Establishing the occurrence of major and minorglucosinolates in Brassicaceae by LC-ESI-hybrid linearion-trap and Fourier-transform ion cyclotron resonancemass spectrometry

Glucosinolates (GLSs) are sulfur-rich plant secondary metabolites which occur in a variety of cruciferous vegetables and among various classes of them, genus Brassica exhibits a rich family of these phytochemicals at high, medium and low abundances. Liquid chromatography (LC) with electrospray ionization in negative ion mode (ESI-) coupled to a hybrid quadrupole lineariontrap (LTQ) and Fouriertransformioncyclotronresonancemass spectrometer (FTICRMS) was employed for the selective and sensitive determination of intact GLSs in crude sample extracts of broccoli (Brassica oleracea L. Var. italica), cauliflower (B. oleracea L. Var. Botrytis) and rocket salad (Eruca sativa L.) with a wide range of contents. When LTQ and FTICR mass analyzers are compared, the magnitude of the limit of detection was ca. 5/6-fold lower with the FTICR MS. In addition, the separation and detection by LC–ESI-FTICR MS provides a highly selective assay platform for unambiguous identification of GLSs, which can be extended to lower abundance (minor) GLSs without significant interferences of other compounds in the sample extracts. The analysis of Brassicaceae species emphasized the presence of eight minor GLSs, viz. 1-methylpropyl-GLS, 2-methylpropyl-GLS, 2-methylbutyl-GLS, 3-methylbutyl-GLS, n-pentyl-GLS, 3-methylpentyl-GLS, 4-methylpentyl-GLS and n-hexyl-GLS. The occurrence of these GLSs belonging to the saturated aliphatic side chain families C4, C5 and C6, presumably formed by chain elongation of leucine, homoleucine and dihomoleucine as primary amino acid precursors, is described. Based on their retention behavior and tandem MS spectra, all these minor compounds occurring in plant extracts of B. oleracea L. Var. italica, B. oleracea L. Var. Botrytis and E. sativa L. were tentatively identified

Mass spectrometric evidence for collisionally induced removal of H2 from monoanions of 10B nido-carborane derivatives investigated by electrospray ionization quadrupole linear ion trap and Fourier transform ion cyclotron resonance mass spectrometry

Some newly synthesized 10B nido-carborane derivatives, i.e., 7,8-dicarba-nido-undecaborane monoanions ([7-Me-8-R-C2B9H10]-K+, R = H, butyl, hexyl, octyl and decyl), have been fully characterised and examined by electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry with liquid chromatographic separation (LC/ESI-FTICR-MS). These boron-containing compounds exhibit abundant molecular ions ([M]−) at m/z 140.22631 [CB9H14]−, m/z 196.28883 [CB9H22]−, m/z 224.32032 [CB9H26]−, m/z 252.35133 [CB9H30]− and m/z 280.38354 [CB9H34]− at the normal tube lens voltage setting of −90 V, which was an instrumental parameter value selected in the tuning operation. Additional [M–nH2]− (n = 1−4) ions were observed in the mass spectra when higher tube lens voltages were applied, i.e., −140 V. High-resolution FTICR-MS data revealed the accurate masses of fragment ions, bearing either an even or an odd number of electrons. Collision-induced dissociation of the [M–nH2]− ions (n = 0–4) in the quadrupole linear ion trap (LTQ) analyzer confirmed the loss of hydrogen molecules from the molecular ions. It is suggested that the loss of H2 molecules from the alkyl chain is a consequence of the stabilization effect of the nido-carborane charged polyhedral skeleton

Fatty acid neutral losses observed in tandem mass spectrometry with collision-induced dissociation allows regiochemical assignment of sulfoquinovosyl-diacylglycerols

A full characterization of sulfoquinovosyldiacylglycerols (SQDGs) in the lipid extract of spinach leaves has been achieved using liquid chromatography/electrospray ionization-linear quadrupole ion trap mass spectrometry (MS). Low-energy collision-induced dissociation tandem MS (MS/MS) of the deprotonated species [M - H]- was exploited for a detailed study of sulfolipid fragmentation. Losses of neutral fatty acids from the acyl side chains (i.e. [M - H - RCOOH]-) were found to prevail over ketene losses ([M - H - R'CHCO]-) or carboxylates of long-chain fatty acids ([RCOO]-), as expected for gas-phase acidity of SQDG ions. A new concerted mechanism for RCOOH elimination, based on a charge-remote fragmentation, is proposed. The preferential loss of a fatty acids molecule from the sn-1 position (i.e. [M - H - R1COOH]-) of the glycerol backbone, most likely due to kinetic control of the gas-phase fragmentation process, was exploited for the regiochemical assignment of the investigated sulfolipids. As a result, 24 SQDGs were detected and identified in the lipid extract of spinach leaves, their number and variety being unprecedented in the field of plant sulfolipids. Moreover, the prevailing presence of a palmitic acyl chain (16:0) on the glycerol sn-2 position of spinach SQDGs suggests a prokaryotic or chloroplastic path as the main route for their biosynthesis

GRAPHICAL RESOLUTION OF HUMIC STRUCTURES

Advanced techniques have been recently used to obtain information on Natural Organic Matter (NOM). However, the current knowledge of the chemical structure of humic substances (HS) is still incomplete. These substances appear to be too complex mixtures of charged organic molecules, and their characterization remains one of the most stimulating challenges in modern environmental science. Knowledge of the chemical composition of NOM is of great importance for the definition of soil and water properties because it has a significant impact on the understanding of numerous molecular and global-scale processes. This study aims to apply two-dimensional graphical methods to resolve homologous series in mass spectra of humic extracts (Suwannee River, Nordic Aquatic, and Soil) obtained using FT-ICR / MS (Thermo LTQ FT, 7 Tesla) in negative ionization mode. Electrospray ionization (ESI) coupled with ultra-high resolution mass spectrometry offered by Fourier transformed ion cyclotron resonance (FT-ICR / MS) has emerged with great promise as it can provide an overview of the NOM composition and details on a molecular scale. NOM's very high-resolution FT-ICR spectra can be extremely complicated. These spectra usually contain many peaks at each nominal mass and thousands of peaks across the entire spectrum. Each peak can represent a chemically distinct compound. This complexity poses an analytical challenge to the study of spectra for structural interpretation. Two-dimensional graphing methods, such as Kendrick and van Krevelen graphs, have been successfully applied to very high-resolution mass spectra, allowing peaks to be sorted into complicated spectra from their homologous relatives across the mass range. In van Krevelen plots, ionic signals corresponding to structural similarities between homologous series of compounds involved in the loss or gain of functional groups are found on straight lines. We identified many interesting homologous regions and compared the three humic standards with each other. Finally, we recognized the structural relationships of the homologous series obtained through Kendrick graphs. The results showed homologous series in the Suwannee River and Nordic Aquatic samples compared to the soil-extracted samples (soil-FA and soil-HA). In particular, homologous series signals related to methylation/demethylation, hydrogenation/dehydrogenation, hydration/dehydration and oxidation/reduction processes were lower in the soil-FA van Krevelen diagrams. On the contrary, the differences were not so evident in all the homologous series for the soil-HA samples