Quantitative analyses of fullerene and polycyclic aromatic hydrocarbon mixtures via solvent‐free matrix‐assisted laser desorption/ionization mass spectrometry

We explore the feasibility of reliable quantitative matrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS) analyses via solvent‐free sample preparation, as this procedure provides the unique convenience of being applicable also to insoluble samples. As quantitative MALDI measurements are even more complicated for species ionized by cation attachment, we investigated model systems, such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes, which undergo photoionization and do not require additional cationizing salts. Our quantitative approach rests upon applying the standard‐addition method in MALDI for the quantitative characterization of binary mixtures. Two different systems are tested. Set 1 is composed of hexakis(dodecyl)hexabenzocoronene and hexakis(dodecyl)hexaphenylbenzene, which represent the product and precursor of a cyclodehydrogenation reaction, and Set 2 is a mixture of C60 and C70 fullerenes. In Set 1, severe anomalies could be detected due to a strong influence of the matrix/analyte ratio on the correlation between signal intensity and analyte amount. This can be related to the strong intermolecular interactions among the hexabenzocoronene (HBC) aromatic cores hampering the desorption step and to intermolecular charge transfers, which influence the ionization probability. Minor interferences to the quantitative MALDI characterization are encountered in the analysis of C60 and C70 fullerenes. The spherical shapes of C60 and C70 buckyballs prevent strong aggregation. Thus, no molecule‐dependent anomalies in their desorption‐photoionization behaviour are recognized. Copyright © 2008 John Wiley & Sons, Ltd.     

Multisample preparation methods for the solvent-free MALDI-MS analysis of synthetic polymers

A limitation of any current approach using solvent-free MALDI mass spectrometry is that only one sample at a time can be prepared and transferred to the MALDI-plate. For this reason, multiple-sample preparation approaches for solvent-free MALDI MS analysis of synthetic polymers were developed that are simple and practical. One approach multiplexed sample preparation by simultaneously preparing multiple samples. With this approach, as many as 384 samples could be prepared by addition of analyte, matrix, salt, and 1-mm metal beads to each well of a 384-well disposable bacti plate, capping the plate with the lid and homogenizing all samples simultaneously using a common laboratory vortex device. Besides the time savings achieved by a single vortex step for multiple samples, an additional advantage of this method relative to previously reported solvent-free preparation methods is that the mixing volume per sample is reduced, which allows a reduction in the amount of analyte required. This method, however, still requires the transfer of each homogenized sample to the MALDI plate for subsequent analysis. Here we report a novel approach that combines multiple simultaneous solvent-free sample preparation with automatic sample transfer to the MALDI target plate. This approach reduces the possibility of cross-contamination, the amount of sample and matrix consumed for an analysis, and the time required for preparation of multiple samples. These methods were shown to provide high-quality mass spectra for various synthetic polymer standards with M(n) values to 10 kDa. The methods are efficient in that small sample amounts are required, the sample/salt/matrix ratio is not critical, and the time necessary to achieve sufficient homogenization of multiple samples is less than 5 min.     

Photophysical and theoretical insights on non-covalently linked fullerene-zinc phthalocyanine complexes

The photo-physical aspects of non-covalently linked assemblies of a series of fullerenes, namely, C60, C70, tert-butyl-(1,2-methanofullerene)-61-carboxylate (1) and [6,6]-phenyl C70 butyric acid methyl ester (2) with a designed zinc phthalocyanine (ZnPc), viz., zinc-1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (3) in toluene medium are studied employing absorption spectrophotometric, steady state and time resolved fluorescence spectroscopic measurements. Of central interest in these investigations is the preferential binding of various fullerenes with ZnPc in toluene. The ground state interaction between fullerenes and 3 is first evidenced from UV-Vis measurements. Steady state fluorescence experiment reveals efficient quenching of the excited singlet state of 3 in presence of both underivatized and derivatized fullerenes. K values for the complexes of C60, C70, 1 and 2 with 3 are determined to be 6500, 22,230, 47,800 and 54,770 dm3 mol(-1), respectively. The magnitude of K suggests that 3 preferentially binds C70 and derivatized C70 in comparison to C60 and 1. Time resolved emission measurements establish that C(70)-3 and 2-3 complexes are stabilized much more in comparison to C(60)-3 and 1-3 systems in terms of charge separation process. Semi empirical calculations employing third parametric method substantiate the strong binding of C70 and its derivative with 3 in terms of heat of formation values of the respective complexes, and at the same time, determine the orientation of bound guest (here fullerenes) with the molecular plane of 3.     

