The specifics, since they change the answer: MKM · QST · Spain.
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.
So which one, and on what grounds?
The short version: same compound, same laboratory, same method, ideally same week — otherwise you are comparing laboratories rather than suppliers.
Cost per milligram of actual peptide is the honest price comparison, which means dividing by measured content rather than by label claim.
Sample size matters. One order each is an anecdote about six vials; three orders each over a year is the beginning of a comparison.
Inter-laboratory spread on identical peptide material is routinely half a per cent to a per cent by RP-HPLC, which is the noise floor for any cross-laboratory comparison.
Name the laboratory and the dates or the comparison cannot be reproduced.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsA single member running three suppliers on one method is worth more than thirty members running one supplier each.
To compare properly: order the same compound at the same nominal strength from each supplier, submit all samples to the same laboratory in the same submission if possible, and ask for the same test set on each.
It helps to be literal here: publish the method with the result. A comparison without the laboratory named and the submission dates given is not reproducible by anyone.
Use a fixed documentation checklist rather than an impression.
edited 31 Mar 2026 by t_oyelaran — reworded for clarity after a comment
Answer first: comparing suppliers is only meaningful if the comparison holds the laboratory, the method and the compound constant, and most published comparisons hold none of them.
Content is the more discriminating measurement than purity for supplier comparison, because purity clusters tightly among competent suppliers and content does not.
Submitting to different laboratories introduces a method difference that commonly exceeds the supplier difference. Gradient slope alone can move a reported purity figure by a per cent in either direction.
Content assay results across the published datasets vary considerably more between suppliers than purity does, which makes content the more discriminating axis.
Price per milligram of measured peptide, not per milligram of label claim.
The relevant point is that two competent laboratories disagree by half a per cent on identical material, which is larger than most of the differences people argue about.
Lead time and lane behaviour are supplier properties too and are easier to compare than analytical ones, because they need no laboratory at all.
The caveat is that a comparison is a snapshot of the lots compared, and lots change.
Compare content, not purity. Purity clusters and content does not.
The honest answer is that most claimed differences between reputable suppliers are inside inter-laboratory noise.
Compare documentation on a fixed checklist rather than by impression: lot specificity, method section, chromatogram availability, quantified content, water and counter-ion figures, and whether the code is on the vial.
Comparisons drawn from different laboratories at different times are weaker evidence than most people treat them as.
One laboratory, one method, one submission. Otherwise it is not a comparison.
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.