ELECTRON IONISATION MASS

Comparison with mass-spectral libraries is the easiest way of interpreting mass spectra (and often the only way for non-mass spectroscopists). The NIST/EPA/NIH (NIST 02) electron-ionisation mass spectral library... 

The MAB ion source offers several advantages over El for PyMS. By eliminating excessive fragmentation, characteristic of electron ionisation, and by producing highly reproducible mass spectra MAB (Kr) greatly simplifies the analysis of pyrolysis data. Furthermore, MAB ionisation, when combined to MS/MS, provides a useful tool for structural elucidation of pyrolysis products. The ability for selective ionisation can be very useful to reduce the background combination in techniques such as GC-MS, LC-MS or SFC-MS. 

M.P. Colombini, F. Modugno, E. Ribechini, Chemical study of triterpenoid resinous materials in archaeological findings by means of direct exposure electron ionisation mass spectrometry and gas chromatography/mass spectrometry, Rapid Communications in Mass Spectrometry, 20, 1787 1800 (2006). 

Figure 3.8 Mass spectra of 7 oxo dehydroabietic acid obtained by DE MS using (a) electron ionisation at 70 eVand (b) chemical ionisation with isobutane...

Direct Mass Spectrometry in the Electron Ionisation Mode... 

Results obtained by Direct Mass Spectrometry using the Electron Ionisation Mode on Archaeological Samples and Wax Sculptures... 

The mode of operation of an ion trap can be described in the following way the ions are generated in the central part of the filter by electron ionisation using a short electron pulse. A radiofrequency voltage is then applied to the annular electrode, which confines the ions in the source where they follow complex trajectories in the presence of a low helium pressure of about 0.01 Pa. The mass spectrum is obtained by increasing the radiofrequency amplitude, which destabilises ions of increasing mass. The increase in voltage causes the ions to increase the amplitude of their... 

Electron ionisation is still the most widely used technique for the analysis of volatile molecules. It is considered to be a hard ionisation process, which leads to reproducible spectra that can be compared to a library of mass spectra for compound identification. In this technique, ionisation occurs in the ion source by the collision of the sample molecules with electrons that are emitted from a filament by a thermoionic process (Fig. 16.15). 

Figure 16.25—Electron ionisation mass spectrum of butanone.

Rather limited use has been made of mass spectrometry in the study of organotin compounds,23-24 though MS linked to gas-liquid chromatography is now being used for the identification of organotin compounds, particularly in environmental studies. Most of the early work involved electron ionisation (El), but in recent years, other techniques such as chemical ionisation (Cl),25 fast atom bombardment (FAB),26, 27 field desorption,28 surface ionisation,29 and, particularly, electrospray (ES),30 31 have been used. 

Mass spectroscopy [electron ionisation (El), chemical ionisation (Cl), electrospray ionisation (ESI), fast atom bombardment (FAB), matrix-associated laser desorption ionisation (MALDI), inductively coupled plasma-mass spectrmetry (ICP-MS, cf and ), etc]... 

The mass spectral data selected for multivariate analysis represented a suite of 30 microlayer and bulk surface seawater surfactant samples (fraction FI Frew et al. (2006)) including seven from waters off Monterey and Santa Barbara, California and 23 collected along a transect from Delaware Bay on the U. S. east coast to the Sargasso Sea. Sample extracts were analysed in triplicate by desorption-electron ionisation (DEI) mass spectrometry (Boon 1992 Frew et al. 2006). Individual DEI mass scans were summed over the full desorption/pyrolysis interval, reduced to integer masses, and averaged for processing by multivariate analysis. Elasticities were estimated quasi-statically from surface pressure-area (El A) isotherm measurements in a KSV 2200 Langmuir film balance (Frew et al. 2006). The elasticity data were subsampled at fixed surface pressure intervals (0.5 mN m"1) for comparison with the results of the multivariate analysis. 

A tubular stainless steel reactor (I.D. 104 mm) heated by an electrical oven at atmospheric pressure is used for the oxidation of toluene [Fig. 1]. The toluene is dosed with an HPLC pump (LKB2150) to an evaporator at 320 °C and then mixed to the O2 and N2 flows which are controlled with mass flow controllers (Bronkhorst High-Tech B. V ). Nitrogen is used as diluent. The catalyst fixed-bed preceded by quartz beads is maintained between quartz wool. The temperature of the fixed-bed is measured with a K-type thermocouple (Philips AG). The outlet gases are cooled in three consecutive condensers. The liquid products are collected and analysed by gas chromatography with a flame ionisation detector for quantification (Perkin-Elmer Autosystem gas chromatograph, capillary column Supelco SPB-1, 30 m x 0.53 mm I.D. X 0.50 jum film thickness) and with an electron ionisation detector for identification (Hewlett-Packard, G1800A, GCD System, capillary column HP-5, 30 m x 0.25 mm I.D. x 0.25 fm film thickness). The experiments are carried out at a conversion less than 5 per cent. 

The sample introduction system used will depend on the type of sample and whether a chromatographic separation is required. The usual technique for separated and relatively pure samples is the direct insertion probe which carries a small amount of solid sample through an air lock into the high vacuum of the mass spectrometer. The sample would then be vapourised by applying gentle heat, and the vapours ionised in an ionisation source. Samples already in the gas phase, such as vapours or the eluent from gas chromatography, can be introduced directly into the ionisation source at low flows ( 1 mLmin ). Usually two ionisation techniques are used for these samples, electron ionisation (El) or chemical ionisation (Cl). 

The choice of an ionisation method depends on the analyte characteristics and the required type of analytical information. Classically, hard ionisation methods such as electron ionisation (El) or chemical ionisation (Cl) make use of their fragmentation capabilities to gain stractural information, typically of small organic molecules. In contrast, soft ionisation techniques such as electrospray ionisation or laser desorption are used to obtain mass spectra of intact molecules with little or no fragmentation, being capable of analysing complex multi-component mixtures. 

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