Three different faces Of L-threonine (L-Thr) crystals growing from supersaturated solutions enriched with D-Thr were observed under an optical microscope to make clear the mechanism of optical purity decrease during preferential crystallization operation, in particular the effect of washing the seed crystals. The solution concentration corresponded to those in the later period of optical resolution processes. On the {210} faces small platelike crystals Of D-Thr were found to appear by the mechanism of surface epitaxial nucleation. For seed crystals washed with water, such D-Thr crystals were not observed. A closer observation of the surfaces with an atomic force microscope (AFM) revealed that the {210} faces washed with water had changed to the combination of the {100} and {110} faces. With a forced nucleation method on the {210}, {100}, and {110} faces, deposits Of D-Thr particles showed very different appearances, indicating that surface nucleation occurred only on the {210} faces. On the basis of these observations, the purity decrease in the later period of batch preferential crystallization was explained as the surface nucleation of the undesired isomer.
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