Calculating D-lactide content by probability using gas chromatographic data

A new method to calculate the d-lactide content in the intermediate for the synthesis of poly(lactic acid) (PLA) has been developed using capillary gas chromatography (GC). Separating d-lactide from l-lactide using conventional GC is challenging. However, the content of two lactide enantiomers can be obtained through probability calculation from the GC data. The reactivity of l-lactic acid is equivalent to that of d-lactic acid during polycondensation to prepare low molecular weight PLA prepolymers. Racemization reactions occur randomly during the production of the prepolymers and subsequent depolymerization of the prepolymers for the preparation of a cyclic lactic acid dimmer (lactide). The proportions of generated lactide stereo isomers in crude lactide conform to the laws of probability. The stereochemical structure of lactide is not altered during its purification. Thus, the ratio of l- to d-lactide is a constant. Therefore, the d-lactide content in refined lactide can be obtained. The results are in good agreement with the testing data obtained using GC-HPLC. The residual standard deviation, δ^=0.29%, indicates that the probability principle is suitable for calculating the isomer content in lactide. The relative standard deviation of the measurements for d-lactide content is less than 8.0%, indicating that this method is suitably precise.