Hard ionisation

Ionisation11 method Hard ionisation Soft ionisation ... 

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])...

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). 

Interpretation of mass spectra depends on the type of mass spectrometer and ionisation technique used. Hard ionisation methods such as El produce molecular ion fragmentation, which can be used to identify diagnostic fragmentation patterns and functional groups. Softer ionisation techniques such as ESI and MALDI provide pseudomolecular ion formation, and rules in accordance with spectral information can be used to identify corresponding molecular structure and elemental composition. Table 13.3 lists some of the types of information that can be provided by mass spectrometry, and Table 13.4 gives dehnitions of molecular masses that are highly relevant in mass spectrometry. 

Hard ionisation techniques commonly fragment molecular ions, leading to the loss of neutral species and the formation of fragmentation ions. Some common species lost in mass spectra, and possible chemical inferences that can be drawn from this information, are shown in Table 13.10. In contrast, examples of common fragment ions that are formed are listed in Table 13.11. 

The choice of an ionisation method depends on the analyte characteristics and the required type of analytical information. Classically, hard ionisation methods such as electron ionisation (El) or chemical ionisation (Cl) make use of their fragmentation capabilities to gain stractural information, typically of small organic molecules. In contrast, soft ionisation techniques such as electrospray ionisation or laser desorption are used to obtain mass spectra of intact molecules with little or no fragmentation, being capable of analysing complex multi-component mixtures. 

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