Mass spectrometry allows a user to determine the molecular mass of a molecule and, by looking at the mass of the common occurring fragments, deduce certain other things about the structure. It is commonly used in tandem with IR spectroscopy and NMR spectroscopy.
Mass spectrometry is often coupled to gas chromatography when studying certain chemicals like polymers which can display a wide distribution of molecular sizes. Such coupling allows for greater separation of molecules and for more accurate determination of the distribution of sizes.
The most common technique consists of the following steps:
atomisation of a substance
ionisation (either by bombardment with high energy electrons or addition of H
acceleration of the charged species with an electric field,
deflection by a magnetic field where the radius of deflection depends on the mass to charge ratio (m/z)
detection on a generator
Typically the detector is kept in a fixed position and the magnetic field strength to which the ions are exposed changed slowly. The radius of deflection depends on the magnetic field strength so as the magnetic field changes, fragments with different m/z values will hit the detector. The intensity of the signal received by the detector is directly proportional to the number of fragments with that m/z value. This allows a graph of intensity v. m/z value to be plotted.