Mass spectrometry

Need to know

What you need to know

• **1. Ionisation:** The sample is first vaporised and then injected into the ionisation chamber. Here, it is bombarded with a stream of high-energy electrons from an 'electron gun'. These electrons knock off electrons from the atoms or molecules of the sample, creating positive ions. For a molecule M, this is represented as $M + e^- \rightarrow M^+ + 2e^-$.
• **2. Acceleration:** The newly formed positive ions are accelerated by a series of negatively charged plates, creating a fine beam of ions all having the same kinetic energy.
• **3. Deflection:** The beam of ions passes into a strong magnetic field, which is perpendicular to their direction of travel. The magnetic field deflects the ions into a curved path. The degree of deflection depends on the mass-to-charge ratio ($m/z$). Lighter ions are deflected more than heavier ions, and more highly charged ions are deflected more than singly charged ions.
• **4. Detection:** By varying the strength of the magnetic field, ions of different $m/z$ ratios can be directed towards a detector. When an ion hits the detector, it accepts an electron, generating a tiny electrical current. The size of the current is proportional to the number of ions arriving, giving the relative abundance of that ion.

Explanation

The Molecular Weighing Scales

1. First, the sample is vaporised and bombarded with high-energy electrons. This knocks an electron off each atom or molecule, creating a positive ion.
2. Next, these positive ions are accelerated by an electric field, ensuring they all have the same kinetic energy before the next stage.
3. The fast-moving ions then enter a magnetic field, which deflects them. Lighter ions are deflected more than heavier ions.
4. Finally, a detector counts the ions at each deflection angle. This data is used to generate a mass spectrum, plotting abundance against mass-to-charge ratio ($m/z$).