Membrane introduction mass spectrometry

An overview of membrane introduction mass spectrometry (MIMS) is presented and comparisons are made with other direct sample introduction techniques. Special attention is given to the unique advantages and the limitations of newer variants on the MIMS technique, including affinity MIMS, reverse-phase and trap MIMS. The salient features of the interfaces used in MIMS are summarized and the various membrane materials commonly used are delineated. The applicability of MIMS is illustrated via discussion of

1. (i) bioreactor monitoring (represented by yeast fermentation),

2. (ii) environmental monitoring (illustrated by analysis of contaminated ground water samples) and

3. (iii) on-line chemical reaction monitoring (exemplified by the photolysis of aryl esters).

The applicability of MIMS to the analysis of environmental samples, including complex mixtures in water, air and soil, is noted.     

Determination of the site of protonation of substituted benzenes in water chemical ionization mass spectrometry

The site of protonation of a substituted benzene may be determined using chemical onization mass spectrometry with D₂O as a reagent gas. The observation of extensive exchange of the ring hydrogens for deuteriums is linked to protonation on the benzene ring. The lack of this exchange coupled with the formation of cluster ions (the association of the protonated species with one or more D₂O molecules) is evidence of protonation on the substituent rather than the ring. Aniline, benzaldehyde and nitrobenzene are observed to protonate at the substituent while toluene, bromobenzene, biphenyl and iodobenzene protonate on the ring. The dimethylbenzenes protonate on the ring while the diaminobenzenes protonate at one of the substituents. The dihydroxybenzenes, as well as a number of other compounds in which an oxygen is attached directly to the ring, protonate predominantly at the substituent although a small amount of exchange of one ring hydrogen is observed.     

On-line monitoring of chloramine reactions by membrane introduction mass spectrometry

Membrane introduction mass spectrometry has been applied to on-line monitoring of the reaction of monochloramine with hydrogen chloride. The detection limit for monochloramine introduced by a sheet-membrane direct-insertion probe and measured by electron impact ionization and selected ion detection was found to be 0.7 mg/l. Formation of dichloramine, trichloramine and molecular chlorine in response to the addition of hydrogen chloride to the monochloramine solution was measured on-line. The flow-through membrane introduction mass spectrometry method for detection of chloramines and characterization of their chemistry has minimal memory effects, high molecular specificity, high speed of analysis owing to fast response times, and low detection limits.     

Evaluation of nitric oxide chemical ionization mass spectra of substituted benzenes

Nitric oxide chemical ionization mass spectra of substituted benzenes obtained with the Townsend discharge technique were studied. There were four kinds of base peaks in the mass spectra, i.e. [M + NO]⁺˙, M⁺˙, [M ? H]⁺ and [M ? OR]⁺ (R = H, CH₃). The formation of the specific ion [M + NO]⁺˙ was highly dependent on the kind of substituent, and it was produced more abundantly in the case of substitutions involving electron-accepting groups. The measure of [M + NO]⁺˙ production was evaluated from the value of the ratio [M + NO]⁺˙/M⁺˙. In mono-substitutions, it was clarified that the ratios of [M + NO]⁺˙/M ⁺˙ were correlated with the Hammett substituent constant s?_p or the electrophilic substituent constant s?_p⁺. Monosubstitutions (C₆H₅R) and toluene substitutions (CH₃C₆H₄R) could be classified into six groups in terms of base peak species, [M + NO]⁺˙/M⁺˙ ratios and substituents. In disubstitutions, the mass spectral patterns were governed by the combination of substituents. Mass spectral distinctions among ortho, meta and para isomers could be made in many cases.     

Flow-injection systems for sample introduction in mass spectrometry

A feasibility study of flow injection/mass spectrometry is described. Flow-injection systems can be used either for direct introduction of an analyte to the mass spectrometer, or to convert the analyte to a suitable form for detection. This ability, along with the members separation, provides enhanced selectivity and scope. The selectivity and sensitivity inherent in mass spectrometry serve to increase the utility of flow-injection methods. Determination of arsenic and chloroform are discussed.     

