Solvent-free MALDI-MS: developmental improvements in the reliability and the potential of MALDI in the analysis of synthetic polymers and giant organic molecules.

A dry sample preparation strategy was previously established as a new method for matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS), so-called solvent-free MALDI-MS. In this contribution, we examine systems that have been shown problematic with conventional solvent-based MALDI approaches. Problems frequently encountered are solubility, miscibility, and segregation effects during crystallization as a result of unfavorable analyte and matrix polarities. In all cases studied, solvent-free MALDI-MS simplified the measurement and improved the analysis. Solvent-free MALDI-MS enables more reliable results in well-known problematic systems such as polydimethylsiloxane with its segregation effects. However, even in highly compatible analyte/matrix systems such as polystyrene and dithranol, there were undesirable suppression effects when employing THF as solvent. Generally, the solvent-free method allows for more homogeneous analyte/matrix mixtures as well as higher shot-to-shot and sample-to-sample reproducibility. As a result, less laser power has to be applied, which yields milder MALDI conditions, reduced background signals, and provides better resolution of the analyte signals. Solvent-free MALDI-MS proved valuable for the characterization of nanosized material, e.g., fullereno-based structures, which indicated having an increased fragmentation-susceptibility. New analyte/matrix combinations (e.g., polyvinylpyrrolidone/dithranol) are accessible independent of solubility and compatibility in common solvents. An improved quantitation potential is recognized (e.g., insoluble polycyclic aromatic hydrocarbon against soluble dendrite precursor). The rapid and easy measurement of industrial products demonstrates the solvent-free method capable for improved throughput analysis of a variety of compounds (e.g., poly(butylmethacrylate) diol) in routine industrial analysis. Hence, this new MALDI method leads to qualitative and quantitative improvements, making it a powerful tool for analytical purposes, which may also prove to be valuable in future automation attempts.

Soft deposition of organic macromolecules with fast atom bombardment mass spectrometry.

In order to investigate the requirements for soft deposition of intact positively charged organic macromolecules, an homogenous series of modal compounds such as polyphenylene dendronized perylenes (PDPs), C(80)H(52), C(200)H(132) and C(320)H(212) and a series of derivatives involving perylene derivative, C(98)H(104)N(8)O(4), terrylene derivative, C(78)H(82)N(6)O(4) and quaterrylene derivative, C(140)H(138)N(10)O(8), were used for soft-landing experiments on a metallic or matrix coated surface using fast atom bombardment mass spectrometry. Soft-landing can be achieved at impact energies below 180 eV with no production of fragments. The deposition rate shows strong energy dependence with similar behavior of the different organic compounds. A single isotope of the molecule was selected and soft-landed at increased resolution.

Characterization of an insoluble poly(9,9-diphenyl-2,7-fluorene) by solvent-free sample preparation for MALDI-TOF mass spectrometry.

The application of solvent-free sample preparation for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) allowed the characterization of an insoluble fraction of poly(9,9-diphenylfluorene) that was previously hindered by the lack of suitable characterization methods. The MALDI mass spectrometric analysis gives valuable mechanistic information about the heterogeneous polymerization process of the insoluble high molecular weight fraction of the polymer. The fragmentation appearing even under moderate desorption and ionization conditions of this rigid backbone analyte is identified as a multiple loss of the bulky phenyl side groups and can be avoided by applying the new MALDI matrix 7,7,8,8-tetracyanoquinodimethane. A specialized fragmentation study by postsource decay MALDI-TOF MS reveals a molecular weight dependent change in fragmentation mechanism from an exclusive cleavage of side groups from long polymer chains to an additional cleavage of the polymer backbone of short polymer chains.

New aspects in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: a universal solvent-free sample preparation.

A method of solvent-free sample preparation is shown to be of universal applicability for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Results obtained were compared with those of traditional solvent-based sample preparation for MALDI-MS in order to demonstrate their similarities with respect to accuracy, sensitivity and resolution for polymers such as polystyrene and poly(methyl methacrylate) in a mass range from 2 to 100 kDa. The results revealed that there is fundamentally no difference in the quality of the obtained mass spectra, and we conclude that the mechanism of desorption and ionization remains unchanged. However, the solvent-free sample preparation turned out to have some advantages over the traditional method in certain cases: quick and easy applicability is shown for polyetherimide avoiding time-consuming optimization procedures. In particular, industrial pigments that are insoluble in common solvents were characterized without interfering signals from fragments. The method even showed improvements with respect to reproducibility and mass discrimination effects in comparison to traditional sample preparation. Additionally, this contribution provides new insight regarding the analyte/matrix preorganization for the desorption step which now appears to be independent of crystallinity.

Recalcitrance of poly(vinylpyrrolidone): evidence through matrix-assisted laser desorption-ionization time-of-flight mass spectrometry.

The aerobic biodegradability of an extensively used synthetic polymer was monitored the first time on a laboratory-scale fixed-bed bioreactor (FBBR) applying matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS). Polymeric poly(vinylpyrrolidone) (PVP) was spiked at concentrations of 10 mg l(-1) onto the FBBR run with river water and the biodegradation monitored after lyophilization of aliquots of the test liquor applying MALDI-TOF-MS. The latter proved to be a powerful tool for qualitative screening purposes of PVP in a molecular mass range <20 kDa in particularly yielding a high sensitivity and shot-to-shot reproducibility. The sample-to-sample reproducibility was enhanced applying the anchor target device. Post-source decay-MALDI-TOF-MS fragmentation investigations determined the unknown end groups of PVP unambiguously. Poor biodegradability of PVP can be assumed, since even after 30 days, no oxidation of the terminal groups and no difference in the repeating units was observed. A decrease in the molecular mass distribution can be drawn back rather to adsorption of PVP in the FBBR other than to biodegradation. This was further investigated performing an adsorption experiment with sewage sludge as solid matrix and analyses of the aqueous phase and sludge samples. Extrapolating these results to the situation in wastewater treatment plants, it is highly likely that PVP is eliminated from the dissolved phase by adsorption onto sludge particles.