Mass analyser basic principles

Principles and Characteristics Analytical multistage mass spectrometry (MSn) relies on the ability to activate and dissociate ions generated in the ion source in order to identify or obtain structural information about an unknown compound and to analyse mixtures by exploiting two or more mass-separating steps. A basic instrument for the currently most used form, tandem mass spectrometry (MS/MS), consists of a combination of two mass analysers with a reaction region between them. While a variety of instrument set-ups can be used in MS/MS, there is a single basic concept involved the measurement of the m/z of ions before and after a reaction in the mass spectrometer the reaction involves a change in mass and can be represented as ... 

As there are a great variety of sources, several types of mass analysers have been developed. Indeed, the separation of ions according to their mass-to-charge ratio can be based on different principles (Table 2.1). All mass analysers use static or dynamic electric and magnetic fields that can be alone or combined. Most of the basic differences between the various common types of mass analyser lie in the manner in which such fields are used to achieve separation. 

Mass analysis, i.e., the separation of ions according to their m/z in either time or space, can be achieved in a number of ways. This section does not discuss the basic principles of the most important types of mass analysers [1-2], but focusses on aspects and developments important in relation to LC-MS. An excellent discnssion on the various mass analysers and their advantages and hmitations is given by Bruimee [32]. 

The basic principle of the method requires that the molecules to be analysed can be ionised in a gaseous phase after desorption from the matrix on which it was previously adsorbed. The energy that provokes desorption and ionisation of the species to be analysed is provided by a laser pulse working in the ultraviolet domain. Each ionised molecule coming from the sample and desorbed from the matrix has an acceleration that is inversely proportional to its own mass and proportional to its charge. This means that the smallest ions reach the detector first and the heaviest ions reach the detector last, as shown in Figure 2.3. 

Although the basic principle of mass spectrometry (MS) was discovered as early as 1910 by Sir J. J. Thomson, it was not untU the end of World War II that MS was first developed for analyses of gas and hydrocarbon mixtures [1], Almost all of the mass spectrometers at that time were made by researchers or specialists. The first commercial mass spectrometer manufactured by Consolidated Engineering Corporation (CEC) was delivered to the Atlantic Refining Company in 1946 for analysis of hydrocarbon fractions in gasoline boiling range [2]. Since then, the petroleum industry has pioneered the use of MS in chemical research. Many advances in MS were driven by the needs of the petroleum industry for analyzing components in complex mixtures. 

In this section we will explain the operating principles of the three types of mass analyser used so far in PTR-MS studies, namely quadrupole, ion trap and TOF-MS. However, before doing so some basic definitions relevant to mass spectrometry are introduced. 

The quadrupole mass filter was the type of mass analyser used in the earliest PTR-MS instruments and it is still the most popular choice at the time of writing. The popularity of the QMS stems from its relatively compact size and reasonable price. In this section the basic operating principles of QMS systems will be described along with a discussion of some of their performance characteristics. 

The mass analyser described in the preceding section is sometimes referred to as a linear quadrupole, since the electrodes consist of parallel rods. Another type of mass spectrometer based on the use of a quadrupolar electric field is the three-dimensional (3D) quadrupole ion trap, which is sometimes also known as a Paul trap. This is a device that has been in existence for several decades, but it is only in the past twenty years or so that it has moved from a small number of research laboratories out into the commercial domain. Ion traps are now popular devices for a whole range of mass spectrometry applications, their popularity being enhanced by their very compact size and the ease with which they can be used in MS experiments (see Section 3.5.3.2). Several PTR-MS instruments have been constructed with a quadrupole ion trap and so a description of the basic operating principles... 

The principle underlying time of flight (TOF) mass spectrometers is based on the relationship that exists between mass and velocity at a given kinetic energy. The instrument, which uses pulsed ionisation, measures the time taken by each mass to travel the length L of a field-free analyser tube. The basic equation (I6.l l) used in linear TOF analysers is obtained by eliminating the velocity v from equation (16.5) in conjunction with the relationship L = vt ... 

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