Mass Spectrometry Flashcards
What is Electron Ionization (EI)?
EI is done when a sample is bombarded by a high energy electron beam.
This beam ejects an electron which causes a radical cation to form.
The cation then fragments into a radical and a different cation
The cation is then detected from the spectrometer
What is Chemical Ionization (CI)?
A sample is placed into an ionization chamber with a carrier gas (H2, NH3, CH4)
The gas is ionized by high energy electrons and creates a primary Ion
The primary ion reacts with the gas again to form a secondary ion
The secondary ion then attaches itself to the sample to give two quasi-molecular ions which are detected by the spectrometer
What is the molecular ion?
The molecular ion [M+] is the ion thats generated upon the removal of one electron from the molecule.
It is a radical cation that contains a positive charge.
The weight of the molecular ion is the actual molecular weight of the original molecule
What is the mass of an ion in Mass Spec (MS)?
In MS, the mass of an ion is its TRUE mass
It is the sum of the masses of each atom in that single ion.
It does NOT correspond to the mass calculated in the periodic table as that is a weighted average of of the masses of all the isotopes in an element.
How do you find the exact mass of an ion in mass spec?
you simply use the mass of the most common isotope of each element and multiply it by the number of elements in the compound
What is the Nitrogen Rule?
Molecular ions containing an odd number of Nitrogen’s must have an odd number m/z
Compounds lacking Nitrogen or those that have an even amount of Nitrogen will have an even number m/s
What are the isotopic signatures of Chlorine and Bromine?
Certain isotopes like Chlorine and Bromine have signature intensity patterns.
The ratio of Chlorine isotopes is roughly 3:1. This pattern is the same for MS peak intensities
The ratio of Bromine isotopes is roughly 1:1. This pattern is the same for MS peak intensities
What is High Resolution Mass Spec good for?
It can very accurately determine m/z values up to 4 decimal places long.
What is Stevenson’s Rule?
The most probable fragmentation is the one that leaves the positive charge on the fragment with the lowest ionization energy
Relative ion stability is based on regular stuff: full octet, favourable charge localization, resonance delocalization, and absence of unpaired electrons
The initial ionization event relies on ejecting an electron from a molecule. How easy is it to eject one from a molecule?
non-bonding electron pairs are the easiest way to eject an electron.
If there are none, than it is easier to eject an electron from a pi orbital.
It is hardest to eject an electron from a sigma orbital and will not be done unless there are no non-bonding pairs or pi bonds.
What is alpha-cleavage?
Essentially, the bond next to the ion being radicalized is easiest to cleave as the resulting charge is stabilized by the heteroatom (N, O, S)
What is a Retro Diels-Alder (elimination) reaction?
Essentially, is the cleavage of a ring. The fragment with the lowest ionization energy (according to Stevensons rule) will preferentially take on the positive charge
What is McLafferty Rearrangement?
A gamma-Hydrogen attaches itself to the radical cation of a carbonyl, alkene, or imine compound to form a new radical cation and an alkene
What is an Onium Reaction?
It is a secondary fragmentation that can occur after an alpha cleavage
How is Benzyl or Allyl cleavage different from the rest?
Benzyl (Or allyl) cleavage forms an intermediate Tropylium ion that then turns into a cyclopentane.
