The mass resolution of a time-of-flight (TOF) mass spectrometer is directly proportional to its total flight path length. Therefore, a long flight path is essential to achieve high mass resolution. In most TOF instruments, the length of the flight path is fixed, and the parameter is proportional to the size of the instrument whether linear or reflectron type. Thus, the maximum flight path that can be achieved in a typical laboratory instrument is of the order of a few meters. It is difficult to achieve very high resolution by simply extending the length of the flight tube. In order to circumvent this fundamental limit, it is necessary to place the ions in a closed orbit and to pass the ions around the same orbit many times.
The first multi-turn TOF mass spectrometer, the ‘MULTUM Linear plus’, was constructed as a laboratory model for the COSAC project of the ROSETTA space mission. Mass resolution greater than 350000 was achieved after 500 cycles. After this instrument, some improved multi-turn systems were developed.