Peptide purity, as reported on most Certificates of Analysis, is an HPLC area percent: the main peak’s UV signal divided by the signal of all detected peaks, usually read at 210 to 220 nm. It measures the intended sequence relative to peptide-related impurities, not the share of the powder that is peptide. Water and counterions are measured by other tests, so vetting research peptides properly means reading HPLC purity, mass spectrometry identity, net peptide content and, where relevant, endotoxin results together.
These checks apply to any supplier’s research peptide catalog, and they are the practical basis for deciding where to buy research peptides.
Reversed-phase HPLC separates a peptide from related impurities before a UV detector measures each peak. Bachem’s quality-control guide describes the usual purity figure as UV detection at 210 to 220 nm, with the area of the main peak taken relative to the total area of all peaks (Bachem). USP’s peptide reference-standard work monitored peptides and impurities at 220 nm (McCarthy et al. 2023).
Three limits follow from that definition.
USP scientists showed how the two numbers combine when assigning value to peptide reference standards. Their worked example has HPLC impurities totalling 0.92% of detected area and acetic acid at 5.58% w/w, with trace TFA, residual solvents and residue on ignition. The resulting purity is 0.93 mg of peptide (free base) per mg of material on an anhydrous basis, and each lab then corrects that figure further for its measured water content (McCarthy et al. 2023). In that theoretical example, material about 99% pure by HPLC works out to roughly 93% peptide by weight before water is even counted.
Net peptide content (NPC) is the percentage by weight of the powder that is peptide (target plus peptidic impurities); the rest is mostly counterions and water. Because NPC includes peptidic impurities, it is not the same as purity. Bachem notes that a low NPC should be expected for sequences with many basic amino acids, “even if they are extremely pure”, because of salt formation, and for hydrophilic peptides that absorb moisture (Bachem).
Bachem measures NPC from nitrogen content by elemental analysis; amino acid analysis is the other common route. Water is measured by Karl Fischer titration, and acetate or TFA by HPLC or ion chromatography.
In standard SPPS, cleavage from the resin and HPLC purification both use trifluoroacetic acid, so cationic peptides are obtained as trifluoroacetate salts. A 2020 review in Pharmaceuticals summarizes reports that residual TFA can interfere with physicochemical characterization and affect experiments on mammalian cells, sometimes inhibiting cell proliferation and in other cases stimulating growth, as reported for glioma cells (Sikora et al. 2020).
The same review notes that exchanging TFA for another counterion is common, that the usual HCl repeat-lyophilization approach does not remove TFA completely, and that reported efficiency reaches up to 98% after several repetitions. For cell-based work, the counterion listed on a CoA is worth checking before a peptide is weighed into an assay.
| Component of the vial | How it is measured | Shows up in HPLC purity? |
|---|---|---|
| Target peptide | HPLC main peak; AAA or nitrogen for amount | Yes, as the main peak |
| Peptide-related impurities (deletions, diastereomers, adducts) | HPLC side peaks; LC-MS for identity | Yes, unless they co-elute |
| Counterion (acetate, TFA, chloride) | HPLC or ion chromatography | No |
| Water | Karl Fischer titration | No |
| Residual solvents | Residual solvent test (USP <467>) | No |
| Endotoxin | LAL (gel-clot or photometric techniques) | No |
Mass spectrometry measures mass-to-charge ratio and confirms that the main component has the expected molecular mass. Changes to the primary sequence from truncation, insertion, substitution or deletion typically produce detectable mass shifts (Zeng et al. 2015). High-resolution instruments go further. In that FDA study, experimental monoisotopic masses for salmon calcitonin and its impurities matched theoretical values within 10 ppm, and deamidated species were identified by their +0.984 Da shift.
MS has blind spots. Racemized peptides have the same mass as the intended sequence, so they are “indistinguishable by mass spectrometry” (Adams et al. 2017). Leucine and isoleucine are isobaric too. The USP reference-standard program pairs LC-MS/MS for mass and sequence with separate chirality testing and amino acid analysis for exactly these cases (McCarthy et al. 2023).
Amino acid analysis (AAA) hydrolyzes the peptide in strong acid, then separates and quantifies the free amino acids. USP scientists describe it as a way to determine “the amino acid composition or content of peptides” (McCarthy et al. 2023). It does not provide sequence information (Bachem). USP authors describe AAA as gradually replaced by HPLC and MS for routine identity, yet still useful when leucine and isoleucine must be distinguished. They used it to confirm the leucine and isoleucine ratio in bivalirudin, a 20-residue peptide containing one of each (McCarthy et al. 2023).
Endotoxin (bacterial lipopolysaccharide) is not detected by HPLC, MS or AAA. FDA has accepted the Limulus amebocyte lysate (LAL) test in place of the rabbit pyrogen test for more than 30 years (FDA 2026), and its guidance builds on USP chapter <85> “Bacterial Endotoxins Test”. That chapter is harmonized with the European and Japanese pharmacopoeias and allows gel-clot and photometric (turbidimetric or chromogenic) techniques (Franco et al. 2018). FDA’s 2026 Edition 2 Q&A allows validated alternative assays but names the compendial gel-clot method as the reference if results conflict (FDA 2026).
