Measuring Atomic Mass
Mass spectrometry can be used to find the relative atomic mass, relative molecular mass, relative isotopic abundance and molecular structure.
There are 5 steps in a mass spectrometer:
1. Vaporisation: The sample is tyrned into gas (vaporised) using an electrical heater.
2. Ionisation: The gas particles are bombarded with high-energy electrons to ionise them. Electrons are knocked off of the particles, leaving positve ions.
3. Acceleration: The positive ions are accelerated by an electric field.
4. Defelction: The positive ions' paths are altered with a magnetic field. Lighter ions have less momentum and are deflected more than heavier ions. For a given magnetic field, only ions with a particular mass/charge ration make it to the detector.
5. Detection: The magnetic field strength is slowly increased. As this happens, different ions (ones with a higher mass/ratio) can reach 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.
Interpreting Mass Spectra
If the sample passed through the mass spectrometer is an element, each line will represent a different isotope of the element.
The y-axis gives the abundance of ions, often as a percentage.
For an element, the heigthof each peak gives the relative isotopic abundance
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.
Calculating Relative Atomic Mass
1. 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.
2. Add up these totals.
3. Divide by 100 since % were used.
4. If the relative abundance is not given as a %, the total abundance may not add up to 100. In this case just divide by the total relative abundance instead of 100.
Calcualting Relative Molecular Mass
A molecualr ion, M+(g), is formed when the bombarding electrons remove 1 electron from the molecule.
This gives the peak in the spectrum with the highest mass and furthest to the right beside isotopes.
The mass of M+ gives Mr for the molecule.
But bombarding with electrons makes some molecules break up into fragments.
These all show up on the mass spectrum, making a fragmentation pattern.
Fragmentation patterns can be used to identify molecules and even their structure.
Mass spectrum: A chart produced by a mass spectrometer giving information on relative isotopic mass and relative abundance of isotopes.
Relative isotopic abundance: The amount of each isotope present in a sample.