Mass spectrometry (MS) is an extremely valuable analytical technique in which the molecules in a test sample are converted to gaseous ions that are subsequently separated in a mass analyser according to their mass-to-charge (m / z) ratio and then detected. The mass spectrum is a plot of the relative abundances of the ions at each m / z ratio. Note that it is the mass-to-charge ratio (m / z), and not the actual mass, that is measured. For example, if a biomolecule is ionised in positive ion mode, the instrument measures the m / z after the addition of one proton (i.e. 1.0072772984 Da for exact mass or 1.0078 Da for average mass). Similarly, for a biomolecule ionised in negative ion mode, an m / z after the loss of one proton is measured.
The essential features of all mass spectrometers are:
Several techniques exist to generate ions and are discussed below. Of these, the development of electrospray ionisation (ESI; Section 15.2.4) and matrix-assisted laser desorption ionisation (MALDI; Section 15.2.5), has effectively expanded the detectable mass range, enabling the measurement of almost any biomolecule. Mass analysers separate ions by use of either a magnetic or an electric field (Section 15.3); detectors produce a measurable signal in the form of either a voltage or a current that can be transformed by a computer into data to be analysed (Section 15.4). The symbol M r is used to designate relative molecular mass. As a relative measure, M r has no units.
The treatment of mass spectrometry in this chapter will be rather non-mathematical and non-technical. Mass spectrometry has a wide array of applications, including drug discovery and the sciences of proteomics (Chapter 21) and metabolomics (Chapter 22).
This chapter will focus on the fundamental principles of mass spectrometry. The intention is to give an overview of the different types of instrumentation that are available and discussion of their applications, complementary techniques and the advantages/disadvantages of each system. Sample preparation and data analysis will also be covered. A further reading list is provided, covering more technical and mathematical aspects of mass spectrometry.