Mass Spectrometry
- Mass Spectrometry is widely used to determine and identify the elements present in samples and to determine their concentrations.
- Mass Spectrometry is also used to measure the relative atomic mass of an element and to measure the relative molecular mass of a substance.
Mass Spectrometry Process
- There are four key stages in the process for Mass Spectrometry.
- Ionisation
- Acceleration
- Deflection
- Detection
Stage 1 Ionisation
- Electro Ionisation is the most common type of ionisation.
- The sample is bombarded by electrons which come from a heated filament.
- The electrons run in a stream between the cathode and anode.
- When the sample passes through the electron stream, the high energy electrons in the stream knock electrons out of the sample to form ions.
Stage 2 Acceleration
- Acceleration is a simple step where the ions are placed between a set of charges parallel plates.
- The ions will then be repelled by one plate and attracted to the other.
- There is a slit cut in the plate which the ions are attracted to. the force of attraction and repulsion forces the ions through the slit at an accelerated rate.
- The speed of acceleration can be adjusted by changing the charge on the plates.
Stage 3 Deflection
- Ions are deflected by the magnetic field surrounding the instrument.
- The amount of deflection depends on the mass and charge of the ions.
- The heavier ions and ions with a positive charge of 2 or more, are deflected the least (Ion stream C)
- The lightest ions and ions with 1 positive charge are deflected the most (Ion Stream A)
- The ions at the correct mass and charge travel to the detector. (Ion Stream B)
- The mass to charge ratio (m/z) is determined from the ion that hits the detector.
Step 4 Detection
- When the ion stream reached the detector the hit a wire. On hitting the wire they become neutralised by an electron jumping from the metal wire to the ion.
- The amplifier picks up on this current being created between the wire and the ion and amplifies the signal being detected.
- The computer picks up on this and converts it to mass/charge ratio and a spectrum is produced.
Applications of Mass Spectrometry
- Mass Spectrometry as a technique can be coupled with other techniques such as HPLC and GC.
- As it is used in the identification of compounds it is used in all areas of science.
- Some of its uses are:
- Trace Gas Analysis
- Pharmaceutical Industry
- Space Exploration
- Forensic Toxicology
- Archaeological Dating.