Isotopes & Mass Spectra (Edexcel International AS Chemistry): Revision Note

The Mass Spectrometer

• The mass spectrometer is an instrument used to determine the relative isotopic mass and the relative abundance of each isotope

• Relative isotopic mass is the mass of an individual isotope relative to the mass of carbon-12

• Apart from finding the masses of isotopes, mass spectrometry is a vey useful tool for detecting illegal drugs, forensic science, space exploration and carbon-14 dating

The components of a mass spectrometer

How the mass spectrometer works

• A sample is injected into the spectrometer where it is heated and vaporised

• The atoms or molecules are then bombarded by high energy electrons created by an electron gun

• The high energy electrons collide with the sample material and remove electrons creating positive ions

• The ions are then accelerated by attraction towards negatively charged plates

• Many ions strike the plate and are discharged, but a few will pass through a gap in the plates into a curved section of the spectrometer known as the flight tube

• The flight tube is encased in electromagnets which create a strong electromagnetic field that deflects the path of the ions in a curve

• The ions pass onto a charged detector plate where every strike is recorded as a small current which is then amplified 

• By varying the electric and magnetic fields all the charged particles can be deflected to the detector which gives information about the mass/charge ratio and abundance of each ion

Isotopes & Mass Spectra

• Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons

  • These are atoms of the same elements but with different mass numbers

• Therefore, the mass of an element is given as relative atomic mass (A_r) by using the average mass of the isotopes

  • The relative atomic mass of an element can be calculated by using the relative abundance values

A_r =

• The relative abundance of an isotope is either given or can be read off the mass spectrum

Mass spectra of molecules

• When a compound is analysed in a mass spectrometer, vaporised molecules are bombarded with a beam of high-speed electrons

• These knock off an electron from some of the molecules, creating molecular ions:

• The relative abundances of the detected ions form a mass spectrum: a kind of molecular fingerprint that can be identified by computer using a spectral database

• The peak with the highest m/z value is the molecular ion (M⁺) peak which gives information about the molecular mass of the compound

• This value of m/z is equal to the relative molecular mass of the compound

The M+1 peak

• The [M+1] peak is a smaller peak which is due to the natural abundance of the isotope carbon-13

• The height of the [M+1] peak for a particular ion depends on how many carbon atoms are present in that molecule; The more carbon atoms, the larger the [M+1] peak is

  • For example, the height of the [M+1] peak for an hexane (containing six carbon atoms) ion will be greater than the height of the [M+1] peak of an ethane (containing two carbon atoms) ion

Mass Spectra - Predictions

• You can also predict how a mass spectrum might appear for a given compound, e.g. ethanol, CH₃CH₂OH

  • The methyl, CH₃⁺, fragment has a mass of 15.0

  • The ethyl, CH₃CH₂⁺, fragment has a mass of 29.0

  • The base ion, CH₂OH⁺, fragment has a mass of 31.0

  • The whole molecule has a mass of 46.0

• Predicting mass spectra becomes more complex with the inclusion of halogen isotopes such as chlorine and bromine

Chlorine

Chlorine exists as two isotopes, ³⁵Cl and ³⁷Cl

• A compound containing one chlorine atom will therefore have two molecular ion peaks due to the two different isotopes it can contain

  • ³⁵Cl = M⁺ peak

  • ³⁷Cl = [M+2] peak

  • The ratio of the peak heights is 3:1 (as the relative abundance of ³⁵Cl is 3x greater than that of ³⁷Cl)

• A diatomic chlorine molecule or a compound containing two chlorine atoms will have three molecular ion peaks due to the different combinations of chlorine isotopes they can contain

  • ³⁵Cl + ³⁵Cl = M⁺ peak

  • ³⁵Cl + ³⁷Cl = [M+2] peak

    • There is an alternative of ³⁷Cl + ³⁵Cl doubling the [M+2] peak

  • ³⁷Cl + ³⁷Cl = [M+4] peak

  • The ratio of the peak heights is 9:6:1

    • This ratio can be deduced by using the probability of each chlorine atom being ³⁵Cl or ³⁷Cl

    • ³⁵Cl + ³⁵Cl = 

    • ³⁵Cl + ³⁷Cl =  but this doubles for the ³⁷Cl + ³⁵Cl option, therefore, 

    • ³⁷Cl + ³⁷Cl = 

• The presence of bromine or chlorine atoms in a compound gives rise to a [M+2] and possibly [M+4] peak 

Mass spectrum of compounds containing one chlorine atom (1) and two chlorine atoms (2)

Bromine

• Bromine too exists as two isotopes, ⁷⁹Br and ⁸¹Br

• A compound containing one bromine atom will have two molecular ion peaks

  • ⁷⁹Br = M⁺  peak

  • ⁸¹Br = [M+2] peak

  • The ratio of the peak heights is 1:1 (they are of similar heights as their relative abundance is the same!)

• A diatomic molecule of bromine or a compound containing two bromine atoms will have three molecular ion peaks

  • ⁷⁹Br + ⁷⁹Br= M⁺ peak

  • ⁷⁹Br+ ⁸¹Br = [M+2] peak

  • ⁸¹Br + ⁸¹Br= [M+4] peak

  • The ratio of the peak heights is 1:2:1

Mass spectrum of compounds containing one bromine atom