I am comparing three suppliers on documentation rather than on price.
I want the working, not the result — I need to be able to redo it with different numbers.
I care about the precision as well as the value — I want to know how many figures are real.
Where is my error, and what is the correct working?
In practice, wastage from expired reconstituted vials is a real line item and nobody includes it.
Worked example. Supplier A: £60 for a 10 mg vial, content 96 per cent, so 9.6 mg for £60, or £6.25/mg before carriage. Supplier B: £52 for the same nominal vial, content 88 per cent, so 8.8 mg for £52, or £5.91/mg. B still wins here, but the gap has narrowed from thirteen per cent on the label to five per cent in reality.
| Observation | Implication | How to check |
|---|---|---|
| Lot number not on the vial | Certificate cannot be tied to your material | Photograph vial and certificate together |
| No method section | The number is not reproducible | Request column, gradient, wavelength |
| Purity to two decimals, no chromatogram | False precision | Request the trace |
| Test date before manufacture date | Certificate belongs to a different lot | Compare dates |
| Identical figures across lots | One certificate reused | Compare two lots side by side |
| “Sterile filtered” with no sterility test | Process claim substituted for a result | Ask for the sterility report |
Specifically, carriage amortises across the order. Twenty-five pounds of carriage on one vial is £2.50/mg on a 10 mg vial; on ten vials it is £0.25/mg. That single term explains most of the case for larger, less frequent orders.
Syringe dead-space volumes are published per design, with fixed-needle insulin syringes under 5 microlitres and conventional luer designs at 35 microlitres or more.
Larger orders are cheaper per milligram and concentrate lot risk. Price both.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe relevant arithmetic is that a fifteen per cent price advantage disappears against a ten per cent content shortfall plus a testing cost.
Wastage from a reconstituted vial discarded at the end of its in-use period is a genuine cost, and it is a function of the diluent volume chosen at reconstitution rather than of anything the supplier did.
On the detail: cost per milligram is the wrong metric entirely if you are optimising for confidence rather than price, and it is worth saying which one you are doing before you build the spreadsheet.
Nothing here is medical advice, and research-use compounds are not approved for human use.
Divide by measured content, not by label claim. That is the whole correction.
Answer first: compare cost per milligram of measured peptide, not per milligram of label claim, because content varies enough to reverse a comparison.
Independent testing costs roughly the price of one to two vials at the services this community uses. On a two-vial order that is a fifty to a hundred per cent surcharge; on a twenty-vial order it is five per cent.
Change one number and it reverses: if B assays at 82 per cent, that is 8.2 mg for £52, or £6.34/mg, and the cheaper vial is now the more expensive peptide.
Published content assay results across the independent services show nominal and measured content differing by one to ten per cent, which is the term that makes label-price comparisons unreliable.
Larger orders reduce cost per milligram and increase exposure to a single lot, which is a real trade rather than a free win.
Decide whether you are optimising cost or confidence before you build the model.
Specifically, this is a spreadsheet question and doing it properly changes conclusions more often than people expect.
Dead-space loss is small with fixed-needle insulin syringes — a few microlitres per draw — and substantial with detachable-needle luer syringes at 35 to 100 microlitres. Across twenty draws that is up to two millilitres of solution.
Independent testing prices at the services this community uses are published and are stable enough to model.
Fixed-needle syringes save more peptide than most price differences do.
edited 27 Oct 2025 by tenth_of_a_unit — tightened the wording; no substantive change
The honest answer is that the cheapest headline price is frequently not the cheapest outcome.
The full calculation: (unit price + carriage share + testing share) ÷ (nominal mg × measured content fraction × (1 − dead-space and wastage fraction)). Every term after the first is routinely omitted.
Carriage on international consignments scales sub-linearly with weight, which is the quantitative basis for order consolidation.
A spreadsheet built on label claim rather than measured content is precise about the wrong number.
Include carriage and testing as per-milligram terms. They dominate small orders.
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.