Mass analyzers and detectors

Schematic diagram of a double-focusing mass spectrometer. 

Schematic diagram of a time-of-flight reflectron mass analyzer. 

Another form of mass analyzer is Fourier Transform Ion Cyclotron Resonance (FTICR-MS) [6]. This separates ions according to their cyclotron frequency, /, in a fixed magnetic field, according to the following equation. 

The final mass analyzer we consider is the orbitrap [7], which when introduced in 2005 represented the most significant technological advance in this field for around 30 years. It consists of two electrodes, the outer barrel-shaped, the inner spindle-like, between which there is an electric field into which the ion 

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The first part of this book is dedicated to a discussion of mass spectrometry (MS) instrumentation. We start with a list of basic definitions and explanations (Chapter 1). Chapter 2 is devoted to the mass spectrometer and its building blocks. In this chapter we describe in relative detail the most common ion sources, mass analyzers, and detectors. Some of the techniques are not extensively used today, but they are often cited in the MS literature, and are important contributions to the history of MS instrumentation. In Chapter 3 we describe both different fragmentation methods and several typical tandem MS analyzer configurations. Chapter 4 is somewhat of an outsider. Separation methods is certainly too vast a topic to do full justice in less than twenty pages. However, some separation methods are used in such close alliance with MS that the two techniques are always referred to as one combined analytical tool, for example, GC-MS and LC-MS. In effect, it is almost impossible to study the MS literature without coming across at least one separation method. Our main goal with Chapter 4 is, therefore, to facilitate an introduction to the MS literature for the reader by providing a short summary of the basic principles of some of the most common separation methods that have been used in conjunction with mass spectrometry. 

Figure 14.1 Schematic view of a mass spectrometer. Its basic parts are ion source, mass analyzer, and detector. Selected principles realized in modern mass spectrometers are assigned El—electron impact. Cl—chemical ionization, FAB—fast atom bombardment, ESI—electrospray ionization, MALDI—matrix-assisted laser desorption/ionization. Different combinations of ion formation with mass separation can be realized.

Mass spectrometry is an analytical technique that measures the mass-to-charge ratio (,m/z) of gas-phase ions formed from molecules ranging from inorganic salts to proteins. The mass spectrometer is a device or instmment that measures the mass-to-charge ratio of gas-phase ions and provides a measure of the abundance of each ionic species. To measure the m/z of ions, the mass analyzer and detector must be maintained under high-vacuum conditions and calibrated using ions of known m/z. As explained in the following section, some ion sources can be maintained at atmospheric pressure, while others require vacuum conditions. 

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. 

Figure 7-3 Block diagram of the components of a chromatograph-mass spectrometer system.The mass analyzer and detector are always under vacuum,The ton source may be under vacuum or under near-atmospheric pressure conditions, depending on the ionization mode.The computer system is an integral part of data acquisition and output.

The choice of an ionization source must be consistent with the mass spectrometer inlet system, as described above, and the ionization characteristics of the analyte. Likewise, the appropriate choice of mass analyzer is highly application-, compound-, and matrix-dependent, and each mass analyzer requires certain characteristics from its ion detector The variety of individual ionization, mass analyzer, and ion detector systems that can be integrated creates a myriad of unique mass spectrometer systems, each with certain advantages and disadvantages. To add even more complexity to the situation, modular configuration of these individual components (ionization source, mass analyzer, and detector) is not limited... 

A picture of the ion source, mass analyzer and detector is shown in Figure 13.4, and the entire instrument in Figure 13.5. The instrument uses a 1 in x 1 in sample... 

As shown in Fig. 19.1, the ion source, mass analyzer and detector are incorporated into a vacuum system. This vacuum must be sufficient (i.e., the mean free path must be sufficiently long) to prevent collisions between particles prior to analysis. 

The basic instrument design is shown in Figure 40.19 with the four components source, tandem accelerator, mass analyzer, and detector. The key to excellent detection limits is the use of all four of them for mass discrimination. 

Desorption methods have been developed since around 1970. They have not only expanded the scope of mass spectrometry, but have also changed the strategies of sample introduction and brought attention to solid and liquid phase chemistry as well as to particle and gas phase chemistry. They have also stimulated the rapid development of new ion sources, mass analyzers, and detectors with higher mass range capabilities and improved sensitivity. 

Improvements in the instrumentation, ionization sources, high-resolution mass analyzers, and detectors [67-69], in recent years have taken mass spectrometry to a different level of HPLC-MS for natural product analysis. Mass spectrometry detection offers excellent sensitivity and selectivity, combined with the ability to elucidate or confirm chemical structures of flavonoids [70-72]. Both atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) are most commonly used as ionization sources for flavonoid detection [73-76]. Both negative and positive ionization sources are applied. These sources do not produce many fragments, and the subsequent collision-induced dissociation energy can be applied to detect more fragments. Tandem mass spectrometry (MS , n> 2) provides information about the relationship of parent and daughter ions, which enables the confirmation of proposed reaction pathways for firagment ions and is key to identify types of flavonoids (e.g., flavones, flavonols, flavanones, or chalcones) [77-80]. 

The function of a mass spectrometry is divided into three major units ionization source, mass analyzer, and detector. The method of sample introduction to the ionization source depends on the ionization procedure. Many ionization... 

In its simplest form, the mass spectrometer has five components (Fig. 8.1), and each will be discussed separately in this chapter. The first component of the mass spectrometer is the sample inlet (Section 8.2), which brings the sample from the laboratory enviromnent (1 atm) to the lower pressure of the mass spectrometer. Pressures inside the mass spectrometer range from a few millimeters of mercury in a chemical ionization source to a few micrometers of mercury in the mass analyzer and detector regions of the instrument. The sample inlet leads to the ion source (Section 8.3), where the sample molecules are transformed into gas phase ions. The ions are then accelerated by an electromagnetic field. Next, the mass analyzer (Section 8.4) separates the sample ions based on their mass-to-charge (miz) ratio. The ions then are counted by the detector (Section 8.5), and the signal 418... 

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