Electron-impact ionisation

Principles and Characteristics Electron impact (El) ionisation is the original ionisation method (1918). Before 1980, mass spectrometry was merely restricted to electron impact (El), with chemical ionisation (Cl) being applied mainly for those samples which resist generation of satisfactory El data. Nowadays, El is still a widely used universal and nonselective ionisation method. In El, the sample is introduced as a vapour 

Supersonic molecular beam (SMB) mass spectrometry (SMB-MS) measures the mass spectrum of vibra-tionally cold molecules (cold El). Supersonic molecular beams [43] are formed by the co-expansion of an atmospheric pressure helium or hydrogen carrier gas, seeded with heavier sample organic molecules, through a simple pinhole (ca. 100 p,m i.d.) into a 10 5-mbar vacuum with flow-rates of 200 ml. rn in. In SMB, molecular ionisation is obtained either through improved electron impact ionisation, or through hyperthermal surface ionisation 

High flow-rate sampling at atmospheric pressure Fast and ultrafast GC-MS 

Enhanced molecular ion implies reduced matrix interference. An SMB-El mass spectrum usually provides information comparable to field ionisation, but fragmentation can be promoted through increase of the electron energy. For many compounds the sensitivity of HSI can be up to 100 times that of El. Aromatics are ionised with a much greater efficiency than saturated compounds. Supersonic molecular beams are used in mass spectrometry in conjunction with GC-MS [44], LC-MS [45] and laser-induced multiphoton ionisation followed by time-of-flight analysis [46]. 

For the El ion source, the generated total ion stream is directly proportional to the gas pressure in the impact field, which provides a basic condition for quantitative analysis. Compounds can only safely be quantified if influences on the sensitivity of detection, such as ion-molecule reactions and competition in the ionisation process, can be excluded by experimental evidence. 

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Rabin, 1., Schulze, W. and Jackschath, C. (1992) Electron impact ionisation potentials of gold and silver clusters,... 

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

Electron impact ionisation (El) stands for extensive fragmentation, but also produces molecular ions. The other ionisation methods shown in Table 6.10 mainly generate quasi-molecular ions for various compound classes. Protonation of organic compounds is one of the most fundamental processes of Cl, FAB and ESI mass spectrometry. Apart from electrospray (ESI), which... 

Table 6.12 shows the main characteristics of thermal electron impact ionisation. Electron impact can be used for analysing a wide variety of volatile organic... 

Table 6.12 Main characteristics of thermal electron impact ionisation...

GC-IMR-MS is based on gentle ionisation of gas molecules by ion-molecule reactions (IMR) [242]. Such reactions between reaction ions and sample gases produce a significantly smaller excess of energy than does electron impact ionisation. Thus, IMR provides a... 

Atmospheric pressure spray with electron impact ionisation Atomic plasma emission spectrometry... 

Standard deviation/mean] X 100/1) diode array detector electron impact (ionisation)... 

Mass spectrometry (MS) has been shown as the most valuable tool for the identification and characterisation of AHLs with detection levels in the pi-comole range. The major fragmentation ions in the electron impact ionisation-MS (EI-MS) of the AT-acyl and AT-(3-oxoacyl) classes of AHLs are summarised in Fig. 2 [37]. 

The most common ionisation mode used for GC/MS is electron ionisation (El), sometimes alternatively described as electron impact ionisation. Here, the compound is vaporised into the ion source. Electrons are emitted from a heated filament and accelerated to a kinetic energy of normally 70 eV through the sample vapour. This is much higher than the ionisation potential of organic compounds, so interaction of the sample molecules with electrons results in ionisation by loss of an electron. 

Mass spectra obtained under electron impact ionisation conditions... 

One other approach is direct-inlet MS. A prerequisite for mass analysis is ionisation, a process that critically influences the sensitivity and selectivity of the experiment. Electron impact ionisation (El) causes considerable fragmentation. Because of overlapping fragment and parent ions, the molecular information is difficult to deconvolute, and little chemical information can be extracted. 

Fig. 15.14 Analytical techniques for time-resolved headspace analysis. An electronic nose can be used as a low-cost process-monitoring device, where chemical information is not mandatory. Electron impact ionisation mass spectrometry (EI-MS) adds sensitivity, speed and some chemical information. Yet, owing to the hard ionisation mode, most chemical information is lost. Proton-transfer-reaction MS (PTR-MS) is a sensitive one-dimensional method, which provides characteristic headspace profiles (detailed fingerprints) and chemical information. Finally, resonance-enhanced multiphoton ionisation (REMPI) TOFMS combines selective ionisation and mass separation and hence represents a two-dimensional method. (Adapted from [190])...

Gas Chromatography Coupled with Electron Impact Ionisation Mass Spectrometry... 

Chemical ionisation, electron impact ionisation and field ionisation have been used to obtain molecular fragmentation patterns in speciation studies involving GC-MS. However, the systems used often lacked sensitivity. This area has expanded recently as inexpensive, smaller and more sensitive instruments have become available (Zufiaurre et al., 1997). Tributyl tin and triphenyl tin compounds have recendy been determined in seawater using GC negative ion chemical ionisation MS (Mizuishi et al., 1998). Of particular interest are those mass spectrometers in which the sample is broken down into elemental form. 

From electron impact ionisation mass spectra of alcohols and amines, it is well known that onium ions - oxonium ions (CR1R2OR3)+ and immonium ions (CR1R2NR3R4)+, respectively - play important roles in determining fragmentation and thereby the detailed appearance of the spectra. Onium ions are usually the results of a-cleavages of the molecular ions, and they may fragment further... 

Electrolytic conductivity detector, 185 Electron capture detector, 184 Electron impact ionisation, 254 Elentol, 707 Elimination of drugs, by the body, 280 by the kidneys, 282 by the liver, 281 Elimination rate constant, 281 Elixicon, 1011... 

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