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Real World

The Theme of Engineering in Stoichiometry

The Mass Spectrometer

Often times in a research lab, a chemist might make something but he/she might not know exactly what that something is. Sure…they'll have a pretty good idea, but how do they know for sure? There are tons of techniques and machines that scientists have developed to identify and study compounds galore.

One of these machines is called the mass spectrometer. This machine is the most direct and accurate method for determining atomic and molecular masses of known and unknown complexes alike. Who needs a microscopic scale to measure weights when you have a mass spectrometer?


A mass spectrometer (if you're wondering, ICP stands for "Inductively Coupled Plasma").

How does one of these fancy gizmos work? We're glad you asked. In a mass spectrometer, a stream of high-energy electrons bombards a gaseous sample of a given compound. Picture a dodgeball match with a single player left on one team, but 20 players on the other team…and the other team has all the balls. It's a total dodgeball nightmare. It's time to bombard that poor lonely player. When the electrons hit the gaseous atoms (or molecules) they kick off an electron from the surface. This produces a positively charged ion. It's like that poor dodgeball player getting hit in the head…as a consequence his glasses fall off.


Mass Spectrometry is kinda like dodgeball.

Now there are positively charged ions flying through the machine. Let's say they have a mass of m and a charge of e. These particles are accelerated as they fly through the machine by two oppositely charged plates. At some point the ions encounter a magnet, which alters the path of the ions. The charge-to-mass ratio (e/m) of the particles dictates how much their path is altered. Ions with a smaller e/m achieve a wider curve, while ions with a larger e/m achieve a smaller curve.


Inside a Mass Spectrometer.

Eventually the ions reach their final destination, a detector. The detector and its partner in crime, a computer, can help translate the path of the ion to the ion's molecular weight.

It makes us wonder who comes up with this stuff. The mass spectrometer was first developed way back in the 1920's by the English physicist Francis William Aston.6 While his machine was extremely crude compared to today's modern devices, it was still able to roughly calculate some molecular weights. Ah, now you know the truth behind how masses are experimentally calculated in chemistry. Sorry, no teeny tiny scales here, but the mass spec is pretty ingenious.


Francis William Aston

FYI: F. W. Aston even won a Nobel Prize in 1922 for his pioneering work in this field!6 Check out more about this dude here.

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