Choice of ionization technique

The use of different electromigration-based separation techniques hyphenated with MS has become a standard technique in modem pharmaceutical analysis. With the possibility of several commercially available instraments, including various interfaces, the coupling between CE and MS can now be easily achieved. Therefore, numerous issues can be resolved according to the wide choice of operation techniques afforded in CE and the various ionization modes and/or analyzers. CE—MS has emerged as a good alternative for trace... 

Ionization of the analyte is the first crucial and challenging step in the analysis of any class of compounds by mass spectrometry. The key to a successful mass spectrometric experiment lies to a large extent in the approach to converting a neutral compound to a gas-phase ionic species. A wide variety of ionization techniques have become available over the years, but none has universal appeal. In some techniques, ionization is performed by ejection or capture of an electron by an analyte to produce a radical cation [M+ ] or anion [M ], respectively. In others, a proton is added or subtracted to yield [M - - H]+ or [M — H] ions, respectively. The adduction with alkali metal cations (e.g., Na+ and K+) and anions (e.g., Cl ) is also observed in some methods. The choice of a particular method is dictated largely by the nature of the sample under investigation and the type of information desired. Table 2.1 lists some of the methods currently in vogue. Some methods are applicable to the atomic species, whereas others are suitable for molecular species. Also, some methods require sample molecules to be present in the ion source as gas-phase species, whereas others can accommodate condensed-phase samples. The methods that are applicable to molecular species are the subject of the present chapter those applicable to atomic species are described in Chapter 7. 

During the last few decades, an increasing number of different breath analysis techniques have been developed for the analysis of VOCs. These techniques include gas chromatography/flame ionization detection (GC/ FID), gas chromatography-mass spectrometry (GC-MS) (with quadrupole mass spectrometry, ion trap mass spectrometry, time-of-flight (TOF) tube mass spectrometry, and ion mobility spectrometry), soft ionization flow tube mass spectrometry (SIFT-MS), chemiluminescence, electronic nose, and a large variety of optical absorption detection techniques. The multitude of methods and techniques used in breath analysis reflects not only its strength, but also its weakness. On one hand, there is a choice of sensitive techniques suitable to measure almost any compound on the other hand, it makes it very hard to compare all the various results. 

In natural product investigations, the most crucial decisions usually involve the choice of (i) derivative and (ii) ionization technique. The factors governing the choice of ionization method are the requirements for an abundant molecular ion (particularly in quantitative studies) and for diagnostic fragment ions. The derivative is chosen to fit in with the ionization method e.g. aromatic substituents to confer molecular stability in positive-ionization if necessary and perfluorinated derivatives to confer electron capture properties in negative ionization) and also, of course, to give suitable GC properties. Further discussion can be found in the sections on individual natural product classes below. 

The previous discussion demonstrates that measurement of precise isotope ratios requires a substantial amount of operator experience, particularly with samples that have not been examined previously. A choice of filament metal must be made, the preparation of the sample on the filament surface is important (particularly when activators are used), and the rate of evaporation (and therefore temperature control) may be crucial. Despite these challenges, this method of surface ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other chapters in this book discuss practical details and applications. 

In line with the policy of Advances to provide periodic coverage of major developments in physical methodology for the study of carbohydrates, A. Dell (London) here surveys the use of fast-atom-bombardment mass spectrometry in application to carbohydrates. This technique has achieved rapid prominence as the soft ionization technique of choice for structural investigation of complex carbohydrate sequences in biological samples. The author s extensive personal involvement in this field makes her chapter a critical, state-of-the-art overview for the specialist, as well as a valuable primer for the reader unfamiliar with this technique. 

For human studies, the choice of stable isotopes is limited because radioisotopes are associated with ionization radiation and thus with some potential harmful effects for humans. Studying the bioavailability of compounds labeled with stable isotopes requires complex techniques such as gas chromatography coupled with mass spectrometry (GC-MS), liquid chromatography coupled with MS (LC-MS), and atmo-... 

Universal and selective detectors, linked to GC or LC systems, have remained the predominant choice of analysts for the past two decades for the determination of pesticide residues in food. Although the introduction of bench-top mass spectrometers has enabled analysts to produce more unequivocal residue data for most pesticides, in many laboratories the use of selective detection methods, such as flame photometric detection (FPD), electron capture detection (BCD) and alkali flame ionization detection (AFID) or nitrogen-phosphorus detection (NPD), continues. Many of the new technologies associated with the on-going development of instrumental methods are discussed. However, the main objective of this section is to describe modern techniques that have been demonstrated to be of use to the pesticide residue analyst. 

The method of choice for the measurement of ionization constants is potentio-metry [35,112-119]. Special circumstances warrant the determination of the pKa by UV spectrophotometry [120-143], capillary electrophoresis (CE) [144-147], and a chromatographic technique [148]. In principle, UV and CE methods are more sensitive and less sample-demanding than is the pH-metric method. That not withstanding, the latter method is preferred because it is so much better developed,... 

Molecules can be small, like CH4, large, or very large, like some biopolymers with molecular weights of millions of daltons. They can be organic, inorganic, polar, apolar, etc. Mass spectrometry can study all of them. However, one ionization technique cannot ionize all kinds of molecules but different ionization techniques are now available. Their choice is based on the chemico-physical properties of the molecule, such as molecular weight, polarity, thermal stability, etc. 

Recent innovations in ionization techniques have allowed the development of ambient mass spectrometry. Mass spectra can be determined for samples in their native environment without sample preparation. Although the ambient mass spectrometry technique is still in its infancy, its potential for serving as a tool of choice for high-throughput bioanalysis is very encouraging. 

In the past, PTRC screening was mainly based on gas chromatography-mass spectrometry (GC-MS) [116]. The choice of GC-MS was based on a number of good reasons (separation power of GC, selectivity of detection offered by MS, inherent simplicity of information contained in a mass spectrum, availability of a well established and standardized ionization technique, electron ionization, which allowed the construction of large databases of reference mass spectra, fast and reliable computer aided identification based on library search) that largely counterbalanced the pitfalls of GC separation, i.e., the need to isolate analytes from the aqueous substrate and to derivatize polar compounds [117]. 

However, some obstacles still have to be removed before HPLC-MS is fully accepted as a routine technique in the forensic labs. First of all the cost of equipment is still high, despite the reduction trend in the last years, as compared to GC-MS. Second, some problems have to be tackled, i.e., the susceptibility of ion sources (and particularly ESI) to matrix effects on analyte s ionization efficiency (suppression or enhancement) and the scarce reproducibility of MS fragmentation. The third obstacle is actually the other side of the coin of versatility the wide choice of technical alternatives makes HPLC-MS still far from being a highly standardized, one-button technique as GC-MS. 

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