天然有机化合物X. 糖苷快原子轰击质谱中的碱金属加合离子应 用技术的研究

在对糖苷的FAB分析技术的研究中发现,如果在样品中同时加入适量的NaCl和LiCl水溶液进样时,其FAB谱中会出现[M+Na]^+和[M+Li]^+两个强峰,两峰之质量差为16(Na和Li原子量之差值),并且发现其谱图中找不到明显的碎片离子和Na^+及Li^+的加合离子,而只有糖苷分子和Na^+及Li^+的加合离子。所以很容易在FAB谱中识别它们。因而不需联谱解析而仅凭FAB谱就可准确、快速地定出糖苷的分子量。此技术国内外尚未见报道。     

糖苷混合物FAB质谱分析技术的研究

我们曾阐明了糖苷单-化合物分子量的简明, 快速确定方法, 根据糖苷化合物中引入碱金属离子的FAB正离子的加合离子峰而难于找到碎片的加合离子峰, 现在我们在糖苷混合物中, 同时引入两种碱金属离子Na^+和Li(两元素原子量之差为16), 由此得到的糖苷混合物的FAB正离子谱中, 就出现两个一组的多组强峰, 且每组的两峰之质量差为16, 每组中低质量峰为[M+Li]^+峰, 高质量为[M+Li]^+峰, 根据两峰已标出的质量数便可轻易地定出各成分的分子量来。谱图中出现的强峰组数就是糖苷混合物的成分数。     

Continuous-flow fast atom bombardment mass spectrometry

The continuous-flow fast atom bombardment probe performs equally well with or without a high-performance liquid chromatography column producing clean spectra containing little or no background noise. Its function as a liquid chromatography-mass spectrometry interface for labile and involatile samples has been illustrated with reference to dansylated amino acids. The versatility of the new probe has been exemplified by on-line enzymatic peptide sequencing.     

Chemical reactions in fast atom bombardment mass spectrometry

Examples of various chemical reactions occurring in the matrix or in the selvedge region in fast atom bombardment (FAB) mass spectrometry are discussed. These are categorized as oxidations and reductions; substitutions; clusterings and additions; and sample decomposition or transformation. Some reactions observed showed significant time behaviour and in one case it was possible to determine rate constants. These data suggest that chemical reactions can be accelerated significantly by fast atom bombardment.     

Continuous-flow fast atom bombardment mass spectrometry of oligonucleotides

Although frit-fast atom bombardment (frit-FAB) and continuous-flow FAB mass spectrometry have become standard methods for the analysis of peptides and peptide mixtures, these techniques have not been applied previously to the analysis of oligonucleotides. Mobilephase composition, flow rate, and sample size were optimized for the analysis of oligonucleotides by negative ion frit-FAB mass spectrometry (a type of continuous-flow FAB mass spectrometry). With a mobile phase consisting of methanol/water/triethanolamine (80:20:0.5, v/v/w), flow injection frit-FAB analysis of oligonucleotides showed lower limits of detection compared to standard probe FAB mass spectrometry. For example, in order to obtain a signal-to-noise ratio of 3:1, 38 prnol of d(GTIAAC) were required for frit-FAB mass spectrometry and 62 pmol were required for standard probe FAB mass spectrometry. The largest difference between frit-FAB and standard probe FAB was observed for d(pC)₅, for which the limit of detection by frit-FAB was approximately 11-fold lower than by standard FAB mass spectrometry. Adjustment of the mobile phase to pH 7 with trifluoroacetic acid increased the limit of detection (reduced sensitivity) a minimum of sixfold. Equimolar mixtures of two or three oligonucleotides produced deprotonated molecules in identical relative abundances whether analyzed by frit-FAB or standard probe FAB mass spectrometry. Finally, frit-FAB liquid chromatography mass spectrometry was demonstrated by separating mixtures of oligonucleotides on a β -cyclodextrin high-performance liquid chromatography column with a mobile phase containing methanol, water, and triethanolamine.     

Characterization of iridoids by fast atom bombardment mass spectrometry followed by collision-induced dissociation of

Fast atom bombardment mass spectra of a series of naturally occurring and synthetically modified iridoid glycosides were studied using lithium cationization and collision-induced dissociation of the resulting [M+Li]+ ions. Lithium cationization leads to the unambiguous determination of the molecular masses of these compounds. Collision-induced dissociation of the lithiated molecular ions give valuable structural information regarding the nature of the substituent on both the aglycone and the sugar moieties. The characteristic fragmentation pathways identified are (1) elimination of neutral molecules comprising the substituents on either the aglycone or sugar moieties, (2) formation of lithiated aglycone and their fragment ions, (3) formation of lithiated sugar and their fragment ions, (4) fragmentation corresponding to the cleavage of the aglycone or sugar ring and (5) fragmentation characteristic of the substituents present in either the aglycone or sugar parts of the molecule. Elimination of two acyloxy radicals from the lithiated molecular ion is a characteristic fragmentation in the case of acyloxy derivatives.     

