Using solid‐state nuclear magnetic resonance to rationalize best efficiency of 2,6‐dihydroxybenzoic acid over other 2,X‐dihydroxybenzoic acid isomers in solvent‐free matrix‐assisted laser desorption/ionization of poly(ethylene glycol)

Abstract

RationaleAmong isomers of dihydroxybenzoic acid (DHB), 2,5‐DHB is often the most efficient matrix in matrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS) for a great variety of compounds. Yet, when performing solvent‐free MALDI, 2,6‐DHB yields better results for poly(ethylene glycol [PEG]). This intriguing feature is explored here using solid‐state nuclear magnetic resonance (NMR).MethodsTernary mixtures were prepared by grinding 2,X‐DHB (X = 3–6), poly(ethylene glycol) (Mn = 2000 g mol−1) and lithium fluoride (LiF) in a matrix/analyte/salt molar ratio of 50/1/10 for 16 min under a controlled atmosphere. After mixing, a few grains were applied to the MALDI target for MS analysis, whereas the major part of the ground sample was transferred into rotors to perform 13C, 7Li, and 19F NMR experiments.ResultsLithiated PEG chains are mainly formed with 2,6‐DHB in solvent‐free MALDI, but their abundance increases with 2,3‐DHB and 2,4‐DHB when water uptake is favored by a humid atmosphere. Solid‐state NMR shows that grinding 2,6‐DHB‐based samples in atmospheric conditions leads to a solid phase in which the matrix, PEG, and salt molecules exhibit a high mobility compared with systems involving other 2,X‐DHB isomers. This mobile environment would favor (as a solvent) LiF dissociation and best promote PEG cationization.ConclusionsComplementary data in 13C, 7Li, and 19F NMR spectra are consistent with the formation of a solid phase of high mobility composed of 2,6‐DHB, PEG, and the two salt components that ultimately favor the production of lithiated PEG chains.

Design of Experiments for Matrix-Assisted Laser Desorption/Ionization of Amphiphilic Poly(Ethylene Oxide)-b-Polystyrene Block Copolymers

“…Even worse results were obtained with the 5-CSA/AgI couple that mainly generates [AgI] n Ag + salt clusters in MALDI ( Supplementary Figure 2A ). This particular result shows that the analyte, alone or in conjunction with the matrix, actually contribute to the extent of salt dissociation upon grinding, as previously demonstrated in the case of PEG with DHB matrices ( Pizzala et al, 2021 ). Accordingly, one way to further improve the DoE performance should be to include specific properties of the copolymer as additional variables.…”

“…MALDI experiments were also considered as unsuccessful (hence designated by a 100% laser fluence) when displaying signals that do not correspond to the expected oligomers adducted with the cation from the supplemented salt. Indeed, sodium is a common matrix pollutant which was shown to efficiently compete for PS or PEG cationization in MALDI, even when present at concentration levels far below that of the added salt ( Pizzala et al, 2021 ).…”

“…In contrast, the reason why matrices of low molecular weight are often found in matrix/salt associated with high fluence (r = –0.53) is more puzzling. Yet, using solid state NMR, our group has recently shown that, in the case of 2,x-DHB isomers (with x = 3–6), MALDI of PEG was influenced by matrix/polymer aggregates formed in the solid state upon grinding ( Pizzala et al, 2021 ). A similar study should be conducted to find out whether such aggregates are also formed with alternative matrices and, if so, whether the number of molecules in such aggregates varies with the size, and hence the molecular weight, of the matrix.…”

“…To gain insights in the MALDI process, morphology and molecular interactions within MALDI samples have been studied using a variety of techniques operating on solid state analytes such as scanning electron microscopy ( Doktycz et al, 1991 ; Horneffer et al, 2003 ), time-of-flight secondary ion mass spectrometry ( Hanton et al, 1999 ), crystallography ( Mele and Malpezzi, 2000 ), MS imaging ( Hanton et al, 2008 ) or solid state nuclear magnetic resonance ( Pizzala et al, 2009 ; Major et al, 2012 ; Pizzala et al, 2021 ). Although providing useful new information, reported findings could not be clearly related to molecular properties of components in the solid mixture, and hence could not allow construction of predictive rules.…”