For research purposes only. Not for human use. This guide explains the analytical methods used to assess synthetic research peptides. It is not medical advice and does not describe administration of any compound.
Why a Purity Figure Needs a Method Behind It
A research peptide arrives with a number: 98%, 99%, "≥99% by HPLC". The number is only as good as the method that produced it, and the method is often not the one a reader assumes. HPLC purity measures what fraction of the UV-absorbing material in a vial elutes as the main peak. It does not measure how much peptide is in the vial, whether the main peak is the right peptide, whether that peptide has the right stereochemistry, or whether the vial contains anything that does not absorb UV at all. Each of those questions needs a different instrument, and a certificate of analysis that answers only the first one has left the others open.
The gap is not theoretical. Verbeke and colleagues ordered a set of research-grade quorum-sensing peptides with a requested purity of at least 95% and re-tested them in-house; only 44% met the requested purity, most impurities were sequences missing one or more amino acids, and one sample's main compound had a different structure from the ordered peptide (Verbeke et al., 2015). De Spiegeleer's group profiled obestatin from five manufacturers after research groups failed to reproduce the peptide's reported receptor activity, and found that one product was an entirely different peptide and two-thirds of the rest fell below 95% purity or carried individual impurities above 1% (De Spiegeleer et al., 2008). Both papers concluded that a supplier's CoA is a starting point for in-house quality control, not a substitute for it.
Reversed-Phase HPLC: What the Purity Number Means
Reversed-phase HPLC separates peptides by hydrophobicity. The sample is loaded onto a column packed with silica particles carrying C18 (or C8 or C4) alkyl chains and eluted with a gradient of increasing organic solvent, usually acetonitrile in water with 0.1% trifluoroacetic acid or formic acid as an ion-pairing agent. More hydrophobic species stick longer and elute later. A UV detector at 210 to 220 nm (where the peptide backbone amide absorbs) records absorbance against time, and the software integrates every peak.
"Purity by HPLC" is the main peak's area divided by the total integrated area, expressed as a percentage. Three consequences follow from that definition.
It is a relative measure. A vial that is 40% peptide and 60% mannitol by weight can still show 99% HPLC purity, because mannitol does not absorb at 214 nm and never appears on the trace. Choules and colleagues found exactly this in two commercially sourced custom peptides: quantitative NMR detected 20% and 43% w/w undeclared mannitol that LC-UV methods had been blind to (Choules et al., 2020).
It depends on what co-elutes. Two species that leave the column at the same time are one peak. Deletion sequences (the target peptide missing one residue), deamidated forms and, above all, diastereomers formed by racemisation during synthesis often have retention times within seconds of the parent. Petersson and colleagues screened 30 column and mobile-phase combinations specifically because peptide isomers have the same mass-to-charge ratio and cannot be resolved by MS alone, so they must be separated chromatographically; their 2D-LC-MS approach uses a second, orthogonal reversed-phase dimension to expose impurities hiding under the main peak (Petersson et al., 2023). A single-dimension trace at one wavelength on one column is the floor, not the ceiling, of what purity analysis can see.
It says nothing about identity. A clean single peak at the expected retention time is consistent with the right peptide. It is also consistent with a different peptide of similar hydrophobicity, which is how the "totally different peptide" in the obestatin study passed the manufacturer's own check.
Reading the chromatogram
A CoA that reports HPLC purity should attach the trace. On it, look for the main peak's retention time and shape (a shoulder or a split top indicates co-elution), the number and size of minor peaks, where the minor peaks sit relative to the main peak (deletion sequences and deamidated forms usually elute slightly earlier; oxidised methionine elutes earlier still; acetylated or protecting-group-retaining species elute later), the baseline (a rising baseline in gradient elution is normal, but a noisy one hides small peaks), and the injection solvent front at the start, which should not be integrated as an impurity. The integration table beneath the trace should list each peak's area percentage; if the table is absent, the purity figure cannot be checked against the picture.
Mass Spectrometry: Confirming Identity
Mass spectrometry measures the mass-to-charge ratio of ionised molecules and is the method that ties a peak to a sequence. Electrospray ionisation (ESI) is standard for peptides, producing multiply charged ions whose deconvoluted mass is compared with the theoretical mass for the sequence and its modifications. Coupled to HPLC as LC-MS, it identifies each chromatographic peak on the fly, so an impurity at 0.8% can be named as "minus one glycine" rather than left as an anonymous shoulder. MALDI-TOF is the common alternative: fast, tolerant of salts, and well suited to confirming a lyophilised powder's dominant mass, but less useful for resolving impurity structure.
