Experimental and Computational Study of Aminoacridines as MALDI(-)-MS Matrix Materials for the Analysis of Complex Samples

Monoaminoacridines (1-, 2-, 3-, 4-, and 9-aminoacridine) were studied for suitability as matrices in the negative ion mode matrix-assisted laser desorption/ionization mass spectrometry (MALDI(-)-MS) analysis of various samples. This is the first study to examine 1-, 2-, and 4-aminoacridine as potential matrix material candidates for MALDI(-)-MS. In addition, spectral (UV-Vis absorption and fluorescence), proton transfer-related (basicity and autoprotolysis), and crystallization properties of these compounds were characterized experimentally and/or computationally. For testing the capabilities of these aminoacridines as matrix materials, four samples related to cultural heritage materials-stearic acid, colophony resin, dyer's madder dye, and a resinous case-study sample from a shipwreck-were analyzed with MALDI(-)-MS. A novel algorithm (implemented as an executable Python script) for MS data analysis was developed to compare the five matrix materials and to help mass spectrometrists rapidly identify peaks originating from the sample and matrix material. It was determined that all five of the studied aminoacridines can successfully be used as matrix materials in MALDI(-)-MS analysis. As an interesting finding, in several cases, the best mass spectra were obtained by using a relatively small amount of matrix material mixed with an excess amount of sample. 3- and 4-aminoacridine outperformed the other aminoacridines in the ease of obtaining acceptable spectra, average number of ions identified in the mass spectra, and low dependence of the sample-to-matrix mass ratio on experimental results.

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)

RATIONALE

Among 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).

METHODS

Ternary mixtures were prepared by grinding 2,X-DHB (X = 3-6), poly(ethylene glycol) (M_n = 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 ¹³ C, ⁷ Li, and ¹⁹ F NMR experiments.

RESULTS

Lithiated 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.

CONCLUSIONS

Complementary data in ¹³ C, ⁷ Li, and ¹⁹ F 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.

MALDI ionization mechanisms investigated by comparison of isomers of dihydroxybenzoic acid

Matrix-assisted laser desorption/ionization (MALDI) ion formation mechanisms were investigated by comparison of isomers of dihydroxybenzoic acid (DHB). These exhibit substantially different MALDI performance, the basis for which was not previously understood. Luminescence decay curves are used here to estimate excited electronic state properties relevant for the coupled chemical and physical dynamics (CPCD) model. With these estimates, the CPCD predictions for relative total ion and analyte ion yields are in good agreement with the data for the DHB isomers. Predictions of a thermal equilibrium model were also compared and found to be incompatible with the data. Copyright © 2015 John Wiley & Sons, Ltd.