Why do two laboratories report different purity for the same vial?

The lot number on the vial matches the certificate, which at least rules out the easy problem.

I keep seeing this stated as a fact with no explanation attached, and unexplained facts make me suspicious.

My background is quantitative but not chemical, so I can follow an equation more easily than a hand-wave.

Is the standard explanation correct, and if so, what is the evidence for it?

In practice, peak shape carries as much information as peak area does, and a badly tailing peak or a shouldered peak is telling you something about the sample or the column that matters.

System suitability checks on replicate injections of a known standard establish whether the method was in control — if the peak area varies by more than two per cent between replicates, something is wrong.

The 214 nanometre wavelength is chosen because it corresponds to the amide backbone absorption, making response roughly proportional to the number of peptide bonds.

Inter-laboratory studies using identical methods on identical material show precision well within half a per cent when the method is locked down, pointing to method variability as the primary source of disagreement.

If two labs give different numbers, the method difference is the first hypothesis, not lab quality.

Start by understanding what the detector is measuring and what that means about how the molecule needs to be prepared and handled before injection.

Reverse-phase stationary phases use C18 or C8 chains bonded to silica, and the pore size of the silica matters more for a peptide of this chain length than the ligand length does.

The relevant detail is that proline-rich sequences are particularly problematic because the isomerisation kinetics are in the same timescale as the separation, leading to split or broadened peaks at low temperature.

Peptide impurities from solid-phase synthesis include deletion sequences, truncations from premature cleavage, racemised residues from epimerisation and oxidised variants, each of which may have different chromatographic behaviour.

One qualification: the limit of detection on a reversed-phase HPLC is set by the noise on the baseline, which for these molecules is usually in the range of a tenth of one per cent or less, and anything smaller is not reproducibly detectable.

Ask for the chromatogram and the system suitability data, not just the number.

edited 5 Feb 2026 by s_kalniete — removed a claim I could not source

To be exact about it, the limit of this technique for these molecules is reaching a point where small improvements require proportionally large investments in method development.

Trifluoroacetic acid at 0.1 per cent is the standard ion-pairing agent because it suppresses tailing by neutralising the basic residues, but it also suppresses mass spectrometry signal.

Temperature affects both the viscosity of the mobile phase and the dynamics of molecular interactions, and a method developed at 25 degrees and run at 40 degrees will not behave identically.

The caveat is that HPLC is a purity technique and says almost nothing about whether the main peak is actually your target compound — that is why identity confirmation from mass spectrometry or peptide mapping matters.

The practical summary: specify the method, run the same method on every sample you compare, and use orthogonal techniques to confirm the result.

Put another way, high-performance liquid chromatography is a separation technique, not an identification technique, and it answers the question "how much of the detected signal is a single species" not "which species is it."

Wider-pore phases — 300 Angstrom rather than 100 Angstrom — have faster mass transfer and narrower peaks for peptides above three kilodaltons, which is almost every peptide you will see.

The selectivity of a reverse-phase separation depends on the hydrophobicity of the side-chain profile, which is why two peptides can co-elute even if they differ by a residue.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.