Concretely: 96.8% · VendorInvestigate · 99.4%.
I think I have a problem. I am not yet sure whether it is a real problem or a measurement artefact.
I want to know whether this is recoverable or whether the honest answer is to write it off.
Is this recoverable, and how would I tell?
2.6 percentage points, and the direction is the informative half. VendorInvestigate reports 96.8 per cent and the certificate 99.4, so the independent figure is lower. As impurity that is 3.2 per cent against 0.6 — 5.33 times as much unassigned area. A gentler gradient resolves impurities that a steeper one hides beneath the main peak, so the better method routinely reports the worse number; 2.6 points is comfortably inside what method choice alone produces on identical material. Ask both parties for the method section before you decide which figure is wrong, because the answer is often neither.
Stated carefully, 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.
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.
I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.
Compare purity within a single laboratory on the same method, never across laboratories.
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 BiochemGradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.
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.
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.
Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
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.
Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
The underlying point is that 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.
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.
edited 28 Mar 2026 by p_mkhize — added the citation requested in comments
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.