Size-exclusion chromatography–electrospray-ionization mass spectrometry to characterize end group and chemical distribution of poly(lactide-co-glycolide) co-polymers

The characterization of the microstructure of in vivo degradable polyesters is gaining increased interest thanks to their high-performance applications, such as drug delivery systems. The design of such material requires a high level of understanding of the critical material attributes of the polyesters, such as molecular-weight distribution (MWD), chemical-composition distribution (CCD), and end-groups (functionality-type distribution, FTD). Size-exclusion chromatography (SEC) hyphenated with mass spectrometry (MS) yields SEC-MS, which is an effective method for analyzing the microstructure of polymers. While the MWD can be determined by size-exclusion chromatography hyphenated with ultraviolet spectrometry and refractive index (SEC-UV/RI), the CCD and FTD can be determined by SEC-MS. However, previous applications of SEC-MS have failed to address the risk of polymer fragmentation during the analysis process. It is crucial to establish whether SEC-MS methods can be applied to biodegradable polymers and to recognize if fragmentation processes occurred during the SEC separation or during the ESI-MS process. In this study, we investigate whether SEC-MS methods can be applied to PLGA biodegradable polyesters. The research demonstrates that the choice of alkali metal salt used during ionization strongly influences the stability of PLGA during SEC-MS analysis. CsI was found to minimize fragmentations during ESI-MS, simplifying the MS spectra and allowing isomeric PLGA structures to be distinguished. The resulting method facilitates end-group and CCD determination. Additionally, when combined with selective degradation, the described method can provide insights in the “blockiness” of the polymer and support the development of sequence-controlled PLGA synthesis.