The Secrets of Time-of Flight Mass Spectrometry Revealed

Time-of-flight mass spectrometry (ToF MS) developed quite late in the 20 century compared with the classical magnet-sector instruments used by J.J. Thomson, F.W. Aston and others. This is astonishing considering the relatively easy concept of these instruments. But it can be understood if one realizes the technical problems arising from the necessary pulsed ion production and fast signal acquisition. The first mass spectrometer using a magnetic field was developed in 1919 but it was not before 1948 that the first ToF mass spectrometer appeared and it then took five more years to reach a resolving power of more than 50. Another reason for the slow development of ToF mass spectrometers was the lack of ionization sources for big molecules that would exploit the main advantage of these instruments: the fundamentally unlimited mass range. With the advance of methods that produce ions from large, fragile molecules [plasma desorption (PD), fast-atom bombardment (FAB), field desorption (FD), laser desorption (LD), matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI)] the importance of ToF mass spectrometry has grown (for an introduction to ion sources, see Reference 5). Today, especially ESI and MALDI, coupled with ToF mass spectrometers, are used extensively in studies of biomolecules, polymers and other large molecules. The first successful attempts to increase the mass resolving power of the time-of-flight technique were performed by Wiley and McLaren in 1957. They used a two-grid ion source which allows an optimization of space and energy focusing on the ions produced. Soon after this major invention, time-of-flight mass spectrometers became commercially available through the Bendix Corporation. However, it needed another ten years before B.A. Mamyrin introduced the concept of a reflecting field to improve the mass resolving power of the time-of-flight mass spectrometer. With this technique, the mass resolving power could be raised from a few hundred to approx 2000. Today, linear ToF mass spectrometers that routinely offer a mass resolving power of around 6000 can be built. Specially designed reflector instruments can have mass resolving powers exceeding 15000 and more. The possible mass range reaches several million Dalton. It should be noted by the reader that, as well as the introduction of the reflector (sometimes also called an ion mirror), other techniques, such as velocity compaction and post-source pulse focusing have been developed during the last two decades. The great success of ToF instruments during the last decade is due to several important features which are more or less unique in mass spectrometry, for example: • ideal compatibility where the ionization method is spatially confined or pulsed • acceptance of a high initial energy spread of ions (several hundreds of eV and more) • complete mass spectrum for each ionization event • high transmission • unlimited mass range (in principle) • very small sample size (approaching 10 g or even less) • fastest scanning mass spectrometer with repetition rates up to 500 kHz The following article will introduce the reader to the principle of ToF mass spectrometry and provide a mathematical description of the time of flight of ions in an instrument that is made of exclusively linear electrical fields. This seems to be a serious restriction but most of the instruments used today employ only, or mostly, linear electrical fields.