Analytical Techniques - Mass spec.

Spectrometry is used to mesure the relative atomic masses of atoms, and also, is a frequent method of measuring the relative atomic masses of compounds too. As a recap: the compound enters as a Vapour. It is then Ionised by at electron gun that bombards it with high speed electrons. After this the compound is Acclereated through the spectrometer as a beam of ionoised molecules, and then they are deflected into the scanner by altering the strength of the magnetic. This depends on the mass and charge of the compound. The compounds are then detected via their mass and charge.

When compounds are in a mass spec, they get ionised, as I said above. The ion that they form at this point is the molecular ion. The molecular ions are really unstable and can break down into fragments. When it goes through the spectrometer, a spectrum is produced. Generally, the largest peak is the molecular ion (as it is the biggest thing going into the detctor (not having broken down [FRAGMENTATION]). This is how you can tell the molecular formula. As, it will be the same as the molecule's relative mass, and the molecule is the highest peak.

There are lots of peaks in the mass spec of a compound, because of the fragmentation taking place. They break down into smaller parts, and these smaller molecules are also detected, and, obviously, they will have different masses, and therefore, different readings as it is relative to m/z ratio.

High resolution mass spec tells us the Mr to 3 decimal places. This means that by adding up the atoms (to three decimal places) we can work out the molecular formula.