The effects of molecular weight distribution and sample preparation on matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of petroleum macromolecules

To date there have been no systematic, quantitative investigations of the effect of sample preparation on the matrix-assisted laser desorption/ionization time-of-flight (MALDI) mass spectrometry response for polydisperse systems. To this end, the interrelationships between sample preparation, analyte molecular weight distribution (MWD) and solubility, and signal response were investigated for mixtures of alkylated polycyclic aromatic hydrocarbon (PAH) oligomers, the constituents of petroleum pitch that serve as precursors for advanced carbon materials. These PAH oligomers served as a useful analyte system for study, as their solvent solubilities decrease significantly with each increasing oligomeric unit. Molecular weight standards consisting of relatively pure dimer and trimer cuts of the starting M-50 petroleum pitch were produced using a dense-gas/supercritical extraction (DGE/SCE) technique and were then used to produce oligomeric mixtures of well-defined composition for study. Both traditional, solvent-based and newer, solvent-free sample preparation methods were evaluated, and their effects on both homogeneity and signal response were determined. While solvent-free sample preparation methods produced homogeneous samples and reproducible results regardless of the MWD of the analyte, solvent-based samples that contained more than one oligomeric cut produced non-homogeneous samples and poor reproducibilities. The differing solubilities of dimer, trimer, and tetramer oligomers in a given solvent (e.g., CS(2) or toluene) were found to be the cause of the inhomogeneities observed in solvent-based sample preparation. A quantitative analysis study performed with dimer/trimer mixtures over a wide range of compositions via solvent-free sample preparation indicates that linear, reproducible calibration curves can be generated and used to calculate the molecular composition of unknown dimer/trimer mixtures with confidence.     

A comparison of EDI with solvent-free MALDI and LDI for the analysis of organic pigments

To evaluate the applicability of EDI to material analysis as a new ionization method, a comparison of EDI with solvent-free matrix-assisted laser desorption ionization (MALDI) and laser desorption ionization (LDI) was made for the analysis of organic pigments, e.g. Pigment Yellow 93, Pigment Yellow 180, and Pigment Green 36, as test samples, which are poorly soluble in standard solvents. In EDI, the samples were prepared in two ways: deposition of suspended samples in appropriate solvents and dried on the substrate, and the direct deposition of the powder samples on the substrate. No matrices were used. Both sample preparation methods gave similar mass spectra. Equally strong signals of [M + H](+) and [M - H](-) ions were observed with some fragment ions for azo pigments in the respective positive or negative mode of operation. For the powder sample of the phthalocyanine pigment PG36, M(+*) and [M + H](+) in the positive mode and M(-*) in the negative mode of operation were observed as major ions. Positive-mode, solvent-free MALDI gave M(+), [M + H](+) and [M + Na](+) and negative mode gave [M - H](-) depending on the sample preparation. As solvent-free MALDI, EDI was also found to be an easy-to-operate, versatile method for the samples as received.     

Extending the solvent-free MALDI sample preparation method

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data can be used to determine the polymer average molecular weights, repeat units, and end groups. One of the key issues in traditional MALDI sample preparation is making good solutions of the analyte and the matrix. Solvent-free sample preparation methods have been developed to address these issues. Previous results of solvent-free or dry prepared samples show some advantages over traditional wet sample preparation methods. Although the results of the published solvent-free sample preparation methods produced excellent mass spectra, we found the method to be very time-consuming, with significant tool cleaning, which presents a significant possibility of cross contamination. To address these issues, we developed an extension of the solvent-free method that replaces the mortar and pestle grinding with ball milling the sample in a glass vial with two small steel balls. This new method generates mass spectra with equal quality of the previous methods, but has significant advantages in productivity, eliminates cross contamination, and is applicable to liquid and soft or waxy analytes.     

