Peptide mixtures: equal mass is not equal molar amount

“Mix equal amounts of each peptide” sounds clear until someone asks what “amount” means. Equal milligrams and equal micromoles can produce different mixtures. Decide which comparison the study needs before asking a supplier to prepare a pool.

Mass counts weight; moles count molecular entities

A mole is the SI unit for amount of substance. Different molecules can have different molar masses, so the same weight does not necessarily contain the same number of molecules. The BIPM unit definition provides the basis for this distinction. [1]

An equal-mass example

Imagine peptide A has a molar mass of 1,000 g/mol and peptide B has a molar mass of 2,000 g/mol. Assume the weighed quantities represent the defined pure peptides. One milligram of A is 1 micromole; one milligram of B is 0.5 micromole. Mixing those equal masses gives a 2:1 molar ratio of A to B. These are invented numbers, not product specifications.

An equal-molar version of that mixture

To use 1 micromole of each, the calculated quantities are 1 mg of A and 2 mg of B. The total is 3 mg, with a 1:1 molar ratio. Neither design is automatically better. One controls the mass contribution; the other controls the specified molecular amount. The experiment determines which is useful.

Powder composition affects the calculation

Real supplied powders can contain water, counterions or other components. A molar pool specification needs an appropriate peptide-quantity basis, not an assumed conversion from an HPLC area percentage. Ask how component quantities are assigned and which records support them. If the basis is uncertain, keep that uncertainty visible.

Write the mixing rule into the order

List each sequence and final molecular form, then specify the quantity per component and whether the ratio is by mass or moles. Include the requested total quantity and delivery format. NHD’s enquiry form lets you submit this component list for assessment; any pooling and quantity-verification work must be agreed separately.

Useful links

• Content versus purity
• Research calculation tools
• Request a defined peptide mixture

For laboratory research, development and manufacturing only. Not for human or veterinary use.

Sources and examples

• BIPM: the mole
• JPT: peptide pools and matrix layouts

Reference records explain the topic; they do not establish NHD stock, a supplied lot’s results or clinical suitability. Examples explicitly labeled as hypothetical are calculations, not test data.

Editorial source check: Codex AI editorial · 2026-10-03