Methods of ionisation

This chapter will be limited to the following methods of ionisation  

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]. 

It would be desirable to ionise all samples by both hard and soft techniques, thereby gaining the maximum information. This is not always possible as certain ionisation techniques may be precluded by the nature of the sample, for example most cationic and anionic surfactants cannot be ionised directly by EL Where species can only be ionised by soft techniques, structural information may be obtained by collisional activation of the molecular ion (or quasimolecular ion, depending on the ionisation mode) to induce fragmentation. This type of technique is denoted as CID in Table 12.1, where ionisation modes appropriate to applications are summarised. 

These are the most widely used ionisation techniques in mass spectrometry both take place in the vapour phase. 

In electron impact ionisation, the sample is bombarded with a beam of highly energetic electrons. Positively charged molecular ions resulting from this process lose their excess energy by fragmentation. Electron ionisation spectra are very reproducible and may be identified by reference to libraries of previously obtained spectra. Commercial libraries contain tens of thousands of spectra. 

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As evident from Scheme 7.13, most modern ionisation techniques have been used for TLC-MS, and no single ionisation method is used exclusively with TLC-MS. Various ionisation methods may be applied that avoid the need to evaporate the sample into an El or Cl source these are based in particular on sputtering (FAB, SIMS) or laser desorption. Several sputtering methods of ionisation do not require the use of a liquid matrix, e.g. TLC-SIMS [797], Recent developments include the use of matrix-assisted laser desorption ionisation (MALDI) and surface-assisted laser desorption ionisation (SALDI). It is obvious that TLC-MS is complemented with TLC-MS11 [800] and TLC-HRMS techniques. Table 7.82 lists the general characteristics of TLC-MS. 

The most common method of ionisation involves Electron Impact (El) and there are two general courses of events following a collision of a molecule M with an electron e. By far the most probable event involves electron ejection which yields an odd-electron positively charged cation radical [M]+ of the same mass as the initial molecule M. 

Another common method of ionisation is Electrospray Ionisation (ES). In this method, the sample is dissolved in a polar, volatile solvent and pumped through a fine metal nozzle, the tip of which is charged with a high voltage. This produces charged droplets from which the solvent rapidly evaporates to leave naked ions which pass into the mass spectrometer. ES is also a relatively mild form of ionisation and is very suitable for biological samples which are usually quite soluble in polar solvent but which are relatively difficult to vaporise in the solid state. Electrospray ionisation tends to lead to less fragmentation of the molecular ion than EL... 

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. 

The most widely used method of ionisation is by electron impact (El) in which the vaporised sample molecules are bombarded with a stream of high energy electrons. Usually, electrons of 70 eV energy are used although only 10 eV is required for ionisation. The excess energy absorbed (up to 10 eV) causes fragmentation of the molecule to produce both negative and positive ions. 

The resolution of the mass spectrometric analysis is a measure of its ability to distinguish between species with different m/z ratios for example, a resolution of 1000 implies that the system can distinguish between species with m/z ratios of 1000 and 1001. Different modes of analysis are used, depending on the specific experimental needs and also on the method of ionisation. 

This method of ionisation is used for most samples which are reasonably stable and reasonably volatile. A number of other ionisation methods exist to tackle less stable or less volatile samples, but all work on the same principle the molecule absorbs energy and breaks down to its more stable ions. 

A major breakthrough in the analysis of proteins and peptides came about with the development of sensitive methods based on the use of mass spectrometry. The importance of these developments was recognised in 2002 with award of the Nobel Prize in Chemistry to John Fenn (electrospray (ESI) ionization) and Koichi Tanaka (matrix-assisted laser desorption/ionization (MALDI) ionization) pioneers in the development of methods of ionisation that made protein and peptide MS a practicable procedure. These developments have led to MS being the method of choice for protein identification and characterisation of protein and peptides (Fig. 

The method used to introduce the sample to the source of the spectrometer will depend on the method of ionisation used (see section 12.3), the physical form of the sample, and any requirement for separation techniques to be applied in line with the MS analysis. 

