Article pubs.acs.org/Macromolecules
Supercharging Synthetic Polymers: Mass Spectrometric Access to Nonpolar Synthetic Polymers Jan Steinkoenig,†,‡,§ Martina M. Cecchini,∥ Samantha Reale,∥ Anja S. Goldmann,†,‡,§ and Christopher Barner-Kowollik*,†,‡,§ †
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George St., QLD 4000, Brisbane, Australia ‡ Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76128 Eggenstein-Leopoldshafen, Germany § Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∥ Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi dell’Aquila, Via Vetoio, Coppito, 67100 L’Aquila, Italy S Supporting Information *
ABSTRACT: We report a mass spectrometric access route to analyze nonpolar poorly ionizing synthetic polymers exploiting supercharging technology by chloride attachment via highresolution electrospray ionization mass spectrometry (HR ESI MS). The novel mass spectrometric procedure allows for the characterization of polyhydrocarbons, which include hardly ionizable polymers such as poly(styrene) (PS) (ranging from 1700 to 18 000 g mol−1) andfor the first time reported using ESI as ionization methodpoly(1,4-butadiene) (PBD) (ranging from 1000 to 10 000 g mol−1). The method is also applied to additional synthetic polymers including poly(2vinylpyridine) (P2VP) and poly(acrylamide) (PAAm). The powerful chloride attachment enables the detection of multiply charged polyhydrocarbons (up to quadruply charged). A systematic assessment of the manipulation of these charge states using supercharging agents (sulfolane, propylene carbonate, and m-nitrobenzyl alcohol) is carried out. Our investigations include an assessment of the influence of different ESI solvents (water, acetonitrile, acetone, tetrahydrofuran, dichloromethane, and methanol), water-doped organic ESI solvents, and the amount of supercharging agent.
■
INTRODUCTION The enormous number of multifunctional polymers,1 advanced polymer architectures,2 and the design of novel ligation techniques3 as well as supramolecular chemistries4 drive the demand to enhance existing characterization techniques and to develop novel analytical methodologies. In addition to size exclusion chromatography5 as a fast and versatileyet relatively mass inaccuratetechnique to determine the molecular weight distribution of synthetic polymers, highresolution mass spectrometry is the most powerful characterization technique to assess the exact chemical structure of macromolecules including their end groups.6 Matrix-assisted laser desorption ionization (MALDI)7 and electrospray ionization (ESI)6 as soft ionization techniques feature specific advantages. For instance, the matrix-based approach in MALDI enables the investigation of polyhydrocarbons and polyaromats such as poly(styrene) (PS), poly(butadiene) (PBD), poly(isoprene), and poly(ethylene).8 In ESI MS, the coupling to liquid chromatography (LC) (e.g., size exclusion chromatog© XXXX American Chemical Society
raphy (SEC)) allows for the assessment of a broad range of polar polymers,9 proteins,10 and natural products,11 which are well soluble in the employed ESI solvent. The electrospray ionization (ESI) process is complex and still not fully understood.12 Especially the charge-to-analyte transfer is subject to ongoing research. Molecular dynamics simulations by Konermann et al.related to proteinshighlight important mechanistic scenarios: (i) the ion evaporation model (IEM) is discussed for species with low molecular weight via ejection of small solvated ion droplets; (ii) the charge residual model (CRM) for large, globular analytes via incremental solvent evaporation close to the Rayleigh limit; and (iii) the chain ejection model (CEM) for nonpolar polymer chains via incremental desolvatation of chain segments until their full release into the gas phase.12 The CEM is considered the dominant ionization process for synthetic polymers and Received: September 18, 2017
A
DOI: 10.1021/acs.macromol.7b02018 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
in particular PS, poly(2-vinylpyridine) (P2VP), poly(acrylamide) (PAAm), andreported for the first time via ESI MSPBD. In addition, the formation of multiple charges allows for charge state manipulation experiments via supercharging,12,21−24 which have already proven successful for poly(ionic liquid)s (PILs)25 representing an entire family of polyelectrolytes. Multiple charging and well-ionizing polymers, for instance PEG17 and poly(methyl methacrylate) (PMMA), are not subject of the present study. Supercharging Technology. One of the most advantageous properties of ESI is the ability to produce multiply charged gas-phase ions, able to ionize even high-molecularweight (bio)macromolecules while keeping their structural integrity (no bond cleavage).26 Many physical properties of gasphase ions change with their charge state, e.g., proton transfer,27 ion−ion reactivity,28 fragmentation pathway,29 and efficiency of electron capture dissociation (ECD).30 Typically, the charge state of gas-phase ions is restricted by several factors including analyte conformation in solution (i.e., reduced Coulomb repulsion afforded by elongated conformations),31 charge competition (i.e., the solvent coordinates stronger to the charge than the analyte itself),32 and instrumental conditions.24 A significant factor for generating multiply charged species is the Lewis basicity of the analyte and, thus, the ability to coordinate to H+ or Na+ (in positive ion mode) or the Lewis acidity of the analyte to transfer the proton to the solvent (in negative ion mode). For instance, polyesters (e.g., P(M)MA) and polyethers (e.g., PEG) exhibit strong signal intensities of multiply charged distributions. Having oxygen as Lewis base in their chemical composition in common, these analytes bind to Na+ and H+.33 In contrast, polyhydrocarbons (e.g., PE, PS, and PBD) do not possess heteroatoms and consequently form mainly singly charged species with low ion intensities. Williams and co-workers discovered the supercharging effect of specific additives in ESI solvents to shift the charge distribution to higher charge states, thereby also increasing the ion intensity significantly.21,34 Their work focused on peptides and proteins, where supercharging facilitates the detection of proteins with high molecular weights by promoting multiply charged species. The subsequent tandem mass spectrometry affords an excellent analytical platform to determine the amino acid sequence (topdown method).35 In contrast to highly polar solution additives such as supercharging agents, supercharging can also be induced with multivalent metal salts (e.g., trivalent lanthanium chloride).22 Currently, the supercharging mechanism, strictly related to the ESI process, is under debate. Four distinct mechanistic scenarios are discussed: (i) The surface tension mechanism (sometimes referred to as the Berkeley mechanism) proposes the retarded formation of the gas-phase ions caused by the nonvolatile supercharging agents. The increased surface tension counters the accumulation of charges in the droplet resulting in delayed Coulomb fission and thus yielding highly charged droplets.23 Recent work questioned the role of the surface tension36,37 and thus the Berkeley mechanism. (ii) The Lewis acid/base mechanism attributes the supercharging affect to the less basic property of these solution additives in comparison to water. Thus, the charge competition between analyte and solvent is reduced, yielding an increased probability of the charge to attach to the analyte.38 (iii) The dipole-based mechanism studied by Douglass et al.39 and Donald and coworkers proposes that supercharging agents interact with the analyte by binding via dipole interactions and, thus, shield the
denatured proteins. Placed in a Rayleigh-charged nanodroplet, the chain end of a polymer migrates immediately to the droplet’s surface followed by the stepwise ejection into the gas phase transferring the charge from the droplet to the chain. Consta et al. have simulated the behavior of charged macroions (i.e., positively charged PEG) in solution.13,14 In positive ion mode, synthetic polymers can be charged by complexation with protons, alkali metal ions (Li+, Na+, K+), ammonium, silver(I), Cu(II), and Co(I).15 Polymers with low coordination tendency (e.g., PS) mainly form singly charged species. Therefore, the detection of multiple charges via ESI enabling the mapping of high molecular weight synthetic polymer is not applicable, restricting the mass range of the polymers to the employed mass analyzer (e.g., Fourier transform (FT) mass analyzers
