Mass spectrometry

You can use a mass spectrometer to find the relative atomic or molecular mass, relative isotopic abundance or molecular structure.

There are five things that happen in a mass spectrometer:

Vaporisation - the sample is vaporised using an electrical heater.

Ionisation - the gas particles are bombarded with high-energy electrons to ionise them. Electrons are knocked off the particles, leaving positive ions.

Acceleration - the positive ions are accelerated by an electric field.

Deflection - the positive ions' paths are altered with a magnetic field.lighter ions have less momentum and are deflected more than heavier ions. 

Detection - for a given magnetic field, only ions with a particular mass/charge ratio make it to the detector. As ions hit the detector they cause a current to flow. The bigger the current produced, the more of that isotope was present in the sample. A mass spectrum is produced.

Mass spectrum - a chart produced by a mass spectrometer giving information on relative isotopic mass and relative abundance of isotopes.

If the sample is an element, each line will represent a different isotope of that element. 

The y-axis gives the abundance of ions, often as a percentage. For an element, the height of each peak gives the relative isotopic abundance (the amount of each isotope present in a sample).

The x-axis units are given as a mass/charge ratio. Since the charge on the ions is mostly 1+, you can often assume the x-axis is simply the relative isotopic mass.

For each peak, read the relative isotopic abundance from the y-axis and the relative isotopic mass from the x-axis.
Multiply them together to get the total mass for each isotope.
Add up these totals.
Divide by the total relative isotopic abundance (100 if a percentage abundance).

You can also get a mass spectrum for a molecular sample. 

A molecular ion is formed when the bombarding electrons remove 1 electron from the molecule.
This gives the peak in the spectrum with the highest mass (furthest to the right, ignoring isotopes).
The mass of the 1+ ion gives the Mr for the molecule.
However, bombarding with electrons makes some molecules break up into fragments, showing up on the mass spectrum making a fragmentation pattern