For reference: 97.6% · PeptideMeter · 95.2%.
An unexpected observation, and I would like a differential rather than reassurance.
The conditions were within what I understood to be the acceptable range, which is why I am asking.
What would you check first, and what would you conclude from each outcome?
The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
| Δ mass (Da) | Most likely cause | Distinguishing feature |
|---|---|---|
| +1 | Deamidation (Asn or Gln) | New peak, slightly earlier retention |
| −17 | Loss of ammonia | Often with deamidation |
| −18 | Dehydration / succinimide | pH-dependent, reversible |
| +16 | Oxidation (Met, Trp) | Earlier retention, light-related |
| −128 | Missing Gln or Lys | Deletion sequence from synthesis |
| 0 | Isomer: racemisation or scrambling | Same mass, shifted retention |
More usefully, sample solvent strength affects peak shape — if you inject in strong solvent on a gradient starting in weak solvent, the solvent peak can distort your main peak or create a false shoulder.
Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.
If you are ranking vendors, specify a method and have all samples tested at the same place.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsStart from what the detector sees, because that tells you what the number means.
Tailing factor measures peak shape, and a badly tailing peak spreads into the region where small impurities live, forcing tangent-skim integration that assigns tail area to the main peak.
Integration of the shoulder — whether you use perpendicular drop or tangent skim — determines what area gets assigned to the main peak versus the impurity table.
Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.
Compare purity within a single laboratory on the same method, never across laboratories.
Read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.
Detection wavelength matters because 214 nm sees the peptide backbone while 280 nm sees only aromatic side chains — so truncation impurities lacking a tryptophan are invisible at 280 nm.
Buffer versus acid in the mobile phase changes the ionisation state of basic and acidic residues, shifting retention and selectivity — same vial, potentially different separation.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
More usefully, area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.
One qualification: achieving purity above roughly 98 per cent on a 30-residue peptide is fighting the chemistry of synthesis, not the quality of the purification.
If you are ranking vendors, specify a method and have all samples tested at the same place.
It helps to be literal here: identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
Retention time is sequence-specific and method-specific, so comparing your result to a supplier value using a different method is meaningless without method documentation.
Compare purity within a single laboratory on the same method, never across laboratories.
edited 22 Oct 2024 by tare_and_weigh — reworded for clarity after a comment
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