Infra-red Spectroscopy and Mass Spectroscopy

These two techniques are very important in identifying unknown chemical compounds. Infra-red helps in determining the structure of a compound by locating exact positioning and presence of various bonds, whereas Mass Spectroscopy helps chemist to identify the relative molecular mass of the compound. Both techniques used together can help a chemist to identify an unknown solution.

Infra-red works by exposing a molecule to radiation.  Within the molecule there are different types of bonds and each bond has its own vibration frequency. For example a single bond will have different vibration frequency than the double bond. So when these various bonds are exposed to radiation they will absorb energies of different frequencies. Therefore their vibration frequency will change as they absorb the radiation and according to the type of bond it is, it will absorb frequencies only in certain ranges.  This gives IR spectra which can be interpreted to show how many different bonds are present within the molecules. This provides information on functional groups of the molecule and how they are bonded. This technique is valuable in distinguishing between compounds such as alcohols, carboxylic acids, esters and carbonyls.

Mass Spectroscopy works by breaking the molecule into small fragments. Each fragment is weighed and abundance of each fragment is shown. These fragments are put back together to give the total mass of the original molecule. Molecules are vaporised and then bombarded with an electron gun. This electron gun knocks off electron from the atoms of the molecule and ionises them. These ions are accelerated by electric field then are passed through a magnetic field where they are deflected according to their mass.  The heavy ions with travel at low speed and will not deflect so much. These ions are then detected and a spectrum is produced with peaks of ion fragments. The spectrum shows the mass to charge ratio of each fragment and how abundant each fragment is produced. This mass spectrum is used to work out the relative molecular mass of the compound.