Soft Ionization Sources

The ion source used for the generation of biomolecular parent ions is critical, and only recently have the so-called soft ionization methods been developed.2 Electron-impact ionization sources fall into the category of hard sources, whereby the sample must be in the vapor phase initially, and the ionization process produces a very large number of fragments. Soft methods were introduced to overcome the problems associated with the thermal instability and involatility of macromolecular analytes. Soft ionization produces few fragments under relatively mild conditions. In Table 15.1 a comparison is shown between the three main soft ionization methods some of these values are strongly dependent on individual mass spectrometer configurations and the desired resolution. 

Fast Atom/lon Bombardment. This ionization technique utilizes a 

TABLE 15.1. Comparison of Soft Ionization Sources Used in Biological MSa 6 

Sensitivity Low. Analyte at High. Analyte at pico- High. Analyte at pico- 

Fragmentation Little Usually none Little or none 

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As shown in Figure 15.1, there are three main components of every mass spectrometer. The ion source is used to produce gas-phase ions by capture or loss of electrons or protons. In the mass analyzer, the ions are separated according to their mlz ratios ions of a particular mlz value reach the detector, and a current signal is produced. This section describes the soft ionization sources, mass analyzers, and detectors that are used in experiments involving biological macromolecules. 

Gas sampling GD was first described by McLuckey et al. as a soft ionization source [490]. It was shown to facilitate the ionization of gaseous compounds while giving both elemental as well as molecular information, as shown for the case of ASH3 produced by hydride generation (Fig. 121) [488] and can be used successfully for the ionization of any atomic as well as molecular vapor. By alternating from soft to harsher conditions, it is possible to change from atomic to molecular information. This can be illustrated by the spectra obtained from ferrocene vapor [605],... 

FIGURE 20-2 Mass spectrum of 1 -decanol from (a) a hard ionization source (electron impact) and (b) a soft ionization source (chemical ionization),... 

MALDl acts as a soft ionization source and generally produces singly charged molecular ions from even very large polymers and biomolecules, although a few multiple-charge ions and some fragment ions may occur (Fig. 9.11). 

Describe how an El source forms ions from analyte molecules. Is this a hard or soft ionization source What are the advantages and disadvantages of this source ... 

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 Cl source is considered a soft ionization source it results in less fragmentation of analyte molecules and a simpler mass spectrum than that resulting from EL Most importantly, the molecular ion is much more abundant using Cl, allowing the determination of the MW. Since proton adduct... 

While the first instance of MS-based enantioselective discrimination was demonstrated by CI-MS, more recent work has focused on the use of fast atom bombardment (FAB), matrix-assisted laser desorption/ ionization (MALDI), and electrospray ionization (ESI) sources. All of these sources are considered to be soft-ionization sources due to the low internal energy they impart on a molecule as it is converted to a gas phase ion. The result is a lack of fragmentation, but in cases where source parameters are properly tuned, noncova-lent complexes can also be preserved from the condensed phase and transferred into the gas phase. 

Tandem-in-Space Spectrometers. In tandem-in-space instruments, two independent mass analyzers are used in two different regions in space. The triple quadrupole mass spectrometer is the most common of these instruments. In commercial triple quadrupole instruments, such as the instrument illustrated in Figure 20-2,3, the sample is introduced into a soft ionization source, such as a Cl or FAB source. The ions are then accelerated into quadrupole 1 (Q), which is an ordinary quadrupole mass filter. The selected fast-moving ions pass into quadrupole 2 (q), which is a collision chamber where dissociation of the ions selected by quadrupole 1 occurs. This quadrupole is operated in a radio-frequency-only mode in which no dc voltage is applied across the rods. This mode basically traps the precursor and product ions in a relatively high concentration of collision gas so that CAD can occur. Quadrupole 3 (Q) then allows mass analysis of the product ions formed in the collision cell. The configuration is known as the QqQ configuration. 

ESI can be considered a complementary method to MALDI. As with MALDI, electrospray ionization of biomolecules yields protonated or cationized molecular ions with little or no fragmentation, and it is also referred to as a soft ionization source. A particular advantage of ESI compared to MALDI is that the analyte is sampled from the solution phase. Under these conditions, ESI is readily coupled to high performance liquid chromatography (HPLC) or capillary electrophoresis (CE) separation systems. Such combinations permit online LC-MS or CE-MS experiments. 

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