Question

Describe the differences between a linear TOF mass spectrometer and an orthogonal TOF instrument?

What are the advantages and limitations of these two geometries?

Answer 1

Principle of Time of Flight (TOF) MS:

Ions of different mass (accelerated by the same field, V), have different velocities and thus different flight times.

The time that it subsequently takes for the ion to reach a detector at a known distance is measured. This time will depend on the velocity of the ion, and therefore is a measure of its mass-to-charge ratio. From this ratio and known experimental parameters, one can identify the ion.

The larger the mass the slower the ion since kinetic energy K.E. = zeV = 1/2 (mv²) where V is the extraction pulse potential.

v = (2zeV/m)^1/2

time of flight: t = L/v = L[m/(2zeV)]^1/2 L is the length of field free drift zone (m) and is the measured time-of-flight of the ion (s).

m/z = 2eVt²/L²

In the linear TOF mass spectrometer, the ion beam can start from an ion source and be directed straight in to ion source.

Alternatively, an ion beam produced by some other analzyer can be deflected (by electric field) in to a TOF analyzer placed at right angles (Orthogonal) to the beam.

Advantages/disadvantages of Linear and Orthogonal TOF:

Common advantages are :

i) High ion transmission

ii) Highest practical mass range of all MS analyzers (linear >10⁵ Da, reflectron <10⁴ Da )

iii) Detection limit

iv) Very good scan-to-scan reproducibility for linear and reflectron Tof.

v) Very fast aquisition time (up to kHz)

Limitations:

i) The main limitation of Linear TOF is resolution which lies with the method of ion production and separation in the instrument. Not all ions of the same m/z value arrive at the detector simultaneously due to a distribution of the kinetic energies they acquire in the accelerating voltage and are not all accelerated from the same point within the ion source. Therefore, even for ions of the same m/z value, there is a distribution of arrival times at the detector.

ii) The accuracy and resolution of Orthogonal TOF analyzers are much heigher as compare to Linear TOF analyzers.

Reason: Ions having a greater amount of kinetic energy (faster ions) will penetrate the reflectron to a greater depth and so spend slightly longer in the device than ions with lower kinetic energy. In this way isobaric ions can be caused to bunch together to reduce the distribution of flight times and greatly enhance the resolving power of the instrument.

iii) The Orthogonal TOF analyzers are expensive as compare to Linear TOF analyzers because in the former an electric field is required additionally for focusing the ion beam towards detector.

iv) one more limitation of using Orthogonal TOF analyzers is a slight decrease in the sensitivity of the analyser due to ion loss and dispersion of the ion beam.