Mass Spectrometry

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Relative Atomic Mass

• In this context: weighted average of all isotopes occurring naturally of an element, relative to $\frac{1}{12}$ of the mass of a Carbon-12 atom in its ground state
• We use this when determining composition of elements from mass spectrometry
• Determined from mass spectrums
• Calculated as $Ar(X)=\frac{(\%a\times m(A))+(\%b\times m(B))...(\%z\times m(Z))}{100}$
• Where $\%a$ is percentage abundance of isotope A. Represented as whole numbers, e.g. 9% would be shown as 9.00

Mass Spectrometry

• Analytical technique used to determine the relative abundance of isotopes and the types of isotope present for a specific element

Process

1. Vaporisation: Sample containing atoms or molecules of interest is injected into the mass spectrometer. The sample, usually in a aqueous or organic solution, is vaporised by a heater
2. Ionisation: Sample is bombarded by high-energy electrons that have enough energy to displace electrons off the atom, turning them into cations.
3. Acceleration: The ions are accelerated through charged plates
4. Deflection: The ions are deflected by an magnetic field. The degree to which it is deflected depends on its speed, mass and charge
   1. Mass: Heavier ions will move more slowly, and hence be deflected less, and vice versa
   2. Charge: Ions with more charge are deflected more, as they are affected more by the magnetic field due to their greater charge
5. Detection: The number of particles and the amount of deflection is measured. A mass spectrum is produced from this.
   1. Mass-to-charge ratio is measured (m/z). Since the charge of cation formed in the mass spectrometer is almost always +1, the mass-to-charge ratio of a cation is usually equal to the mass of the cation. Can sometimes be represented as “amu”
   2. Frequency of each isotope is measured. The mass spectrum is produced based off of these 2.