Ionization methods choice

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. 

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. 

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. 

Very large branched alkanes, such as 24,24-diethyl-19,29-dioctadecylhepta-tetracontane, CgvHng, for example, pose difficulties to obtaining useful mass spectra and even 15 eV El does not anymore allow for the detection of their molecular ions. [80] Beyond C40 alkanes, especially in case of mixtures such as hydrocarbon waxes or polyethylenes of low molecular weight, field desorption and matrix-assisted laser desorption/ionization are the ionization methods of choice (Chaps. 8, 10). 

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

Nowadays, ESI is the leading member of the group of atmospheric pressure ionization (API) methods and the method of choice for liquid chromatography-mass spectrometry coupling (LC-MS, Chap. 12). [10-13] Currently, ESI and MALDI (Chap. 10) are the most commonly employed ionization methods and they opened doors to the widespread biological and biomedical application of mass spectrometry. [5,10,11,13-17] Moreover, ESI serves well for the analysis of ionic metal complexes [18,19] and other inorganic analytes. [20-22]... 

MS has a great potential to be established as a key method in almost all fields of IL research. In parhcular, soft ionization methods like ESI and APCl and to a lower extent MALDl MS are the methods of choice. Potential applications of MS lie, for example, in the analysis of reactions occurring in ILs, that... 

APCI and APPI are probably the ionization methods of choice for unconjugated, underivatized steroids. They are related techniques in APCI a corona discharge initiates the ionization process, while this is initiated by ultraviolet light in APPI. There are probably no absolute rules regarding sensitivity, it being very much dependent on the instrument design. A particular source on a particular instrument may be... 

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. 

Without authentic standards or a spectroscopic detection scheme, compound identification is difficult. Our choice of a desorption/ionization method is potassium ionization of desorbed species (K+IDS) with mass spectrometric detection (n. 12) which provides a rapid qualitative tool for compound identification. Using K+IDS, molecular weight data is available and fragmentation is minimal. Ions appear as M[K], the mass of the analyte plus 39 Da, the mass of potassium. Hence, structure identification is possible based on a knowledge of starting materials and the molecular weight data afforded by K+IDS. 

The instrumentation and interfaces that had been used up to 1998 in CWC-related LC/MS analysis were summarized previously (4). At that time, sources that operate at atmospheric pressure, using electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI), were displacing their predecessors that used thermospray ionization or continuous flow fast atom bombardment. Atmospheric pressure ionization (API), either ESI or APCI, is now the method of choice in CWC-related analysis and will be the focus of this current review. A small number of recent applications involving alternative types of ionization are also included. For earlier applications of LC/MS to chemical weapons (CW) analysis, using thermospray and other ionization methods, the reader is referred to our previous review (4). The other major trend has been the increasing availability and ease of use of less-expensive bench-top quadrupole and... 

Most instruments with an API source offer a choice of APCI or ESI. Detailed descriptions of these ionization methods are provided elsewhere a 3). Both rely on the formation of ions at atmospheric pressure in a source region separated from the high vacuum mass analyzer. Ions are transported to the analyzer through one or more differentially pumped skimmers or a heated capillary. Both APCI and ESI require aerosolization of the liquid eluent from the liquid chromatography (LC) column. The major difference lies in the method and phase of ionization. 

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... 

Tandem mass spectrometry (MS/MS) is a new analytical technique applied to problems in food and flavor analyses. Rapidity of analysis, a high discrimination against chemical noise, and the ability to analyze mixtures for functional groups are attributes of MS/MS that make it attractive for such problems. Sanple collection and pretreatment differ frcm methods used in GC/MS. Correct choice of an ionization method is paramount. Daughter ion MS/MS spectra are used for conpound identification via comparison with those of authentic compounds, and parent and neutral loss spectra are useful in functional group analysis. Applications to direct analysis of volatiles emitted from fruits and to spice analyses are considered. 

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... 

The optimal HPLC separation conditions for the nemadectins precluded the use of "buffer only" ionization. The Finnigan TSP-46 thermospray source is equipped with a filament and a discharge electrode as choices for an external ionization method. Both positive and negative ion detection are available. The best choice for an ionization method and for the mode of ion detection would have to provide adequate sensitivity for minor component analysis, unambiguous molecular weight information, and sufficient fragmentation to differentiate between components with similar retention times and identical molecular weights. 

Like the QIT, ionization in the FT-ICR maybe performed either internally or externally to the cell. For several reasons, external ionization has become the technique of choice. External ionization allows the use of virtually any ionization method, and commercially available instruments are typically designed as general purpose instruments incorporating interchangeable ion sources. All three major suppliers of FT-ICR instruments (lonSpec, Bruker Daltonics, and Finnigan) offer external sources. 

Spoilt for Choice - Which Ionization Method to Choose 357... 

In this section, we will look briefly at factors to be considered in selecting an ionization method, the choice of a scan mode, how mass spectral data can be used to identify an unknown compound or a known compound, and the factors to be considered in setting up a quantitative mass spectral assay. 

For high-molecular-weight samples, most commonly proteins and peptides, but also polysaccharides and synthetic polymers, the choice of an ionization method will be limited to ESI (Section 9.10.2.2.4) and/or MALDI (Section 9.10.2.2.7) (see also Table 5 for a comparison of ESI and MALDI). As mention above, MALDI is a solid-phase-based ionization technique and ESI is a flow-based liquid technique. Both readily generate... 

Microdialysis coupled to mass spectrometry is a powerful technique for on-and off-line analysis, providing information on pharmacokinetics, drug transport, and metabolite formation. With the widespread availability of LC/ESI/MS instruments there has been a shift toward liquid chromatography and away from gas chromatography and flow injection. Electrospray is the ionization method of choice for most applications and tandem mass spectrometry has grown in popularity. Thermospray and cfFAB applications have been used with microdialysis, but are older, obsolete techniques. 

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... 

In the past decade, mass spectrometry (MS) has become the method of choice for quality control of synthetic peptides. Historically, plasma desorption (PD) and fast atom bombardment (FAB) were the first ionization methods used for the mass analysis of nonderivatized peptides. More recently, electrospray ionization (ESI) MS and matrix-assisted laser desorption ionization (MALDI) MS have found widespread utility for peptide analysis. Both of the latter methods yield protonated molecules and, thus, provide direct molecular weight information. As will be covered later, ESI can be employed with a variety of mass analyzers, including quadrupole, magnetic sector, ion trap, and time-of-flight (TOF) analyzers. On the other hand,... 

Inductively coupled plasma has become the ionization method of choice for elemental mass spectrometry. It was initially developed as the excitation source for multi-element optical spectrometers, because at typical plasma temperatures of 5000-10,000°C virtually all elements on the periodic chart emit detectable light. Most molecules are also atomized at these temperatures, which makes inductively coupled plasma ideal for mass spectrometry monitoring of elemental composition as well. Fassell and co-workers introduced the first inductively coupled plasma interfaced to a mass spectrometer in 1980 (Houk et al., 1980). Elemental mass spectrometry normally requires only low-resolution analysers because unit mass resolution is typically required (i.e. the mass difference between elements, which is always equal to or greater than 1 Da). 

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). 

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