Mass analyzers principles

It is a long-stressed platitude that the right tool makes all the difference anyone who has ever tried to fix an inch hex bolt with a metric wrench will confirm this from personal experience. Well, what applies to screwdrivers is in fact not different to high-tech analysis equipment in your lab, and it is particularly correct for mass spectrometry. Currently, five different mass analyzer principles are established in the market for LC/MS apphcations ... 

Mass resolution describes the capability of an MS to distinguish ions with different m/z values. It is defined by the M/AM equation in which M is the m/z ratio of a mass peak and AM is the full width of a peak at half its maximum height. The mass resolution of an instrument often correlates with its accuracy. Mass range indicates the m/z range at which the mass analyzer best functions. For example, quadrupole mass analyzers exhibit a mass range of up to 4000 m/z, while the mass ranges of TOF extend up to 100,000. The operating principles of common MS instruments are discussed below. 

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. 

A tandem-in-space mass spectrometer consists of an ion source, a precursor ion activation device, and at least two nontrapping mass analyzers. The first mass analyzer is used to select precursor ions within a narrow m/z range. Isolated precursor ions are allowed to enter the ion activation device, for example, a gas-filled collision cell, where they dissociate. Created fragments continue on to the second mass analyzer for analysis. The second mass analyzer can either acquire a full mass fragment spectrum or be set to monitor a selected, narrow, m/z range. In principle the second mass analyzer could be followed by more ion activation devices and mass analyzers for MSn experiments. However, due to rapidly decreasing transmission and increasing experimental... 

Achieving high resolving power and high m/z measurement accuracy is one way of decreasing uncertainty when the determination of unknown analyte identity is the object of an experiment. But like many techniques, an increase in experimental or interpretive confidence does not come without some cost (e.g., instrument size, complexity, price, etc.). However, exact m/z measurements (and their associated elemental formula information) are but one type of information that can be derived from mass spectrometers. In the sections that follow, a variety of mass analyzers will be described in terms of their basic principles, functionality and applications. 

This book is tailored to be your guide to mass spectrometry - from the first steps to your daily work in research. Starting from the very principles of gas phase ion chemistry and isotopic properties, it leads through design of mass analyzers, mass... 

Under the headline of instmmentation we shall mainly discuss the different types of mass analyzers in order to understand their basic principles of operation, their specific properties and their performance characteristics. Of course, this is only one aspect of instmmentation hence topics such as ion detection and vacuum generation will be addressed in brief. As a matter of fact, sample introduction is more closely related to particular ionization methods than to the type of mass analyzer employed, and therefore, this issue is treated in the corresponding chapters on ionization methods. The order of appearance of the mass analyzers in this chapter neither reflects the ever-changing percentage they are employed in mass spectrometry nor does it strictly follow a time line of their invention. Instead, it is attempted to follow a trail of easiest understanding. 

From its very beginnings to the present almost any physical principle ranging from time-of-flight to cyclotron motion has been employed to construct mass-analyzing devices (Fig 4.1). Some of them became extremely successful at the time they were invented, for others it took decades until their potential had fully been recognized. The basic types of mass analyzers employed for analytical mass spectrometry are summarized below (Tab. 4.1). 

Different mass analyzers may impose unique technical requirements when interfaced to LC. Understanding the operating principles and technical properties of both LC/MS interfaces and mass analyzers is deemed beneficial. A brief overview of the history of the development of LC/MS interfaces is given in Section II, which is followed in Section III by a summary of working principles and characteristics of commonly used mass analyzers. 

A brief description of the working principles of commonly used mass analyzers is given below. For a more comprehensive discnssion of the principles of these mass analyzers, excellent reviews can be fonnd in the literature. ... 

In principle, a mass spectrometer may be divided into four different central constituent parts (1) the inlet system, (2) the ion source, (3) the mass analyzer, and (4) the ion detector (see Fig. 1.8). 

In principle, the neutral desorbed products of dissociation can be detected and mass analyzed, if ionized prior to their introduction into the mass spectrometer. However, such experiments are difficult due to low ejfective ionization efficiencies for desorbed neutrals. Nevertheless, a number of systems have been studied in the groups of Wurm et al. [45], Kimmel et al. [46,47], and Harries et al. [48], for example. In our laboratory, studies of neutral particle desorption have been concentrated on self-assembled monolayer targets at room temperature [27,28]. Under certain circumstances, neutrals desorbed in electronically excited metastable states of sufficient energy can be detected by their de-excitation at the surface of a large-area microchannel plate/detector assembly [49]. Separation of the BSD signal of metastables from UV luminescence can be effected by time of flight analysis [49] however, when the photon signal is small relative to the metastable yield, such discrimination is unnecessary and only the total yield of neutral particles (NP) needs to be measured. 

Two types of mass analyzers have been used extensively in atmospheric applications quadrupole mass filters and time-of-flight (TOF) instruments. The use of ion traps is also being increasingly explored for this application. For the fundamental principles of mass... 

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.

The atom-probe field ion microscope is a device which combines an FIM, a probe-hole, and a mass spectrometer of single ion detection sensitivity. With this device, not only can the atomic structure of a surface be imaged with the same atomic resolution as with an FIM, but the chemical species of surface atoms of one s choice, chosen from the field ion image and the probe-hole, can also be identified one by one by mass spectrometry. In principle, any type of mass analyzer can be used as long as the overall detection efficiency of the mass analyzer, which includes the detection efficiency of the ion detector used and the transmission coefficient of the system, has to be close to unity. 

Dynamic ion separation systems are based on another physical principle and use the different flight time of ions with different masses and different velocity (e.g., in ToF mass analyzers). In addition, in dynamic ion separation systems there is a time dependent variation of one or more system parameters, e.g., changing of electrical or/and magnetic field strengths, which means the ion motion during the measurement procedure is crucial for the mass spectrometric analysis. 

The quadrupole mass filter (QMF) is a mass analyzer on whose operation use of an MEM is not necessarily dependent. The ion currents produced are of sufficient magnitude to be measured by means of a Faraday cage and a suitable amplifier such as a vibrating-reed electrometer. The QMF is a true M/z filter which requires no magnetic fields. Since first being proposed by Paul and Steinwedel (30), the QMF has been investigated extensively, and the principles and methods of operation are well known (see, for example, ref. 31). 

Figure 7.8 Excitation (a) and detection (b) of the ion cyclotron motion within an FTMS mass analyzer cell. Reprinted from Marshall, A.G. and Flendrickson, C.L., Fourier transform ion cyclotron resonance detection principles and experimental configurations. International Journal of Mass Spectrometry, 215, 59-75. Copyright (2002), with permission from Elsevier.

Fig. 9 Functional Principle of the Synchronous Ion Shield Mass Analyzer...

In the geometric separation method, ions having different m/e ratios are separated according to their geometric position at the collecting spot. The magnetic mass analyzer, the quadrupole mass analyzer, and the ion trap are based on these principles of operation. The magnetic mass spectrometer will not be discussed in this chapter as it has not been used for the detection of explosives. 

---