Determination of copolymer ratios of poly(lactide-co-glycolide) using near-infrared spectroscopy

The near infrared (NIR) spectroscopic technique was used to determine copol ymer ratios of polylactide-co-glycolide samples. Appropriate quantities of DL-polylactic acid and lactic-co-glycolic acid polymers with 86:14, 75:25, 64:36 and 52:48 lactide to glycolide ratios were dissolved in methylene chl oride to obtain 5% (w/w) solutions. NIR spectra of the samples were obtaine d from the solutions using a Polyol Analyze(TM) operated in the transmittan ce mode. Linear regression calibration models were generated at 2130 and 22 88 nm from the second derivative spectral data obtained from the NIR techni que. The lowest and highest standard errors of calibration (SEC) at 2130 nm were 1.29 and 1.63%, whereas those obtained from the calibration models ge nerated at 2288 nm were 2.00 and 2.03%, respectively. Partial least squares (PLS) calibration models were also generated from the second derivative sp ectral data from 1100 to 2500 nm. The lowest and the highest SEC for the mo dels were 1.46 and 1.53%, respectively The calibration models were then use d to predict the lactide contents of the unknown (test) samples. The highes t percent error of prediction was 2.56% for samples with 86% lactide conten t when the linear regression calibration at 2130 nm was used, whereas the h ighest percent error of prediction was 1.56% for samples with 64% lactide c ontent when the linear regression calibration at 2288 nm was used. The high est percent error of prediction was 1.73% for samples with 75% lactide cont ent when the two-factor PLS calibration model was used. (C) 1999 Elsevier S cience B.V. All rights reserved.