The Purity of Biochemicals Produced by Crystallization

As has been discussed throughout this book, crystallization can generally result in very large purification factors as well as being useful as a concentration step that removes most of the solvent from the desired solute. Impurities within crystals that limit purification efficiency arise from  

Direct incorporation into the lattice structure (solid-solution formation) and surface adsorption. 

Quantitative and even qualitative information on impurity incorporation in pharmaceutical crystals is quite limited in the open literature, but the general trends are expected to follow the behaviors described in Chapter 3. Given that the molecular size of most biological compounds are larger than inorganic substances the resulting crystal structures are rather more open and better able to accommodate interstitial impurities including solvent. 

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