Photoelectron spectra of substituted benzenes

Photoelectron spectra of several substituted dimethylanilines, nitrobenzenes, acetophenones and nitrosobenzenes have been studied with a view to examine the electronic effects of substituents on the various φ and n levels. The results are discussed in the light of molecular orbital calculations and electronic absorption spectra. Correlation of substituent effects on the IE's with π-electron densities and Hammett substituent constants has enabled rationalization of all available data on mono- and disubstituted benzenes. The IE's generally increase with the electron-withdrawing power of the substituents.     

Substituent effects in mass spectrometry: The correlation of the ionisation potentials of substituted benzenes with σp+

It is demonstrated that the ionisation potentials of mono- and di-substituted benzenes are related to the sum of Brown's σ_p⁺ of the substituents for both the meta and para isomers. From this it is concluded that the charge, in substituted arenes, lies predominantly on the aromatic ring rather than on either of the substituents. An empirical equation relating the IP's of mono- and disubstituted benzenes to the sum of σ_p⁺ is presented.     

An introduction to quadrupole-time-of-flight mass spectrometry

A brief introduction is presented to the basic principles and application of a quadrupole-time-of-flight (TOF) tandem mass spectrometer. The main features of reflecting TOF instruments with orthogonal injection of ions are discussed. Their operation and performance are compared with those of triple quadrupoles with electrospray ionization and matrix-assisted laser desorption/ionization (MALDI) TOF mass spectrometers. Examples and recommendations are provided for all major operational modes: mass spectrometry (MS) and tandem MS (MS/MS), precursor ion scans and studies of non-covalent complexes. Basic algorithms for liquid chromatography/MS/MS automation are discussed and illustrated by two applications.     

Environmental applications of membrane introduction mass spectrometry

The purpose of this review is to highlight the versatility of membrane introduction mass spectrometry (MIMS) in environmental applications, summarize the measurements of environmental volatile organic compounds (VOCs) accomplished using MIMS, present developments in the detection of semi-volatile organic compounds (SVOCs) and forecast possible future directions of MIMS in environmental applications.     

Regiospecific ortho oxyfunctionalization of substituted benzenes with singlet oxygen

A new procedure for regiospecific ortho oxyfunctionalization of phenylacetones, phenylacetic acid esters and phenylacetaldehyde has been developed. The method involves tert-butyldimethylsilylation, singlet oxygenation and successive reduction.     

用Fourier变换质谱研究苯胺衍生物的自身化学电离

报导了3-苯甲酰氧基苯胺和P-溴代苯胺在FT质谱中的自身化学电离, 采用多共振离子消除技术研究离子-分子反应机理. 着重探讨系列离子m/z 200, 290, 380和470的生成机理.     

Interaction of pyromellitic diimide derivatives with beta-cyclodextrin and anthracene-appended beta-cyclodextrin: rim binding vs inclusion complexation

Complex formation of pyromellitic diimide derivatives with beta-cyclodextrin and anthracene-appended beta-cyclodextrin was studied with use of induced circular dichroism and 1H NMR spectroscopies. It is revealed that pyromellitic diimides form rim-binding type complexes with beta-CD and in these complexes the pyromellitic diimides lie just outside of the narrow rim of the CD. With anthracene-appended beta-CD the pyromellitic diimides form true inclusion complexes. Implications of the formation of rim-binding type complexes are also discussed.     

Detection of substituted benzenes in water at the pg/ml level using solid-phase microextraction and gas chromatography-ion trap mass spectrometry.

