Mass spectrometry soft ionisation techniques

Tandem mass spectrometry or ms/ms was first introduced in the 1970s and gained rapid acceptance in the analytical community. The technique has been used for stmcture elucidation of unknowns (26) and has the abiUty to provide sensitive and selective analysis of complex mixtures with minimal sample clean-up (27). Developments in the mid-1980s advancing the popularity of ms/ms included the availabiUty of powerhil data systems capable of controlling the ms/ms experiment and the viabiUty of soft ionisation techniques which essentially yield only molecular ion species. 

Applications Early MS work on the analysis of polymer additives has focused on the use of El, Cl, and GC-MS. The major drawback to these methods is that they are limited to thermally stable and relatively volatile compounds and therefore are not suitable for many high-MW polymer additives. This problem has largely been overcome by the development of soft ionisation techniques, such as FAB, FD, LD, etc. and secondary-ion mass spectrometry. These techniques all have shown their potential in the analysis of additives from solvent extract and/or from bulk polymeric material. Although FAB has a reputation of being the most often used soft ionisation method, Johlman el al. [83] have shown that LD is superior to FAB in the analysis of polymer additives, mainly because polymer additives fragment extensively under FAB conditions. 

Several years later, the next step in the application of MS-MS for mixture analysis was developed by Hunt et al. [3-5] who described a master scheme for the direct analysis of organic compounds in environmental samples using soft chemical ionisation (Cl) to perform product, parent and neutral loss MS-MS experiments for identification [6,7]. The breakthrough in LC-MS was the development of soft ionisation techniques, e.g. desorption ionisation (continuous flow-fast atom bombardment (CF-FAB), secondary ion mass spectrometry (SIMS) or laser desorption (LD)), and nebulisation ionisation techniques such as thermospray ionisation (TSI), and atmospheric pressure ionisation (API) techniques such as atmospheric pressure chemical ionisation (APCI), and electrospray ionisation (ESI). 

Recent advances in mass spectrometry have produced a number of soft ionisation techniques such as fast atom bombardment (FAB) or electrospray ionisation. The major advantage of these techniques is that they are less likely to break the sample into small fragments and are more likely to produce a molecular ion. This is particularly important in the analysis of macromolecules such as proteins and nucleic acids. 

Particularly with overtly reactive labelling reagents such as halocarbonyl compounds, it is desirable to have confirmation that the labelling is active site directed. Only one molecule of the reagent should be incorporated per active site inactivated (these days this readily established by mass spectrometry with soft ionisation techniques such as electrospray) and the kinetics of inactivation in the presence of excess inactivator should be first order (if A is activity, then At = oexp(—kobsO- particular, quantitative protection by a reversible... 

The two chapters that were selected for this topic one on GC-ion trap mass spectrometry, by SabUer and Fujii and the other by Schroder on LC-MS in environmental analysis give an excellent contribution to the application of GC-MS and LC-MS to environmental analysis. Both chapters include many practical aspects and examples in the environmental field and also cover the historical perspective of the techniques and show the perspective on ionisation and scanning modes. Advances achieved in GC-ion trap by the use of external ion sources and GC/MS/MS possi-bihties are discussed. The LC-MS chapter provides an overview of the first applications of LC/MS interfacing systems, such as moving belt, direct Uquid introduction (DLI) and particle beam (PB), and then on the more recent soft ionisation techniques, like thermospray and atmospheric pressure ionisation interfacing systems. 

The technique of electrospray ionisation (ESI) has proved a highly valuable tool for studying proteins and their complexes in recent years. The fusion of mass spectrometry with the development of gentle ionisation methods such as ESI, have allowed larger, more delicate protein complexes to be analysed. ESI has the capability to ionise and desolvate biomolecules from solution, into the gas phase without input of excessive internal energy to the molecular ions. This soft ionisation technique can allow the preservation of covalent and non-covalent modifications to proteins [100,101]. 

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... 

In the deformulation of PE/additive systems by mass spectrometry, much less fragmentation was observed with DCI-MS/MS using ammonia as a reagent gas, than with FAB-MS [69]. FAB did not detect all the additives in the extracts. The softness and the lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Applications of hyphenated FAB-MS techniques are described elsewhere low-flow LC-MS (Section 7.3.3.2) and CE-MS (Section 7.3.6.1) for polar nonvolatile organics, and TLC-MS (Section 7.3.5.4). 

Although FD was one of the earliest forms of soft ionisation, poor sensitivity and limited applicability have restricted the impact of the approach in the mainstream of mass spectrometry. More recently, many of the application areas of FD and FI have been appropriated by FAB-MS, which is generally considered to be a technique that requires less expertise alternatively, laser desorption is frequently being applied. FD-MS is only used in a handful of laboratories worldwide. The technique has recently been reviewed [107], and is subject of various monographs [108,112],... 

Mass spectrometry is used to identify unknown compounds by means of their fragmentation pattern after electron impact. This pattern provides structural information. Mixtures of compounds must be separated by chromatography beforehand, e.g. gas chromatography/mass spectrometry (GC-MS) because fragments of different compounds may be superposed, thus making spectral interpretation complicated or impossible. To obtain complementary information about complex mixtures as a whole, it may be advantageous to have only one peak for each compound that corresponds to its molecular mass ([M]+). Even for thermally labile, nonvolatile compounds, this can be achieved by so-called soft desorption/ionisation techniques that evaporate and ionise the analytes without fragmentation, e.g. matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). 

A recently introduced technique for the separation of larger molecules is matrix-assisted faser Resorption-ionisation mass spectrometry (MALDI-MS). Developed by Karas et al. [4, 5] in 1988, it has been successfully used to determine the mass of biomolecules up to 500.000 Da. This method is based on the principle that the dissolved specimen is mixed with a matrix, and then crystallizes. After drying, a laser pulse is directed onto the solid matrix to photo-excite the matrix material,resulting in desorption and soft ionisation of the analyte.The molar mass is then determined by the lime ef ilight (TOF). 

Two new independently developed techniques called Dart ° (direct analysis in real time) and Desi (desorption electrospray ionisation) are making a huge impact on mass spectrometry. Together they remove the need for sample preparation and vacuum, speed up analysis time and can work in the open air. The sample is held in a gas or liquid stream at room temperature and the impact induces the surface desorption of ions. The ions then continue into the vacuum interface of the MS for analysis. Samples can be hard, soft or even liquid in nature. Ifa et al. have used Desi to image biological samples in two dimensions, recording images of tissue sections and the relative concentrations of molecules therein. Jeol have launched a commercial Dart ion source for non-contact analysis of materials in open air under ambient conditions. 

Also another soft ionisation mode, vacuum ultraviolet (VUV) photoionisation (PI) has been proposed for structural characterisation of polymeric materials [707]. Photoionisation is not a common technique in mass spectrometry but has been utilised for both PyMS [705] and PyGC-MS [705], A photoionisation system usually consists of a windowless, differentially pumped rare gas resonanee lamp coupled with the ionisation chamber of the mass spectrometer. Argon, krypton, or other inert gases are used in the lamp. Argon produces energies of... 

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