Two-stage acceleration

Figure Bl.7.17. (a) Schematic diagram of a single acceleration zone time-of-flight mass spectrometer, (b) Schematic diagram showing the time focusing of ions with different initial velocities (and hence initial kinetic energies) onto the detector by the use of a reflecting ion mirror, (c) Wiley-McLaren type two stage acceleration zone time-of-flight mass spectrometer.

For efficient removal, these plates should ideally be placed at the point at which ions attain their tightest temporal and spatial focus prior to detection. Although deflection of this type would be challenging with a swept-beam geometry, the space focus plane of the two-stage acceleration design offers an excellent opportunity to deflect the maximum number of ions over the shortest distance in the shortest time. This placement also ensures the most resolution of mass deflection. For an ICP-TOF-MS, the efficiency of removal for this parallel-plate scheme was promising [24]. 

Figure 5.21 Schematic diagram for measuring ion dissociation rates in a two-stage acceleration region. The rates of ions which dissociate in the acceleration region can be determined by measuring either the A+ product ion kinetic energies, or their times of flight.

The concept of the TOF mass analyzer was first proposed by Stephens in 1946 [3], and the first instrument was demonstrated by Cameron and Eggers in 1948 [4]. Since then, several important advances in TOF technology have occurred, including time-lag focusing and two-stage acceleration techniques demonstrated by Wily and McLaren in 1955 [5], reflec-tron configuration demonstrated by Mamyrin et al. in 1973 [6], and curved-field reflectron demonstrated by Cornish and Cotter in 1993 [7]. 

Figure 3.1 Schematic of TOF mass spectrometers equipped with two-stage acceleration ion sources, (a) A linear TOF mass spectrometer and (b) a reflectron TOF mass spectrometer...

To compensate for the energy spread, the first stage of the two-stage acceleration field can be activated shortly after ion production or ejection from the sample. This event typically occurs in the nanosecond to low microsecond range. The method is known as time-lag focusing or delayed extraction technique. In TOF-MS equipped with matrix-assisted laser desorption/ionization (MALDI) ion source, delayed extraction may improve the mass resolving power by a factor of two- to fivefold [9, 10]. Mass resolving power defines the sharpness of spectral features, which is explained in Chapter 5. 

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