An examination of the kinetics of the solution-mediated polymorphic phase transformation between α- and β-forms of L-glutamic acid as determined using online powder X-ray diffraction

Online powder X-ray diffraction, employing a flow-through cell [Hammond et al. Cryst. Growth Des. 2004, 4, 943] and a previously developed cheuaometric method [Chen et al. Anal. Chem. 2005, 77, 6563], was applied for quantitative analysis of the polymorphic phase transformation from the metastable alpha-form to the stable beta-form of L-glutamic acid (LGA). The process of interconversion was monitored in aqueous slurries, using a jacketed 400 mL magnetically stirred reactor as a function of temperature. Calibration studies revealed that the current detection limits are.ca. 0.8 and 0.2 wt%, respectively, for alpha- and beta-forms of LGA as determined for slurries of LGA in methanol and good signal detection can be accomplished using a data acquisition time of 10 s albeit with increased noise levels. Compositions of slurries, in terms of the amounts of the individual solid phases present, were calculated from the areas of individual, discriminating peaks associated with the phases. Analysis of the diffraction peaks arising from the (111) reflection, 2 theta = 18 degrees for the a-forth and (102) reflection, 2 theta = 31 degrees for the beta-form, was used to obtain the rates of phase interconversion. Phenomenologically, the rate law observed for dissolution of the alpha-form was -D-alpha = k(d alpha) and the rate law observed for growth of the beta-form was D-beta = k(g beta)C(beta) where C-beta is the concentration of the beta-form expressed in weight percent. At 45 degrees C, the values of the rate constants k(d alpha) and k(g beta) were 18 x 10(-4) wt% s(-1) and 3 x 10(-4) s(-1), respectively. Activation energies calculated, using the Arrhenius relationship, were found to be 43.9 kJ/mol with k(d alpha 0) calculated to be 25.9 x 10(3) wt% s(-1) for the dissolution of alpha-form and 47.0 kJ/mol with k(g beta 0) calculated to be 16.2 x 10(30) s(-1) for the growth of the beta-form. The activation energy calculated from a different, characteristic peak of the beta-form at 2 theta = 21 degrees corresponding to the reflection from (101) planes, was 32.1 kJ/mol suggesting that there might be some texture effects manifested in the flow field of the crystalline slurry. The overall quantitative accuracy of this method together with potential for improvement is also discussed.