13 Jul

Fundamentos de la espectrometría de masas

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 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 screw of that wheel, for which any simple kitchen scale would serve us. Finally, imagine what you want to weigh is an atom from the surface of the screw. As I would do it? Even the most precise and accurate balance built today would be unable to perform the measurement.

This was the situation chemists faced in the early 20th century. Thanks to John Dalton's atomic theory, they knew that matter was made up of atoms and that the atoms of the same element were the same. But what did an atom look like and how heavy did it weigh? In 1897, J.J. Thomson discovered the electron by studying the behavior of cathode rays, the stream of negatively charged particles that originate at the cathode in a gas-filled vacuum tube. A year later, in 1898, Willy Wien began working with "positive rays", positively charged particles that leave the anode and go to the cathode. Wien observed that a magnetic field could deflect positive rays. Later, 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 on 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 we are going to use the following example. Imagine a person perched on top of a skyscraper on a windy day. That person has several balls of different sizes: a tennis ball, a soccer ball, and a medicine ball. And he is about to throw them into the void one by one. As each ball falls, the wind deflects the trajectory along a curved path. The mass of each ball influences the trajectory since, for example, the medicine ball is heavier than the tennis ball and therefore will be more difficult to move in the wind. So each ball will follow a different trajectory.

The same thing happens in a mass spectrometer, with the exception 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.