Mass Spectrometer (Dual Sector)

Mass spectrometers are machines that give scientists a look at the composition and origin of a material by analyzing and quantifying its atoms and molecules. This tutorial shows how a dual sector mass spectrometer works.

Mass spectrometers (MS for short) are machines that give scientists a look at the composition and origin of a material by analyzing and quantifying its atoms and molecules. They do this by vaporizing the sample (if it’s not already a gas) to make it easier to work with, ionizing it to give it a charge, then seeing how those ions react to a magnetic field. That reaction tells scientists what a particle's mass is. The machine then sorts and counts these particles, revealing the specimen’s chemical makeup. The machine is able to distinguish among isotopes (atoms of the same element that have differing numbers of neutrons and, consequently, slightly different masses). Scientists can learn a lot from knowing how many of which isotopes make up a certain sample.

The single sector MS is made with a single magnetic analyzer. A widely used variation of this is the double sector MS. It has two analyzers – one magnetic, one electrostatic. If you are new to this topic, you may want to read up on the single sector mass spectrometer before continuing.

In the dual sector MS, a sample is ionized by injecting it into an extremely hot plasma. As a result of this type of ionization, ions of the same mass enter the machine at different velocities, and therefore different trajectories. But to make it to the end of the process and get counted, they must all follow the same path. So, after ions are analyzed by mass (with the magnet), they must be analyzed a second time (hence the "dual") by speed (or kinetic energy) with the electrostatic field. The interactive tutorial below illustrates.


Ions of different masses (denoted by different colors) stream through the machine. You can manipulate three sliders: one adjusts the Magnetic Field Strength, another adjusts the Electrostatic Field Strength, and a third controls the Tutorial Speed.

The ions enter the first (magnetic) analyzer, where they interact with the magnetic field and are sorted by mass before a select group of one particular mass proceeds into the electrostatic analyzer (this arrangement is sometimes flipped, but the same principles are at work). Traveling at different speeds, they disperse as they take the turn in the electrostatic analyzer. At this rate, only a few will make it into the Detector cell; the slower ones will crash into the inner wall, the faster ones will crash into the outer wall, and the count will be inaccurate.

This dispersion is corrected as the ions pass through an electrostatic field; an electrode on the inner wall emits negative charges, while an electrode on the outer wall emits positive charges. The positive ions are repelled from the outer wall. To boot, the laws of physics are such that the closer the ion is to that positively charged wall, the more it will feel the repulsion. So, all these particles are drawn into the same middle ground – the same velocity – and travel together into the detector.

For a more thorough explanation of mass spectrometers and their uses, please visit Weighing Atoms: A Race Odyssey.

Last modified on 7 January 2015