Mass spectrometry is a method of analysis which helps us to determine relative atomic masses of unknown substances. It also helps us to determine the abundances of specific isotopes.
Outline the 4 steps in TOF mass spectrometry.
Ionisation
Acceleration
Ion Drift
Detection
What are the two types of ionisation that can happen in TOF mass spec?
Electrospray Ionisation
Electron Impact (preferred method)
Describe the steps involved in ElectroSpray Ionisation:
Dissolve the sample in a polar solvent
Push it through a tiny nozzle at high pressure
Apply a high voltage
Each particle will gain a proton
The sample will be a gaseous sample of positive ions
Describe Electron Impact Ionisation:
Vaporise the sample
Sample is bombarded by high energy electrons
An electron will be removed from each particle.
This will create +1 ions
Describe the Acceleration phase of TOF Mass Spec:
The positive ions move through a negative electric field accelerating toward the negative plate.
Lighter ions and ions with a higher charge achieve a higher speed here.
Describe the Ion Drift phase of Mass Spec:
During ion drift the different ions will move across the flight tube.
Positive ions with smaller m/z values will have the same KE as those with a large m/z values and will move faster.
The ions are distinguished by different flight times.
When they hit the detector plate, the positive ions are discharged by gaining electrons from the plate. This generates a movement of electrons and hence an electric current that is measured. The size of the current gives a measure of the number of ions hitting the plate.
Use electrospray ionisation over electron impact for larger organic molecules
If a molecule is put through a mass spectrometer it will often break up and give a series of peaks caused by the fragments
According to the first rule, electrons always enter an empty orbital before they pair up.
p-orbitals are dumbbell shaped and four of the five d orbitals are cloverleaf shaped.
An s-orbital is spherical with the nucleus at its centre
In time of flight Mass Spec, the apparatus is kept under a high vacuum to prevent any ions that are produced from colliding with molecules in the air
First ionisation energy: the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms
Second ionsation energy: The energy required to remove 1mol of electrons from 1 mole of gaseous ions with a single positive charge
Dalton's model (1803)
John Dalton thought that all matter was made of tiny particles called atoms, which he imagined as tiny solid balls.
Niels Bohr developed Rutherford's nuclear model by proposing that electrons exist in shells. These were orbits with fixed energy When the electrons moved between shells electromagnetic radiation with a fixed energy was emitted or absorbed.
Why is the Bohr model not fully correct?
The lecterns in each shell have slightly different energies
The model now includes sub-shells
These explain experimental ionisation energy trends
First ionisation energy: The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions
Relative atomic mass of an element: The average mass of an atom relative to the mass of 1/12th of the mass of an atom of Carbon-12
Mass number of an isotope: The total number of protons and neutrons in the nucleus of an atom
What does the atomic number tell us about an element? Atomic number = number of protons in an atom
Which letter is used to represent the atomic number of an atom? Z
What letter represents mass number? A
What are isotopes of an element? Different forms of the same element, containing the same number of protons but different numbers of neutrons. They still have the same chemical properties
Orbital: A region around the nucleus that can hold up to two electrons
How many orbitals and electrons do these shells contain? a) 1s- 1 orbital, 2 electrons b) 2p- 3 orbitals, 6 electronsc) 3s- 1 orbital, 2 electrons d) 3d- 5 orbitals, 10 electrons e) 4s- 1 orbital, 2 electrons