Mass Spectrometry

Mass spectrometry is one of the most powerful analytical tools in chemistry. It allows us to determine the relative masses and abundances of isotopes, calculate relative atomic masses, and identify unknown compounds from their fragmentation patterns. For Edexcel A-Level, you need to understand how the instrument works and how to interpret the data it produces.

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How a Mass Spectrometer Works

A mass spectrometer separates particles based on their mass-to-charge ratio (m/z). There are four key stages:

Stage 1: Ionisation

The sample is vaporised (if not already gaseous) and then ionised. The most common method is electron impact ionisation: a beam of high-energy electrons knocks out an electron from each atom or molecule, forming positive ions.

X(g) → X⁺(g) + e⁻

The ions must be positive so they can be accelerated by an electric field.

Stage 2: Acceleration

The positive ions are accelerated by a strong electric field (a potential difference between charged plates). All ions of the same charge gain the same kinetic energy, but lighter ions achieve higher velocities:

KE = ½mv² = qV

Where q is the charge and V is the potential difference.

Stage 3: Deflection