GROUP DYMAMICS

Determination of D-lactide content in lactide stereoisomeric mixture using gas chromatography-polarimetry

Lidong Fenga,b, Xinchao Biana,b, Zhiming Chenb, Sheng Xianga,b, Yanlong Liua,b, Bin Suna,b, Gao Lia, Xuesi Chena,⁎Talanta 164 (2017) 268–274Lidong Fenga,b, Xinchao Biana,b, Zhiming Chenb, Sheng Xianga,b, Yanlong Liua,b, Bin Suna,b, Gao Lia, Xuesi Chena,⁎

An analytical method has been proposed to quantify the D-lactide content in a lactide stereoisomeric mixture using combined gas chromatography and polarimetry (GC- polarimetry). As for a lactide stereoisomeric mixture, meso-lactide can be determined quantitatively using GC, but D- and L-lactides cannot be separated by the given GC system. The composition of a lactide stereoisomeric mixture is directly relative to its specific optical rotation. The specific optical rotations of neat L-lactide were obtained in different solutions, which were −266.3° and −298.8° in dichloromethane (DCM) and toluene solutions at 20 °C, respectively. Therefore, for a lactide sample, the D-lactide content could be calculated based on the meso-lactide content obtained from GC and the specific optical rotations of the sample and neat L-lactide obtained from polarimetry. The effects of impurities and temperature on the test results were investigated, respectively. When the total content of impurities was not more than 1.0%, the absolute error for determining D-lactide content was less than 0.10% in DCM and toluene solutions. When the D-lactide content was calculated according to the specific optical rotation of neat L-lactide at 20 °C, the absolute error caused by the variation in temperature of 20 ± 15 °C was not more than 0.2 and 0.7% in DCM and toluene solutions, respectively, and thus usually could be ignored in a DCM solution. When toluene was used as a solvent for the determination of D-lactide content, a temperature correction for specific optical rotations could be introduced and would ensure the accuracy of results. This method is applicable to the determination of D-lactide content in lactide stereoisomeric mixtures. The standard deviation (STDEV) of the measurements is less than 0.5%, indicating that the precision is suitable for this method.