Why Two Suppliers Charge Very Different Prices for the Same Peptide

The same compound, the same labelled quantity, and a price that differs by two or three times between suppliers. Some of that gap is real cost and some of it is the labelled quantity not meaning the same thing in both places. The useful question is not which listing is cheapest but which figures you are being quoted on.

Costs that genuinely differ

• How hard the molecule is to make. Length is the first factor: each additional residue is another coupling step with its own yield loss, so a 30-residue chain is not three times a 10-residue chain, it is considerably more. Modifications multiply this — a lipid chain, a cyclisation, one or more disulfide bonds, a non-standard or D-amino acid, an amidated terminus. Two products on the same shelf can differ tenfold in synthesis difficulty.
• How far purification was taken. Getting from a crude mixture to 95% is routine. Getting from 95% to 99% throws away product, and the material thrown away is already expensive. A 99% figure costs more than a 95% figure of the same compound, legitimately.
• How much testing was paid for. A report per lot costs money every lot. A report on one lot reused as marketing for every subsequent lot costs money once. No testing costs nothing. This is the single largest avoidable difference between two prices, and the one easiest to check.
• Fill accuracy and overfill. Filling tightly to specification requires better equipment and more rejects than filling loosely. A supplier who fills generously to avoid under-fill complaints is paying for the extra material.
• Packaging and logistics. Crimped vials under inert gas, individual boxes, desiccant, insulated shipping and tracked delivery all cost more than a bag of vials in a padded envelope.
• Scale. Large synthesis runs cost less per milligram. A supplier buying at volume can price below one buying in small quantities, with no quality difference.

Where the labelled quantity stops meaning the same thing

Two 10 mg listings can contain materially different amounts of peptide:

• Gross versus net. If 10 mg is the total weighed solid, it includes counter-ion and residual water. Net peptide content of 80% means 8 mg of peptide. If the other supplier means 10 mg of peptide, their product contains a quarter more, and a 25% price difference is not a discount at all. Net peptide content is the figure that settles this.
• Salt form. Trifluoroacetate and acetate have different masses. The same peptide as a different salt weighs differently for the same number of molecules.
• Purity as a multiplier. 10 mg at 95% and 10 mg at 99% are not the same purchase, and the 4% is in the part you cannot see.
• Actual fill. Nothing on a label guarantees what went into the vial. Only a content figure measured on that lot does.

What an unusually low price usually means

In order of how often it turns out to be the explanation: no per-lot testing; a content figure that was never measured; a lower net peptide content with salt and water counted as product; a purity specification quoted as a minimum that is never verified; or a report that belongs to a different lot than the vial. Outright substitution of a different compound happens but is rarer than simply not measuring anything.

How to compare two listings properly

• Work out the price per milligram of peptide, not per vial: labelled quantity multiplied by net peptide content, divided by price.
• Ask whether the certificate is for this lot, and check the lot number on the report against the vial.
• Check whether purity and content are both reported, or only purity.
• Check whether the chromatogram is included, or only a summary table.
• Include shipping and any customs handling in the comparison, since these do not scale with quantity and change the ranking on small orders.
• For repeat purchasing, ask what happens when a lot changes — whether a new report is issued, and whether you are told.

A supplier who is more expensive and can answer all six questions is often the cheaper one per milligram of verified material. A supplier who is cheaper and cannot answer any of them is selling an unknown quantity of an unverified substance, which has no price per milligram at all.

All products referenced on this site are supplied for laboratory research use only, and are not for human or veterinary use.