Other Ion Sources

A patent describing a thermionic emitter for generating positive ions that incorporated a mixtnre of beta-alumina and inert material snch as charcoal positioned on a filament for heating the mixture has been issued. Two decades later, an alkali cation emitter, based on intercalated alkali ions in a graphite matrix, was proposed. When heated on a red hot filament, this source emits ions from alkali salts, which snbseqnently can be used to form product ions. 

A trnly innovative soft ionization source, based on a nanometer-thick membrane, was developed. The gas sample passes through a porous membrane that is coated on both sides with a metallic conductor film. A low voltage (10 V) produces a large electric field ( 10 V cm- ) that causes soft and efficient ionization of molecnles passing through the membrane. Despite its apparent advantages, this ionization method has not found its way to commercial devices. 

A variety of methods has been developed to produce ions nnder atmospheric pressure conditions that can be introduced into an IMS. Some of these methods are universal and can be employed for several applications, while others are limited to specific types of compounds (like surface ionization) or to certain types of samples (gaseous, liquid, or solid). Some require expensive and large systems (like MALDI), while others are particularly suitable for handheld devices (like the radioactive ion sources). Some are useful for studies of macromolecules, mainly of biological interest (like ESI and MALDI), while others can only be nsed to detect volatile 

Another aspect is the commercial availability of the different ion sources. Some sources can, at present, be considered only as exotic and suitable only for research purposes and have yet to be tested in the field and in the laboratory, while others are mature techniques that are well understood, with advantages and limitations that are well recognized. 

Finally, the Appendix shows schematic representations of the principle of operation of some of the ionization processes presented. Among these are the theory of ESI, atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI). Also shown are schematics of the ionizers and typical experimental conditions for the APCI and APPI sources as well as that of an ESI-APCl mixed source. It should be noted that these schematics are for sources that are interfaced with a mass spectrometer but are similar to IMS interfaces. 

The techniques described thus far cope well with samples up to 10 kDa. Molecular mass determinations on peptides can be used to identify modifications occurring after the protein has been assembled according to its DNA code (post-translation), to map a protein structure, or simply to confirm the composition of a peptide. For samples with molecular masses in excess of 10 kDa, the sensitivity of FAB is quite low, and such analyses are far from routine. Two new developments have extended the scope of mass spectrometry even further to the analysis of peptides and proteins of high mass. 

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In contrast to the other ion sources, the MALDI source may operate under high vacuum or under atmospheric pressure. In the latter case the acronym AP-MALDI (atmospheric pressure matrix assisted laser desorption ionization) is used. 

MALDI, which is LDI utilizing a particular sample preparation). Although the performance of MALDI is superior to LDI in the analysis of many groups of compounds, LDI is still the perferred choice in some important applications, including cmde oil analysis [155], fullerene detection in rocks [156], atmospheric aerosol analysis [157], semiconductors, and surface analysis [158]. Reference 21 is a comprehensive review of the use of LDI (and several other ion sources) in analysis of inorganics. 

The quantification capability is normally limited by the detector and/or the ion source. The MCP that is often utilized in TOF instruments cannot fully handle the ion currents that are produced in MALDI and are often saturated to some extent. With other ion sources, such as SIMS, the detection system is less strained so the detector is less limiting. Instead the ion source will limit the quality in quantification. Magnetic sectors and also qudmpoles are more often utilized when quantification is important. 

Relatively complex hardware compared to other ion sources... 

Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) is a direct mass spectral technique that has found application for the analysis of various phytochemicals. MALDI represents the ion source, whereas TOF is the mass analyzer. The two techniques may be coupled with other ion sources or mass analyzers, but are most commonly used in tandem. In MALDI, the analytes are mixed with a matrix, which is usually an aromatic organic acid, which aids in... 

This reaction explains the advisability found in the literature of unusually high sulfur in mixtures involving oxalates. In calculations of glitter mixtures, the oxalates should be treated as other ion sources for sulfides. 

All mass spectrometers require a sample input system, an ionization source, a mass analyzer, and a detector. All of the components with the exception of some sample input systems or ion source volumes are under vacuum (10" -10" torr for that portion where ions are separated by mass, i.e., the analyzer, or 10 -10" torr in some ion sources, where the ions are initially formed), so vacuum pumps of various types are required. Other ion sources, such as the direct analysis in real time (DART) (discussed in Section 9.2.23), electrospray ionization (ESI) (Sections 9.2.2.3 and 13.1.6.1), or chemical ionization (Cl) (Section 9.2.2.2), operate at atmospheric pressure and use extraction lenses set to a polarity opposite that of the ions to draw them into subsequent stages of the MS instrument. Modern mass spectrometers have all of the components under computer control, with a computer-based data acquisition and processing system. A block diagram of a typical mass spectrometer is shown in Figure 9.4. 

The working mechanism of EBIS is better understood than other ion sources. Because the electron component is a beam in EBIS, usually is calculated (where je is the current density of electron) instead of the factor (described in O Sect. 50.1.1.2). O Figure 50.10 shows the different charge states versus jgCz- In EBIS, the plasma parameters (e.g., electron number density and electron energy) are much better known than in other sources, so the calculations fit experiments better. 

Accessories that permit use ot most of the other ion sources listed in Table 11-1 in conjunction with an ICP torch are available commercially. 

In summary, the ionization efficiency and analytical sensitivity of ICP is higher than other ion sources, and the matrix effect in ICP-MS is much smaller. It can be found that the ICP and other soft ion sources (e.g. ESI or MALDI) are really complementary techniques. Structural information is preferably obtained by means of ESI- or MALDI-MS, whereas ICP-MS is ideal to quantify elements in samples, even in the very low concentration ranges. 

In ion mobility spectrometry (IMS) used for explosive detection purposes, ions are generated from injected sample species, e.g. nitroglycerine, RDX, trinitrotoluene, etc., in the reaction region (includes other ion sources) the ions so generated are injected into a drift mbe/channel where separations occur in an electrical field perpendicular to the bulk gas (air) flow containing the ions. For (R/A/) values (see (3.1.219) and (3.1.220)) below 10 Td (Eiceman et al., 2004), i independent of the electrical field. For higher (B/A values employed in differentiai mobiiity spectrometry (DMS), R >n,g characterized as... 

An HCD is the ion source used in the majority of PTR-MS instruments. However, other ion sources have been explored and exploited in PTR-MS and we briefiy illustrate some of these in this section. For full details the reader is referred to the original publications. 

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