4.2.6.4 Time-of-Flight (TOF) mass filter
While time-of-flight mass filters were one of the first modern MS systems to be developed, they had limited use due to their need for very fast electronics to process the data. Developments in fast electronics and the need for mass filters capable of resolving high mass ranges (such as in geological analysis of isotopes) has renewed interest in time of flight systems.
Entry into the TOF mass filter is considerably different than with other mass filters. The entry has to be pulsed or intermittent in order to allow for all of the ions entering the TOF to reach the detector before more ions are created. With sources that operate in a pulsing fashion such as in ICP or direct insertion of the sample, the TOF functions easily as a mass analyzer. In sources that continually produce ions such as an ICP, the use of a TOF is a bit more difficult. In order to use a TOF system with these continuous sources, an electronic gate must be used to create the necessary pulse of ions. The gate changes the potential on an accelerator plate to only allow ions to enter the TOF mass filter in pulses. When the slit has a positive charge, ions will not approach the entryway to the mass analyzer and are retained in the ionization chamber. When all of the previously admitted ions have reached the detector, the polarity on the accelerator(s) is again changed to negative and ions are accelerated toward the slit(s) and into the TOF mass analyzer. This process is repeated until several scans of each cation peak has been measured. (This type of ionization and slit pulsing will be shown in the animation below).
Prior to developing the mathematics behind TOF separations a simple summary is useful. Mass to charge ratios in the TOF instrument are determined by measuring the time it takes for ions to pass through the “field-free” drift tube to the detector. The term “field-free” is used since there is no electronic or magnetic field affecting the ions. The only force applied to the ions occurs at the repulsion plate and the acceleration plate(s) where ions obtain a similar kinetic energy (KE). All of the ions of the same mass to charge ratio entering the TOF mass analyzers have the same kinetic energy and velocity since they have been exposed to the same voltage on the plates. Ions with different mass to charge ratios will have velocities that will vary inversely to their masses. Lighter ions will have higher velocities and will arrive at the detector earlier than heavier ones. This is due to the relationship between mass and kinetic energy.
KE = mv2/2
The kinetic energy of an ion with a mass m and a total charge of q = ze is described by:
mv2/2 = qVS = zeVS
where VS is potential difference between the accelerator plates, z is the charge on the ion, and e is the charge of an electron (1.60 x 10-19 C). The length (d) of the drift tube is known and fixed, thus the time (t) required to travel this distance is
t = d/v .
By solving the previous equation for v and substituting it into the above equation we obtain
.
In a TOF mass analyzer, the terms in parentheses are constant, so the mass to charge of an ion is directly related to the time of travel. Typical times to traverse the field-free drift tube are 1 to 30 ms.
Advantages of a TOF mass filter include their simplicity and ruggedness and a virtually unlimited mass range. However, TOF mass filters suffer from limited resolution, related to the relatively large distribution in flight times among identical ions (resulting from the physical width of the plug of ions entering the mass analyzer).
Animation 4.6 illustrates how a pulsed accelerator plate/slit acts as a gate for a reflectiveTOF mass filter system.
Animation 4.6. Illustration of a TOF Mass Filter.
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