Electron ionisation

An El spectrum comprises a mixture of ions of types A and C which may contain some molecular ion (Fig. 5.3). To complicate matters, the ions A and C may also fragment further to produce smaller ions. In these processes, the ion which fragments is known as the precursor or parent ion, whilst the smaller ions formed are known as the product ions. Understanding the relationship between precursor and product ions is at the heart of mass spectral interpretation and deducing the structure of the original molecule. A simple example is shown (Fig. 5.4). 

El spectra using 70 eV electrons are very reproducible and form the basis of many libraries of mass spectra available as an option with most data systems. There are some drawbacks. The technique is not suitable for involatile molecules, or compounds which are thermally unstable. Often the molecular ion is very weak or not detected. However, it is an excellent technique to analyse the eluent from Gas Chromatographs and many commercial GC/MS systems utilising El are available. 

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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. 

Recently, a revival of electron ionisation LC-MS has been proposed by development of LC-SMB-MS [529]. In this system, the LC output (50-250 p,Lmin 1) is vaporised at atmospheric pressure and expanded from a supersonic nozzle into the vacuum system... 

Dynamic reaction cell El Electron ionisation/impact... 

LC-ICP-MS Liquid chromatography - LVEI Low-voltage electron ionisation... 

Band System Fe(Cp)2 Fe(MeCp)2 Fe(ClCp)2 Assignment (Electron Ionised)... 

The final system of the metallocene series, Ni(Cp)2, and its dimethyl derivative, sup-ly only a small amount of information from their photoelectron spectra, since only a single peak due to a d-electron ionisation is observed in each case. This band is obviously due to ionisation of a 7r d-electron from the 32 (o2 tt2 54) ground level to yield a single ion state, 2Il(a2 tt 54), and its intensity relative to the ligand ionisation region rules out the possibility of other d-electron ionisations being coincident with it. 

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... 

Saturated aliphatic compounds represent the other extreme. Since they do not have n electrons ionisation, the molecular ion requires removal of electrons from o orbitals and so high energies. Therefore the formation of molecular ion is difficult. Further, since, the formation of molecular ion is difficult and since this is quite unstable, it cleaves easily into more stable carbocations. 

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. 

Both ESI and APCI spectra can look relatively simple in most cases, just showing the pseudo-molecular ion MH or adduct ion in the positive mode, and deprotonation or adduct ions in the negative mode. With API techniques we are dealing with even-electron (non-radical) MH ions as opposed to odd-electron M species that result from electron ionisation. Once an ion has achieved an even-electron state, it is unlikely to revert to an odd-electron state, as this is energetically unfavourable. This means that fragmentations from MH should... 

When a 300 V potential is applied between the electrodes, the electrons ionise neon (or argon). Some of the ions possess enough kinetic energy to strip atoms away from the cathode, which becomes equivalent, at the surface, to an atomic gas. Using M(S) for the element M in its metallic state (cathode) and M(G) when it is in its atomic state, emission corresponds to a series of steps ... 

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 16.15—Electron ionisation (El). The collision of an electron with a sample molecule m produces ionisation that leads to formation of a parent ion and fragment ions. Ions that result from the reaction m/ and raj are also called secondary or daughter ions. Since they carry no charge, neutral fragments produced during decomposition, (ra, m[ and m ), are not detected. An illustration of electron ionisation of benzene is shown. Also shown is a schematic of the ionisation chamber (ion source). Using a parallel magnetic field can increase the effective path of an electron in the ion source, which increases ionisation efficiency.

The pseudomolecular ion MH + (which is not a radical ion) is usually more stable than the M + ion produced by electron ionisation. Hence, fewer fragment ions are observed in chemical ionisation and so it is called a soft ionisation technique. 

Figure 16.25—Electron ionisation mass spectrum of butanone.

Electric sector, 295 Electrochromatography, 119 Electrode potential, 348 Electromagnetic separator, 294 Electromigration, 114, 117 Electron capture detector, 36 Electron ionisation, 307 Electro-osmosis, 115 Electro-osmotic flow, 114 Electrophoregram, 113 Electrophoretic mobility, 114 Electrospray, 312 ELISA, 336... 

Alteration of instrumental conditions may also provide evidence to confirm the recognition of the molecular ion. The use of maximum sensitivity may show up a very weak molecular ion. Alternatively, if the energy of the electron beam is decreased the intensity of the fragment ions will decrease relative to the molecular ion this also applies to fragment ions arising from impurities. Alternative methods of ionisation such as chemical ionisation and field ionisation are very much more likely to produce a molecular ion cluster than the electron ionisation method, and should be used if they are available. 

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