Spectrometry ionisation process

Principles and Characteristics Ionisation processes are the basis for mass-spectrometric detection. Each of the ionisation techniques occupies its own position in mass spectrometry. The optimum performance of any ionisation method (and therefore the result) will depend critically on the characteristics and reliability of the mass spectrometer. Ionisation may occur in the gas, liquid or condensed phase, and may be either hard or soft , i.e. with or without extensive... 

The sensitivity of the mass spectrometry method depends upon the effieieney of the ionisation process, the size of the analytical blank and the precision of the isotopic ratios. Rosman et al. (30) report sample-processing blanks of 0.10 0.05 pg when measuring Antarctic ice. These blanks are exceptionally low and difficult to maintain. 

For the determination of drugs in plasma, extraction and concentration are also necessary in most cases. Matrix effects can influence the chromatographic separation in GC- or LC/MS, and also the ionisation process in ESI mass spectrometry [45, 46]. 

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... 

Several techniques have been used to follow explosive reactions on a shorter time scale, such as mass spectrometry [6,7,31,46] and emission spectroscopy [11-14]. Mass spectrometry seems more universal as it can identify many species relatively easily. But due to the inherent instability of especially explosive molecules and some of the decomposition intermediates, the ionisation process can generate secondary species. This influence of the ionisation process is difficult to avoid. Also, the time resolution achieved in mass spectrometric techniques is not high enough, unless sophisticated and expensive techniques are used. On the other hand, emission spectroscopy is a fast, non-intrusive, sensitive detection technique. Furthermore, there is little spectroscopic information about the decomposition reactions of explosives. It is this technique (emission spectroscopy) that has been used in the experiments described below to investigate fast (explosive) decomposition reactions induced by a nanosecond laser light pulse. 

For mass spectrometry the species of interest need to be ionised. Mass spectrometric techniques may be divided into methods with simultaneous evaporation (atomisation) and ionisation processes in the ion source (such as SSMS, ICP-MS, SIMS, LMMS) and methods with post-ionisation processes... 

Electron impact ionisation (El) stands for extensive fragmentation, but also produces molecular ions. The other ionisation methods shown in Table 6.10 mainly generate quasi-molecular ions for various compound classes. Protonation of organic compounds is one of the most fundamental processes of Cl, FAB and ESI mass spectrometry. Apart from electrospray (ESI), which... 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

Flegal and Stokes [59] have described a sample processing technique necessary for avoiding lead contamination of seawater samples prior to lead stable isotope measurements by thermal ionisation mass spectrometry. Levels down to 0.02 ng/kg were determined. 

Flegal and Stukas [406] described the special sampling and processing techniques necessary for the prevention of lead contamination of seawater samples, prior to stable lead isotopic ratio measurements by thermal ionisation mass spectrometry. Techniques are also required to compensate for the absence of an internal standard and the presence of refractory organic compounds. The precision of the analyses is 0.1 -0.4% and a detection limit of 0.02 ng/kg allows the tracing of lead inputs and biogeochemical cycles. 

For quantitative analysis of organic compounds in general by means of liquid chromatography-electrospray ionisation mass spectrometry (LC-ESI-MS), one should be aware of two major factors, which may strongly impact on the outcomes. These are directly associated with the process of ion generation in the interface. 

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])...

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