Choice of ionization method

There are several methods of producing gas-phase inorganic ions, the starting materials in mass spectrometric studies. The properties of the source of the ions required for study are important in the choice of ionization method. The production of bare metal ions from an involatile nonmolecular source requires a large amount of energy deposited on the surface of the material. The processes that occur after the initial ionization process may also affect the ions finally observed (e.g., clustering). At the other end of the ionization energy spectrum, gas-phase ions of a complexity similar to those observed in the condensed phases require a soft ionization process. A brief description of some of the ionization methods follows. 

Another factor to consider in the choice of ionization method is the formation of adducts with sodium or other metal ions, in addition to the protonated molecule. Although these may help confirm the molecular mass, they tend to lower the signal-to-noise ratio of the protonated molecule, resulting in higher detection limits in trace analysis and causing problems in quantitative analysis. Sodium... 

Another selection of ionization mode is the choice of ionization method— that is, ESI or APCI. This also depends on the structures of compounds. ESI works best for most polar molecules, while APCI performs better with low to moderately polar molecules. Alternating between ESI and APCI modes is suitable for detection of unknown compounds so that the best ionization method can be selected for signal optimization. As in the case of polarity mode selection, HPLC conditions also contribute to the performance of ESEAPCI (see later sections for discussions). 

MS operation High vacuum in mass analyser (< 10 Pa) Free choice of ionization method (El, Cl or other) Free choice of Cl reagent gas conditions Both positive and negative ion mode Low interference from solvents and solvent impurities... 

An evalnation of the applications of the moving-belt interface, which was the most widely nsed LC-MS interface based on analyte enrichment, demonstrates both the strength and the weakness of this approach. The advantage of complete analyte enrichment, i.e., solvent removal prior to MS introduction, is a free choice of ionization method. Thns, El mass spectra can be acquired. The disadvantage is that the analytes are transferred in solid state, i.e., on a solid support like the belt, requiring vaporization prior to the ionization. Therefore, the analytes should have sufficient vapour pressure and thermal stability. 

Choice of Ionization Method. Both TSP and DLI can only operate to give Cl spectra while the MBI can be operated under El, Cl (choice of reagent gases), NCI. FAB and SIMS. The ability to obtained El spectra has been stressed for library searches since under soft ionization conditions the spectra usually exhibit only protonated molecule ions and adduct ions with the reagent species and little or no fragmentation. In this respect, the... 

MS is becoming the detection system of choice for LC by virtue of its flexibility and high selectivity for individual solutesHowever, LC-MS is always less sensitive than GC-MS as a result of the need to transfer the analytes from the liquid phase into a high-vacuum gas phase. Other limitations of LC-MS combination include the inability to use nonvolatile buffers, the narrow optimum range for eluent flow rate influence of the proportion of organic modifier on the sensitivity, and the narrow choice of ionization methods.Nevertheless, LC-MS has been widely accepted as an advantageous choice for the determination of carbamate pesticides in water matrices, which is more robust and flexible in the absence of derivatization. Thermospray and particle-beam interfaces are probably most commonly used for offline and online determination of carbamates in Atmospheric pressure sources such as... 

FIGURE 2.6 The choice of ionization method is often determined by the polarity of the analyte. 

In conclusion, the choice of ionization method and mass analyzer usually depends on the nature of the analyte and the particular purpose of the experiment, but they should be compatible with... 

Central to any measurements in mass spectrometry is the ionization of the sample molecules, and transfer of those ions into the vacuum required for operation of the mass spectrometer. The choice of ionization methods available to analytical and organic mass spectrometrists has expanded greatly in the past ten years, and now includes means for the ionization of nonvolatile as well as volatile molecules. Any of several methods might be chosen to address a particular problem, and each might provide satisfactory results. Since there is no single ionization method used exclusively with TLC/MS, this section of the chapter contains an overview of the most common methods of sample molecule ionization in mass spectrometry. 

