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Mass Spectroscopy

Key Concepts

Mass Spectrometers use the difference in mass-to-charge ratio (m/z or m/e) of ionised atoms and molecules to separate them from each other.
A mass spectrum is the record of the quantity of ions of particular mass-to-charge ratio.
Peak heights on a mass spectrum are proportional to the number of ions of each mass.
Mass Spectroscopy can be used to determine:

relative atomic mass of an element
molecular mass of a molecule
molecular structure (fragmentation)

The Mass Spectrometer

Step 1 sample is vaporised before entering mass spectrometer.

Step 2 Sample enters the low pressure ionisation chamber where an electron beam ionises the sample.

Step 3 Ions are accelerated to high speeds by an electric field.

Step 4 Ions pass through a perpendicular magnetic field which bends the path of the ions dependent on their mass-to-charge ratio.

Step 5 Ions with a particular mass reach the collector. Different masses can be collected by changing the magnetic or electric field.

Step 6 The detector identifies the mass of each ion from its path. Data is recorded as a mass spectrum.

Determining Relative Atomic Mass

The ionisation of a sample of naturally occuring mercury in a mass spectrometer produces a mass spectrum similar to the one shown below:

From this we can identify seven isotopes of mercury with mass numbers (A):

196, 198, 199, 200, 201, 202, and 204.

Because the height of each peak is proportional to the number of ions of each mass, the relative heights of each peak correspond to the percentage abundance of each isotope.

This data is often presented in a table:

Mass Number (A) Isotopic Mass (amu) % Abundance

196 195.165 0.14

198 197.967 10.04

199 198.967 16.83

200 199.968 23.12

201 200.970 13.23

202 201.970 29.79

204 203.973 6.85

This data can be used to calculate the relative atomic mass (r.a.m.) of the element.

r.a.m. = (% isotope 1 x mass isotope 1) + (% isotope 2 x mass isotope 2) + (% isotope 3 x mass isotope 3) + (% isotope 4 x mass isotope 4) + (% isotope 5 x mass isotope 5) + (% isotope 6 x mass isotope 6) + (% isotope 7 x mass isotope 7)] ÷ 100

r.a.m. (Hg) = [(195.165 x 0.14) + (197.967 x 10.04) + (198.967 x 16.83) + (199.968 x 23.12) + (200.970 x 13.23) + (201.970 x 29.79) + (203.973 x 6.85)] ÷ 100

r.a.m. (Hg) = [27.3231 + 1987.3907 + 3348.615 + 4623.2601 + 2658.8331 + 6016.6863 + 1397.21505] ÷ 100

r.a.m. (Hg) = 20059.3234 ÷ 100

r.a.m. (Hg) = 200.5932 = 200.59 to two decimal places

Determing Molecular Mass and Molecular Structure

When a molecule is bombarded with the high-energy electron beam it fragments into smaller ions which are then accelerated through the magnetic field before being collected and detected.
Fragments can be identified by their mass-to-charge ratios.
The highest molecular mass (m/z) peak is usually the parent molecule minus an electron, called the molecular ion and given the symbol M⁺.
Some molecules, such as some alcohols, do not display molecular ion peaks, instead they have a peak at m-1 corresponding to the loss of one hydrogen atom from the parent alcohol.
Small peaks surround a main peak due to the natural isotopic abundances of ¹³C, ²H, etc.

Common Mass Spectrum Fragments

m Fragment

14 CH₂

15 ^.CH₃

17 ^.OH

26 ^.CN

28 H₂C=CH₂

29 ^.CH₂CH₃ ^.CHO

31 ^.OCH₃

35 ^.Cl

43 CH₃C^.=O

45 ^.OCH₂CH₃

Consider pentane, C₅H₁₂.

Mass Spectrum of Pentane

m Fragment loss of

72 CH₃CH₂CH₂CH₂CH₃

57 ^.CH₂CH₂CH₂CH₃ CH₃ (m-15)

43 ^.CH₂CH₂CH₃ CH₂ (m-14)

29 ^.CH₂CH₃ CH₂ (m-14)

The highest m/z peak at 72 corresponds to the molecular mass of the pentane, CH₃CH₂CH₂CH₂CH₃, molecule.

MM (C₅H₁₂) = (5 x 12) + (12 x 1) = 60 + 12 = 72

The peak at 57 represents the loss of a CH₃ (m=15) fragment from pentane, ie 72 - 15 = 57.
57 is the mass of the CH₂CH₂CH₂CH₃ remaining fragment.

The peak at 43 represents the loss of a CH₂ fragment (m=14), ie, 57 - 14 = 43.
43 is the mass of the CH₂CH₂CH₃ remaining fragment.

The peak at 29 also represents the loss of a CH₂ fragment (m=14), ie, 43 - 14 = 29.
29 is the mass of the CH₂CH₃ remaining fragment.

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