Evaluation of a Standardless Method of Determination of Molecular Weight and Polydispersity of a Polystyrene Sample by Thermal Field-Flow Fractionation

Coupling thermal field-flow fractionation with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the analysis of synthetic polymers

Thermal field-flow fractionation (ThFFF) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) have been coupled to yield a powerful combination of techniques for polymer analysis. Thermal FFF's high molecular weight (MW) selectivity and sensitivity to chemical composition are used to separate polydisperse polymers and polymer mixtures into the narrow polydispersity and homogeneous chemical composition fractions essential for MALDI-TOFMS analyses. On the other hand, MALDI-TOFMS's ability to directly measure molecular weight alleviates the need for polymer standards for ThFFF. In this first-time coupling of ThFFF and MALDI-TOFMS, compatibility issues were addressed and optimum conditions and procedures were identified and developed to maximize the capabilities of the combined technique. Depending on the polymer MW and the method of MALDI sample deposition, fractions from 1-10 ThFFF runs were combined for MALDI-TOFMS analysis. Binary solvents were used to enhance ThFFF retention and resolution of low-MW (<15-kDa) polymers, and methods were developed to allow routine MALDI-TOFMS analyses of polystyrene polymers up to 575 kDa. Overall, the MW compatibility of the two techniques was extended from several kilodaltons to several hundred kilodaltons. Polymer fractions were collected after separation by ThFFF and analyzed either by MALDI-TOFMS or reinjection into the ThFFF system. Good agreement was observed between the MW distribution data obtained by MALDI-TOFMS and ThFFF. The application of ThFFF/MALDI-TOFMS to polydisperse polymers and polymer mixtures was demonstrated. This combined technique was also shown to be a viable means for preparing standards from the original polymer sample.