Mass Spectrometry Flashcards
What is the peak furthest to the right on a mass spectrum and what does it tell us?
Parent Peak or Molecular Ion (M+); it tells us the molecular weight of the molecule.
What is the tallest peak on a mass spectrum and what does it tell us?
Base Peak; it tells us the most stable ion created by the fragmented molecule.
What causes the molecule to ionize and fragment?
A beam of electrons
What does m/z stand for?
Mass to charge ratio of an ion
What determines the curvature of an ion’s path as it is passed through the varying magnetic field?
Officially, the m/z; however, since the majority of ions have a +1 charge, we can assume that the curvature of their paths are dependent only on mass.
What is the ration of carbon 12 to carbon 13? What will this cause on a mass spectrum?
98.9 : 1.1. It will cause a small M+1 peak.
What is the ratio of chlorine 35 to chlorine 37? What will this cause on the mass spectrum?
75.5 : 24.5. So molecules with chlorine will have an M+2 peak that is 1/3 as tall as the M peak.
What is the ratio of bromine 79 to bromine 81? What will this cause on a mass spectrum?
50.5 : 49.5. So molecules with bromine will have an M=2 peak that is the same height as the M peak.
How does a radical cation typically fragment in a mass spectrometer?
Fragments to give the most stable cation and a radical.
Where do alkanes most often fragment? Why?
At branched points, because this creates inductively stabilized secondary and tertiary cations.
What does a peak at M-15 usually symbolize?
Loss of a methyl radical.
What should you suspect if you see a molecule that has an even molecular weight fragment and an even MW?
Loss of a neutral molecule
What should you suspect if you have a spectrum with a peak at M+2 that is 1/3 the height of M and peaks at M-36 and M-38?
A molecule containing Cl and loss of HCl
What sort of molecule do you have if you observe an M-15 peak and peaks at increments of additional losses of 14?
Straight chain alkane
What is the fragmentation mechanism for an alkene?
Radical mechanism (single-hooked arrows).
What do alkenes tend to fragment to yield?
An allylic cation (very stable, should be the base peak).
What does a peak for a tropylium ion indicate?
A benzyl group
What does a peak for a phenyl cation indicate?
A benzene ring
What is the m/z for a tropylium ion?
91
What is the m/z for a phenyl cation?
77
For what functional groups will cleavage be observed adjacent to the alpha carbon?
Ethers, alcohols, and amines
Where will cleavage be observed if there are two alpha carbons on a hetero atom?
Adjacent to both carbons
What is diagnostic of an alcohol?
Loss of H20 (M-18)
What is diagnostic of a compound containing nitrogen?
Odd MW (parent peak) and even molecular weight fragments.
