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Quantitative Methods: Part 2. Diluting Solutions

Author(s): David R. Caprette, PhD

Example: Prepare a Protein Sample to a Predetermined Concentration and Volume

To work with macromolecules, we often must dilute a solution to some predetermined concentration, and we usually need some particular amount of solution. For example, it is common to conduct a protein assay to determine the concentration of an unknown protein mixture, and then to dilute the unknown to a predetermined amount and concentration in order to run it on a gel.

This time, suppose you already have determined the final desired concentration and volume (C2 and V2). You know the starting concentration, C1, from the protein assay. However, you must do the algebra to find out the volume, V1, of the starting solution you will need to dilute to the final desired volume.

In this example, the protein assay gave a concentration of 11 milligrams/milliliter unknown protein sample. You need 100 µl at a concentration of 1 mg/ml. Since three of the four variables are known, you only need to solve for the fourth variable, namely the volume of 11 mg/ml protein with which to start. To avoid confusion I have converted the concentrations to micrograms per microliter. One milligram per milliliter is the same as one microgram per microliter or 1 gram per liter.

V1 = (100 µl)(1 µg/µl)/11 µg/µl = 9 µl