What the purity percentage on a COA means
The purity percentage on a certificate of analysis (COA) comes from a high-performance liquid chromatography (HPLC) chromatogram. It is calculated as the area of the main peak belonging to the target peptide relative to the total area of all detected peaks: (main peak area / sum of all peak areas) × 100. Reverse-phase HPLC separates molecules by hydrophobicity, and detection is usually by UV at roughly 214–220 nm, where the peptide bond absorbs. It is important to understand that this is a relative measure among UV-absorbing species — not the absolute peptide content of the sample. So "99%" tells you what fraction of the UV signal belongs to the target, not necessarily what fraction of the mass in the vial is actually peptide.
Why ≥98–99% is the research standard
For reliable research data, ≥98% is usually cited as the minimum, and ≥99% HPLC purity is often required. The reason is not cosmetic: at lower values (e.g. 95%), the remaining fraction consists of synthesis by-products that can interfere with sensitive biological assays and signalling pathways. Every percent of impurity is another variable the researcher does not control. Because the impurity profile can differ between batches, a peptide that behaved a certain way in a pilot experiment may behave differently once re-synthesised — a common and frustrating source of irreproducibility. A higher declared purity reduces this variability and makes comparison easier, both batch-to-batch and lab-to-lab.
What impurities hide in the remainder
The gap to 100% is not empty space — it is made of specific substances. Synthesis impurities include deletion and truncated sequences that are missing one or more amino-acid residues due to incomplete coupling, plus oxidized forms (most often at methionine, tryptophan or cysteine) and deamidated products. Beyond peptide impurities there is non-peptide mass: residual counter-ions (TFA or acetate from purification) and bound water. This fraction often does not absorb UV and therefore stays invisible on HPLC. For synthetic glucagon, for example, LC-UV indicated purity above 970 mg/g, while a mass-balance approach revealed an actual content of 896 mg/g — alongside 103 mg/g TFA and 50 mg/g water. Declared HPLC purity and actual peptide mass content are therefore not the same thing.
Why even small impurities skew results
An impurity is not necessarily inert. Active contaminants can compete for the same binding sites, add background signal, or trigger a toxic response — and thereby distort an assay. In sensitive in vitro systems, as little as ~1% impurity can produce a false-positive or false-negative outcome. A clear example is residual TFA counter-ions, which in cell experiments have been shown to either promote or inhibit cell proliferation and to increase toxicity. The consequences are measurable: estimates suggest roughly half of US preclinical experiments are irreproducible (about $28 billion per year), with 36% ($10.4 billion) attributed to poor-quality biological reagents and reference materials. Purity is therefore not paperwork but a precondition for interpretable data.
Purity and identity: why HPLC needs mass spectrometry
Purity tells you how clean the sample is; identity tells you whether the right molecule is actually in it — and both are required. HPLC-UV is only semi-quantitative for the peptide bond, and oxidized forms, for instance, can co-elute with the main peak and stay invisible to UV while mass spectrometry (MS) detects them. Identity is therefore confirmed by MS: the measured mass (often as [M+H]+) must match the theoretical mass calculated from the sequence. Liquid chromatography with high-resolution MS detects impurities below 0.1% while simultaneously confirming peptide composition. Modern reagent-quality guidelines list precisely identity (MS), purity and homogeneity as core parameters. A credible COA therefore combines an HPLC purity figure with MS identity confirmation.
For research use only
This article is educational and analytical in nature. All peptides are intended exclusively for research purposes (Research Use Only) — they are not for human or veterinary use and are not intended for diagnosis, prevention or treatment. Nothing here constitutes medical advice or dosing instructions. Peptide quality can only be verified through documentation: peptid.si provides laboratory-verified purity and a publicly accessible certificate of analysis (COA) for every batch. Before you use any figures in your research, check the HPLC purity and MS identity confirmation for the specific batch in the COA Vault — the number on the label is only meaningful if a document supports it.
References / Links
- Erckes V, Streuli A, Chamera Rendueles L, et al. (2025). Towards a Consensus for the Analysis and Exchange of TFA as a Counterion in Synthetic Peptides and Its Influence on Membrane Permeation. Pharmaceuticals (Basel). MDPI
- Wang X, Zhang F, Li H, Xiao P (2020). Purity determination of synthetic glucagon using a mass balance approach. Scientific Reports. Nature
- de Marco A, Berrow N, Lebendiker M, et al. (2021). Quality control of protein reagents for the improvement of research data reproducibility. Nature Communications. Nature
- Zeng K, Geerlof-Vidavisky I, Gucinski A, et al. (2015). Liquid Chromatography-High Resolution Mass Spectrometry for Peptide Drug Quality Control. The AAPS Journal. SpringerLink
