ABSTRACT
The simultaneous nature of sampling ions in TOF offers distinct advantages over traditional scanning (sequential) quadrupole technology for ICP-MS applications in which large amounts of data need to be captured in a short span of time. To understand the benefits of this mass separation device, let us first take a look at its fundamental principles. All TOF mass spectrometers are based on the same principle: the kinetic energy (KE) of an ion is directly proportional to its mass (m) and velocity (V). This can be represented by the following equation:
KE = ½ mV2
Therefore, if a population of ions-all with different masses-is given the same KE by an accelerating voltage (U), the velocities of the ions will all be different, depending on their masses. This principle is then used to separate ions of different mass-to-charge ratios (m/e) in the time (t) domain over a fixed flight path distance (D), represented by the following equation:
m/e U 2
2 = 2 t
D
This is schematically shown in Figure 9.1, which shows three ions of different massto-charge ratios being accelerated into a “flight tube” and arriving at the detector at different times. It can be seen that, depending on their velocities, the lightest ion arrives first, followed by the medium mass ion, and finally the heaviest one. Using flight tubes of 1 m length, even the heaviest ions typically take less than 50 µs to reach the detector. This translates into approximately 20,000 mass spectra per
second-three orders of magnitude faster than the sequential scanning mode of a quadrupole system.
