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.     

Pulsed double resonance in the pure quadrupole regime: off-resonant DRRF technique for detection of rare quadrupolar nuclei

A scheme for the detection of rare quadrupolar nuclei based on off-resonant pulse excitation of the abundant quadrupolar nuclei coupled to the rare ones in the sample has been outlined and demonstrated. Pure quadrupole resonance spectrum of ²³Na in poly-crystalline samples of sodium chlorate has been thus recorded at room temperature. The present double resonance scheme in the pure quadrupole regime is marked by a substantial reduction in spectrometer transmitter power and hardware requirements.     

On-line flow injection analysis of volatile organic compounds in seawater by membrane introduction mass spectrometry

The combination of flow injection analysis with membrane introduction mass spectrometry for analysis of volatile organic compounds (VOCs) in seawater is examined and is compared to measurements made in water. Membrane introduction mass spectrometry is performed using a benchtop ion trap mass spectrometer, and characterization of various aspects of the flow injection and ion trap combination for the analysis of volatile organic compounds (including anthropogenic halocarbons) in seawater is carried out. The analyte responses are shown to be linear over several orders of magnitude (e.g. for methylene chloride), independent of seawater pH (e.g. for chlorobenzene) and independent of matrix effects for the VOCs studied. A comparison of the performance of a microporous (Teflon) membrane with that of an amorphous silicone membrane is made, and the former is shown to provide lower detection limits which are in the parts-per-trillion range (300 ppt for chlorobenzene, 190 ppt for trans-1,2-dichloroethene). The microporous membrane provides faster response times by a factor of four to five for relatively more polar compounds, such as chlorobenzene. An analysis of a seven-component mixture demonstrates the ability of this on-line combination to allow multicomponent analysis of mixtures of some complexity.     

Reduced fluidity of dipalmitoyl phosphatidic acid membranes by salicylic acid

This paper presents DSC and NMR study of how the kerotolytic drug, salicylic acid (SA), affects the thermotropic and morphological behavior of a model membrane, dipalmitoyl phosphatidic acid (DPPA). The membrane-drug system has been studied in the multilamellar vesicular (MLV) and in the unilamellar vesicular (ULV) forms, for SA/DPPA molar ratios from 0 to 0.5. The mode of interaction of SA molecules with DPPA is similar in MLV and ULV. Chain-melting transition becomes sharper and shifts to higher temperatures in the presence of the drug, implying an enhanced co-operativity of the acyl chains. NMR and DSC data indicate that the drug molecules are located in the aqueous interfacial region neighboring the lipid headgroups. The membrane becomes more rigid in the presence of the drug molecules, due to a stronger interaction between the lipid headgroups leading to reduced permeability. ULVs are destroyed by even a short equilibration at room temperature, whereas prolonged equilibration of the MLV only leads to a slightly reduced interaction between the lipid headgroups due to sequestering of the drug molecules in the interfacial aqueous region.     

On-Line Monitoring of the Photolysis of Benzyl Acetate and 3,5-Dimethoxybenzyl Acetate by Membrane Introduction Mass Spectrometry

Membrane introduction mass spectrometry (MIMS) allows on-line monitoring of the products of photolysis (254 nm) of benzyl acetate in aqueous methanol and 3,5-dimethoxybenzyl acetate in water. The reaction mixture is continuously exposed to a silicone membrane through which analyte molecules permeate into a triple quadrupole mass spectrometer for qualitative and quantitative analysis. Ionization is achieved by either isobutane or ammonia chemical ionization, and ions characteristic of the reactant ester and its products are monitored simultaneously and continuously. Three products, benzyl methyl ether, ethylbenzene, and bibenzyl are observed in the benzyl acetate photolysis. Two products, 3,5-dimethoxybenzyl alcohol and 3,5-dimethoxyethylbenzene, are formed in the photolysis of 3,5-dimethoxybenzyl acetate. Quantitation is achieved through calibration using external standard solutions and, in the case of benzyl methyl ether, tandem mass spectrometry is used to verify product identification. During the photolysis of benzyl acetate, benzyl methyl ether and ethylbenzene are produced at onset with similar efficiencies. For the 3,5-dimethoxy ester photolysis, performed in aqueous solution, the efficiency of formation of the polar product 3,5-dimethoxybenzyl alcohol is about 300 times greater than that of the nonpolar product 3,5-dimethoxyethylbenzene. The results show that the relative reaction rates are dependent on the solvent and on the photon intensity and are consistent with earlier off-line experiments by Pincock et al. which showed that the photolysis proceeds through both ion and radical pair intermediates. To the best of our knowledge, the work reported here describes the first analysis of the photochemistry of an aralkyl ester in water and the first use of on-line mass spectrometry in a mechanistic study.     

