Electron ionisation mode

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 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... 

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])...

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... 

Illustrated in Figure 24.4 is the output characteristic of a pentacene OFET with Au drain-source electrodes and a 200 nm Si02 dielectric [32]. The OFET exhibits unipolar p-type behaviour with a hole mobility = 0.165 cmWs, a threshold of = -4.5 V as well as an On/Off ratio of >10. These parameters have been derived from the respective transfer characteristics. The absence of an s-shaped feature in the linear range of the characteristic indicates ohmic contacts between the Au electrodes and the pentacene active layer. This is attributed to the good matching of the ionisation potential of the organic semiconductor and the Au work frmction. However, employing a Ca drain-soirrce metallisation, with an otherwise identical OFET device structure, the transistor did not exhibit any current in the electron accumulation mode. This is unexpected, since the metal work frmction is well matched to the electron affinity of pentacene. 

The chemical ionisation mass spectral data indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation process of polyethylene oxalate resulting in the formation of cyclic oligomers. These products are not stable in the electron ionisation (El) mode and are therefore not directly observed in the El mass spectrum. 

Ballistreri and co-workers [46] studied the thermal decomposition of some totally aromatic, totally aliphatic polyhydrazides or polyoxamides by direct Py-MS using both chemical ionisation and electron impact modes ... 

Direct pyrolysis in the ion source of a mass spectrometer (DPy-MS) operating both in electron impact and chemical ionisation modes was used in these studies. Flash Py-GC-MS was also used in the case of polythiomethylene to confirm the DPy-MS results. The overall evidence indicated that the primary thermal decomposition of these polymers yielded a wide range of cyclic sulfides by an intramolecular exchange process. A 3-CH hydrogen transfer reaction, occurring in parallel with the former process, produced primary pyrolysis compounds with SH end-groups. 

Analysis of volatiles produced in the headspace of bread and strawberries packed samples was performed by using a Perkin Elmer TurboMass Gold GC-MS (Boston, MA, USA) equipped with a split/splitless injector and a quadrupole mass spectrometer operating in electronic impact (El) ionisation mode (70 eV). A SPB-5 capillary column (30 m x 0.25 mm x 0.25 pm Supelco, Bellefonte, PA, USA) was used. The column temperature was programmed from 40 C (hold 10 min) to 120 C (hold 1 min) at 5 C min , to 140"C at 2 C min" (hold 0 min) and to 230"C at 5 C min (hold 8 min). Helium was used as carrier gas at a flow rate of 1 ml min. ... 

On- and off-line PyGC-MS approaches were discussed by Boon [708]. The first directly coupled PyGC-MS system, using a Curie-point pyrolyser, was described by Simon et al. [762]. The use of flash pyrolysis has increased dramatically with introduction of fused silica GC columns. In PyGC-MS the type of ionisation mode is usually either El or CL Electron impact ionisation at the normal ionising voltage (70 eV) causes extensive fragmentation. 

Figure 4.8 shows the typical pyrogram of a vinylidene chloride/vinyl chloride copolymer. The identification of all four trimers was accomplished by comparing retention times with those of standard compounds, as well as identification by Py-GC/MS in the electron ionisation (El) mode. 

The system was used to study a wide range of polymeric materials. The sensitivity is sufficiently high to allow samples of 5 ug or less to give adequate electron-impact spectra, but in the chemical ionisation mode larger samples are necessary. Mass pyrograms are usually characteristic of the sample type and frequently allow discrimination between samples of similar composition. 

Barcelo [13] characterised selected pesticides by negative ion chemical ionisation thermospray high performance liquid chromatography-mass spectrometry. Ions observed in the negative ion chemical ionisation spectra corresponded to mechanisms of anion attachment ([M + acetate], electron capture ([M] ) and dissociative electron capture ([M-R] ). Sensitivity was lower in the negative ion chemical ionisation mode than in the positive ion mode. 

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. 

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