Direct electron ionization

If the analyte is exposed to energetic electrons the method is called direct electron ionization (DEI) or desorption electron ionization (DEI), and accordingly it is termed direct or desorption chemical ionization (DCI) if the analyte is immersed into the reagent gas under conditions of chemical ionization (Chap. 7). 

Some analyte fragmentation can be induced with APCI-MS and ESI-MS by collision-induced dissociation (CID) on octapole, hexapole, or cone devices at the input of the mass spectrometer. The newly developed direct electron ionization interface (DEI) involves the direct introduction of a nano-LC system working with a mobile phase flow rate of between 0.3 and 1.5 /rl/min into a mass spectrometer equipped with an electron ionization interface. It has been used to determine and identify several OPPs in water samples. Electron ionization generates spectra that can be interpreted using commercially available documentation (Wiley or NIST). 

Cappiello, A., Famiglini, G., Palma, P., and Mangani, F., Trace level determination of organophosphorus pesticides in water by the new direct-electron ionization LC/MS interface. Anal. Chem., 74, 3547-3554, 2002. 

APCI), atmospheric pressure photoionization (APPI), and direct electron ionization (El). Sancho et al. [4] discuss the advantages and limitations of the various mass-analyzer technologies, and Anacleto et al. [5] describes the applications of various MS sources. 

The above direct process does not produce a high yield of ions, but it does form many molecules in the vapor phase. The yield of ions can be greatly increased by applying a second ionization method (e.g., electarn ionization) to the vaporized molecules. Therefore, laser desorption is often used in conjunction with a second ionization step, such as electron ionization, chemical ionization, or even a second laser ionization pulse. 

It is very obvious from the peaks and troughs displayed in Fig. 1 that the anticipated dissymmetry between forward and backward electron ejection directions (relative to the photon beam direction) is borne out by experiment. Moreover, one sees that the dissymmetry lies in opposite directions for ionization of the two energetically accessible orbitals observed here. 

An example of a system that can be evalnated nsing this approach is methylene (CH2). The ionization energy of CH2 has been measnred directly from electron ionization thresholds to give valnes of 10.35 + 0.15 and 10.2 + 0.2eV. However, very accurate measurements of the ionization energy have been made more recently... 

Regert, M. and C. Rolando (2002), Identification of archeological adhesives using direct inlet electron ionization mass spectrometry, Anal. Chem. 74(5), 965-975. 

M. Regert, C. Rolando, Identification of archaeological adhesives using Direct Inlet Electron Ionization Mass Spectrometry, Analytical Chemistry, 74, 965 975 (2002). 

M.P. Colombini, F. Modugno, E. Ribechini, Direct exposure electron ionization mass spectra metry and gas chromatography/mass spectrometry techniques to study organic coatings on archaeological amphorae, Journal of Mass Spectrometry, 40, 675 687 (2005). 

Figure 2.1 Mass spectrometric approach. Dl, direct inlet GC, gas chromatography HPLC, high performance liquid chromatography CZE, capillary zone electrophoresis El, electron ionization Cl, chemical ionization ESI, electrospray ionization DESI, desorption electrospray ionization APCI, atmospheric pressure chemical ionization MALDI, matrix assisted laser desorption ionization B, magnetic analyzer E, electrostatic analyzer...

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

Electron ionization (El) was introduced in 1921 by Dempster, who used it to measure lithium and magnesium isotopes [31]. Modern El sources are, however, based on the design by Bleakney [32] and Nier [33, 34], who both worked in Prof. J. T. Tate s laboratory. In El ions are produced by directing an electron beam into a low pressure vapor of analyte molecules. 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

Direct photo-ionization or photo-induced electron transfer from marine and terrestrial DHS to a variety of polyaromatic electron acceptors have been documented by time-resolved and steady-state laser flash kinetic spectroscopy studies under conditions which facilitate extrapolation to the environment. 

The quadrupole ion trap (QIT) creates a three-dimensional RF quadrupole field to store ions within defined boundaries. Its invention goes back to 1953, [103-105] however, it took until the mid-1980s to access the full analytical potential of quad-mpole ion traps. [137-140] The first commercial quadmpole ion traps were incorporated in GC-MS benchtop instruments (Finnigan MAT ITD and ITMS). Electron ionization was effected inside the trap by admitting the GC effluent and a beam of electrons directly into the storage volume of the trap. Later, external ion sources became available, and soon a large number of ionization methods could be... 

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9]... 

Employing a direct exposure probe (DEP) may be helpful in case of analytes that cannot be evaporated from a sample vial without complete decomposition. [44] Here, the analyte is applied from solution or suspension to the outside of a thin wire loop or pin which is then directly exposed to the ionizing electron beam. This method has also been termed in-beam electron ionization. Early work describing the direct exposure of a sample to the electron beam came from Ohashi, [45,46] Constantin, [47] and Traldi. [48,49]... 

Traldi, P. Direct Electron Impact - a New Ionization Technique Org. Mass Spectrom. 1982,17,245-246. 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

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