For purely chemical or structural work, endotoxin is usually less relevant. For cell-based immune assays and animal studies it can decide the result. Schwarz et al. found that primary human CD1c+ dendritic cells were activated by lipopolysaccharide at levels equivalent to the contamination they measured in some commercial recombinant proteins, and concluded that “even low endotoxin contamination may generate erroneous data” with LPS-sensitive cells (Schwarz et al. 2014). If a study involves immune readouts, an endotoxin result in EU per mg, with the method named, belongs on the CoA or should be generated in-house.
A structured review of peptide-related impurities groups them by origin (D’Hondt et al. 2014):
Minor impurities can change an experiment. In synthetic amyloid-β42, a single-residue deletion (Aβ42Δ39, missing valine 39) was identified as the main contaminant limiting both cytotoxicity and fibrillation of the peptide (Adams et al. 2017). Every coupling cycle is another chance for this kind of error, so identity and impurity data for any research peptide, retatrutide included, should come from the specific lot rather than a generic reference sheet.
FDA’s May 2021 guidance on ANDAs for certain highly purified synthetic peptides (glucagon, liraglutide, nesiritide, teriparatide and teduglutide) recommended sensitive, high-resolution methods such as UHPLC-HRMS, identification of each peptide-related impurity at 0.10% of the drug substance or greater, and no new specified peptide-related impurity above 0.5% (FDA 2021). The concern it names is immunogenicity from sequences related to, but different from, the active peptide.
On July 28, 2026, FDA withdrew that guidance because it “no longer reflects FDA’s current scientific thinking” and issued revised draft product-specific guidances that address impurity thresholds (FDA 2026). The 0.10% and 0.5% figures remain a useful picture of how closely drug-grade peptides are characterized, but they are no longer current FDA guidance, and they applied only to generic drug applications.
CoA layouts vary between suppliers. The table below lists the fields most often included, what each one tells a researcher, and what a complete entry looks like.
| CoA field | What it tells you | What to check |
|---|---|---|
| Product name and sequence | Which molecule was tested | Full sequence, including modifications such as C-terminal amide or acetylation |
| Lot or batch number | Traceability to one synthesis | Matches the number on the vial label |
| Molecular formula and theoretical mass | The expected value for MS | Consistent with the sequence and salt form stated |
| Appearance | Physical form, e.g. white lyophilized powder | Agrees with what arrived |
| Identity by MS | Observed molecular mass | A numeric observed mass beside the theoretical one, plus the spectrum |
| Purity by HPLC | Main peak area as % of total | Wavelength, column and gradient stated; chromatogram attached with integrated peaks |
| Net peptide content | Peptide share of the powder by weight | Method named (AAA or nitrogen); may not be reported on research CoAs |
| Counterion | Salt form (acetate, TFA, chloride) | Identity and ideally % w/w |
| Water content | Residual moisture | Karl Fischer result in % w/w |
| Endotoxin | Bacterial LPS level | Numeric EU per mg and LAL method, if claimed at all |
| Test date, lab and signature | Who ran the analysis and when | Date after synthesis; named laboratory; responsible analyst or QA sign-off |
| Storage conditions | Recommended handling | Temperature and protection from moisture and light |
Good manufacturing practice is a quality system, not a purity number. FDA’s ICH Q7 guidance for active pharmaceutical ingredients exists to help ensure that APIs “meet the quality and purity characteristics that they purport, or are represented, to possess” (FDA ICH Q7). The guidance pursues that goal within an appropriate quality management system.
The cross-contamination with unrelated peptides reported by D’Hondt et al. is the kind of failure those controls are meant to prevent. Research-grade peptides are not necessarily made under GMP, which is why lot-specific analytical data carries more weight when evaluating them. The trade-offs are covered in more detail in the GMP and non-GMP peptide supplier guide.
A vial of BPC-157 deserves the same lot-level documentation as any longer or modified peptide.
No. HPLC purity compares UV peak areas of peptidic species. Peptide content is a weight measure that also accounts for counterions, water and solvents. In the USP worked example above, material with under 1% HPLC impurities came to about 0.93 mg peptide per mg on an anhydrous basis. This mass-balance figure counts only the target peptide, so it sits slightly below the net peptide content, which also includes peptidic impurities.
It depends on the experiment. Endotoxin is usually less relevant for chemical and structural studies. For cell-based immune assays it can matter, because low LPS levels have been shown to activate sensitive immune cells.
TFA is used in standard SPPS cleavage and HPLC purification, so the TFA salt is the default product. Exchange to acetate or chloride adds processing steps. The Sikora review notes that TFA is often exchanged for chloride because of chloride’s higher biocompatibility.
At Peptides For Sale, each batch is tested by HPLC and mass spectrometry, with a Certificate of Analysis per batch, so the HPLC and MS checks above can be applied to the lot received. Browse the full catalog of research peptides.
Research use only: This article is for educational and informational purposes. All products referenced are sold strictly for in vitro laboratory research and are not intended for human or veterinary use, diagnosis, or treatment. Nothing here is medical advice. Research should be conducted by qualified professionals in compliance with applicable laws, regulations and institutional policies.
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