Spectrometry soft ionisation

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. 

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],... 

Direct polymer compound analysis by soft ionisation, tandem MS/MS and high-resolution (AC-MS) mass spectrometry, has been reviewed [236]. 

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). 

R. Quantitative determination of noncovalent binding interactions using soft ionisation mass spectrometry Int. J. Mass Spectrom. 2002, 216, 1-27. 

HRTG-EI/SI QMSHigh resolution thermogravimetry-electron impact/soft ionisation quadrupole mass spectrometry... 

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). 

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... 

Deoxygenation involves three enzyme activities the 4,6-dehydratase, the 3,5-epimerase and the reductase. The 4,6-dehydratases work by a broadly similar mechanism to the 5 -adenosylhomocysteine hydrolase. They contain one tightly bound NAD(P)" molecule per active site. This oxidises the 4-position to a ketone and an iilcB reaction drives off the 6-OH. The glucosenone is then reduced by the NAD(P)H to the 4-keto-6-deoxyhexose. The intermediacy of the glucosenone has been verified by rapid quench/soft ionisation mass spectrometry. ... 

Schulten H.-R. (1994), A chemical structure for humic acid. Pyrolysis-gas chromatography/mass spectrometry and pyrolysis-soft ionisation mass spectrometry evidence. Humic Substances in the Global Environment and Implications on Human Health, Elsevier Science, 43-56. 

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. 

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... 

Barber M, Bordoli RS, and Sedgyvick RD (1980) Fast atom bombardment mass spectrometry. In Morris HR (ed.) Soft Ionisation Biological Mass Spectrometry. London Heyden. 

Games, D. E., 1981, Combined liquid chromatography-mass spectrometry with a moving-belt interface, in Soft Ionisation Biological Mass Spectrometry (H. R. Morris, ed.), pp. 54-68, Heyden and Son, London. 

Soft Ionisation, Tandem (MS-MS) and High Resolution (Atomic Composition-MS) Mass Spectrometry... 

HPLC-ESI-MS (Electrospray Ionisation mass spectrometry) uses a soft ionisation method, which yields the quasi molecular ion, in positive or negative ionisation mode or cation adducts, but a very restricted amount of fragmentation. The identification of unknown compounds is a major challenge in plant metab-... 

Mass spectrometry of intact glycosphingolipids by direct probe insertion and soft ionisation procedures has become one of the major means of structure determination for these compounds [460,896,974]. information is thereby obtained on the nature of both the iipid and carbohydrate moieties, aithough the presence of different moiecuiar species hampers the interpretation of the spectra greatiy. Detailed discussion of this aspect is beyond the scope of this book. 

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