The specifics, since they change the answer: 97.6% · PeptideMeter · 95.2% · Janoshik.
I am trying to choose between two options that are usually discussed as though only one exists.
I am not optimising for price, but I am not indifferent to it either.
Which axes does this decision turn on?
97.6 and 95.2 are 2.4 percentage points apart, which sounds small until you restate it as impurity. PeptideMeter is leaving 2.4 per cent of the detected area unassigned and Janoshik 4.8 per cent — a factor of 2 between them. The impurity fraction is the quantity that moves when a method changes, and it is the one worth arguing about; the headline is just its complement. Comparable means same column chemistry, same gradient slope, same detection wavelength, same integration convention. Until you have those four from both laboratories, 97.6 and 95.2 are two measurements of slightly different quantities that happen to share a unit.
Purity is a method-dependent figure, and that is not a limitation of the measurement, it is a property of what the measurement actually answers.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
| 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 |
Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemUnderstanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
Temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.
Concretely, 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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
The short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.
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.
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.
Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.
The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.
Compare purity within a single laboratory on the same method, never across laboratories.
Identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
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.
edited 1 Nov 2024 by tobias_maartens — expanded the table to cover the lower concentration
Mechanically, area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.
Column pore size affects mass transfer — a 100 Angstrom packing on a 5 kDa peptide restricts diffusion, broadening the peak and potentially hiding small impurities in the shoulders.
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.
In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.