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We now know how to analyze pure compounds, but what if we have a mixture? Spectrophometry becomes quite complex when dealing with multiple species of compounds at once. In order to purify a compound we can separate if from a mixture based on its intrinsic chemical properties. Remember that fluorescein is negatively charged at a pH above pKa of the carboxyl group. We can take advantage of this fact and use its attraction to positive charges to separate it from other molecules. In ion-exchange chromatography, we will use a stationary phase with a positive charge, allowing negatively charged molecules to bind and positively charged species to flow through. We can then disrupt this interaction and retrieve our now-purified molecule, and use spectrophotometric analysis of our purified fractions to determine how well we were able to separate our molecules.