How mass spectrometry works I

13 JUL

How mass spectrometry works I

Fundamentals of mass spectrometry

The principles behind mass spectrometry are somewhat abstract, so let’s start with a mental exercise. Imagine you want to weigh a fully loaded truck. The easiest thing to do would be to drive the vehicle to a heavy truck scale. Next, we want to weigh a wheel. We could do this with a normal scale. Next, we want to weigh a bolt on that wheel, for which any simple kitchen scale will do. Finally, imagine that what you want to weigh is an atom on the surface of the bolt. How would you do it? Even the most precise and accurate balance built today would be unable to make the measurement.

This was the situation facing chemists in the early 20th century. Thanks to John Dalton’s atomic theory, they knew that matter was made up of atoms and that atoms of the same element were equal. But what did an atom look like and how much did it weigh? In 1897, J.J. Thomson discovered the electron by studying the behaviour of cathode rays, the stream of negatively charged particles originating at the cathode in a gas-filled vacuum tube. A year later, in 1898, Willy Wien began working with “positive rays”, positively charged particles leaving the anode and heading towards the cathode. Wien observed that a magnetic field could deflect the positive rays. Then, in 1907, Thomson began deflecting positive rays with electric and magnetic fields and discovered that he could determine the mass of the particles by measuring the distance at which they were deflected.

In 1919, Francis Aston improved Thomson’s methods and equipment, leading to the first mass spectrometer, which was literally a piece of equipment that weighed atoms and molecules. Today it is also used to measure the molecular weights of compounds, but also to identify and quantify substances present in a sample.

Understanding mass spectrometry

To understand the basic principles of mass spectrometry, let us use the following example. Imagine a person on top of a skyscraper on a windy day. This person has several balls of different sizes: a tennis ball, a football and a medicine ball. He is about to throw them one by one into the void. As each ball falls, the wind deflects the trajectory along a curved path. The mass of each ball influences the trajectory because, for example, the medicine ball is heavier than the tennis ball and will therefore be more difficult for the wind to move. So each ball will follow a different trajectory.

In a mass spectrometer, the same thing happens, except that instead of the balls, it is the atoms that are being deflected and it is the electric or magnetic fields that deflect the trajectory of those atoms.