Question

If an ion fragments in the drift tube of a TOF, how would you know? Keyboard Shortcuts

Answer 1

• In a time-of-flight mass spectrometer the ions are formed in a similar manner by electron bombardment, and the resulting ions accelerated between electrically charged plates.

• Again, the sample must be a gas or vapourised and is bombarded with an electron beam or laser beam to knock off electrons to produce positive ions.

• However, the method of separation due to different m/e (m/z, mass/charge) values is then dependent on how long it takes the ion to travel in the drift region' i.e. the region NOT under the influence of an accelerating electric field.

• The ions are accelerated in the same way between positive to negative plates in an electric field of fixed strength i.e. constant potential difference.

• The smaller the mass of the ionised particle (ionized atom, fragment or whole molecule) the shorter the time of flight in the drift region where no electric field operates.

• This is because for a given accelerating potential difference, a lighter particle is accelerated more to a higher speed than a heavier ion, so the 'time of flight' down the tube is shorter.

• Therefore the ions are distinguished by different flight times NOT by different masses being brought into focus with a magnetic field as described in section 4a BUT the separation by time of flight is still determined by the m/e (m/z) value of the ion.

• The general principles of the separation are required knowledge but the mathematics is NOT needed by A level students, but if you are interested, a simplified summary is given below

  • t = K_inst√(m/q)

  • t = time of flight, m = mass of ion, q = charge on ion,

  • K_inst = a proportionality constant based on the instrument settings and characteristics e.g. the electric field strength, length of analysing tube etc.

  • Therefore t is proportional to the square root of the mass of the ion for particles carrying the same charge - the bigger the mass the longer the 'flight time'.

  • The first equation is derived partly from the extra mathematics outlined below.

  • KE = qV, the kinetic energy imparted to the ion is given by its charge x the potential difference of the accelerating electric field.

  • The acceleration, for a fixed electric field, results in an ion having the same kinetic energy (KE) as any other ion of the same charge q but the velocity v of the ion depends on the m/e (m/z) value.

  • v = d/t (or t = d/v), where v = velocity of accelerated particle in the drift region, d = length of tube in the drift region. (or t = d/v)

  • KE = ¹/₂mv², so the bigger m, the smaller is v in the drift region and hence the basis of detecting ions of different mass by different 'flight times'.

  • The diagram makes the method look simple, but far from it, the instrument works in a pulsed manner i.e. pulsed electric field, and some pretty sophisticated electronics are used to analyse the signals from the detector and the software calculates the mass of the ion based on the drift flight time.

• Ultimately the data for analysis and subsequent calculations is the same as that derived from a deflection mass spectrometer described in method