Mechanochemical Reactions of Fullerenes under High-Speed Vibration Milling

Since a novel technique called "high-speed vibration milling" (HSVM) was first applied to the Reformatskytype reaction of C60 in 1996, [1] this technique has been applied to various kinds of fullerene functionalizations including the preparation of C120. [2] Most recent reactions of fullerenes under HSVM conditions will be discussed: (1)reaction of C60/C70 with N-alkylglycines and aldehydes (Scheme 1); (2) reaction of C60 with active methylene compounds in the presence of bases (Scheme 2); (3) reaction of C6o with diazo compounds (Scheme 3); (4) reaction of C6o with anthracene derivatives (Scheme 4).     

Dimethyl azo(bisisobutyrate) and C(60) produce 1,4- and 1, 16-Di(2-carbomethoxy-2-propyl)-1,x-dihydro

Thermal decomposition of dimethyl azo(bisisobutyrate) in a solution containing C(60) produced 1,4- and 1, 16-di(2-carbomethoxy-2-propyl)-1,x-dihydro[60]fullerenes in yields of 21% and 27%, respectively, based on reacted C(60). The structure of this first 1,16-dialkyl-1,16-dihydro[60]fullerene was assigned from (13)C 2D INADEQUATE NMR spectra. The 1,16-isomer has first and second electrochemical reduction potentials shifted positively by 0. 18 V relative to those of the 1,4-isomer. From the close similarity of all spectral, chromatographic, and electrochemical data, the previously unassigned isomer of 1,x-di(2-cyano-2-propyl)-1, x-dihydro[60]fullerene, which was obtained from azo(bisisobutyronitrile) and C(60), is also a 1,16-isomer.     

Synthesis and structure of nitrogen bridged calix[5]- and -[10]-pyridines and their complexation with fullerenes

Azacalix[5]pyridine, a heteroatom bridged calixaromatic with an odd number of arene units, and azacalix[10]pyridine, a giant molecular belt, were selectively synthesized based on a 2 + 3 macrocyclic coupling strategy; both novel macrocyclic hosts formed strong 1 : 1 complexes with fullerenes C60 and C70 in a size-selective manner with association constants up to 1.3 x 10(5) +/- 0.03 x 10(5) M(-1).     

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.     

Synthesis and structure of multicomponent crystals of fullerenes and metal tetraarylporphyrins

The preparation of fullerene complexes with metal tetraarylporphyrins in the presence of excess ferrocene (Cp(2)Fe) results in the formation of new solvent-free and multicomponent molecular crystals. New isomorphous complexes of C(60) with PyZnTPP (ZnTPP identical with zinc 5,10,15,20-tetraphenyl-21H,23H-porphyrinate) and PyCoTPP (CoTPP identical with cobalt(II) 5,10,15,20-tetraphenyl-21H,23H-porphyrinate) containing Cp(2)Fe and the isostructural C(70) complex with PyZnTPP have been prepared. The crystal structures of the new layered C(60) complexes CoTMPP x C(60) (obtained in the presence of Cp(2)Fe) and CoTMPP x 2C(60) x 3C(7)H(8) (obtained in the absence of Cp(2)Fe) have been described (CoTMPP identical with cobalt(II) 5,10,15,20-tetrakis(p-methoxyphenyl)-21H,23H-porphyrinate). Cobalt atoms of the PyCoTPP and CoTMPP molecules are weakly coordinated to C(60) with Co...C(C(60)) distances in the 2.64-2.82 A range, whereas zinc atoms of PyZnTPP, as well as cobalt atoms of the CoTMPP molecules in the solvent-free phase, form only van der Waals contacts with fullerenes. Different packing arrangements in the crystals of fullerene-porphyrin complexes have been discussed.     

