Mass spectrometry 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... 

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])...

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). 

The process described above imphes a chemical reaction between the substance of interest, M, and an electron, e% followed by further non-reversible dissociation reactions. The latter is true for all modes of ionisation, as shown in the following sections. Hence, it must be noted that mass spectrometry is a destructive method of analysis. Fortunately, the technique is very sensitive and only a minute amount of material is required to perform an analysis (often of the order of few picograms or less). Furthermore, as shown later in the section dealing with linked-scanning techniques, a single sample introduction step is often sufficient to allow for the performance of several experiments. 

One of the most serious drawbacks that has been observed in the ionisation process with TSP, APCI, ESI interfaces, and also with FAB, is the soft ionisation of the analytes which mostly leads to molecular ions or molecular adduct ions. Though molecular mass information is provided, there is little or no structural information at all observable with PBI or electron impact (El) MS. This soft ionisation is clearly disadvantageous for any identification of environmental contaminants, since it generates either considerably less or no fragments at all, and hence is unable to confirm the presence of such compounds of environmental concern. With the commercial availability of tandem devices, tandem mass spectrometry (MS/MS) helped to overcome these identification obstacles via coUision-induced dissociation (CID) in MS/MS mode or via ion trap in MS mode. Today, even bench-top machines provide the possibility of MS . However, when TSP began to become the method of choice in environmental analysis and became commercially available, MS/MS technology was still quite expensive. Users of TSP ionisation with spectrometers not amenable for MS/MS had the possibility to record... 

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. 

Besides the universal detector systems, for example electron capture, flame ionisation and thermal conductivity usually coupled with gas chromatographic columns, various other detectors are now being used to provide specific information. For example, the gas chromatograph/mass spectrometer couple has been used for structure elucidation of the separated fractions. The mechanics of this hybrid technique have been described by Message (1984). Other techniques used to detect the metal and/or metalloid constituents include inductively coupled plasma spectrometry and atomic absorption spectrometry. Ebdon et al. (1986) have reviewed this mode of application. The type and mode of combination of the detectors depend on the ingenuity of the investigator. Krull and Driscoll (1984) have reviewed the use of multiple detectors in gas chromatography. 

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