- 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
||sample is vaporised before entering mass spectrometer.
||Sample enters the low pressure ionisation chamber where an electron beam ionises the sample.
||Ions are accelerated to high speeds by an electric field.
||Ions pass through a perpendicular magnetic field which bends the path of the ions dependent on their mass-to-charge ratio.
||Ions with a particular mass reach the collector. Different masses can be collected by changing the magnetic or electric field.
||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)
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|
Consider pentane, C5H12.
|Mass Spectrum of Pentane
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.