AQA A Level Chemistry

Revision Notes

3.6.2 Mass spectrometry

Interpreting a Mass Spectrum

  • Mass spectroscopy is an analytical technique used to identify unknown compounds
  • The molecules in the small sample are bombarded with high energy electrons which can cause the molecule to lose an electron
  • This results in the formation of a positively charged molecular ion with one unpaired electron
    • One of the electrons in the pair has been removed by the beam of electrons

Interpreting Mass Spectra equation 1

  • The molecular ion can further fragment to form new ions, molecules, and radicals

 

Analytical Techniques Fragmentation, downloadable AS & A Level Chemistry revision notes

Fragmentation of a molecule in mass spectroscopy

 

  • These fragmented ions are accelerated by an electric field
  • Based on their mass (m) to charge (e) ratio, the ion fragments are then separated by deflecting them into the detector
    • For example, an ion with mass 16 and charge 2+ will have a m/e value of 8
  • The smaller and more positively charged fragment ions will be detected first as they will get deflected the most and are more attracted to the negative pole of the magnet
  • Each fragment corresponds to a specific peak with a particular m/e value in the mass spectrum
  • The base peak is the peak corresponding to the most abundant ion

Isotopes

  • 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 number
    • For example, Cl-35 and Cl-37 are isotopes as they are both atoms of the same element (chlorine, Cl) but have a different mass number (35 and 37 respectively)
  • Mass spectroscopy can be used to find the relative abundance of the isotopes experimentally
  • The relative abundance of an isotope is the proportion of one particular isotope in a mixture of isotopes found in nature
    • For example, the relative abundance of Cl-35 and Cl-37 is 75% and 25% respectively
    • This means that in nature, 75% of the chlorine atoms is the Cl-35 isotope and 25% is the Cl-37 isotope
  • The heights of the peaks in mass spectroscopy show the proportion of each isotope present

Analytical Techniques Mass Spectrum Boron, downloadable AS & A Level Chemistry revision notes

The peak heights show the relative abundance of the boron isotopes: boron-10 has a relative abundance of 19.9% and boron-11 has a relative abundance of 80.1%

Worked Example

Calculating m/e ratio

In a sample of iron, the ions 54Fe2+ and 56Fe3+ are detected.

Calculate their m/e ratio and determine which ion is deflected more inside the spectrometer.

Answer


    • 56Fe3+ has a smaller m/e ratio and will therefore be deflected more.
    • It also has the largest positive charge and will be more attracted to the negative pole of the magnet within the mass spectrometer.

Exam Tip

A small m/e value corresponds to fragments that are either small or have a high positive charge or a combination of both.

Deducing Molecular Formula

  • Each peak in the mass spectrum corresponds to a certain fragment with a particular m/e value
  • The peak with the highest m/e value is the molecular ion (M+) peak which gives information about the molecular mass of the compound
  • The molecular ion is the entire molecule that has lost one electron when bombarded with a beam of electrons

Interpreting Mass Spectra equation 1

  • The [M+1] peak is a smaller peak which is due to the natural abundance of the isotope carbon-13
  • The amount of naturally occurring C-13 is a little over 1%, so the [M+1] peak is very small
  • 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

Worked Example

Analysing mass spectra

Determine whether the following mass spectrum corresponds to propanal or butanal

Analytical Techniques Spec 1_Mass Spectrometry, downloadable AS & A Level Chemistry revision notes

Answer

    • The mass spectrum corresponds to propanal as the molecular ion peak is at m/e = 58
    • Propanal arises from the CH3CH2CHO+ ion which has a molecular mass of 58
    • Butanal arises from the CH3CH2CH2CHO+ ion which has a molecular mass of 72

Fragmentation

  • The molecular ion peak can be used to identify the molecular mass of a compound
  • However, different compounds may have the same molecular mass
  • To further determine the structure of the unknown compound, fragmentation is used
  • Fragments may appear due to the formation of characteristic fragments or the loss of small molecules
    • For example, a peak at 29 is due to the characteristic fragment C2H5+­­
    • Loss of small molecules give rise to peaks at 18 (H2O), 28 (CO), and 44 (CO2)

Alkanes

  • Simple alkanes are fragmented in mass spectroscopy by breaking the C-C bonds
  • M/e values of some of the common alkane fragments are given in the table below

m/e Values of Fragments Table

Analytical Techniques Table 1_Identifying Molecules using Fragmentation, downloadable AS & A Level Chemistry revision notes

Analytical Techniques Alkane Spectrum, downloadable AS & A Level Chemistry revision notes

Mass spectrum showing fragmentation of alkanes

Halogenoalkanes

  • Halogenoalkanes often have multiple peaks around the molecular ion peak
  • This is caused by the fact that there are different isotopes of the halogens

 

Analytical Techniques Halogenoalkane Spectrum, downloadable AS & A Level Chemistry revision notes

Mass spectrum showing different isotopes of bromine in the molecular ion

Alcohols

  • Alcohols often tend to lose a water molecule giving rise to a peak at 18 below the molecular ion
  • Another common peak is found at m/e value 31 which corresponds to the CH2OH+­­ fragment
  • For example, the mass spectrum of propan-1-ol shows that the compound has fragmented in four different ways:
    • Loss of H to form a C3H7O+ fragment with m/e = 59
    • Loss of a water molecule to form a C3H6+ fragment with m/e = 42
    • Loss of a C2H5 to form a CH2OH+ fragment with m/e = 31
    • And the loss of CH2OH to form a C2H5+ fragment with m/e = 29

Analytical Techniques Alcohols Spectrum_2, downloadable AS & A Level Chemistry revision notes

Mass spectrum showing the fragmentation patterns in propan-1-ol (alcohol)

Worked Example

Ion fragmentation

Which of the following statements about the mass spectrum of CH3Br is correct?

A. There is one peak for the molecular ion with an m/e value of 44.

B. There is one peak for the molecular ion with an m/e value of 95.

C. The last two peaks have abundances in the ratio 3:1 and occur at m/e values of 94 and 96.

D. The last two peaks are of equal size and occur at m/e values of 94 and 96.

Answer

The correct answer is option D

    • Bromomethane (CH3Br) can produce 3 peaks
      • CH381Br → [CH381Br]+ + e at m/e 96
      • CH379Br → [CH379Br]+ + e at m/e 94
      • CH3Br → [CH3]+ + Br at m/e 15
    • The last two peaks (which correspond to the molecular ion peak) therefore are equal in size and occur at m/e values of 94 and 96

Analytical Techniques Answer Worked example - Ion fragmentation, downloadable AS & A Level Chemistry revision notes

Worked Example

Alcohol fragmentation

Which alcohol is not likely to have a fragment ion at m/e at 43 in its mass spectrum?

A. (CH3)2CHCH2OH

B. CH3CH(OH)CH2CH2CH3

C. CH3CH2CH2CH2OH

D. CH3CH2CH(OH)CH3

Answer

The correct answer is option D

    • Because a line at m/e = 43 corresponds to an ion with a mass of 43 for example:
      • [CH3CH2CH2]+
      • [(CH3)2CH]+
    • 2-butanol is not likely to have a fragment at m/e = 43 as it does not have either of these fragments in its structure.
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