Mass Spectrometer

A mass spectrometer is a machine that can be used to analyse elements or compounds. We need to know about a Time Of Flight (ToF) Mass Spectrometer.

Mass Spec can give you information about relative atmoic mass, relative molecular mass and the relative abundance of isotopes.

1) Electrospray ionisation

The sample is dissolved in a polar solvent and pushed through a small nozzle at high pressure. A high voltage is applied to it, causing the particles to lose an electron. The ionised particles are seperated from the solvent, leaving a gas made up of positive ions.

2) Acceleration

The positive ions are accelerated by an electric field. The particles need to be positively charged to be accelerated by the field. The electric field gives the same kinetic energy to all the ions. The ions with a lower mass/charge ration expereince a greater acceleration. They are given as much energy as heavier ions, but they are lighter so accelerate faster.

3) Ion Drift

The ions leave the electric field with a constant speed and constant kinetic energy. They enter a region with no electric field and drift through it at the same speed as they left the electric field. So the ions with lower mass/charge rations will be drifting at higher speeds.

4) Detection

Because ions with a lower mass/charge ratio travel through the drift region at higher speets, they reach the detector n less time than ions with a higher mass/charge ratio. The detector detects the current created with the ions hit it and records how long they took to pass through the spectrometer. This date is them used to calculate the mass/charge values needed to produce a mass spectrum.

A mass spectrum is a type of chart produced by a mass spectrometer.

If the sample is an elemet, each line represents a different isotope of the element. The y-axes gives the abundance of the ions, often as a percentge. The height of each peak gives the relative isotopic abundance. The x-axis units are give nas a mass to charge ratio. Since the charge on the ions is mostly 1+, you can usually assume the x-axis is simply the relative isotopic mass.

Mass spectrometry can be used to identify elements. Elements with different isotops produve more than one line is a mass spectrum because the isotopes have different masses. This produces characteristic patterns which can be used as fingerprnts to identify certain elements.

1) For each peak, read the % relative isotopic abundance from the y-axis and the lrelative isotopic mass from the x-axis. Multiply them together to get the total relative mass for each isotope.

2) Add up these totals

3) Divide by 100 (since % were used)

You can also get a mass spectrum for a molecular sample. A molecular ion, M+ is formed when 1 electron is removed from the molecule. This gives a peak in the spectru mwith a mass/charge ration equal to the Mr of the molecule.