Field ionisation

Principles and Characteristics The pioneering technique of field ionisation (FI) was the first soft ionisation technique, introduced in 1954 [105]. For FI analysis of a reasonably volatile sample, the compound under investigation is volatilised by heat close to the emitter, so that its vapour can condense on to an emitter needle. Hence, 

Applications FI-MS and FD-MS have some attractive features that make them both complementary to other desorption/ionisation methods and unique in their applications. FI has found wide application mainly for the 

Direct probe FI-MS of PP samples containing various phosphites was used to study the effect of a sequence of extrusion passes at 260 °C [109]. 

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Beckey, H.D., Principles of Field Ionisation and Field Desorption Mass Spectrometry, Pergamon Press, Oxford, 1977. 

In this chapter, we have chosen from the scientific literature accounts of symposia published at intervals during the period 1920 1990. They are personal choices illustrating what we believe reflect significant developments in experimental techniques and concepts during this time. Initially there was a dependence on gas-phase pressure measurements and the construction of adsorption isotherms, followed by the development of mass spectrometry for gas analysis, surface spectroscopies with infrared spectroscopy dominant, but soon to be followed by Auger and photoelectron spectroscopy, field emission, field ionisation and diffraction methods. 

R. Gomer, Field Emission and Field Ionisation, Oxford University Press, Oxford, 1961. 

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. 

Pyrolysis Field ionisation (FI) Quadrupole (Q) Daly detector... 

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]. 

Table 6.16 summarises the main characteristics of FI-MS. FT uses high voltages and was once restricted to sensitive double-focusing magnetic sector instruments of relatively high cost. Field ionisation is considered to be the softest ionisation mode. The reproducibility of the non-standard techniques, such as FI-MS and FD-MS, is less well assessed than that of EI-MS. A noticeable drop in FI use occurred after the mid-1980s because of the advent of FAB and other desorption/ionisation methods. FI-MS is only used in a few laboratories worldwide. 

The two closely related techniques of field desorption [108] and field ionisation [105] are appropriate for... 

FI-MS, FIMS Field ionisation mass spectrometry spectrometry... 

Brunnee, C. A Combined Field Ionisation-Electron Impact Ion Source for High Molecular Wdght Samples of Low Volatility. Z Naturforsch., B 1967,22,121-123. 

Femtosecond infrared strong field ionisation of metal clusters produced by kHz laser ablation... 

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. 

In field ionisation microscopy (FIM), helium at low pressure is introduced into the above system and the polarity of the applied potential difference is reversed. Helium atoms in the vicinity of the now positively charged metal tip are stripped of an electron and the resulting helium ions are accelerated radially to the negatively charged fluorescent screen. 

Schlichting, A., and Leinweber, P. (2009). New evidence for the molecular-chemical diversity of potato plant rhizodeposits obtained by pyrolysis-field ionisation mass spectrometry. Phytochem. Anal. 20,1-13. 

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. 

Schulten, H. R., and Lehmann, W. D. (1978). Quantitative field ionisation and field desorption mass spectrometry in life sciences. In Quantitative Mass Spectrometry in Life Sciences (A. P. De Leenheer, R. R. Roncucci, and C. Van Peteghem, eds.), Vol. 2, pp. 63-82. Elsevier, Amsterdam. 

The N2 rotational distribution also showed a sub-thermal distribution (Fig. 27) with a rotational temperature of 450 K, similar to the translational temperature. Despite the cold translational and rotational distributions, the vibrational co-ordinate is excited, with excess population in the high vibrational states. Remarkably, this result had been suggested previously on the basis of threshold ionisation measurements of N2 desorbed from Pd covered field ionisation tips [129]. Unlike the translational energy distributions observed for desorption from Ru(0001) [103], the energy release on Pd(l 1 0) does depend on the vibrational state, (E) decreasing rapidly for excited N2(u) states [127]. A cold translational distribution is indicative of desorption from a bound state, where cooling of the adsorbate hindered... 

Ofrier soft ionisation techniques include field desorption, field ionisation, atmospheric pressure ionisation, and fast atom bombardment and each increases the rmolecular weight but because of the production of cluster ions such as (M + Na), (M + 2Na), 2M, etc., they are not used in the investigation of unknowns. 

Mass Spectrometry. Mass spectrometry was conducted on the coke concentrates using a VG instrument in which the probe was heated from ambient to 5mass range 50-600 were recorded every 5 s. Spectra were recorded in both electron impact (El) and chemical ionisation (Cl, with ammonia) modes. Field ionisation (FI) spectra of some of the deactivated catalysts from the n-hexadecane MAT runs were obtained at the Stanford Research Institute as described elsewhere (16). 

If the removal of specific impurities is fundamental to obtaining reliable results, equally important is the minimisation of the residual water concentration in a cationic system. This experimental aspect deserves some ccanments. Allhcxigh the attainment of dry conditions diould be a sine qua non to anyone working in cationic polymerisation, the actual moisture level acceptable in a given study can vary appreciably with such factors as the type of initiation, the nature and concentration of the catalyst, etc. Thus,bare-cation polymerisations initiated by 7-rays or field ionisation as well as studies on direct initiation by Lewis acids require the maximum degree of dryness attainable by present techniques ( M of residual water). On the other hand, certain systems... 

Field ionisation where a positively-charged tipped or bladed electrode produces a local field which is sufficiently strong to abstract an electron from the monomer. 

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