In-plume thermodynamics of the MALDI generation of fluorofullerene anions

The mechanism of formation of fluorofullerene (FF) negative ions derived from the compounds C(60)F(18), C(60)F(36), and C(60)F(48) was studied by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (ToF) mass spectrometry (MS). A combined experimental/theoretical approach provides compelling evidence of nondissociative, thermodynamically controlled electron transfer from matrix-derived negative ions to the FF analyte as the main secondary-ionization process. Consistent with this thermochemical model, analyte parent molecular ion yield and degree of fragmentation for a particular MALDI experiment was found to depend on the nature of the matrix material (the five matrices investigated were sulfur, trans-2-[3-{4-tert-butylphenyl}-2-methyl-2-propenylidene]malononitrile, 9-nitroanthracene, 2,6-bis((furan-2-yl)methylene)cyclohexanone, and 2,6-bis((thiophen-2-yl)methylene)cyclohexanone). For mixtures of C(60)F(n) compounds with different n values and therefore different electron affinitites, unwanted electron-transfer reactions, which can lead to the suppression of C(60)F(n)(-) ions with low n values, were successfully blocked for the first time by judicious choice of the matrix. Therefore, reliable qualitative MS analysis of FF mixtures with wide ranges of composition is now possible.     

Solvent-free MALDI-MS for the analysis of biological samples via a mini-ball mill approach

An efficient, low sample load mini-ball mill (MBM) sample preparation procedure was developed for solvent-free MALDI analysis of peptides and proteins. Picomole sample amounts can be handled conveniently, with 30 s grinding times being sufficient. Matrix purity and molar analyte/matrix ratios are not as critical as with methods employing solvent. Ammonium salt is employed for protonation of the peptide and suppression of sodiation. This strategy allows for peptide mapping and other biochemical manipulations to be performed prior to MBM sample preparation and mass analysis. The analysis of bovine serum albumin (66 kDa) yielded good results, indicating that higher molecular weight proteins are accessible. A semi-solvent-free strategy by the MBM sample preparation method is also described.     

Application of sulfur as a matrix for laser desorption/ionization in the characterization of halogenated fullerenes.

The application of sulfur as a matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) analysis of highly chlorinated and fluorinated fullerenes is reported. Control over fluorofullerene fragmentation which resulted in the domination of the molecular peak C(60)F(36)(-) was achieved, with the optimal matrix-to-analyte ratio found to be 1000:1. We suggest the possible mechanism of the molecular ion formation according to the charge transfer between the sulfur anions and C(60)F(36). For the first time the LDI and MALDI mass spectra of the highly chlorinated fullerene C(60)Cl(x)(x(max) approximately 32) are presented. The formation of odd chlorine ions (positive and negative) is observed. We conclude that use of sulfur as a matrix leads to a significant decrease in fragmentation of the halogenated fullerenes.     

Stabilities of multiply charged dimers and clusters of fullerenes

The authors find even-odd variations as functions of r (...+[C60]2(r+)([C60C70](r+)) electron-transfer collisions. This even-odd behavior is in sharp contrast to the smooth one for fullerene monomers and may be related to even-odd effects in dimer ionization energies in agreement with results from an electrostatic model. The kinetic energy releases for dimer dissociations [predominantly yielding intact fullerenes [C60]2(r+)-->C60(r1+)+C60(r2+) in the same (r1=r2) or nearby (r1=r2+/-1) charge states] are found to be low in comparison with the corresponding model results indicating that internal excitations of the separating (intact) fullerenes are important. Experimental appearance sizes for the heavier clusters of fullerenes [C60]n(r+) (n>3 and r=2-5) compare well with predictions from a new nearest-neighbor model assuming that r unit charges in [C60]n(r+) are localized to r C60 molecules such that the Coulomb energy of the system is minimized. The system is then taken to be stable if (i) two (singly) charged C60 are not nearest neighbors and (ii) the r C60(+) molecules have binding energies to their neutral nearest neighbors which are larger than the repulsive energies for the (r-1) C60(+)-C60(+) pairs. Essential ingredients in the nearest-neighbor model are cluster geometries and the present results on dimer stabilities.     

