Direct measurement of critical micelle concentrations of mPEG-PLA copolymers based on ultrafiltration combined with liquid chromatography tandem mass spectrometry.

The critical micelle concentration (CMC) is a core parameter dictating the formation and stability of polymeric micelles in drug delivery. Conventional CMC determination methods, relying on indirect measurements, are constrained by issues such as limited sensitivity or potential interference. To overcome these limitations, this study established a method based on ultrafiltration coupled with liquid chromatography-tandem mass spectrometry (UF-LC-MS/MS) for directly determining the CMC of methoxy poly(ethylene glycol)-block-poly(D,L-lactide) (mPEG-PLA) with varying PLA chain lengths. The LC-MS/MS method exhibited good linearity, accuracy, and precision for the quantification of mPEG2000-PLA2000, mPEG2000-PLA3000, and mPEG2000-PLA5000. The optimized and validated ultrafiltration process ensured selective separation of micelles from unimers. By identifying the inflection point in the concentration of free mPEG-PLA in the tested sample, the CMC values were determined as 5.77 ± 0.16 μg/mL for mPEG2000-PLA2000, 3.02 ± 0.16 μg/mL for mPEG2000-PLA3000, and 1.86 ± 0.04 μg/mL for mPEG2000-PLA5000. These results were comparable to those from the conventional pyrene fluorescence probe assay, demonstrating that UF-LC-MS/MS is a robust tool for CMC determination of mPEG-PLA copolymers, with strong potential for broader application to other amphiphilic polymers.