Fast Determination of the Lignin Monomer Compositions of Genetic Variants of Poplar via Fast Pyrolysis/Atmospheric Pressure Chemical Ionization Mass Spectrometry.

The proportional content of the phenylpropanoid monomeric units (4-hydroxyphenyl (H), guaiacyl (G), and syringyl (S)) in lignin is of paramount importance in germ plasm screening and for evaluating the results of plant breeding and genetic engineering. This content is usually determined using a tedious and slow (2 days/sample) method involving derivatization followed by reductive cleavage (DFRC) combined with GC/MS or NMR analysis. We report here a fast mass spectrometric method for the determination of the monomer content. This method is based on the fast pyrolysis of a lignin sample inside the ion source area of a linear quadrupole ion trap mass spectrometer. The evaporated pyrolysis products are promptly deprotonated via negative-ion mode atmospheric pressure chemical ionization ((-)APCI) and analyzed by the mass spectrometer to determine the monomer content. The results obtained for the wild-type and six genetic variants of poplar were consistent with those obtained by the DFRC method. However, the mass spectrometry method requires only a small amount of sample (50 µg) and the use of only small amounts of three benign chemicals, methanol, water, and ammonium hydroxide, as opposed to DFRC that requires substantially larger amounts of sample (10 mg or more) and large amounts of several hazardous chemicals. Furthermore, the mass spectrometry method is substantially faster (3 min/sample), more precise, and the data interpretation is more straightforward as only nine ions measured by the mass spectrometer are considered.

Exploring the Reaction Mechanisms of Fast Pyrolysis of Xylan Model Compounds via Tandem Mass Spectrometry and Quantum Chemical Calculations.

A commercial fast pyrolysis probe coupled with a high-resolution tandem mass spectrometer was employed to identify the initial reactions and products of fast pyrolysis of xylobiose and xylotriose, model compounds of xylans. Fragmentation of the reducing end by loss of an ethenediol molecule via ring-opening and retro-aldol condensation was found to be the dominant pyrolysis pathway for xylobiose, and the structure of the product-ß-d-xylopyranosylglyceraldehyde-was identified by comparing collision-activated dissociation of the ionized product and an ionized authentic compound. This intermediate can undergo further decomposition via the loss of formaldehyde to form ß-d-xylopyranosylglycolaldehyde. In addition, the mechanisms of reactions leading to the loss of a water molecule or dissociation of the glycosidic linkages were explored computationally. These reactions are proposed to occur via pinacol ring contraction and/or Maccoll elimination mechanisms.

Molecular-Level Understanding of the Major Fragmentation Mechanisms of Cellulose Fast Pyrolysis: An Experimental Approach Based on Isotopically Labeled Model Compounds.

Evaluation of the feasibility of various mechanisms possibly involved in cellulose fast pyrolysis is challenging. Therefore, selectively 13C-labeled cellotriose, 18O-labeled cellobiose, and 13C- and 18O-doubly-labeled cellobiose were synthesized and subjected to fast pyrolysis in an atmospheric pressure chemical ionization source of a linear quadrupole ion trap/orbitrap mass spectrometer. The initial products were immediately quenched, ionized using ammonium cations, and subsequently analyzed using the mass spectrometer. The loss or retention of isotope labels upon pyrolysis unambiguously revealed three major competing mechanisms-sequential losses of glycolaldehyde/ethenediol molecules from the reducing end (the reducing-end unraveling mechanism), hydroxymethylene-assisted glycosidic bond cleavage (HAGBC mechanism), and Maccoll elimination. Important discoveries include the following: (1) Reducing-end unraveling is the predominant mechanism occurring at the reducing end; (2) Maccoll elimination facilitates the cleaving of aglyconic bonds, and it is the mechanism leading to formation of reducing carbohydrates; 3) HAGBC occurs for glycosides but not at the reducing end of cellodextrins; 4) HAGBC and water loss are the predominant reactions for fast pyrolysis of 1,6-anhydrocellodextrins; and 5) HAGBC can proceed after reducing-end unraveling but unraveling does not occur once the HAGBC reaction pathway is initiated. Moreover, hydrolysis was conclusively ruled out for fast pyrolysis of cellobiose, cellotriose, and 1,6-anhydrocellodextrins up to cellotetraosan. No radical reactions were observed.

Speciation of CuCl and CuCl2 Thiol-Amine Solutions and Characterization of Resulting Films: Implications for Semiconductor Device Fabrication.

