Mass spectrometry

An analytical technique used to identify different isotopes and find the overall relative atomic mass of an element

Time of flight mass spectrometry (TOF)

Records time taken for different ions of an isotope to reach a detector

Spectra: shows which isotope is present and the abundance of it.

Conditions needed:

  • Vacuum needed - in order for other particles to not interfere, cannot test a sample if there are other particles present.

  • Vaporisation - sample must be in a gaseous state, so they can move freely

Stages

1)  Ionisation

charging the sample so that it is ionised - this is necesary so the machine can detect the particles at the end.

this is done by passing a high voltage over the chamber. this gives the sample a +1 charge - this is so the mass to charge (m/z) ratio = its actual mass, therefore it is easier to read and interpret the graph.

Different methods of ionisation:

  • Electron impact - electrons fired at particles, knocking off an e-, leaving behind a +1 ion

  • Electrospray ionisation - Sample dissolved in volatile (ability to turn into a gas) solvent, gains a proton (H+), this leaves behind a +1 ion.

2) Acceleration

positively charged ions are then accelerated into a negatively charged plate, using an electric field so they all have the same KE

  • +1 ions are repelled by the positive plate and attracted to the negative plate

3) Drift region (flight tube)

this is where the ions move in the drift chamber/ flight tube.

  • lighter ions move faster

  • heavier ions move slower

In older variations:

there is bend in the machine responsible for deflecting ions using a magnetic field onto a curved path. the radius of the path is dependent on the charge and the mass of the ion. Basically the lighter ions arrive first, then it goes up depending on how many isotopes there are and their masses. The bend is neseassary due to spatial seperation or else all the ions would arrive at the same time and detection would be more harder - (which is literally the whole point)

4) Detection

+1 ions gain an electron at the detector (they are discharged). creating an electrical current ( flow of charge) that is detected.

  • the time taken is recorded - allows us to find mass

faster = lighter ion

slower = heavier ion

  • size of current is detected - allows us to find the abundance of particles

After all this a mass spectrum (graph) is produced and the abundances of different isotopes can be seen.