Reversed-phase high-performance liquid chromatography is the default purity method for synthetic peptides, and the number it produces — "99.2%" — is the number most often quoted as proof of quality.
It is a real measurement. It is also narrower than most people reading it assume. Understanding the boundary is what lets you tell a strong certificate from a weak one.
How the separation works
A liquid mobile phase carries the dissolved sample through a column packed with a hydrophobic stationary phase, usually C18 — silica particles with eighteen-carbon chains bonded to the surface.
Components partition between the two phases according to hydrophobicity. More polar components spend more time in the mobile phase and exit early; more hydrophobic components are retained longer. A gradient — typically increasing acetonitrile against water, with an acid modifier — steadily raises the mobile phase's eluting strength so that even strongly retained material comes off in reasonable time.
A detector at the column outlet records absorbance as material passes. For peptides, 214 nm is standard, because the amide bond of the peptide backbone absorbs there. 280 nm is sometimes used instead, but it detects only aromatic side chains — tryptophan, tyrosine, phenylalanine — and a peptide lacking those is nearly invisible at that wavelength.
Reading the chromatogram
- a
- solvent front — unretained material, not the compound
- b
- baseline noise — a real detector is never silent
- c
- main peak — its share of total area is the purity figure
- d
- shoulder — a closely related impurity that did not fully resolve
- e
- tailing — real peaks tail; perfect symmetry is a tell
The x-axis is retention time; the y-axis is absorbance. Each peak is material leaving the column at a particular time.
Purity is calculated as the area of the main peak divided by the total area of all peaks, expressed as a percentage. This is the single most important thing to understand about the number: it is a relative measure, not an absolute one.
Peak shape carries information too. Real peaks show mild tailing. A shoulder on a main peak often indicates a closely related impurity that did not fully resolve. Baseline noise is normal and its absence is suspicious.
What the number cannot tell you
It does not confirm identity. A chromatogram shows that one component dominates. It does not establish that the component is the intended molecule. Retention time is suggestive when compared against a reference standard run under identical conditions, but it is not proof — different compounds can co-elute. Identity requires mass spectrometry.
It cannot see what does not absorb. The detector reports UV absorbance at one wavelength. Material without a chromophore at that wavelength contributes nothing to total peak area — and therefore cannot reduce the purity percentage, no matter how much of it is present. Inorganic salts, many counterions, and residual solvents fall into this category. A sample can be substantially salt by mass and still return a high HPLC purity figure, because the salt was never counted.
It may not resolve close impurities. The impurities SPPS generates most often — deletion sequences missing one residue, or diastereomers where a single residue racemised — are structurally very similar to the target. Under a given gradient some will co-elute with the main peak and be integrated into it.
It says nothing about several things that matter separately:
| Question | Method that answers it |
|---|---|
| Is this the right molecule? | Mass spectrometry |
| How much water is in it? | Karl Fischer titration |
| How much peptide is actually in the vial? | Gravimetric analysis, amino acid analysis |
| Residual solvents? | Gas chromatography |
| Endotoxin? Sterility? | Separate microbiological assays |
None of these is covered by a purity chromatogram, and a certificate reporting only HPLC is silent on all of them.
Why net peptide content diverges from purity
A lyophilized peptide is not pure peptide by mass. It typically also contains counterions from synthesis and purification (commonly trifluoroacetate from TFA-based mobile phases), plus residual water.
HPLC purity and net peptide content are therefore different figures, and the gap between them can be substantial. A material reported at 99% purity by HPLC may be materially less than 99% peptide by weight — the two numbers answer different questions, and only one of them is usually quoted.
What a strong certificate looks like
- HPLC purity with its method: column, gradient, and detection wavelength stated
- Mass spectrometry confirming identity against theoretical mass
- The chromatogram itself, not just a transcribed number
- Explicit acknowledgement of what was not tested
A certificate that reports "99%" with none of this is not necessarily wrong. It is simply unverifiable, which for most purposes amounts to the same thing.
For research use only · Not for human or veterinary use · No compound discussed here is FDA-approved for any indication