m-nitrobenzyl alcohol electrochemistry in fast atom bombardment mass spectrometry

The efficacy of m-nitrobenzyl alcohol (NBA) as a solvent (matrix) for fast atom bombardment (FAB) mass spectrometry of a group of pyrazolate-bridged dirhodium A-frame complexes has been assessed. Although NBA is frequently used to mitigate the formation of artifacts in FAB/MS of organometallics and other materials susceptible to bombardment-induced reactions, substantial evidence indicates that such reactions cause the formation of artifacts in the spectra obtained here. Parallel absorption spectroscopic studies have established that NBA is capable of inducing both oxidation and reduction reactions independent of ion bombardment, depending on analyte reduction half-wave potential (E_1/2). From the known electrochemistry of the complexes studied, it can be estimated that 1020 mV > E_1/2 > 500 mV for the reaction of NBA serving as a reducing agent, while 500 mV > E_1/2 > 424 mV for the reduction potential of NBA. However, in the presence of bombardment the former E_1/2 must be at least as low as 356 mY, and the latter E_1/2 must be at least as high as 1188 mY. The kinetics of redox reactions involving NBA, and therefore their influence on the appearance of FAB mass spectra, will be highly sample-dependent. However, this study illustrates an important potential role for redox reactions when NBA is used as a solvent, especially in the presence of bombardment in FAB/MS. Although analyte reaction products could be identified, substantial efforts aimed at identifying NBA oxidation and reduction products did not yield any definitive results due to the complexity of product mixtures.     

Negative-ion fast atom bombardment mass spectrometry of N-phosphoamino acids

The fragmentation patterns of N-phosphoamino acids in negative-ion fast atom bombardment mass spectrometry (FABMS) showed different characteristics to those in positive-ion FABMS. Six typical N-diisopropyloxyphorphorylamino acids all had intense [M ? 1]^? peaks, and they underwent similar fragmentation pathways. In general, the elimination of one alkene molecule followed by the loss of one molecule of alcohol occurred. They also favoured an N → O rearrangement reaction, followed by fragmentation to (RO)₂ PO₂^? and (RO) (HO)PO₂^?.     

Collision-induced dissociation of ester enolate ions using fourier transform mass spectrometry

A series of ester enolates was investigated by the technique of collision-induced dissociation (CID) using a Fourier transform mass spectrometer. Two primary modes of fragmentation were observed which led to CID products characteristic of the acyl and alkoxyl moieties of the ester enolates. An investigation of the fragmentation mechanism revealed that the primary fragmentation mode appears to be a sensitive function of the structure and the proton affinities of the two possible product ions. In most cases the anion (ketenyl or alkoxide) with the lower proton affinity was observed as the threshold product, suggestive of a proton-bound dimer intermediate. Interesting secondary dissociations were observed to occur from the primary product ions, as alkoxide ions fragmented to enolate ions or other stabilized anions.     

Mechanism for dehalogenation reactions in fast atom bombardment mass spectrometry

The mechanism of a dehalogenation reaction that occurs during fast atom bombardment (FAB) mass spectrometry was examined using halogenated nucleosides as model compounds. For aglycone-halogenated nucleosides, an inverse linear relationship exists between the extent of FAB dehalogenation and the calculated electron affinity of an individual nucleoside. The degree of dehalogenation for a given nucleoside also varies inversely with the calculated electron affinity of most FAB matrices. The observed dehalogenation reaction can be completely inhibited when matrices with positive electron affinities, such as 3-nitrobenzyl alcohol and 2-hydroxyethyl disulfide, are used. High-performance liquid chromatographic analysis of the bulk glycerol matrix following exposure to the FAB beam indicates measurable amounts of dehalogenated product, suggesting that this reaction occurs in the condensed phase prior to gas-phase ion formation. A dehalogenation mechanism involving thermal electron capture and subsequent negative charge stabilization is consistent with these observations.     

Positive ions and negative ions fab (fast atom bombardment) mass spectrometry of some bisazolium salts

Significant information on the structure of various bisazolium salts of the type CA₂ (C⁺⁺ 2A^?) were revealed by the study of the FAB spectra of their positive and negative ions. Thus only one spectrum, whatever it consists of the positive or of the negative ions spectrum, enables the identification of the cation C⁺⁺ and of the anion A^?. Moreover, the same information are collected from the CAD spectrum of the adduct ion CA⁺ allowing the identification of such a compound in a mixture.     

