Mass Spectrometry

The Mass Spectrometer

Mass spectrometers can be used to determine all the isotopes present in a sample of an element and to therefore identify elements.Calculating relative atomic massThe relative atomic mass quoted on the periodic table is a weighted average of all the isotopes of the element.For each isotope the mass spectrometer can measure a m/z (mass/charge ration) and an abundance.If asked to give a species from a mass spectrum then give charge and mass number.

f57e976e-6b00-4bac-ab18-afd0b81fedb4.gif (image/gif)

Caption: : Spectra for magnesium from mass spectrometer

Sometimes two electrons may be removed from a particle forming a 2+ ion. ^24Mg^2+ with a 2+ charge would have an m/z of 12 (24/2).Use this equations to work out RAM:RAM = isotopic mass x %abundance/100For above example - RAM = [(78.7 x 24) + (100.13 x 25) + (11.17 x 26)]/100 = 24.3Or if relative abundance is used instead of percentage abundance:RAM = isotopic mass x relative abundance/total relative abundance

Mass Spectra for Cl2 and Br2

Cl has two isotopes: Cl^35 (75%) and Cl^37 (25%).Br has two isotopes: Br^79 (50%) and Br^81 (50%).These lead to the following spectra caused by the diatomic molecules -->

436ef7a7-b544-430b-8bb3-3f7d48b97e86.gif (image/gif)

5c5ad04c-21db-4374-ac03-3c0d353879df.gif (image/gif)

Caption: : The first two lines in each spectra show the masses of the two isotopes.

Caption: : The other lines are the combinations of these values added together: e.g. 79 + 79 = 158 The different column heights show the ratio of each combination.

Measuring the Mr of a molecule

If a molecule is put through a mass spectrometer it will often break up and give a series of peaks caused by the fragments. The peak giving the largest m/z, however, will be due to the complete molecule and will be equal to the Mr of the molecule. This peak is called the parent or molecular ion.

dda84718-6cad-494d-946b-dc8c6cd2bf32.gif (image/gif)

Uses of mass spectrometers

Mass spectrometers have been used: in planetary space probes so tat elements on other planets can be identified (they can have different compositions of isotopes) for drug testing in sports to identify chemicals in the blood and breakdown products from drugs in the body for quality control in the pharmaceutical industry and to identify molecules from sample with potential biological activity radioactive dating to determine the age of fossils and human remains