[reddagon5]

If you have, say, 20 mM of sarin in 60 mM of buffer, how would you figure out the concentration of sarin? Would it just be 20mM / (60 mM+20 mM)?

[Corribus]

The molar concentration of the substance is just the number of moles of the substance in the number of liters of solvent. So, a concentration of 20 mM of sarin means 20 mM of sarin in 1 L of solvent. It doesn't matter what the solvent is, whether it is water, ethanol, or buffer.