Lian and colleagues' 2021 review sets out what LC-MS can and cannot do for synthetic peptide therapeutics. Impurities originate from the starting materials, the synthesis and storage; LC-MS identifies most of them by mass and fragmentation, with the acknowledged exception of structural isomers and epimers, which share a mass with the parent and need chromatographic or enzymatic approaches instead (Lian et al., 2021). On a CoA, an MS result should state the observed mass numerically alongside the theoretical mass. "MS: conforms" or "MS: pass" without a number cannot be checked. For modified peptides (fatty-acid conjugates such as semaglutide, PEGylated species, copper complexes) the observed mass should match the full modified structure; a match to the bare sequence means the modification is missing.
Stereochemical Purity: The Impurity Both HPLC and MS Miss
Solid-phase synthesis racemises a small fraction of residues at each coupling step, and D-amino-acid contamination in the starting materials adds more. The resulting epimers have the identical mass and nearly identical hydrophobicity to the target, so they pass a standard LC-MS check. Badgujar and colleagues review the regulatory requirements and the analytical methods for it, including the acid-hydrolysis and sample-preparation steps that precede chiral analysis, and note how few published reports exist (Badgujar et al., 2024). Regulators require this analysis for pharmaceutical peptides; research-grade CoAs almost never include it. For peptides where a single D-residue changes receptor binding (which is most bioactive peptides), the absence of a stereochemical result means the biological activity of the vial has not been confirmed by analysis.
Net Peptide Content: How Much Is in the Vial
Lyophilised peptides are salts, not free peptides. Every basic residue (lysine, arginine, histidine) and the N-terminus carry a counter-ion, almost always trifluoroacetate from the HPLC purification or acetate if the supplier has exchanged it. Hettiarachchi and Ridge developed a capillary electrophoresis method to measure exactly these acids in synthetic peptide samples, noting that both arise from the synthesis and purification reagents (Hettiarachchi & Ridge, 1998). Counter-ions plus bound water routinely make up 10% to 30% of vial mass for a short peptide and more for arginine-rich sequences.
The measurement that accounts for this is net peptide content, determined by amino acid analysis: the peptide is hydrolysed to free amino acids, which are quantified against standards and summed. Qasrawi and colleagues describe a current LC-MS implementation using isotopically labelled internal standards that quantifies even methionine and cysteine without derivatisation (Qasrawi et al., 2023). A CoA that reports HPLC purity and a gross weight but not net peptide content has not told the reader how many milligrams of peptide the vial holds. For a "10 mg" vial at 99% HPLC purity and 75% net peptide content, the answer is 7.4 mg, and any concentration calculated from the label is 35% high. The cost calculator accepts net peptide content so that per-milligram prices reflect peptide rather than salt.
Elemental, Solvent and Microbial Contaminants
None of the methods above detects lead, arsenic, residual solvents or endotoxin, and all four have been found in peptides sold outside regulated supply chains. Janvier and colleagues screened the ten falsified peptide drugs most often seized on the Belgian market, bought from three suspected illegal internet pharmacies, for the active ingredient, related impurities, small-molecule contaminants, elemental impurities and residual solvents. Purity of the cysteine-containing peptides ranged from 5% to 75%; one sample was contaminated with lead; several contained arsenic at up to ten times the ICH limit for parenteral drugs, all of it in the more toxic inorganic form (Janvier et al., 2018). The same Belgian laboratory had earlier published a 30-minute LC-MS/MS screening method that identifies 25 peptides in seized preparations using the five-identification-point standard borrowed from sports drug testing (Vanhee et al., 2015), and in 2020 reported finding Selank and Semax in seized preparations and developing a method for ten nootropic research peptides sold online (Vanhee et al., 2020). For semaglutide specifically, Ashraf and colleagues' 2024 test purchases found endotoxin in every sample and measured purity between 7.7% and 14.4% against labels claiming 99% (Ashraf et al., 2024).
Elemental impurities are measured by ICP-MS, residual solvents by headspace GC, endotoxin by the LAL or recombinant factor C assay. A research-grade CoA that includes any of these is unusual; one that includes none is normal, and the reader should treat those attributes as untested rather than clean.
Why Impurities Matter Beyond the Number
Deletion sequences and epimers can carry their own biological activity or block the target's. De Groot and colleagues review a further mechanism: peptide impurities can introduce new T-cell epitopes and provoke immune responses the parent peptide would not, which is why the FDA's abbreviated pathway for generic synthetic peptides requires sponsors to characterise any impurity not present in the originator product (De Groot et al., 2023). Kuril and colleagues summarise the resulting regulatory expectation that identity, purity, impurity profile and physicochemical properties be demonstrated with several orthogonal methods rather than one (Kuril et al., 2024). For a laboratory, the practical translation is that an unexplained result in a peptide experiment is as likely to be an impurity as a mechanism until the material has been characterised.