Real Time Radiography (RTR) is an advanced method of radiography in which the image is formed while the job is exposed to ionising radiation. RTR is often applied to objects on assembly lines for rapid inspection. Accept-or-reject decisions may be made immediately without the delay or expense of film development. The main advantages of RTR are thus, reduction in inspection cost and processing time. 

The most accurate method of deriving from /igobs. is to use the equation k ih. = /i2obs.(i+/) the ionisation ratio of the compound under study being determined directly at the required acidity and temperature. In the cases where the temperature at which rates are measured is not 25 °C the way in which Aafb. depends upon acidity will be given correctly, but again there will remain the difficulty that the slope to be expected at this temperature other than 25 °C is not known. 

The most widely used method of analysis for methyl chloride is gas chromatography. A capillary column medium that does a very good job in separating most chlorinated hydrocarbons is methyl siUcone or methyl (5% phenyl) siUcone. The detector of choice is a flame ionisation detector. Typical molar response factors for the chlorinated methanes are methyl chloride, 2.05 methylene chloride, 2.2 chloroform, 2.8 carbon tetrachloride, 3.1, where methane is defined as having a molar response factor of 2.00. Most two-carbon chlorinated hydrocarbons have a molar response factor of about 1.0 on the same basis. 

In addition to the wet and optical spectrometric methods, which are often used to analyse elements present in very small proportions, there are also other techniques which can only be mentioned here. One is the method of mass spectrometry, in which the proportions of separate isotopes can be measured this can be linked to an instrument called a field-ion microscope, in which as we have seen individual atoms can be observed on a very sharp hemispherical needle tip through the mechanical action of a very intense electric field. Atoms which have been ionised and detached can then be analysed for isotopic mass. This has become a powerful device for both curiosity-driven and applied research. 

Electrodeposition This method of paint application is basically a dipping process. The paint is water-based and is either an emulsion or a stabilised dispersion. The solids of the paint are usually very low and the viscosity lower than that used in conventional dipping. The workpiece is made one electrode, usually the cathode, in a d.c. circuit and the anode can be either the tank itself or suitably sized electrodes sited to give optimum coating conditions. The current is applied for a few minutes and after withdrawal and draining the article is rinsed with de-ionised water to remove the thin layer of dipped paint. The deposited film is firmly adherent and contains a minimum of water and can be stoved without any flash-off period. This process is used for metal fabrications, notably car bodies. Complete coverage of inaccessible areas can be achieved and the corrosion resistance of the coating is excellent (Fig. 14.1). 

Apparatus. A gas chromatograph equipped with a flame-ionisation detector and data-handling system. The use of a digital integrator is particularly convenient for quantitative determinations, but other methods of measuring peak area may be used (Section 9.4). 

Cox PA (1975) Fractional Parentage Methods for Ionisation of Open shells of d and f Electrons. 24 59-81... 

Obviously, by far the best method of performing SIM is to use a means of sample introduction which generates sample peaks of relatively short peak widths (as in GC or LC) that can be integrated - as opposed to the probe methods of sample introduction which deliver the sample into the ionisation source at a near-constant rate over long periods of time. 

Obvious practical requirements are that the ionisation method has to be available needs to be compatible with the mass spectrometer being used and able to handle the chemical compound class of investigation. Table 6.10 shows the compatibility of ionisation type, and mass spectrometer. 

Figure 6.5 Comparison of the selectivity of ionisation methods in mass spectrometry...

ToF analysers are able to provide simultaneous detection of all masses of the same polarity. In principle, the mass range is not limited. Time-of-flight mass analysis is more than an alternative method of mass dispersion it has several special qualities which makes it particularly well suited for applications in a number of important areas of mass spectrometry. These qualities are fast response time, compatibility with pulsed ionisation events (producing a complete spectrum for each event) ability to produce a snapshot of the contents of the source volume on the millisecond time-scale ability to produce thousands of spectra per second and the high fraction of the mass analysis cycle during which sample ions can be generated or collected. 

The ideal mass-spectrometric interface should allow for a range of ionisation methods (Tables 7.23 and 7.24). The ionisation of organic molecules for use with chromatographic outlets include El, Cl, APCI for samples that can be vaporised prior to ionisation alternative ionisation techniques using TSP, ESP or FAB are needed for labile, high-MW or ionic samples. 

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