A label on a peptide vial makes three claims: what the material is, how pure it is and how much of it there is. Analytical chemists have published methods for testing each claim, and at least one group has applied such methods to vials bought from online shops. This article summarises that literature, sets out what documentation it suggests a researcher should request, and states what Skinni provides. All compounds Skinni supplies are for laboratory research use only and are not for human or veterinary use.
In short
- Karongo and colleagues, writing in the Journal of Chromatography A, described the standard quality-control method for synthetic peptides, reversed-phase liquid chromatography, and showed that closely related impurities can co-elute with the main peak and go undetected.
- Huo and colleagues identified one process-related impurity and thirty-two degradation products in a single synthetic antimicrobial peptide, Cbf-14, and noted that impurities of this kind are not declared for custom synthetic peptides.
- Breindahl and colleagues analysed melanotan II vials from three online shops. Every shop claimed the same content, and measured content ranged from roughly 43 to 88 percent of that figure. Vials from two shops contained unknown impurities at 4.1 to 5.9 percent.
- The literature points to four things worth checking on a certificate: how identity was confirmed, which chromatographic method gave the purity figure, whether impurities were profiled, and whether measured content was compared with the label.
- Skinni documents each compound by CAS number, formula and molecular weight, ships with lot traceability, and sends lot-specific certificates of analysis and safety data sheets to verified accounts on request. The site facts describe no independent third-party certification beyond this.
What an analysis of a synthetic peptide is asked to establish
Peptide quality control rests on two questions: is the material the intended sequence, and what else is in the vial alongside it. Karongo, Ikegami, Stoll and colleagues, writing in the Journal of Chromatography A in 2020, described the usual answer to the second question. It is a generic reversed-phase liquid chromatography method using a C18 stationary phase and an acetonitrile gradient with trifluoroacetic acid in the mobile phase, read by ultraviolet detection. The authors reported that this method works well for a wide variety of impurities and is the common basis of quality control for synthetic and therapeutic peptides.
Identity is a separate measurement. Breindahl, Evans-Brown, Hindersson and colleagues, in Drug Testing and Analysis in 2015, gave a worked example. They confirmed the identity of melanotan II by two checks together: the correct chromatographic retention time, and the relative abundance ratios of five qualifying fragment ions produced by tandem mass spectrometry. They used a separate liquid chromatography method with ultraviolet detection to quantify the peptide and its impurities, and validated it against guidelines for assessing active substances in authorised medicinal products.
Together these papers show that a single purity percentage on its own does not establish identity. A certificate is more informative when it shows how identity was confirmed as well as how purity was calculated.
Why one chromatogram can miss what matters
The weakness of the generic method, as Karongo and colleagues described it, lies in impurities whose sequences closely resemble the target. If such an impurity was not separated during preparative purification, it may co-elute with the main peak in the quality-control run as well. The chromatogram then shows a single clean peak that contains more than one species. To address this, the group developed a two-dimensional method. The first dimension used the common acidic gradient. Selected fractions then passed to a second separation run under basic pH, followed by a diode array detector, a charged aerosol detector and a quadrupole time-of-flight mass spectrometer.
A related problem arises with buffers. Xu, Du, Lv and colleagues, in the Journal of Separation Science, examined dalbavancin hydrochloride, a semi-synthetic lipoglycopeptide. Its pharmacopoeial impurity method relies on non-volatile phosphate buffers that are incompatible with mass spectrometry. Their heart-cutting two-dimensional system separated the related substances under phosphate conditions, then transferred target fractions online to mass-spectrometry-compatible conditions. By accurate mass and fragmentation patterns they identified twenty-two components, five major components and seventeen impurities, and confirmed them against reference standards where these were available.
Neither paper concerns a research-supply catalogue. Both make the same practical point, however: a reported purity figure means no more than the method's ability to separate what it is measuring.
The impurities that synthesis and storage produce
Huo, Xu, Lu and colleagues, in Analytical and Bioanalytical Chemistry in 2022, profiled Cbf-14, a designed antimicrobial peptide derived from the cathelicidin family. They used liquid chromatography with quadrupole time-of-flight tandem mass spectrometry, combined with stress testing. The group identified one process-related impurity and thirty-two degradation products, and synthesised seven of them to confirm their structures. The degradation products fell into five classes: hydrolysates, isomers of the parent peptide, acetylated isomers, aldimine derivatives and oxidised forms. The authors noted that impurities of this kind have not been declared for custom synthetic peptides.
Roberts, Mattei, Howard and colleagues, in Frontiers in Pharmacology in 2024, listed the families of impurity that solid- and liquid-phase synthesis can generate: amino acid insertions, deletions and side-chain modifications. Working on salmon calcitonin, they applied in silico and in vitro methods to assess whether such impurities alter the immunogenicity risk profile of a synthetic generic product.