What the Platform Can and Cannot Tell You
Peptides.so indexes 10,863 priced listings from 119 suppliers. None of the 181 supplier rows in the database has a non-zero testing score, and none has a certificate-of-analysis URL on file. Seven carry badges and four are marked verified, but those flags reflect platform onboarding rather than independent analysis. The platform therefore cannot rank suppliers by documentation quality, and this guide does not attempt to. The platform does show price. The platform-wide median list price is $71.50, and the compound pages (BPC-157, semaglutide, GHK-Cu and others) show per-supplier prices. Price does not predict purity in either direction in the published test-purchase studies, so the price tables are for budgeting, not for quality screening.
Requesting a batch-matched CoA with the chromatogram, the numerical MS result and a net peptide content figure before ordering is the platform-independent check. The supplier checklist lists what to ask for and the CoA interpretation guide walks through a certificate line by line.
Frequently Asked Questions
What does "99% purity by HPLC" mean? That 99% of the UV-absorbing material eluting from the column was in the main peak. It says nothing about the amount of peptide in the vial, the identity of the main peak, its stereochemistry, or non-absorbing contaminants.
Can a vial be 99% pure and mostly not peptide? Yes. HPLC purity is relative to the peptide-like material. A vial diluted with mannitol or heavy in trifluoroacetate can show 99% while containing far less than the labelled mass of peptide. Net peptide content by amino acid analysis is the measurement that answers "how much".
Why does the CoA need a numerical mass? Because "conforms" cannot be checked. The observed mass, compared with the theoretical mass for the full sequence including any modification, is what confirms identity; a match to the unmodified sequence means a conjugate is missing its side chain.
What is a deletion sequence? The target peptide minus one or more residues, produced when a coupling step fails during synthesis. Verbeke's study found most impurities in research-grade peptides were of this type.
Which purity level does a study need? There is no universal threshold. De Spiegeleer's group used 95% purity with no individual impurity above 1% as the bar for in vitro and in vivo experiments. Receptor-binding and cell-signalling work is more sensitive to epimers and deletions than gross physical assays.
Can I test a peptide myself? Analytical HPLC with UV detection is the most accessible check and confirms the number of peaks and their proportions. LC-MS for identity and amino acid analysis for content are usually contracted out. Stereochemical, elemental and endotoxin testing require specialised laboratories.
References
1. Verbeke F, et al. Quality evaluation of synthetic quorum sensing peptides used in R&D. J Pharm Anal. 2015. PMID 29403929
2. De Spiegeleer B, et al. Impurity profiling quality control testing of synthetic peptides using liquid chromatography-photodiode array-fluorescence and liquid chromatography-electrospray ionization-mass spectrometry: the obestatin case. Anal Biochem. 2008. PMID 18342612
3. Choules MP, et al. NMR reveals an undeclared constituent in custom synthetic peptides. J Pharm Biomed Anal. 2020. PMID 31671336
4. Petersson P, Buckenmaier S, Euerby MR, et al. A strategy for assessing peak purity of pharmaceutical peptides in reversed-phase chromatography methods using two-dimensional liquid chromatography coupled to mass spectrometry. Part I. J Chromatogr A. 2023. PMID 36841023
5. Lian Z, Wang N, Tian Y, et al. Characterization of Synthetic Peptide Therapeutics Using Liquid Chromatography-Mass Spectrometry: Challenges, Solutions, Pitfalls, and Future Perspectives. J Am Soc Mass Spectrom. 2021. PMID 34110145
6. Badgujar D, et al. Enantiomeric purity of synthetic therapeutic peptides: A review. Chirality. 2024. PMID 38448043
7. Hettiarachchi K, Ridge S. Capillary electrophoretic determination of acetic acid and trifluoroacetic acid in synthetic peptide samples. J Chromatogr A. 1998. PMID 9764489
8. Qasrawi DO, Petrotchenko EV, Borchers CH. Amino acid analysis for peptide quantitation using reversed-phase liquid chromatography combined with multiple reaction monitoring mass spectrometry. Anal Bioanal Chem. 2023. PMID 37468754
9. Janvier S, et al. Impurity profiling of the most frequently encountered falsified polypeptide drugs on the Belgian market. Talanta. 2018. PMID 30029448
10. Vanhee C, et al. Analysis of illegal peptide biopharmaceuticals frequently encountered by controlling agencies. Talanta. 2015. PMID 26003685
11. Vanhee C, et al. The occurrence of putative cognitive enhancing research peptides in seized pharmaceutical preparations. Drug Test Anal. 2020. PMID 31667971
12. Ashraf AR, et al. Multifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription. J Med Internet Res. 2024. PMID 39509151
13. De Groot AS, et al. Immunogenicity risk assessment of synthetic peptide drugs and their impurities. Drug Discov Today. 2023. PMID 37467878
14. Kuril AK, et al. Analytical considerations for characterization of generic peptide product: A regulatory insight. Anal Biochem. 2024. PMID 39089363
Research Disclaimer
The methods and compounds described here relate to laboratory analysis of research chemicals supplied for research use only. Nothing on this page is medical advice, and no part of it describes administration of any compound to people or animals.