# HPLC Purity Figures Decoded: What a Peptide Certificate Is Really Claiming > A purity percentage has a narrow, precise meaning. Knowing its limits tells you when 98% is plenty and when the chromatogram matters more than the number. Web page: https://peptidemedixeu.com/en/learn/hplc-purity-explained/ · Peptide Medix EU · 3 min read · Updated: 2026-08-09 Expressed as the main peak's area divided by the total area of every peak in a [chromatogram](https://peptidemedixeu.com/en/blog/read-peptide-coa-like-chemist/), HPLC purity states that "≥99% by HPLC" leaves related impurities contributing under 1% of what the detector registered. It is the usual purity measure for [research peptides](https://peptidemedixeu.com/en/learn/research-catalog-type-peptides/) because reversed-phase HPLC pulls a target sequence away from its nearest chemical relatives, such as deletion sequences, oxidised variants and truncations, more effectively than any other routine technique. The figure has a tightly defined scope, though, and nearly all misuse of it comes from stretching that definition. Below we cover how the measurement is made, what separates ≥98% from ≥99% in practice, and what a purity number deliberately declines to claim. ## The measurement itself Reversed-phase HPLC pumps the dissolved peptide through a column packed with hydrophobic particles, usually C18, while the mobile phase moves progressively from water towards acetonitrile. Each species leaves the packing once the solvent is strong enough to displace it, with more hydrophobic species leaving later, and passes a UV detector typically set around 214-220 nm where the backbone absorbs. What emerges is a plot of signal against time. Software integrates the area of every peak, and [purity equals main-peak](https://peptidemedixeu.com/en/glossary/purity/) area divided by total peak area, times 100. Since retention depends on the column, the gradient, temperature and mobile-phase additives, purity figures compare strictly only within a single method, which is why a proper certificate publishes its conditions. ## Where the impurities come from Solid-phase synthesis adds one residue at a time, and no coupling step is perfectly efficient. The typical by-products are deletion sequences missing one [residue](https://peptidemedixeu.com/en/learn/peptide-sequences-explained/), truncated chains where synthesis stopped early, species that were not fully deprotected, and degradation products formed afterwards such as [oxidised methionine](https://peptidemedixeu.com/en/glossary/oxidation-methionine/) (+16 Da) or deamidated variants. These relatives differ from the target by only slight changes in hydrophobicity, precisely the property reversed-phase HPLC resolves, which is why it serves as the purity workhorse. Longer chains offer more chances to go wrong: keeping a 31-residue molecule such as semaglutide at ≥99% is a considerably tougher synthetic proposition than a pentadecapeptide like BPC-157, and tougher again than a tripeptide like GHK-Cu. ## What the step from ≥98% to ≥99% actually buys Moving from 98% to 99% looks like a single percentage point but halves the permitted impurity load, from 2% down to 1%. Whether that counts depends on the question being asked: in a binding assay where a deletion sequence might act as a partial agonist, the impurity allowance sets the experiment's noise floor, whereas in a coarse rodent phenotype it may be immaterial. Vials in this catalogue are specified at ≥99% by HPLC with a lot-matched certificate, covering both [lyophilized peptide vials](https://peptidemedixeu.com/en/collections/injectable-vials/) and the raw material used in finished formats. ## What the number leaves out - Identity. A single tidy peak proves homogeneity rather than correctness, since the wrong sequence can equally be 99% pure. Establishing identity is mass spectrometry's job, which is why the two certificate sections must be read together. - Net peptide content. Purity describes a share of the peptide fraction, so water, counter-ion and residual solvent in the lyophilized powder fall outside the calculation entirely, letting 99% purity sit alongside perhaps 80% peptide by gross mass. - Anything invisible to the detector. Inorganic salts and any substance that neither absorbs at the chosen wavelength nor elutes within the window simply never appears as a peak. Endotoxin likewise requires its own assay rather than emerging from HPLC. - Aggregation and biological activity. A chemically pure peptide may still be aggregated or misfolded, and activity is demonstrated in the assay, never in the chromatogram. ## Reviewing a chromatogram critically 1. Look for one dominant, symmetrical main peak, since shoulders or splitting hint at a co-eluting relative that integration may be concealing. 2. Check the baseline is flat, because drift or humps distort the area ratios in either direction. 3. See that small peaks are shown and integrated rather than cropped away, as a truncated time axis is a classic way of flattering a figure. 4. Confirm the method is stated: column, gradient, wavelength and injection mass. Without a method there is nothing to compare. 5. Verify the trace belongs to your lot and matches the vial, and remember it can be checked independently, as described in third-party testing explained. ## Putting the figure to work Purity informs three practical choices. Sourcing comes first: comparing suppliers by purity only makes sense where methods are comparable and lots verifiable, and the fuller diligence list is in the vendor checklist. Quantification comes second: for molarity-critical work, pair purity with net peptide content using molecular weight, moles and [molarity before committing](https://peptidemedixeu.com/en/glossary/molarity/) anything to a calculation. Stability tracking comes third, since purity is also the property that decays; re-running a chromatogram after months in storage is how that loss gets quantified, which makes storage discipline under [how to store peptides](https://peptidemedixeu.com/en/learn/how-to-store-peptides/) an HPLC-visible variable in the end. The number is a snapshot taken at the moment of analysis, and handling determines how long it remains accurate. ## Frequently asked questions ### What is 99% HPLC purity really saying? That when the material was run on a reversed-phase column with UV detection, the main peak made up at least 99% of the total integrated area, leaving related impurities such as deletion sequences, truncations and oxidation products under 1% combined. It compares detector areas inside the peptide fraction and says nothing about the vial's gross contents. ### Does 99% beat 98% in any meaningful way? In terms of impurity allowance, yes: 98% permits up to 2% of related impurities while 99% permits up to 1%, a halving. Whether it matters depends on the assay, with quantitative receptor and binding work benefiting most because near-identical impurities may retain partial activity. For coarse phenotypic models the difference is often slight, though it also reflects overall synthesis and purification control. ### Could a 99% pure peptide still be the wrong molecule? It could. HPLC purity quantifies homogeneity, meaning how much of the material is a single species, not identity. An incorrect, truncated or differently modified sequence can give an equally clean single peak, which is why a trustworthy certificate combines HPLC purity with mass spectrometry, where observed mass against theoretical mass provides the identity check. ### Why do laboratories report different purities for one lot? Because the figure depends on the method. Columns, gradients, temperatures and detection wavelengths resolve impurities differently, so a shoulder that separates under a shallow gradient may merge into the main peak under a steep one. Minor gaps between in-house and third-party numbers are expected; wide discrepancies with matching lot numbers deserve investigation. ### Are water, salts and counter-ions counted in HPLC purity? They are not. Bound water, residual solvent and counter-ions such as TFA or acetate contribute to the gross lyophilized mass yet produce no peptide peaks in the chromatogram. This is the purity against net-peptide-content distinction: a vial may be 99% pure while only around 80% of the labelled milligrams are peptide. Use net content for molar calculations whenever the certificate reports it. ### Why detect peptides at 214-220 nm? Because the peptide bond absorbs strongly in that far-UV range, making every peptide species visible whatever its sequence, unlike detection at 280 nm which relies on tryptophan and tyrosine content. Detecting at 214-220 nm keeps peak areas roughly proportional to the quantity of backbone present, which is exactly what a purity ratio requires. --- For laboratory research use only. This page is provided for scientific and educational information. Materials referenced here are sold strictly for in-vitro laboratory research by qualified professionals — not for human or veterinary use, and nothing on this page is medical advice.