Solid-phase microextraction (SPME) is combined with gas chromatography-ion trap mass spectrometry (GC-IT-MS) for the analysis of benzene, toluene, ethyl benzene and xylene isomers (BTEX) in water. SPME is a recent technique for extracting organics from an aqueous matrix into a stationary phase immobilized on a fused-silica fiber. The analytes are thermally desorbed directly in the injector of a gas chromatograph. The wide linear dynamic range (five orders of magnitude) and pg sensitivity of the ion trap mass spectrometer in its full scan mode is an ideal detector for identifying and quantifying the analytes extracted with an SPME device. The combined method SPME-GC-IT-MS, using fibers coated with a 100-microns polydimethylsiloxane coating, showed a limit of quantitation (LOQ) of 50 pg/ml benzene in water. This corresponds to 5 pg of benzene absorbed onto the fiber. The limit of detection (LOD) was 15 pg/ml benzene. For o-xylene spiked at 50 pg/ml in water 50 pg were absorbed by the fiber indicating an LOQ and LOD 10 times better than for benzene. The detection limits obtained exceed the requirements of both the United States Environmental Protection Agency method 524.2 and the Ontario Municipal/Industrial Strategy for Abatement program, which range from 30 to 80 pg/ml and 500 to 1100 pg/ml, respectively. The linearity of the method extended over five orders of magnitude. Relative standard deviation ranged from 2.7 to 5.2% for 15 ng/ml BTEX in water and from 5.5 to 7.5% for 50 pg/ml BTEX in water. SPME-GC-IT-MS was used to evaluate the contamination level in laboratory, potable and wastewater sources.     

Relationship of two-dimensional predominance-zone diagrams with conditional constants for complexation equilibria

A method for the construction of two-dimensional predominance-zone diagrams for non-redox chemical species is proposed. The theory for systems containing the species L, ML and ML₂ and buffer X is developed. The chemical interactions of L, M, ML and ML₂ with X are considered through complexation coefficients dependent only on X, whereas the chemical interactions between the M/L species are treated with conditional constants which depend only on X. The method can be extended to systems which contain the species ML₃, ML₄, …, ML_n. This permits the construction of two-dimensional predominance-zone diagrams for non-redox chemical species, with interpretation and applications similar to those of Pourbaix diagrams.     

Electrospray ionization tandem mass spectrometry determined regioisomeric structures of disubstituted beta-cyclodextrin derivatives

Electrospray ionization tandem mass spectrometry was applied to the analysis of regiospecifically disubstituted beta-cyclodextrins. Each disubstituted regioisomer showed unique fragmentation, distinguishing each of them unambiguously. These results demonstrate that mass spectrometry is the most straightforward method among those reported in the assignment of regioisomeric structures of these oligosaccharides.     

An investigation of tautomeric equilibria by means of mass spectrometry

Changes in the mass spectra with inlet temperature were used in this work to demonstrate the dependence of keto-enol tautomerism of acetylacetone, 3-methyl acetylacetone and 3-allyl acetylacetone on temperature. The largest dependence of temperature were shown by the ion [M ? 42]^+. arising from a McLafferty type rearrangement and by the ion [M ? Me]⁺ resulting from simple α-cleavage. The ion [M ? 42]^+. peak increases with the temperature of the inlet system while the ion [M ? Me]⁺ peak decreases. By assuming that the ion [M ? 42]^+. represents the keto form and that the ion [M ? Me]⁺ represents the cis-enol form (stabilized by the hydrogen bond) one sees that the direction of the intensity variation of these peaks with temperature is in accord with the expected change of keto-enol tautomerism with temperature. A quantitative correlation on the basis of the above assumptions is also approached. Recording of the mass spectra of these three β-diketones at different energies of the incident electrons enables us to estimate whether or not the particular ions present in the mass spectra result from energetically favourable processes. The variation of the intensities of the peaks with the temperature of the inlet at different electron energies is also discussed.     

Pd-mediated synthesis of substituted benzenes fused with carbocycle/heterocycle

A new Pd-catalyzed one-pot multicomponent coupling reaction for the construction of benzene ring fused with carbocycle or heterocycle under a Cu-free condition is described.