Solid state nuclear magnetic resonance as a tool to explore solvent-free MALDI samples

Abstract

Solid-state Nuclear magnetic resonance (NMR) was used here to explore structural characteristics of samples to be subjected to matrix-assisted laser desorption/ionization (MALDI) and prepared without the use of any solvent. The analytical systems scrutinized in NMR were mixtures of a 2,5-dihydroxybenzoic acid (2,5-DHB) matrix and caesium fluoride (CsF), used as the cationization agent in synthetic polymer MALDI mass analysis, at different molar ratios (1:1, 5:1, and 10:1). Complementary information could be obtained from 13 C, 133 Cs, and 19 F NMR spectra. Grinding the matrix together with the salt in the solid state was shown to induce a strong modification in the molecular organization within the MALDI sample. The evidenced mechano-induced reactions allow strong interactions between the matrix and the cation, up to the formation of a salt, and only occur in the presence of some water molecules. Addition of a poly(ethylene oxide) polymer as the analyte did not further modify the observed molecular organizations. Although relative matrix and salt concentrations in the scrutinized samples were unusual for MALDI analysis, mass spectra of good quality could be obtained and revealed that cation attachment on polymers during the MALDI process is not a matrix-independent event since a lower ionization efficiency was obtained from highly organized solid samples, mostly consisting of 2,5-DHB caesium salt species. (J Am Soc Mass Spectrom 2009, 20, 1906 -1911) © 2009 American Society for Mass Spectrometry M atrix-assisted laser desorption/ionization (MALDI) is widely used as an efficient ionization method to characterize synthetic polymers by time-of-flight mass spectrometry (TOF MS) [1,2]. A key point in the success of a MALDI-MS analysis is sample preparation, that is, proper matrix and cation selection as well as solid-state organization. The matrix actually plays multiple roles in the MALDI process and should accordingly present specific properties. Some criteria are clearly defined, such as a strong absorptivity at the employed laser wavelength or good vacuum stability, while other requirements, such as a good miscibility of the matrix with the analyte in the solid state, are not easily related to physico-chemical parameters of the matrix. As a result, prediction for the optimal matrix-polymer system is still not possible and sample preparation methods are usually developed from published protocols that were shown to work for a given polymeric system [3]. In particular, solvent-free MALDI was recently proposed as an alternative to the conventional dried droplet method [4,5].Despite MALDI being a widely used technique, mechanisms underlying this desorption/ionization process are still not fully understood. To investigate the MALDI process, many studies have focused on sample morphology using different imaging experiments. Sample heterogeneity and distribution of molecules within the MALDI solid sample have been explored using microscopy techniques, such as optical microscopy [6,7], scanning electron microscopy [8 -...

Evidence of dynamical local scale distribution heterogeneities in CsF-AF (A=Li, Na, K and Rb) molten salts

“…76 CsF crystallizes in P m -3 m space group and Cs is surrounded by 6 F. 77 In solid CsF, the 133 Cs chemical shift has been determined using MAS NMR and is equal to +175ppm. 78,79 In pure molten CsF, 133Cs δ = +88ppm and the coordination number N +-has been estimated to 5.6. 80 Though it may be rather tricky to define a coordination number in these systems and to The second important feature seen in figure 5 is the quasi-plateau at low x LiF (<0.4) for the CsF-LiF system.…”

A perspective on MALDI alternatives—total solvent‐free analysis and electron transfer dissociation of highly charged ions by laserspray ionization

“…Grinding time also varies and can be as short as a few seconds63, 64 to about 10 min 29. NMR spectroscopy has been recently used to demonstrate morphology changes with extended grinding time periods (∼20 min) 65, 66. Low solubility and high‐mass compounds typically require more vigorous grinding and may fail with low energy mixing devices such as laboratory vortexers.…”

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)

“…The characterization of solid‐state structures at the atomic level can also be performed using solid‐state nuclear magnetic resonance (NMR), which can reveal differences between samples that look identical when analyzed by other solid‐state techniques. However, only a few reported studies have used solid‐state NMR to scrutinize molecular organization in MALDI samples 39–42 . NMR can explore various morphological systems, such as those obtained for MALDI samples, as a function of crystallization conditions 40 .…”

“…NMR can explore various morphological systems, such as those obtained for MALDI samples, as a function of crystallization conditions 40 . NMR data permitted us to show that mechano‐induced reactions occurred on grinding the matrix (2,5‐DHB) together with the salt (CsF) to prepare a solid binary mixture for MALDI of PEG; strong interactions between the matrix and the cation led to the formation of a salt in the presence of some water molecules 39 . Moreover, an NMR study of 2,5‐DHB/PEG/CsCl ternary mixtures allowed three main matrix phases to be identified as a function of the grinding time, and different MALDI activity was measured for each phase 42 .…”