Thiol-amine mixtures are an attractive medium for the solution processing of semiconducting thin films because of their remarkable ability to dissolve a variety of metals, metal chalcogenides, metal salts, and chalcogens. However, very little is known about their dissolution chemistry. Electrospray ionization high-resolution tandem mass spectrometry and X-ray absorption spectroscopy were employed to identify the species formed upon dissolution of CuCl and CuCl2 in 1-propanethiol and n-butylamine. Copper was found to be present exclusively in the 1+ oxidation state for both solutions. The copper complexes detected include copper chlorides, copper thiolates, and copper thiolate chlorides. No complexes of copper with amines were observed. Additionally, alkylammonium ions and alkylammonium chloride adducts were observed. These findings suggest that the dissolution is initiated by proton transfer from the thiol to the amine, followed by coordination of the thiolate anions with copper cations. Interestingly, the mass and X-ray absorption spectra of the solutions of CuCl and CuCl2 in thiol-amine were essentially identical. However, dialkyl disulfides were identified by Raman spectroscopy as an oxidation product only for the copper(II) solution, wherein copper(II) had been reduced to copper(I). Analysis of several thiol-amine pairs suggested that the dissolution mechanism is quite general. Finally, analysis of thin films prepared from these solutions revealed persistent chlorine impurities, in agreement with previous studies. These impurities are explained by the mass spectrometric finding that chloride ligands are not completely displaced by thiolates upon dissolution. These results suggest that precursors other than chlorides will likely be preferred for the generation of high-efficiency copper chalcogenide films, despite the reasonable efficiencies that have been obtained for films generated from chloride precursors in the past.

(-)ESI/CAD MSn Procedure for Sequencing Lignin Oligomers Based on a Study of Synthetic Model Compounds with ß-O-4 and 5-5 Linkages.

Seven synthesized G-lignin oligomer model compounds (ranging in size from dimers to an octamer) with 5-5 and/or ß-O-4 linkages, and three synthesized S-lignin model compounds (a dimer, trimer, and tetramer) with ß-O-4 linkages, were evaporated and deprotonated using negative-ion mode ESI in a linear quadrupole ion trap/Fourier transform ion cyclotron resonance mass spectrometer. The collision-activated dissociation (CAD) fragmentation patterns (obtained in MS2 and MS3 experiments, respectively) for the negative ions were studied to develop a procedure for sequencing unknown lignin oligomers. On the basis of the observed fragmentation patterns, the measured elemental compositions of the most abundant fragment ions, and quantum chemical calculations, the most important reaction pathways and likely mechanisms were delineated. Many of these reactions occur via charge-remote fragmentation mechanisms. Deprotonated compounds with only ß-O-4 linkages, or both 5-5 and ß-O-4 linkages, showed major 1,2-eliminations of neutral compounds containing one, two, or three aromatic rings. The most likely mechanisms for these reactions are charge-remote Maccoll and retro-ene eliminations resulting in the cleavage of a ß-O-4 linkage. Facile losses of H2O and CH2O were also observed for all deprotonated model compounds, which involve a previously published charge-driven mechanism. Characteristic "ion groups" and "key ions" were identified that, when combined with their CAD products (MS3 experiments), can be used to sequence unknown oligomers.

Mass spectrometric studies of fast pyrolysis of cellulose.

A fast pyrolysis probe/linear quadrupole ion trap mass spectrometer combination was used to study the primary fast pyrolysis products (those that first leave the hot pyrolysis surface) of cellulose, cellobiose, cellotriose, cellotetraose, cellopentaose, and cellohexaose, as well as of cellobiosan, cellotriosan, and cellopentosan, at 600°C. Similar products with different branching ratios were found for the oligosaccharides and cellulose, as reported previously. However, identical products (with the exception of two) with similar branching ratios were measured for cellotriosan (and cellopentosan) and cellulose. This result demonstrates that cellotriosan is an excellent small-molecule surrogate for studies of the fast pyrolysis of cellulose and also that most fast pyrolysis products of cellulose do not originate from the reducing end. Based on several observations, the fast pyrolysis of cellulose is suggested to initiate predominantly via two competing processes: the formation of anhydro-oligosaccharides, such as cellobiosan, cellotriosan, and cellopentosan (major route), and the elimination of glycolaldehyde (or isomeric) units from the reducing end of oligosaccharides formed from cellulose during fast pyrolysis.

Fast pyrolysis of 13C-labeled cellobioses: gaining insights into the mechanisms of fast pyrolysis of carbohydrates.

A fast-pyrolysis probe/tandem mass spectrometer combination was utilized to determine the initial fast-pyrolysis products for four different selectively (13)C-labeled cellobiose molecules. Several products are shown to result entirely from fragmentation of the reducing end of cellobiose, leaving the nonreducing end intact in these products. These findings are in disagreement with mechanisms proposed previously. Quantum chemical calculations were used to identify feasible low-energy pathways for several products. These results provide insights into the mechanisms of fast pyrolysis of cellulose.