Intense near-infrared optical absorbing emerald green [60]fullerenes

Synthesis of emerald green fullerenes (EF) C60[C(CH3)(CO2Et)2]6 and C60[C(CH3)(CO2-t-Bu)2]6 was performed by using hexaanionic C60 intermediate (C60-6) as a reagent in one-pot reaction for attaching six alkyl ester addends on one C60 cage. These EF compounds exhibit intense optical absorption over 600-940 nm, the longest optical absorption of the C60 cage among many [60]fullerene derivatives synthesized.     

Synthesis of large macrocyclic azacalix[n]pyridines (n = 6 - 9) and their complexation with fullerenes C(60) and C(70)

Large methylazacalix[n]pyridines (n = 6-9) were synthesized effectively from the Pd-catalyzed macrocyclic fragment coupling reactions between alpha,omega-dibrominated and alpha,omega-diaminated linear oligomers. As macrocyclic host molecules, they formed a 1:1 complex with fullerenes C(60) and C(70) with association constants ranging from 3 x 10(4) to 1 x 10(5) M(-1).     

Detection limits of matrix-assisted laser desorption/ionisation mass spectrometry coupled to chromatography--a new application of solvent-free sample preparation

The detection limits of matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS) by semi-online coupling with chromatography were investigated using various mixtures of polyethylene oxides (PEOs) with different end groups. In contrast to the common 'dried-droplet' sample preparation technique, which results in an inhomogeneous sample-to-matrix ratio within the MALDI spot, the used coupling technique offers a very high reproducibility combined with surpassing sensitivity of a few femtograms over a broad range of sample-to-matrix ratios. These results are in correlation with the results of the recently established solvent-free MALDI-TOFMS method utilising the grinding approach and are also of assistance towards the more theoretical aspect of MALDI that suggests that there is no necessity for an analyte incorporation into a matrix crystal for excellent matrix-assistance.     

Regioselective synthesis of 1,4-di(organo)[60]fullerenes through DMF-assisted monoaddition of silylmethyl Grignard reagents and subsequent alkylation reaction

Monoaddition of Grignard reagents, in particular tri(organo)silylmethylmagnesium chlorides, to [60]fullerene took place smoothly in the presence of dimethylformamide to produce (organo)(hydro)[60]fullerenes, C60R(1)H, in good yield (up to 93% isolated yield). The hydrofullerene was then deprotonated to generate the corresponding anion, C60R(-), which was then alkylated to obtain 58pi-electron di(organo)[60]fullerenes, C60R(1)R(2), in good to high yield (up to 93% overall yield). The two-step methodology provides a wide variety of 1,4-di(organo)[60] fullerenes bearing the same or different organic addends on the [60] fullerene core. By changing the addends, one can control the chemical and physical properties of the compounds at the molecular and bulk levels.     

On the thermal stability of [60]fullerene cycloadducts: retro-cycloaddition reaction of 2-pyrazolino[4,5:1,2][60]fullerenes

2-Pyrazolino[4,5:1,2][60]fullerenes undergo a thermally induced retro-cycloaddition process whose efficiency is influenced by the nature of the C-substituent. C-Aryl-N-Aryl-2-pyrazolino[60]fullerenes (2a-d) poorly undergo a thermal retro-cycloaddition reaction even in the presence of a strong dipolarophile or a metal Lewis acid which, in contrast to other fullerene derivatives, shows their remarkable thermal stability. C-Alkyl-N-Aryl-2-pyrazolino[60]fullerenes (2e-f) show a different behavior, being more vulnerable to the presence of copper triflate and leading to the retro-cycloaddition product (pristine C60) in good yield.