3.1.1 Atomic Structure
Isotopes
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons
Relative isotopic mass is the average mass of 1 atom of an isotope compared to 1/12th of the mass of 1 atom of
carbon-12
Isotopes have slightly varying physical properties because they have different masses
Isotopes have similar chemical properties because they have same electronic structure
The chemical properties of isotopes are determined by the number and arrangement of electrons, which doesn’t
change
Mass spectrometry
A mass spectrometer gives accurate information about the relative isotopic mass and relative abundance of isotopes
allowing us to determine its relative atomic mass
A time-of-flight mass spectrometer differentiates molecules based on the time taken for them to travel through a
machine
The 4 stages of TOF mass spectrometry are:
1. Ionisation
2. Acceleration
3. Ion drift
4. Ion detection
The TOF mass spectrometer is kept under a high vacuum to prevent the ions produced from colliding with molecules
from air
Electron impact ionisation:
- Used for elements and substances with low formula masses
- Causes larger organic molecules to fragment
- Hard ionisation
1. Vapourised sample injected at low pressure
2. Electron gun fires high energy electrons at sample
3. This knocks out 1 outer electron forming positive ions with different charges
Electrospray ionisation:
- Used for larger organic molecules
- Softer conditions of technique mean fragmentation doesn’t occur
- Soft ionisation
1. Sample dissolved in volatile, polar solvent
2. Sample injected through fine hypodermic needle giving fine mist
3. Tip of needle has high voltage
4. At tip of needle, sample molecule, M, gains proton, H⁺, from solvent forming MH⁺ ion
, 3.1.1 Atomic Structure
5. 𝑀( ) + 𝐻 → 𝑀𝐻 ( )
6. Solvent evaporates away while MH⁺ ions move towards nega vely charged plate
The mass of the particle doesn’t change after ionisation because only an electron is lost. Electrons have a very small
relative mass of 1/1836 hence when electron is lost it doesn’t affect the mass of the particle
Atoms/isotopes must be ionised for mass spectrometry because particles must be charged to be attracted to the
charged plate or electric field
Acceleration:
1. Positive ions accelerated towards negatively charged plate to give all ions constant kinetic energy
2. Lighter ions and more highly charged ions have faster velocity
Ion drift:
1. Positive ions pass through hole in negatively charged plate forming beam of ions
2. Ions travel along flight tube to detector
3. Heavier ions take longer to travel through flight tube compared to lighter ions
4. Ions distinguished by their different flight times
Ion detection is:
1) When ions with same charge arrive at detector, lighter ions detected first because have faster velocities
2) Flight times of ions recorded
3) Positive ions pick up 1 electron from detector, causing current to flow
Generating a mass spectrum:
- Signal from detector passed to computer
- This generates mass spectrum
- Size of current proportional to abundance of species
The m/z ratio is effectively the mass of the ion because if all the ions produced by electrospray ionisation and most
of the ions produced by electron impact ionisation have a 1+ charge, the m/z ratio is effectively the mass of the ion
because the mass is divided by +1 hence it stays same
If electron impact ionisation is used, in the mass spectrum:
- It will often break up and give series of peaks caused by fragment
- Peak with largest m/z due to complete molecule hence equal to relative molecular mass of molecule
- This peak is called molecular ion
If electrospray ionisation is used, in the mass spectrum:
- Fragmentation won’t occur
- There’ll be one peak equal to mass of MH⁺ ion hence need to subtract 1 to get relative molecular mass of
molecule
Isotopes
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons
Relative isotopic mass is the average mass of 1 atom of an isotope compared to 1/12th of the mass of 1 atom of
carbon-12
Isotopes have slightly varying physical properties because they have different masses
Isotopes have similar chemical properties because they have same electronic structure
The chemical properties of isotopes are determined by the number and arrangement of electrons, which doesn’t
change
Mass spectrometry
A mass spectrometer gives accurate information about the relative isotopic mass and relative abundance of isotopes
allowing us to determine its relative atomic mass
A time-of-flight mass spectrometer differentiates molecules based on the time taken for them to travel through a
machine
The 4 stages of TOF mass spectrometry are:
1. Ionisation
2. Acceleration
3. Ion drift
4. Ion detection
The TOF mass spectrometer is kept under a high vacuum to prevent the ions produced from colliding with molecules
from air
Electron impact ionisation:
- Used for elements and substances with low formula masses
- Causes larger organic molecules to fragment
- Hard ionisation
1. Vapourised sample injected at low pressure
2. Electron gun fires high energy electrons at sample
3. This knocks out 1 outer electron forming positive ions with different charges
Electrospray ionisation:
- Used for larger organic molecules
- Softer conditions of technique mean fragmentation doesn’t occur
- Soft ionisation
1. Sample dissolved in volatile, polar solvent
2. Sample injected through fine hypodermic needle giving fine mist
3. Tip of needle has high voltage
4. At tip of needle, sample molecule, M, gains proton, H⁺, from solvent forming MH⁺ ion
, 3.1.1 Atomic Structure
5. 𝑀( ) + 𝐻 → 𝑀𝐻 ( )
6. Solvent evaporates away while MH⁺ ions move towards nega vely charged plate
The mass of the particle doesn’t change after ionisation because only an electron is lost. Electrons have a very small
relative mass of 1/1836 hence when electron is lost it doesn’t affect the mass of the particle
Atoms/isotopes must be ionised for mass spectrometry because particles must be charged to be attracted to the
charged plate or electric field
Acceleration:
1. Positive ions accelerated towards negatively charged plate to give all ions constant kinetic energy
2. Lighter ions and more highly charged ions have faster velocity
Ion drift:
1. Positive ions pass through hole in negatively charged plate forming beam of ions
2. Ions travel along flight tube to detector
3. Heavier ions take longer to travel through flight tube compared to lighter ions
4. Ions distinguished by their different flight times
Ion detection is:
1) When ions with same charge arrive at detector, lighter ions detected first because have faster velocities
2) Flight times of ions recorded
3) Positive ions pick up 1 electron from detector, causing current to flow
Generating a mass spectrum:
- Signal from detector passed to computer
- This generates mass spectrum
- Size of current proportional to abundance of species
The m/z ratio is effectively the mass of the ion because if all the ions produced by electrospray ionisation and most
of the ions produced by electron impact ionisation have a 1+ charge, the m/z ratio is effectively the mass of the ion
because the mass is divided by +1 hence it stays same
If electron impact ionisation is used, in the mass spectrum:
- It will often break up and give series of peaks caused by fragment
- Peak with largest m/z due to complete molecule hence equal to relative molecular mass of molecule
- This peak is called molecular ion
If electrospray ionisation is used, in the mass spectrum:
- Fragmentation won’t occur
- There’ll be one peak equal to mass of MH⁺ ion hence need to subtract 1 to get relative molecular mass of
molecule
