Ion mobility mass spectrometry as a powerful tool to analyze complex macromolecular systems

Abstract

Modern applications of polymers rely on intricately tailored macromolecules exhibiting very specific properties and therefore require extensive analysis. In this context, IMMS is an immensely powerful technique since it allows for the simultaneous evaluation of the mass - in form of the mass-per-charge ratio (m/z) - and the size - in form of the Collision Cross Section (CCS) - of an analyte. Based on the freely rotating chain model and in combination with molecular modelling, in previous works a set of basic methods for the quantitative evaluation of polymer-IMMS measurements was established. In the past, these methods were used for the evaluation of important physical properties such as the characteristic ratio Cn or the dielectric constant of simple polymers from IMMS measurements. In this work, the methods used for quantitative IMMS evaluation - molecular modelling and physical derivations - were revised and improved with regard to their accuracy and flexibility. Furthermore, the method was extended from simple homopolymer to a wide range of different polymers with varying chain structures and polarities. On top of this, the influence of the topology of macromolecules on their physical properties was investigated using IMMS for triblock copolymer and branched poly (acrylates). Since in IMMS the macromolecules exist in the gaseous phase at low pressures, single molecule molecular modelling can give important information about their conformation and thus their shape. In order to reliably obtain accurate structures via molecular simulations, a protocol employing two well known separate global optimization techniques - Simulated Annealing (SA) and Monte Carlo Basin Hopping (MCBH) - was designed. Consequently, the structures obtained through this SA-MCBH approach were used for theoretical CCS calculation. For this, the highly accurate trajectory method which is based on Lennard-Jones potentials of the analyte ion and the surrounding collision gas was used. The simulation protocol and subsequent CSS calculation was then applied to doubly-charged of poly (ethylene glycol) (PEG) adducts [PEG+2Na+] and resulted in exceptional agreement with experimentally obtained values. In order to be able to reliably evaluate physical properties of polymers from IMMS measurements, the methods used for the quantitative analysis had to be refined and improved with respect to their mathematical and physical derivation. Central to this evaluation is the transformation of the experimentally obtained two-particle CCS into a measure of the size of the polymer coil. This was achieved by translating the CCS into the approximate ion surface projection (AIS) of the analyte using the kinetic radius of the drift gas. Using results from integral geometry, AIS was then directly correlated to the squared projected end-to-end distance of the polymer which enabled a more accurate mathematical derivation of Cn. Finally, the newly designed molecular modelling approach as well as the improved descriptions of AIS and the squared projected end-to-end distance were applied in order to improve the derivation of the dielectric constant. These newly updated methods were then applied to a wide range of polymers. First, PEG and poly (propylene glycol) (PPG) were evaluated with respect to both Cn and the dielectric constant with excellent accuracy. A series of acrylate-based polymers, poly (acrylic acid) (PAA), poly (methyl acrylate) (PMA) and poly (butyl acrylate) (PBA) were also evaluated in order to obtain their Cn with very good results. Even though Cn can be evaluated from the z=1 and z=2 charge state, the evidence suggested that the z=1 state leads to more robust results. For acrylates, a qualitative influence of chain branching on IMMS measurements was also observed. Finally, the quantitative analysis of IMMS data was extended to non-polar polymers leading to the successful Cn evaluation of poly (styrene) and poly (butadiene). In the last part of this work, the methods were used to evaluate two series of triblock copolymers based on PEG and PPG as well as PMA and PBA which exhibited an equal monomer composition but inverted block sequence. These ABBA and BAAB-type triblock copolymers of both systems were analyzed with respect to their Cn. Without exception, the results indicate that there is a significant influence of the block structure on the physical properties. Specifically, it could be demonstrated that the inner block contributes more heavily to the overall properties than the outer block.