Formation of negative ions of monosubstituted group VIB pentacarbonyls during fast atom bombardment mass spectrometry

The first negative-ion fast atom bombardment mass spectra of a related series of monosubstituted Group VIB transition metal pentacarbonyls, M(CO)₅L (M = Cr), Mo or W and L = P(Ph)₃, As(Ph)₃ or Sb(PH)₃), have been obtained. Instead of molecular ion radicals, pseudomolecular adduct ions, [M + H]^? and [M + 15]^?, were detected, with the hydride species being much more abundant. High-resolution measurements and comparison of observed isotope clusters with computer-generated theoretical isotope patterns confirmed that ionization occurred by several mechanisms, including electron capture, charge dissociation and formation of adducts with charged species. Fragmentation consisted primarily of elimination of neutral ligands, i.e. ([MH - L]^?, [MH - CO]^?, [MH - 2CO]^?, etc. B/E and constant neutral loss linked scanning with collisional activation were used to confirm fragmentation pathways and characterize the site of hydride attachment on the transition metal complex. The information obtained demonstrates the utility of fast atom bombardment mass Spectrometry in the analysis of metal carbonyls.     

Sulphonium ions: Collision-activated dissociation using fast atom bombardment ionization and a triple quadrupole mass spectrometer

The low-energy collision-activated dissociation of sulphonium cations, including symmetrical trialkyl R³S⁺, dimethylalkyl (CH₃)₂RS⁺, diphenylalkyl Ph₂RS⁺ and cyclic (CH₂)_nS⁺R, has been recorded using fast atom bombardment ionization and a triple quadrupole mass spectrometer. The general trends are easy loss of sulphide to give [R]⁺, except for R ? CH₃, and loss of alkene to give protonated sulphide if β-hydrogens are available. Loss of alkane, generally found in ammonium compounds, is not observed.     

Analysis of xanthates by negative-ion fast atom bombardment mass spectrometry

A new technique using negative-ion fast atom bombardment mass spectrometry for the analysis of xanthates and related compounds is described. Electron impact and positive-ion fast atom bombardment mass spectrometry produced no structurally related fragment ions or observable molecular ions at the expected m/z values. It was demonstrated that negative-ion fast atom bombardment ionization was the most suitable method of ionization for structure elucidation studies for the compounds described.     

Caesium fluoride as a mass calibrant in fast atom bombardment mass spectrometry

Caesium fluoride was shown to afford advantages over caesium iodide as a mass-calibration compound for fast atom bombardment mass spectrometry.     

Structural determination of saponins extracted from starfish by fast atom bombardment collision-induced dissociation mass spectrometry.

Three saponins were extracted and isolated from starfish by reversed-phase high performance liquid chromatography (HPLC), and analyzed by fast atom bombardment mass spectrometry (FAB-MS). Their molecular weight information could be obtained by the presence of abundant [M+Na]+ ions and weak [M+H]+ ions in FAB-MS spectra. Moreover, high resolution mass measurements of their [M+Na]+ ions were performed at the resolution of 10000 to elucidate the element composition of extracted saponins. The collision-induced dissociation (CID) of sodium-adducted molecules [M+Na]+ yielded diverse product ions via dissociated processes. In the collision-induced dissociation (CID)-MS/MS analysis of [M+Na]+ ion, the sulfate-containing saponins produced characteristic ions such as SO4Na+, [NaHSO4+Na]+, [M+Na-sugar]+ and [M+Na-2sugar]+ ions, whereas the sulfate-free compound showed characteristic ions produced by cleavage of sugar moiety and side chain of aglycone. The fragmentation patterns could provide information on the linkage position of sugar groups in aglycone and sulfate groups.     

Direct microanalysis by negative ion fast atom bombardment mass spectrometry

In negative ion fast atom bombardment (FAB) mass spectrometry, the use of a liquid matrix comprised of 15-crown-5—glycerol (1:10, v/v) enabled the molecular weight determination of small amounts of non-volatile acidic molecules. This ionization process could result in detection limits of less than one-fifth of those obtained using only glycerol as matrix. The function of the matrix as a proton acceptor is emphasized.     

Stereochemical effects in mass spectrometry XIII—Determination of absolute configuration by fast atom bombardment mass spectrometry

It was found that in the fast atom bombardment mass spectra of some asymmetric secondary alcohols and amines, when a pair of enantiomers, such as (2R,3R)- and (2S,2S)-2,3-diacetoxysuccinic anhydride and (2R,3R)- and (2S,3S)-2,3-dibenzoyloxysuccinic anhydride, were used as reagents, the relative abundances of characteristic ions formed by the stereoselective reaction between a sample and a reagent of different configurations were much higher than those of ions formed by a sample and a reagent of the same configuration. The absolute configurations of the sample molecule may be predicted by examination of the mass spectra of the sample measured with reagents of R and S configurations. This approach proved to be a convenient way to determine the absolute configuration of organic molecules at the micromole level by fast atom bombardment mass Spectrometry, and it has advantages over the chemical ionization method reported previously for the analysis of polar and involatile compounds.