Basics by Stefan Kaesdorf
Time-of-flight (TOF) spectrometers combine a relatively simple mechanical setup with extremely fast electronic data acquisition. They can offer mass resolutions > 10000, a mass range of up to 500000 Dalton, an ion optical transmission > 10% and large acceptance volumes. TOF mass spectrometry is based on the fact that for a fixed kinetic energy E the mass m and the velocity v of the ions are correlated:
m = 2*E/v2 
By measuring the time T the ions need for travelling a fixed distance s the velocity v and from  the mass of the ions can be calculated. Typically the length of the flight path in commercially available TOF spectrometer systems is about 1000...2000 mm, the kinetic energies used are ca. 2000 eV, the flight times are 5...100 µs, the width of individual TOF peaks a few ns. In the following some technical details of time-of-flight spectrometers are discussed:
To meet the requirement that all the ions start the travel through the spectrometer the same time two different ionization schemes are used:
- all the ions are created in a static electric extraction field within a very short time interval (several ns). This scheme is used in laser ionization, TOF-SIMS (time-of-flight secondary ion mass spectrometry), laser desorption and laser ablation.
- if a continuous ionization process like electron impact ionization is employed the ions are collected for a certain time interval (normally some µs), afterwards the ionization process is stopped and the ions are extracted into the TOF spectrometer by a high voltage pulse with a rise time < 10ns.
Most TOF spectrometers employ multichannel plate (mcp) detectors which have a time response < 1 ns and a high sensitivity (single ion signal > 50 mV). The large and plane detection area of mcp's results in a large acceptance volume of the spectrometer system. Only few mcp channels out of thousands are affected by the detection of a single ion i.e. it is possible to detect many ions at the same time which is important for laser ionization where hundreds of ions can be created within a few nanoseconds.