What I have: 98.2% · Janoshik · 97.9% · Janoshik.
I have used one of these for a while and I am considering switching, which requires a reason.
What I care about is reproducibility, because a result I cannot repeat is not useful to me.
So which one, and on what grounds?
Worth being precise here: 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.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
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.
Inter-laboratory studies on identical peptide material routinely find half-a-per-cent to a full-per-cent spreads in reported purity on the same sample.
If you are ranking vendors, specify a method and have all samples tested at the same place.
edited 2 Jan 2026 by mala_venkatesh — updated for the 2026 guidance change
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsThe relevant detail is that understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
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 fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
Compare purity within a single laboratory on the same method, never across laboratories.
The underlying point is that the single most important distinction is between what purity measures — the fraction of detected material that is your target — and what you actually want to know — how much of the material in the vial is your target.
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.
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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
Put another way, start from what the detector sees, because that tells you what the number means.
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.
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.
edited 3 Nov 2025 by tandem_gradient — clarified the distinction between purity and content
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.