What I have: SWB · Qingdao Sigma Chemical.
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?
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.
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.
| Step | Value | Note |
|---|---|---|
| Vial price, 10 mg nominal | £34.00 | As advertised |
| Nominal cost per mg | £3.40 | 34 ÷ 10 |
| Measured content | 9.2 mg | Independent content assay |
| Cost per actual mg | £3.70 | 34 ÷ 9.2 |
| Dead-space loss, 20 draws | 4 % | 80 µL of a 2 mL fill |
| Cost per delivered mg | £3.85 | 3.70 ÷ 0.96 |
| First vial, with £110 assay | £14.85 | Testing dominates a single vial |
More usefully, cost per milligram of actual peptide is the honest price comparison, which means dividing by measured content rather than by label claim.
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.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe 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.
Sample size matters. One order each is an anecdote about six vials; three orders each over a year is the beginning of a comparison.
Publish the method with the result. A comparison without the laboratory named and the submission dates given is not reproducible by anyone.
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.
edited 9 Nov 2024 by a_lindgren — expanded the table to cover the lower concentration
Start by asking what you are optimising for, because cost per milligram, documentation depth and lead time do not have a common winner.
Content is the more discriminating measurement than purity for supplier comparison, because purity clusters tightly among competent suppliers and content does not.
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.
Gradient slope, column chemistry and detection wavelength all affect the reported purity figure, which is why the method section is the part that makes results comparable.
No supplier on this site pays for its position, and the storefront links are marked nofollow and sponsored.
Name the laboratory and the dates or the comparison cannot be reproduced.
The honest answer is that most claimed differences between reputable suppliers are inside inter-laboratory noise.
Lead time and lane behaviour are supplier properties too and are easier to compare than analytical ones, because they need no laboratory at all.
Use a fixed documentation checklist rather than an impression.
edited 27 Sept 2024 by m_haraldsen — tightened the wording; no substantive change
The short version: same compound, same laboratory, same method, ideally same week — otherwise you are comparing laboratories rather than suppliers.
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.
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.
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.