how a mass spectrometer works

how a mass spectrometer works

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.

Fundamentals of mass spectrometry

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.

Detecting ions

Atoms, in order to be deflected by magnetic or electric fields, must first be ionised by a bombardment of a beam of electrons, giving rise to positively charged particles. These electrons act like billiard balls, removing electrons from the sample. This is achieved by passing the sample in solution through a capillary to which an electrical potential is applied. At the exit of the capillary, the solution is dispersed as a spray, forming small charged droplets, which evaporate rapidly. This is known as electrospray ionisation (ESI).

The positive ions are then forced out of the ionisation chamber by the force of an electric field provided by a positively charged (repelling) and a negatively charged (attracting) lattice. Because the attractive and repulsive forces act in the same direction, the ions will move rapidly towards the negatively charged lattice. Lighter ions will “travel” faster than heavier ions.

The main job of the mass analyser is to apply an external magnetic field to the ions leaving the ionisation chamber. This external field interacts with the magnetic field generated by the moving particles, which causes the trajectory of each particle to bend slightly. The amount of bending in an ion’s trajectory depends on two factors: the mass and its charge. This is known as the (m/z) ratio, where lighter ions with a larger charge deviate more than heavy ions with a smaller charge. Therefore, each ion follows a path that depends on its mass.

Detecting ions

Atoms, in order to be deflected by magnetic or electric fields, must first be ionised by a bombardment of a beam of electrons, giving rise to positively charged particles. These electrons act like billiard balls, removing electrons from the sample. This is achieved by passing the sample in solution through a capillary to which an electrical potential is applied. At the exit of the capillary, the solution is dispersed as a spray, forming small charged droplets, which evaporate rapidly. This is known as electrospray ionisation (ESI).

The positive ions are then forced out of the ionisation chamber by the force of an electric field provided by a positively charged (repelling) and a negatively charged (attracting) lattice. Because the attractive and repulsive forces act in the same direction, the ions will move rapidly towards the negatively charged lattice. Lighter ions will “travel” faster than heavier ions.

The main job of the mass analyser is to apply an external magnetic field to the ions leaving the ionisation chamber. This external field interacts with the magnetic field generated by the moving particles, which causes the trajectory of each particle to bend slightly. The amount of bending in an ion’s trajectory depends on two factors: the mass and its charge. This is known as the (m/z) ratio, where lighter ions with a larger charge deviate more than heavy ions with a smaller charge. Therefore, each ion follows a path that depends on its mass.

Of all the ions that pass through the mass analyser, there will be many that we are not interested in. Therefore, we filter the stream of ions that are of interest to us by passing them to the detector, which will give us the mass spectrum. This mass spectrum is represented by an X-axis with the mass of the different ions and a Y-axis that gives the relative intensity. Below is an example of a mass spectrum of capsaicin in which we can see its 305 molecular ion (Figure 2). A compound not so unfamiliar to people who enjoy spicy foods. We will talk about this interesting molecule another time.

Of all the ions that pass through the mass analyser, there will be many that we are not interested in. Therefore, we filter the stream of ions that are of interest to us by passing them to the detector, which will give us the mass spectrum. This mass spectrum is represented by an X-axis with the mass of the different ions and a Y-axis that gives the relative intensity. Below is an example of a mass spectrum of capsaicin in which we can see its 305 molecular ion (Figure 2). A compound not so unfamiliar to people who enjoy spicy foods. We will talk about this interesting molecule another time.