Rapid and Simple Determination of Average Molecular Weight and Composition of Synthetic Polymers via ESI‐MS and a Bayesian Universal Charge Deconvolution

RATIONALE

Simple, affordable, and rapid methods for identifying the molecular weight distribution and macromolecular composition of polymeric materials are limited. Current tools require extensive solvent consumption, linear calibrations, and expensive consumables. A simple method for the determination of average molecular weight (M_n, M_w) and chain end groups is demonstrated for synthetic homopolymer standards using direct injection electrospray ionization mass spectrometry (ESI-MS) and an open-sourced CDC algorithm.

METHODS

Five homopolymer standards in the 1-7 kDa molecular weight (MW) range were analyzed using direct injection ESI-MS on a quadrupole/time-of-flight (Q/ToF) mass spectrometer. The samples investigated, viz. two poly (ethylene oxide) (PEO) and two poly (styrene sulfonic acid) (PSS) standards with narrow polydispersity, and one poly(D,L-alanine) (pAla) standard with undefined polydispersity, were chosen to illustrate challenges with ESI-MS quantitation. Using the UniDec program, weight average molecular weights (M_w) obtained from the charge deconvoluted spectra were compared to the reported M_w data of the standards from size exclusion chromatography measurements.

RESULTS

The MW data derived for the PSS, PEO, and pAla standards agreed well with the corresponding reported M_w or MW range values. The method was able to render MW, degree of polymerization (DP), and polydispersity index (PDI) information for polymers with narrow (PSS, PEO) as well as broader (pAla) molecular weight distribution; this feature provides an advantage over MW analysis via matrix-assisted laser desorption/ionization (MALDI) for ESI-compatible materials. PSS standards differing in average MW by only a few repeat units could be confidently distinguished. Additionally, the oligomeric resolution observed for all samples studied unveiled chain end information not available through chromatographic analysis.

CONCLUSIONS

Overall, the free and easy-to-use UniDec CDC algorithm provides a simple, alternative method to measuring MW and DP for polymeric materials without high solvent consumption, expensive ionization sources, or calibration curves. Information about the masses of individual oligomers and the possibility to further characterize these oligomers by tandem mass spectrometry and/or ion mobility techniques constitute additional benefits of this approach vis à vis traditional MW and PDI elucidation through size exclusion chromatography.