Atomic Structure

Mass Spectrometry

Mass Spectrometry

A mass spectrometer is used:

  • to measure the mass of atoms and molecules
  • the relative abundance of different isotopes
  • predict what the structure of complex molecules is

The function of a mass spectrometer

The function of a mass spectrometer is composed of five basic stages:

  • Gaseous material is released into the ionisation chamber.
  • Ionisation: the particles are bombarded with electrons.
  • Acceleration: an electric field is used to accelerate the ions into a uniform speed.
  • Deflection: the ions are deflected by a magnetic field.
  • Detection: as the ions land on the plate they are measured using an electric current. The more abundant an isotope if the bigger the resulting current.

The degree of deflection depends on mass and charge:

  • the greater the mass, the smaller the degree of deflection
  • the greater the charge, the greater the degree of deflection

The degree of deflection is inversely proportional to the m/e ratio.

As the charge tends to be +1 the degree of deflection is usually dependent on the relative mass. Once calibrated, a spectrometer can be used to directly measure different masses.

The more particles that land on one single point, the larger the current and, consequently, peak measured. Therefore, it is possible to measure the relative abundance of isotopes. The mass spectrum can then be used to measure the relative atomic mass (RAM) of an element.

Calculating RAM

A RAM can be calculated using the following equation:

? (percentage abundance of each isotope x mass of each isotope) / 100

All RAM can be found using this equation.

Deducing relative molecular masses

Molecules, as well as atoms, can be put into a mass spectrometer. Inside, the molecule usually breaks up into smaller parts, a process known as fragmentation.

Due to fragmentation a molecular mass spectrum appears more complex. The relative molecular mass can be found by locating the peak in the spectrum which has the largest m/e ratio. In other words, the peak that is furthest to the right of the graph