Using solvent‐free sample preparation to promote protonation of poly(ethylene oxide)s with labile end‐groups in matrix‐assisted laser desorption/ionisation

Abstract

Protonation is usually required to observe intact ions during matrix-assisted laser desorption/ionization (MALDI) of polymers containing fragile end-groups while cation adduction induces chain-end degradation. These polymers, generally obtained via living free radical polymerization techniques, are terminated with a functionality in which a bond is prone to homolytic cleavage, as required by the polymerization process. A solvent-free sample preparation method was used here to avoid salt contaminant from the solvent traditionally used in the dried-droplet MALDI procedure. Solvent-based and solvent-free sample preparations were compared for a series of three poly(ethylene oxide) polymers functionalized with a labile end-group in a nitroxide-mediated polymerization reaction, using 2,4,6-trihydroxyacetophenone (THAP) as the matrix without any added salt. Intact oligomer ions could only be produced as protonated molecules in solvent-free MALDI while sodium adducts of degraded polymers were formed from the dried-droplet samples. Although MALDI analysis was performed at the laser threshold, fragmentation of protonated macromolecules was still observed to occur. However, in contrast to sodiated molecules, dissociation of protonated oligomers does not involve the labile C--ON bond of the end-group. As the macromolecule size increased, protonation appeared to be less efficient and sodium adduction became the dominant ionization process, although no sodium salt was added in the preparation. Formation of sodiated degraded macromolecules would be dictated by increasing cation affinity as the size of the oligomers increases and would reveal the presence of salts at trace levels in the MALDI samples.

MALDI of synthetic polymers with labile end‐groups

“…Other examples of spontaneous dissociation in the MALDI source were reported by Charles and co‐workers for a variety of SG1‐terminated polymers (Mazarin, Phan, & Charles, ; Girod et al, ; Barrere et al, ). Based on data obtained by performing collision‐induced dissociation (CID) of intact electrosprayed alkali adducts to mimic their activation during MALDI, this research group proposed different structures, arising from alternative degradation mechanisms, for the observed modified end‐groups.…”

“…Interestingly, Schulte et al showed that, despite the use of a saturated NaBF 4 salt solution together with 2,5‐DHB, the shortest chains ( M n < 1,000 g mol −1 ) of the nitroxide‐capped PNIPAM studied could also be generated as intact protonated molecules upon MALDI (Schulte et al, ). Trihydroxyacetophenone (THAP) was also shown to enable production of protonated NMP molecules in MALDI, as reported for TEMPO‐capped polyethylene (Lopez et al, ) as well as for small SG1‐terminated PEO by implementing a salt‐free, solvent‐free sample preparation method (Trimpin et al, ) to avoid salt contaminant from the solvent traditionally used in the dried‐droplet procedure (Mazarin, Phan, & Charles, ).…”

“…Based on data obtained by performing collision‐induced dissociation (CID) of intact electrosprayed alkali adducts to mimic their activation during MALDI, this research group proposed different structures, arising from alternative degradation mechanisms, for the observed modified end‐groups. For example, Mazarin et al demonstrated that fragmentation of SG1‐based PEO macroalkoxyamine mainly proceeded via the homolysis of the C–O bond (Mazarin, Phan, & Charles, ). Similar CID experiments performed for an SG1‐capped PEO‐ b ‐PS block copolymer electrosprayed as intact lithiated adducts also evidenced the elimination of the SG1 nitroxide, which was followed by a complete depolymerization of the PS block (Girod, Phan, & Charles, ).…”

Mass spectrometry of nitroxide-terminated poly(4-vinylpyridine): A case of unwanted reactive MALDI

“…The energy level involved in the MALDI process is indeed sufficient to induce spontaneous homolysis of the intrinsically fragile bond between the last monomer and the nitroxide end-group of NMP polymers [18], preventing intact species to be detected. One exception was however reported in the case of protonated species: for SG1-terminated poly(ethylene oxide), protonation of the nitroxide nitrogen was shown to increase the C ON bond dissociation energy and allowed intact species to be generated in the gas phase upon MALDI [24,25]. Here, lack of intact protonated SG1-terminated P4VP hence indicated that the adducted proton was not located in the nitroxide end-group but most probably on a pyridine pendant group, consistent with the high basicity of these moieties.…”

Solvent‐based and solvent‐free characterization of low solubility and low molecular weight polyamides by mass spectrometry: a complementary approach

“…Therefore, there is growing interest in solvent‐free strategies in MALDI sample preparation, with the development of the 'solvent‐free' or 'evaporation grinding' methods . Solvent‐free preparation was also used to analyze fragile compounds when a solvent‐based strategy failed …”