Introduction

1 An Introduction to Mass Spectrometers

Although it is beyond the scope of this book to delve deeply into the theory and physics of mass spectrometers, a brief introduction would appear to be necessary, not only to clarify the nomenclature used for the various techniques and hardware described in the remainder of this monograph, but also to encourage the reader to turn to more complete texts on the subject.

A mass spectrometer, like Caesar's Gaul, can be divided into three fundamental parts, namely the ionization source, the analyser, and the detector (see Figure 1.1). Mass spectrometers are used primarily to provide information concerning the molecular weight of a compound, and in order to achieve this, the sample under investigation has to be introduced into the ionization source of the instrument. In the source, the sample molecules are ionized (because ions are easier to manipulate than neutral species) and these ions are extracted into the analyser region of the mass spectrometer where they are separated according to their mass (m) to charge (z) ratios (m/z). The separated ions are detected and the signal fed to a data system where the results can be studied, processed, and printed out. The whole of the mass spectrometer (except for Atmospheric Pressure Ionization sources) is maintained under vacuum to give the ions a good chance of travelling from one end of the instrument to the other without any interference or hindrance. Nowadays the entire operation of the mass spectrometer and often the sample introduction process are usually under complete data system control and the operator hardly needs to move away from the computer terminal to perform the sample analyses.

Many ionization methods are available and each has its own advantages and disadvantages. The method of ionization used depends on the sample under investigation, the type of mass spectrometer being used, and the available equipment. This book describes the more commonly encountered ionization methods, and aims to provide an account of their set-up and basic operation. Once the ionization method has been set up and has been shown to be operating at its optimum performance, then the operator can start to develop the technique for the particular samples under scrutiny. The optimum performance of any ionization method will depend on the performance and condition of the mass spectrometer, the reliability of any other equipment and materials involved, including gas and liquid chromatographs and chromatography columns, the purity of any solvents or gases used, and the quality of the standard samples. However, it should be remembered that the optimum performance must be established before any sample analyses are undertaken, and this performance should be verified every day, or more frequently if laboratory procedures dictate or if problems are suspected. If the performance is not as good as expected then steps should be taken to retrieve any losses in sensitivity or resolution.

As well as there being a good choice of ionization methods, there are also many different ways of introducing samples into the ionization source depending on the ionization method being used and the type of samples under investigation. For example, single-substance samples can be inserted directly into the ionization source by means of a probe whereas complex mixtures will benefit from some kind of chromatographic separation en route to the ionization source, and this could involve interfacing liquid chromatography (LC), gas chromatography (GC), supercritical fluid chromatography (SFC), or capillary electrophoresis (CE) to the mass spectrometer. The methods of interfacing to the various ionization methods are described in more detail in the relevant chapters.

After the ionization source, the ions proceed to the analyser region, and a mass spectrometer is generally classified by the type of analyser it accommodates. There is a variety of analysers, and the ones referred to in this book are those that are most frequently encountered in organic mass spectrometry, namely the magnetic sector, the quadrupole, and the time-of-flight. Each will be discussed in a little more detail later in this Chapter (see Chapter 1, Section 2). Not all ionization methods are compatible with all of these analysers, as will be revealed where appropriate in the text.

The detector could be one of several possibilities including inter alia photo-multipliers, electron multipliers, microchannel plates, and diode array detectors. On a day-to-day basis, the detector gain should be set at the appropriate level for acquiring data.

The remainder of this Chapter aims to provide a brief overview of a range of mass spectrometers and indicate which ionization methods are appropriate. I have tried to summarize the various ionization methods, instruments, and sample introduction methods in several different ways so that these summaries can be referred to at a later date when reading the more detailed chapters to help put the various topics in perspective. The summaries may appear to overlap, and if this is so, I apologize; it was simply my intention to display the data in a readily accessible manner, emphasizing different significant aspects so that the text would appeal to a variety of readers.

2 The Mass Analyser

Introduction

Resolution

The main function of the mass analyser is to separate, or resolve, the ions formed in the ionization source by their mass to charge ratios (m/z).

The resolution (R) of a mass analyser, or its ability to separate two peaks, is defined as the ratio of the mass of a peak (M1) to the difference in mass between this peak and the adjacent peak of higher mass (M2) (see Figure 1.2), i.e.:

R = M1/M2 - M1 (1.1)

where R = resolution,

M1 = the mass of a peak, and

M2 = the mass of an adjacent, higher mass peak.

In the simplest terms, a singly charged ion at m/z 1000 could be separated from another singly charged ion at m/z 1001 if a resolution of 1000 is available. Similarly, a singly charged ion at m/z 2000 would require a resolution of 2000 to separate it from a second singly charged ion at m/z 2001, whereas a singly charged ion at m/z 100 would need only 100 resolution to separate it from another singly charged ion at m/z 101.

Resolution, when referring to magnetic sector mass spectrometers, is often described by the 'valley definition' where a 'resolution of 10%...