Hi Natasja,

mM is a unit for molar concentration (number of molecules in a volume), depending on the stock solution concentration and the target solution concentration one needs to make an appropriate dilution. For instance if the stock solution is 25 mM and the target solution is 1 mM one would need to dilute the stock 25 times. in other words if you are making 25 ul of the target solution you would need 1 ul of the 25 mM stock solution and 24 ul of other stuff  + water and you will get 25 ul of 1 mM target solution.

ng is a unit for weight - usually ng/ul (wight concentration) is used to describe DNA/RNA concentration since they are usually determined by spectroscopy methods (nanodrop) ie. by light absorption which is proportional to the weight concentration (to the total amount of bases) and not to the molar concentration of the polymer in solution.

"unit" is a unit for enzyme activity - each enzyme has its own defined units - which usually correspond to the amount of molecules which can be used/made by the enzyme solution in unit time (1s, 1 min...). It is a way to standardize the enzyme amount.

Hope this helps,

Milan

Thank you Milan, exactly what I was after!