There are numerous ways in which we may separate and select ions in a mass spectrometer. Magnetic an — Physical Chemistry — Kinetics Chemistry Question

There are numerous ways in which we may separate and select ions in a mass spectrometer. Magnetic and electric fields, as well as radio frequency are often used to separate ions in mass spectrometers. The time–of–flight (TOF) mass spectrometer is the simplest type of common mass analyser and it has a very high sensitivity. With the introduction of matrix–assisted laser desorption / ionization (MALDI) or electrospray ionization (ESI), which is used to introduce and ionize macromolecules such as proteins, DNA, and polymers, the measurement of a large mass range became achievable. It is now possible to desorb and analyze ions with molecular weights upwards of one million atomic mass units (amu; 1 amu = 1.6605×10 –27 kg). In general, the sample ions are generated in a source zone of the instrument, by whatever ionization method is being employed. A high voltage (HV) potential is applied across the source to extract and accelerate the ions from the source into the field–free “drift” zone of the instrument. The fundamental operating principle of TOF is that ions of the same kinetic energy will move with different velocities depending on their m / z value. This can be seen in the following equation, the classical equation for kinetic energy.

KE = ½ m v 2

This relationship may be rearranged to give velocity in terms of kinetic energy and m / z.

v = (2 × KE / m)½

If the distance from the point of ion formation to the detector at some fixed point is L (neglecting the extraction time from the source), the time of flight (t) can be calculated by the following equation:

t = L / v = L / (2 × KE / m)½

In most modern TOF mass spectrometer, ions are generally accelerated to kilo–electron volt (keV) energies. At these energies, flight times will be in the range of a few tens to a few hundreds of microseconds (µs) (depending on the flight distance). The kinetic energy of an ion accelerated by a potential is given by:

KE = z e V

where z is the number of charges on the ion, e is the fundamental unit of charge (1.6022×10 –19 C) and V is the applied accelerating potential in volts.