Mass divided by volume
The NIST Guide to the SI defines mass concentration as the mass of a substance divided by the volume of the mixture. In a lab this is usually written in milligrams per millilitre (mg/mL).
A neutral example: 10 mg of a powder dissolved to make 5 mL of solution gives 10 ÷ 5 = 2 mg/mL. The same 10 mg made up to 2 mL gives 5 mg/mL. Less liquid means a higher concentration.
Units that mean the same thing
From mg/mL to mol/L
Chemists also use amount concentration: moles of substance divided by volume, in mol/L. Molar mass (g/mol) links the two, since moles equal mass divided by molar mass.
A worked example with a made-up compound of molar mass 1,000 g/mol: 2 mg/mL is 2 g/L, and 2 g/L ÷ 1,000 g/mol = 0.002 mol/L, or 2 mmol/L. A compound with double the molar mass would give half the molar concentration at the same mg/mL.
NIST notes that the word "concentration" on its own can be ambiguous, so it is good practice to say which kind is meant.
Powder weight is not peptide weight
A calculation is only as good as the mass you start with. Freeze-dried peptide powder also contains water and counter-ions, so the net peptide content can be well below the labelled powder weight. A concentration based on powder weight then overstates the true peptide concentration. Research peptides are for laboratory research only and are not approved medicines.
Educational information only. Nothing here is medical advice, dosing guidance or a recommendation to use any substance. Research peptides are not approved medicines and are not for human or animal consumption. Speak to a qualified professional and follow the law where you are.
