Mass Spectrometry Flashcards
Describe the 5 stages in detail.
Vaporisation - turn compound into a gaseous state
Ionisation - sample entered into the mass spectrometer is bombarded with electrons to form ions.
Acceleration - the mass spectrometer has two plates, one negatively charged and the other positively charged. These cause a strong electrical field between them. As a result the ions are accelerated through this field.
Deflection - usually the plates are made of iron we creates a magnetic field. This magnetic field causes the ions direction of travel to be bent into a circular path. The degree to which they are bent depends on their mass. (Lighter & more charged the more bent) [so the lower down the number is on the x axis the lighter the ion is and we also know it is smaller]
Detection - ions hit the detector screen which is linked to a computer that produces a mass spectrometry graph showing the number of ions at each particular mass.
4 important bits of knowledge about the final peak?
The M + ion peak.
It represents the largest molecular ion that can be formed from the compound put in the mass spectrometer
You could use the final peak to work out what the full molecular formula of the compound they put in was. (The compound will most likely be the same formula as ion the compound forms but ignoring the charge sign) EG. If the final peak was labelled C3H3O2+ then the compound put it will probably have been C3H3O2. OR you could work out what the compound put in was by looking at the number on the x axis which is the same as the RFM then using trial and error to determine the molecular formula of the compound therefore figuring out what the compound is.
Sometimes the largest ion that can be produced from the compound is very unstable and breaks down before a final peak can be produced.
What are the axis on a mass spec graph and what do they mean?
Y axis = relative abundance (heights of the peaks) tells us the proportions of molecular ions present in relation to each other. Eg. If one peak stopped at 5 and the other stopped at 15 then that means the second peak has three times as many ions produced from the compound than the first peak.
X axis = m/z [mass/charge] ratio. We can assume that this number is often the same as the RFM of the ion.
What do the peaks represent on a mass spec graph?
The different ions that can form form the compound put in the mass spec depending on where the molecule has broken off the compound.
Why do we get the peaks?
Because the ions are charged
What do you do if asked to work out displayed formula of an ion that one of the peaks is representing?
Look at the m/z ratio which is the same as RFM then use this and trial and error to work out what the ion could be.
What compounds are usually put in a mass spectrometer?
Organic compounds
Which two properties determine the degree of deflection of the ion?
Mass (lighter bends more)
Charge (more charged bends more) because more charged means has been bombarded with more electrons so more broken off so small ion which links back to it being smaller again.
Uses of mass spectrometers
Drug testing in sport to identify chemicals in the blood and the breakdown products from these drugs.
Generally, if there’s two tall peaks with one in the middle it will be a molecule containing both ions formed either side.usually this will be a 2+ charge because 2 electrons are knocked off which results in the z value in m/z being 2 so whatever the mass is of the new ion it is divided by 2.
two tall peaks - RHS- 1+ peak, LHS (with smaller m/z) - 2+ peak, middle peak = combination of 79 and 81
Where will the peaks be produced if you have a 2+ ion?
the peak produced by the 2+ ion will have an m/z ratio that is half if the same ion was 1+ (eg. if 2+ was at 15 then the 1+ ion will be at 7.5)
How would the peaks appear if you have a natural mixture of bromine molecules of different isotopic masses?
one Br2 molecule - 79, 79 (tall peak)
another Br2 - 79, 81 (middle short peak)
another Br2 - 81, 81 (tall peak)
How could a 2+ molecular ion be produced and how would this show up on mass spec?
produced when a molecule has two electrons knocked off and hence a 2+ charge
it will have a mass to charge ratio which is half of the equivalent 1+ molecular ion
if you see a peak at an m/z value of 160 and then another peak at m/z of 80 what can you assume?
There are two bromine molecular ions of different charges (one 1+ and another 2+)
if you see a peak at an m/z value of 2x and then another peak at m/z of x what can you assume?
There are two similar molecular ions of different charges (one 1+ and another 2+)
