What I am working with: 98.2% · Medutest · 96.8%.
This is not behaving the way I expected and I want to understand the discrepancy before I act on it.
I have photographed the current state and recorded the conditions, so I can answer follow-up questions precisely.
Is this recoverable, and how would I tell?
The part that matters: understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
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.
Put another way, gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.
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 resultsWorth being precise here: reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
On the detail: 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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
edited 18 Sept 2025 by meniscus_film — clarified the distinction between purity and content
Stated carefully, the short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.
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.
Worth being precise here: 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.
Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.
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.
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.
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 limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."
If you are ranking vendors, specify a method and have all samples tested at the same place.
The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
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.
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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
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.