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. 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

With few exceptions, magnetic sector instruments are comparatively large devices capable of high resolution and accurate mass determination, and suited for a wide variety of ionization methods. Double-focusing sector instruments are the choice of MS laboratories with a large chemical diversity of samples. In recent years, there is a tendency to substitute these machines by TOE or by Fourier transform ion cyclotron resonance (FT-ICR) instruments. 

Busch, K.L. Chemical Noise in Mass Spectrometry. Part 11 - Effects of Choices in Ionization Methods on Chemical Noise. Spectroscopy 2003,18, 56-62. 

The choice of the ionization method depends on both the nature of the sample and the type of information required from the analysis (Table 23.2). A great variety of ionization methods exists that can be classified into six major categories gas-phase ionization, field desorption and ionization, particle bombardment, atmospheric pressure ionization, and the laser desorption. 

The choice of isolation method should be based on all available separation methods (see those listed here and in Section V. C). However, it should be pointed out that scaleup to preparative scale might be necessary. Also, special steps may have to be taken in some cases to circumvent particular problems. For example, it is not possible to utilize a destructive detector, such as a flame ionization detector, to isolate a component in GC. It would be necessary to replace it with a nondestructive detector, such as a thermal conductivity detector, or to use a split stream with accurate timing control that would allow collection of the desired separated component. 

Mass Spectrometric Methods for Capillary SFC-MS. A significant advantage associated with capillary SFC-MS methods, and in contrast to all mechanical (e.g., moving ribbon) HPLC-MS interfaces, results from the flexibility in selection of ionization methods. Although initial studies were conducted using chemical ionization, and it remains the method of choice for most applications, the DFI process is also compatible with electron impact ionization (37). 

TLC-MS techniques will be reviewed first based on the ion source and second by the analyte type. Chromatographic and mass spectrometric procedures will be described in detail to inform the reader of the possibilities and limitations of a particular technique. The selection of ion source is determined by the analyte type that has been resolved on a TLC plate. Appropriate combinations of ionization methods and analyte types allow one to easily obtain and interpret high quality mass spectra. Therefore, the choice of mass spectrometric instrument is a key step in the process of characterization of compounds using TLC-MS techniques. 

The development of such materials is based on a judicious choice of ionizable functional groups, specific crosslinking agents, levels of crosslinking, polymer backbone sequence, and polymer fabrication method. Thus, the field of absorbent materials requires a deep understanding of the fundamentals of swellable, crosslinked polymers (networks), including polymer synthesis, structural characterization, diffusion theory, and polyelectrolytic behavior. 

Following the movement of airborne pollutants requires a natural or artificial tracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source. Limitations placed on the tracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emitted from high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable. On the basis of these criteria, rare earth isotopes, such as those of Nd, were selected as tracers. The choice of tracer, in turn, dictated the analytical method (thermal ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of... 

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. 

The analyst does have some choice of the ionization method to be used El, Cl and FAB are available, subject to certain limitations, and thus both molecular weight and structural information may be obtained from the analyte(s) under investigation. 

If a high -molecular-weight compound is being studied by LC-MS, the analyst has little choice in the ionization method to use, with atmospheric-pressure chemical ionization (APCl) being wholly inappropriate. However, when low -molecular-weight componnds are involved, both electrospray ionization and APCl are potentially of value. 

Another equally important consideration before development of a determinative or confirmatory method is an understanding of the chemical properties of the analyte. Such an understanding becomes the cornerstone of a successful method since the unique chemical properties of each analyte provide the basis for isolation and detection schemes. Table 1 lists some of the important chemical properties that could be considered. For example, knowing the or p/fb of an analyte could influence the choice of a liquid-liquid extraction scheme, solid-phase extraction (SPE) cartridge, mobile phase pH, or mass spectrometric ionization. Knowing the overall polarity of the analyte can be very helpful in the evaluation of an extraction or separation. Currently, computational methods are available to obtain an estimate of the logP... 

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,... 

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