Ionisation field desorption

In the mass spectrometric analysis of drugs and their metabolites new ionisation techniques such as chemical ionisation or field ionisation/field desorption are of great interest. 

In recent years higher field H-n.m.r. spectroscopy and new mass spectroscopic methods (chemical ionisation, field desorption, FAB and Mikes technique) became available. Recently Baldwin etal. (2) applied some of these new mass spectroscopic techniques to the structure determination of some bruceolides. A detailed C-n.m.r. study of a number of quassinoids has been published (Si) and C-n.n.r. data have since then been recorded in most of the papers dealing with constituents of Simaroubaceae. X-Ray analysis has also become more accessible and structures of many quassinoids are now being established by this method. 

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

Recent attention has focused on MS for the direct analysis of polymer extracts, using soft ionisation sources to provide enhanced molecular ion signals and less fragment ions, thereby facilitating spectral interpretation. The direct MS analysis of polymer extracts has been accomplished using fast atom bombardment (FAB) [97,98], laser desorption (LD) [97,99], field desorption (FD) [100] and chemical ionisation (Cl) [100]. 

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

Analysis of polymer additives by mass spectrometry has, for the most part, been limited to molecular weight determination of the solvent-extracted components [4,254], Field desorption is a good ionisation... 

The only mass spectrometric methods available during the era of the first cascade synthesis in 1978 [30] were electron impact (El) and field desorption (FD) mass spectrometry [31]. Fast atom bombardment (FAB) mass spectrometry is limited to fairly low mass ranges and not very suitable for compounds of low polarity. It was not until the development of new and gentle ionisation methods such as MALDI (matrix-assisted laser desorption ionization) [32] and ESI (electrospray ionization) [33] that the conditions were fulfilled for the start of intense research in the field of dendrimer chemistry. The following section will present the special features of these mass-spectrometric methods and their importance in dendrimer analysis. 

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. 

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. 

Rather limited use has been made of mass spectrometry in the study of organotin compounds,23-24 though MS linked to gas-liquid chromatography is now being used for the identification of organotin compounds, particularly in environmental studies. Most of the early work involved electron ionisation (El), but in recent years, other techniques such as chemical ionisation (Cl),25 fast atom bombardment (FAB),26, 27 field desorption,28 surface ionisation,29 and, particularly, electrospray (ES),30 31 have been used. 

Chemical ionisation mass spectra (CIMS NHJ as ionisation reactant) can yield satisfactory spectra from underivatised peptides and field-desorption ionisation has been shown to give intense [M+ 1]+ ions from otherwise involatile peptide derivatives (for example, CH3—CO—Gly—Arg—Arg—Gly—OCH3 Buehler el al., 1974), but less sequence information is gained in these mild ionisation procedures because less fragmentation occurs and there are relatively few peaks in the mass spectrum. MALDF - matrix-assisted laser desorption ionisation MS - is an acronym that is encountered in recent literature for this ionisation technique. 

The molecular ion can be detected by field desorption MS or by negative mode FAB. Upon El ionisation no molecular ion is obtained, but distinct fragment ions are observed. A strong IR absorption at 1240 cm 1 (S=0) is diagnostic. H NMR spectra are only to a small extent influenced by the sulphate group. 

To obtain a molecular ion, mild ionisation techniques are employed, i.e. electron impact (E. I.) with low ionization energy, chemical ionisation (C.I.) or field desorption methods (F.D.). The manner in which the various physical methods have been employed to elucidate the structure of the most important aminoglycoside antibiotics... 

Beckey, H. D. (ed.) Principles of Held ionisation and field desorption mass spectrometry. Oxford Pergamon Press 1977... 

In order to detect intact molecular ions soft ionisation techniques had to be developed. Field desorption (FD) was the first ionisation technique established to produce mass spectra with little or no fragment-ion content FD is based on electron tunnelhng from an emitter biased at a high electrical potential [24]. The filament of the emitter is heated and the sample is evaporated into the gas-phase. Typically, intact molecular ions are detected. This method is limited to relatively low molecular weight compounds, which additionally have to be thermally stable to some extent. 

Focal plane (array) detectors can detect a range of masses simultaneously. This provides a multichannel advantage that can improve the sensitivity for magnetic sectors, and detection limits can be improved if the analysis is limited by the analyte ion current instead of the chemical background level. This is the case for experiments such as MS/MS, electrospray ionisation, and field desorption. Array detectors can be used with pulsed ionisation methods, but the array detectors for commercial magnetic sector mass spectrometers can only detect a portion of the entire mass range at any given instant. 

Hayes and Altenau [34] were the first to report the use of MS to directly characterise antioxidants and processing oil additives in synthetic rubbers. Since then, various MS techniques have been applied to the analysis of rubber and polymer additives either as extracts or on the sample surface by laser techniques as reviewed by Lattimer and Harris [35]. Lattimer reviewed the present situation regarding MS in polymer analysis [36]. Analysis of polymer extracts by MS has proved challenging. Electron impact mass spectra (EI-MS) are often difficult to interpret due to the high concentration of processing oils and the additives in the extract, and excessive fragmentation of the molecular ions. Desorption/ionisation techniques such as field desorption (ED) and fast atom bombardment (FAB) have been found to be the most effective means for analysing polymer and rubber extracts [37, 38]. 

The soft ionisation technique of field desorption (FD) will not be covered in this chapter. For a review of this technique see Lattimer and Schulten [3]. In many applications, FAB and LSIMS are more convenient than FD and for this reason FD suffered a fall in popularity in the 1980s. It does, however, have advantages over FAB in certain applications, for example the analysis of surfactants in the presence of inorganic salts [4]. 

Chemical ionisation with butane as the reagent gas gives spectra with much less fragmentation and with the quasimolecular ion ([M+lf) as the base ion [718]. Others [961] obtained better results with methane as the reagent gas, although caution was necessary in using the data quantitatively, since the ions for saturated and unsaturated species varied appreciably in intensity. Intact wax esters have been quantified without the need for a chromatographic separation by field desorption [640,820-822] and tandem MS [873],... 

Until around 1970, the only ionisation method in common use was electron impact (El). Field ionisation (FI) was developed in the 1950s, but it was never very popular, and chemical ionisation (Cl) was just getting started. These three methods (El, FI, Q) depend upon vaporisation of the sample by heating, which very much limits polymer applications to small, stable oligomers or to polymer degradation products (formed by pyrolysis or other methods). Field desorption (FD), invented in 1969, was the first desorption/ionisation method. FD- and FI-MS are often very useful (particularly for analysis of less polar polymers), but they have never been in widespread use. 

The crude alkaloid extracts can also be examined directly by field desorption (or field ionisation) mass spectrometry (Games et al. 1973). This has proved a particularly useful method for analysing mixtures of similar compounds because little or no fragmentation occurs. Thus one observes essentially only the molecular ions, and this provides a useful crosscheck with the GC-MS results. High reso-... 

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. 

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 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... 

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