Mass Spectrum
Specification: use of data from a mass spectrum to determine relative abundance of isotopes and calculate the relative atomic mass of an element.
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Mass Spectrum Apparatus:
The apparatus of the mass spectrometry consists of different sections. The compound goes through the sample inlet and gets ionized to become a positively charged ion. Then the ion would be accelerated to the detector. The heavier ion would move slower than the smaller ion as it has more mass. This means the first ion detected will have the smallest fragment.
Using a mass spectra to find the relative atomic mass:
This is a mass spectrum of Iron. We can see different Mr ( M/Z) of Iron as Iron has different isotopes.
To find the relative atomic mass of Iron here we need to use each Mr and multiply it by its intensity.
(54x5.8)+(56 x 91.7)+(57 x 2.2)+(58x0.3) / 100 = 55.91 Mr of Iron.
Using a mass spectrum to find the Mr of compound or fragments:
Let’s use 2-chloropropane as an example. It has an Mr of 79.
To find the Mr of 2-chloropropane we need to find the last one or two picks of the spectrum. If there is only one then the last pick will be the Mr of the compound but if there are two picks, use the one before the last one. On the diagram below where there is M+ is where we would find the Mr of 2-Chloropropane. The M+2 would be usually if there is some carbon-13 instead of carbon-12.
After we can see lots of picks on the spectrum before the largest Mr, they are fragments. They are formed when the compound has been accelerated and can break at different places. (Think of a glass breaking, It breaks at different places). The pick with the largest relative abundance is the fragment which is most likely to happen during the acceleration.
For this compound the biggest pick is the fragment with the Mr of about 43. This would be equal to CH3CHCH3 +. The Cl would be what has left the compounds during the acceleration.
When you have fragments you need to add a plus (+) if you are drawing the compound as it’s now an ion.