2]. That’s the full accounting. Most research-grade COAs report a purity figure and stop.

What Each Test Answers

A COA is only as informative as the tests behind it. These are the attributes a complete one addresses, and what each method can establish.

Identity. No single method proves identity. McCarthy and colleagues note that “multiple orthogonal techniques are typically used to verify the chemical structure” [2]. Each one covers a different gap:

• Mass spectrometry gives the molecular mass. It confirms composition but not sequence order, since the same residues in a different order have the same mass.
• Tandem MS (MS/MS) fragments the peptide and can establish the sequence order.
• Amino acid analysis quantifies composition after acid hydrolysis, and distinguishes leucine from isoleucine, which have identical masses [2].
• NMR confirms connectivity.
• Chiral analysis detects D-enantiomers where the L form was intended [2].

Purity and related substances. Reversed-phase HPLC, reported as a percentage of total detected area. Its blind spots are worth knowing: it doesn’t see components that don’t absorb at the detection wavelength, and impurities that elute under the main peak are counted as product. Isomers that differ in the configuration of a single residue can be difficult to resolve.

Water content. Determined by Karl Fischer titration under USP General Chapter 921, Water Determination [2]. For lyophilized material the coulometric variant is standard, since it suits the small quantities of water involved. This matters more than it sounds: peptides are hygroscopic, and absorbed moisture lowers peptide content.

Counterions. Quantified by their own methods, not by HPLC purity. USP has dedicated chapters for both common counterions: General Chapter 503, Acetic Acid in Peptides, and General Chapter 503.1, Trifluoroacetic Acid (TFA) in Peptides. The latter describes TFA as “a common residual process impurity in the preparation of peptides or a counter ion in active pharmaceutical ingredients” [3]. Which counterion a lot carries can change results in cell-based work independently of the peptide.

Residual solvents. Measured by gas chromatography under USP General Chapter 467, Residual Solvents [2].

Non-combustible residue. USP General Chapter 281, Residue on Ignition, captures inorganic residue that other methods miss [2].

Microbiological contamination. USP General Chapter 61 covers microbial enumeration, and General Chapter 62 covers tests for specified microorganisms.

Bacterial endotoxin. The Limulus amebocyte lysate test under USP General Chapter 85, Bacterial Endotoxins Test, reported in endotoxin units per milligram. Lyophilization does not remove endotoxin, and immune cells respond to it at very low concentrations, so it can produce an inflammatory readout unrelated to the compound under study.

The Framework the Pharmacopeia Uses

Synthetic peptides sit awkwardly between small molecules and biologics, and as a result they’re excluded from much of the standard guidance on drug substance specifications. USP published General Chapter 1503, Quality Attributes of Synthetic Peptide Drug Substances, in August 2021 to address that gap. The chapter notes that because synthetic peptides “are not easily classified as small molecules or biological products, they are specifically excluded from many guidance documents” [4].

Its structure is a useful checklist of what a complete specification covers: manufacturing methods, raw materials, general characteristics and specification, peptide content and assay, impurities and related compounds, microbiological contamination, bacterial endotoxins, and specific tests [4]. A companion chapter, General Chapter 1504, covers the starting materials used in peptide synthesis.

A research-grade COA is not a regulatory filing and won’t contain all of this. The value of knowing the framework is that it tells you which attributes exist, so you can see what a given document leaves out.

Who Ran the Test

A number is only as good as the laboratory that produced it, and this is the part of a COA that’s easiest to verify and most often skipped.

The relevant standard is ISO/IEC 17025, which sets general requirements for the competence of testing and calibration laboratories. Two details about it are worth understanding.

Laboratories are accredited, not certified. Accreditation assesses whether a laboratory is competent to perform specific activities within a defined scope.

Accreditation is scope-specific. It works method by method and matrix by matrix, and the accredited methods are listed on a scope document attached to the certificate. A claim that testing was done at “an ISO 17025 accredited lab” tells you little on its own. The question is whether the specific method on your COA falls inside that laboratory’s accredited scope.

Both are checkable. Accreditation bodies maintain public directories of the laboratories they accredit, and the International Laboratory Accreditation Cooperation runs a signatory search that links through to those directories [5]. Look the laboratory up, find its scope, and confirm the method is on it.

Reading the Document Itself

Before trusting any value on a COA, check that the document is internally consistent. A legitimate certificate holds together; a decorative one usually doesn’t.

Things a complete COA states:

• The compound name and full sequence, matching each other
• A lot or batch number that matches the vial in your hand
• The date of analysis, and the laboratory that performed it
• Method parameters for each test, such as column, gradient, and detection wavelength for HPLC
• The chromatogram the purity figure was calculated from, with labeled axes and a peak table that adds up to the stated result
• Which mass is reported, whether average or monoisotopic, and for which ion

Things worth a second look:

• The same chromatogram, test date, or lot number appearing across different products or batches
• A purity figure given to two decimal places with no chromatogram behind it
• A mass value with no indication of charge state or mass type
• A laboratory name you can’t find in any accreditation directory, or no laboratory named at all
• An accreditation logo on the document with no certificate number to look up
• A lot number on the certificate that doesn’t appear on the vial label

What a COA Can’t Tell You

Even a complete, verifiable certificate has limits.

It describes a sample, not your vial, unless the lot number ties the two together. That link is what makes the document mean anything about the material in front of you.

It says nothing about handling after testing. Shipping temperature, time in transit, and storage conditions all affect material that tested clean at the source.

Identity and purity don’t establish suitability. A lot can be correctly identified and highly pure while still being wrong for a particular experiment because of its counterion, its endotoxin level, or a residual solvent.

It’s a snapshot. Peptides degrade, at rates that depend on sequence and storage. A test result from a year ago describes the material as it was a year ago.

What to Ask For

If a COA is missing something you need, it’s reasonable to ask. A supplier that commissioned real testing can usually produce more of it:

• The chromatogram and method parameters behind the purity figure
• The mass spectrum, with the expected value stated alongside the observed one
• Net peptide content, where you’re preparing solutions at a defined concentration
• The counterion or salt form
• The testing laboratory’s name and accreditation certificate number
• An endotoxin result, for work involving immune cells
• Batch-level documentation rather than a generic product-level file

None of this requires running the analysis yourself. It requires knowing which number answers which question, and treating a certificate as a document to be checked rather than a credential to be accepted.

References

1. Bachem. Care and Handling of Peptides. Technical note. bachem.com/knowledge-center
2. McCarthy D, Han Y, Carrick K, Schmidt D, Workman W, Matejtschuk P, Duru C, Atouf F. Reference Standards to Support Quality of Synthetic Peptide Therapeutics. Pharmaceutical Research. 2023;40(6):1317-1328. doi:10.1007/s11095-023-03493-1. PMID: 36949371
3. United States Pharmacopeia. General Chapter 〈503.1〉, Trifluoroacetic Acid (TFA) in Peptides. USP-NF.
4. United States Pharmacopeia. General Chapter 〈1503〉, Quality Attributes of Synthetic Peptide Drug Substances. USP-NF, August 2021.
5. International Laboratory Accreditation Cooperation. ILAC MRA signatory search. ilac.org/signatory-search