Polarographic reduction behaviour of copper in the presence of 1-amino-3-methoxypropane and 2-dimethylaminoethanol

1-Amino-3-methoxypropane (3MPA) and 2-dimethylaminoethanol (DMAE) are potential volatile amines for boiler water treatment and were investigated for their complexation behaviour with copper. Their pK_b values were 3.67 and 4.52 at 25°C and they formed coloured complexes with absorption in the region of 644 and 510 nm, respectively. In the pH range 10–11, the Cu-3MPA complex exhibited stepwise reduction and [Cu(3MPA)₂]⁺ was identified, with a stability constant of 10^9.52. In the pH range 8–10.5, the Cu-DMAE complex exhibited 2e^? reduction and the species [Cu(DMAE)₂(OH)₂] was identified, with a stability constant of 10^20.39. A correlation between visible and ESR spectra and reduction behaviour was established.     

Thermische Reaktionen mit 3-Phenyl-2H-azirinen; 1, 3-dipolare Cycloadditionen und En-Reaktionen

Benzonitrile p-nitrobenzylide ( 5 ) undergoes 1,3-dipolar cyclo-additions in the presence of 3-phenyl-2H-azirines ( 1 ), yielding in benzene at 0° 2-(p-nitrophenyl)-4,5-diphenyl-1,3-diazabicyclo[3.1.0]hex-3-enes ( 7 , scheme 2). Under the basic conditions of the reaction mixture, 7 a and 7 b are partially converted to 2-(p-nitrophenyl)-4,5-diphenyl-1,6-dihydropyrimidines ( 8a, b ) which are dehydrogenated by oxygen to the corresponding pyrimidines 9a and 9b , respectively. 3-Phenyl-2H-azirines ( 1 ) form, on heating at 145° in xylene in the presence of the azalactone 32 (2,4-diphenyl-Δ²-oxazolin-5-one), 4-(aziridin-2′-yl)-2,4-diphenyl-Δ²-oxazolin-5-ones ( 33 , scheme 11). 33 arises from an ene reaction of the enol form of 32 with 1 . Similar ene reactions are observed with the azirines 1 and dimedone ( 37 , scheme 12). Under the ene reaction conditions (xylene, 145°), the non-isolated intermediate primary adducts ( 38a and 38b ) undergo rearrangements of the vinylcyclopane-cyclopentene type to give 6,6-dimethyl-4-oxo-1,3-diphenyl-4, 5, 6, 7-tetrahydroisoindole ( 40 ) and 6, 6-dimethyl-4-oxo-3-phenyl-4, 5, 6, 7-tetrahedroindole ( 42 ), respectively.     

Valence bond calculations of vicinal proton coupling constants in cyclopropane

Vicinal proton couplings in cyclopropane have been calculated using Dirac-Van Vleck and variational methods in valence bond formalism. The couplings may be considered as taking place along a path comprising either three or four intervening bonds. Inclusion of all exchange integrals in the appropriate fragment models leads to a satisfactory ratio of J_cis/J_trans . In this respect valence bond results appear to be superior to those obtained using a molecular orbital approach. The present valence bond results on couplings in cyclopropane are not affected by the choice of models for bonding in cyclopropane, namely the Coulson-Moffitt model or the Walsh model.     

Reduction of Carboxylic Acids by Tetraalkyl Ammonium Borohydride

Tetraalkylammonium borohydride reduces carboxylic acids to the corresponding alcohols in good yields utilizing only stoichiometric quantities of hydride and also in the absence of any Lewis acids.