Mass Spectrometry

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Mass spectrometer used to analyse elements or compounds - time of flight (TOF) mass spectrometer.

Tells you relative atomic mass, relative molecular mass and relative abundance of isotopes.

Electrospray Ionisation:

Sample dissolved in a volatile solvent and pushed through nozzle at high pressure.

High voltage applied to sample, causing particles to lose an electron.

Ionised particles separated from solvent, leaving a gas made up of single positively charged ions.

Acceleration:

Positive ions accelerated by an electric field.

Particles need to be positively charged to be accelerated by the electric field.

Electric field gives the same kinetic energy to all ions.

Ions with lower mass/charge ratio experience greater acceleration.

Ion Drift:

Ions leave the electric field and enter a region with no electric field.

Drift through at the same speed as they left the electric field.

Ions with lower mass/charge ratio will drift at higher speeds.

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Detection:

Ions with lower mass/charge ratio travel through drift region at higher speeds, they reach the detector in less time than ions with a higher mass/charge ratio.

Flight time is recorded.

Positive ions pick up an electron from the detector which causes a current to flow.

Data Analysis:

The signal from the detector is passed to a computer which generates a mass spectrum.

Data used to: measure relative isotopic mass and relative abundance of isotopes in the sample.

Different isotopes that make up an element have different masses so they are detected separately.

Number of peaks = the number of isotopes in the element.

Peak height = relative abundance of isotope.

Horizontal scale gives mass/charge (m/z) ratio - for singly charged ion, the same as mass number.

Peak with the greatest m/z value (furthest right in spectrum) is caused by the molecular ion.

Molecular ion formed when high-energy electrons knock an electron from the molecule, producing a unipositive ion.

Other peaks in mass spectrum caused by fragments from molecular ions.