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How Are Atoms Weighed?
How Are Atoms Weighed?
Atoms are tiny. So how do you actually weigh one?
Mass spectroscopy (MS) doesn’t directly weigh particles, but instead measures their mass-to-charge ratio by tracking how their ions move through an electric field.
On method of MS uses a time-of-flight mass spectrometer, or TOF-MS.
Everything inside a TOF-MS happens in a vacuum.
Otherwise, ions would collide with air molecules and lose their charge or change direction.
There are four stages to the process: ionisation, acceleration, ion drift and detection.
1. Ionisation - turns the sample into positively charged ions. Two methods are commonly used.
Electron impact ionisation vaporises the sample and bombards it with high-energy electrons from an electron gun.
Each particle loses one electron, becoming an [M]⁺ ion.
Electron impact is harsh, but it works well for elements and small molecules.
Electrospray ionisation dissolves the sample in a volatile solvent such as methanol, then pushes it through a fine needle at a high voltage.
The solvent evaporates, each particle is protonated and forms an [M+H]⁺ ion.
Electrospray is a gentle technique, so it suits large or fragile molecules.
2. Acceleration - charged particles are sped up.
The positive ions are pulled through an electric field which exerts force on charged particles. Every ion gains the same kinetic energy (KE) and accelerates.
Because KE is the same for all ions, particles will travel at different speeds depending on their relative isotopic mass. This is the mass of each individual atom compared to 1/12 of a C-12 atom.
Lighter ions move faster, heavier ions move slower.
3. Ion drift - separates the particles.
The accelerated ions enter a long, field-free flight tube.
They travel at constant velocity until they reach the other end.
Lighter, faster ions arrive first. Heavier ones arrive later.
We can calculate how long they take using velocity = distance/time.
The time each ion takes to cross the flight tube (time of flight) is recorded.
4. Detection - the ratio of differently weighted particles is determined.
When ions hit a negatively charged detector plate they each pick up an electron, and this generates a small electric current.
The size of the current is proportional to the number of ions arriving i.e. the percentage abundance.
A mass spectrum is then plotted: mass-to-charge ratio on the x-axis, abundance on the y-axis.
From a mass spectrum, you can identify isotopes, calculate relative atomic masses, work out molecular masses, and determine the structure of unknown compounds.
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