Modification can also occur after manufacture. Cao, Mo, Shannon and colleagues, in the PDA Journal of Pharmaceutical Science and Technology, described an LC/MS approach for identifying and locating product-related impurities that arise from isomerisation, deamidation and oxidation. Their example was aspartate isomerisation in a monoclonal antibody. The group then developed a UV-based assay for routine quantitation, using synthetic peptides and mass spectrometry to assist method development. Their subject was a protein rather than a short peptide, but the chemistry of these modifications is the same.
What analysts found in vials bought online
The Breindahl group bought melanotan II, sold in vials as a skin-tanning product, from three online shops and applied its validated methods to them. Each shop claimed the same content per vial. The measured content ranged from roughly 43 to 88 percent of the claimed figure. Vials from two shops contained unknown impurities at 4.1 to 5.9 percent. Impurities in vials from the third shop were below the quantification limit.
The sample is small: three vendors and one compound. The study does not establish how often mislabelling occurs across the wider market, and the source pool for this article contains no comparable survey of other peptides. It does show directly that a vial's label and its contents can diverge on both quantity and purity, and that the methods to detect this exist.
The authors also placed their findings in context. They noted that people obtaining such products from the illicit market may be exposed to a range of potential harms, because the products are manufactured outside regulated conditions. The human record for melanotan II itself is a separate matter, covered in the-melanocortin-system. That record is thin, and this analytical study adds nothing to it beyond the composition of the vials tested.
What to check on a certificate, and what Skinni provides
Read together, the studies above suggest four questions to ask of any certificate of analysis. Was identity confirmed by mass spectrometry, ideally with fragment-ion data, as Breindahl and colleagues did, rather than by retention time alone? Which chromatographic method produced the purity figure? Karongo and colleagues showed that a single reversed-phase run can hide closely related impurities. Were impurities profiled and characterised, given the range of degradation products Huo and colleagues found in a single peptide? And was measured content compared with labelled content, the measurement on which the online melanotan II vials diverged most?
On its own documentation, Skinni states the following. Each compound in the catalogue is manufactured for laboratory research use and documented by CAS number, molecular formula and molecular weight. Compound pages publish identity, purity method, appearance, solubility, storage and handling. Material ships with lot traceability. Lot-specific certificates of analysis and safety data sheets are sent by email to verified accounts on request. Raw actives come principally from established synthesis partners in Asia. Compounding, filling, labelling and lot assignment take place in the United States.
Skinni's site facts describe no independent third-party certification beyond these documents. For any lot, the practical step is to request the lot-specific certificate through the research desk and read it against the four questions above. All compounds Skinni supplies are for laboratory research use only and are not for human or veterinary use.
References
- Karongo R, Ikegami T, Stoll DR et al.. A selective comprehensive reversed-phase×reversed-phase 2D-liquid chromatography approach with multiple complementary detectors as advanced generic method for the quality control of synthetic and therapeutic peptides. Journal of chromatography. A. 2020;1627:461430.
- Breindahl T, Evans-Brown M, Hindersson P et al.. Identification and characterization by LC-UV-MS/MS of melanotan II skin-tanning products sold illegally on the Internet. Drug testing and analysis. 2015;7:164-72.
- Xu H, Du X, Lv J et al.. Heart‑Cutting Two‑Dimensional Liquid Chromatography/High‑Resolution Mass Spectrometry and Complementary High-Performance Liquid Chromatography Methods for Comprehensive Impurity Profiling and Quality Control of Dalbavancin Hydrochloride. Journal of separation science. 2026;49:e70545.
- Huo Y, Xu K, Lu Y et al.. Characterization of structurally related peptide impurities using HPLC-QTOF-MS/MS: application to Cbf-14, a novel antimicrobial peptide. Analytical and bioanalytical chemistry. 2022;414:6485-6495.
- Roberts BJ, Mattei AE, Howard KE et al.. Assessing the immunogenicity risk of salmon calcitonin peptide impurities using in silico and in vitro methods. Frontiers in pharmacology. 2024;15:1363139.
- Cao M, Mo WD, Shannon A et al.. Qualification of a Quantitative Method for Monitoring Aspartate Isomerization of a Monoclonal Antibody by Focused Peptide Mapping. PDA journal of pharmaceutical science and technology. 2016;70:490-507.
- Lee SL, Yu LX, Cai B et al.. Scientific considerations for generic synthetic salmon calcitonin nasal spray products. The AAPS journal. 2011;13:14-9.
What this article is, and is not
This is a summary of published research, written for qualified professionals evaluating compounds for laboratory work. Every compound discussed is supplied by strictly for in-vitro and laboratory research use. None is a drug, a dietary supplement, or a cosmetic; none is intended for human or veterinary use; and nothing above is medical advice, a treatment recommendation, or a claim that any compound produces any effect in a person. We publish no dosing or administration guidance of any kind.
