Understanding Research Concentrations

A concentration describes how much compound exists in a given volume of solution. The two most common expressions in peptide research are mass concentration (milligrams per milliliter, mg/mL) and molar concentration (millimolar, mM).

Mass concentration is convenient for weighing and measuring. Molar concentration accounts for molecular weight, making it the meaningful unit when comparing compounds of different sizes.

Preparing a target mass concentration is direct: dissolve the desired mass in the desired volume. For example, 10 mg of compound brought to 10 mL of solvent yields a 1 mg/mL solution.

Converting to molar concentration requires the molecular weight: moles equal mass divided by molecular weight. A 1 mg/mL solution of a 1,000 g/mol peptide is 1 mM; the same mass concentration of a 4,000 g/mol peptide is 0.25 mM.

Rigorous work accounts for the vial's net peptide content rather than its gross weight. If a 10 mg vial contains 90% net peptide by weight, it holds 9 mg of actual compound — a 10% difference that matters in precise analytical work.

This is another reason batch-specific documentation matters: the net content figure comes from the same testing that establishes purity.