The principles behind mass spectrometry are somewhat abstract, so let's start with a concrete mental exercise. Suppose you wanted to weigh a fully loaded tractor-trailer. The easiest way would be to drive the rig to a heavy-duty truck scale. Now let's say you wanted to weigh one of the trailer's wheels. A regular bathroom scale could provide that information. Next up, you decide to weigh a lug nut from one of the wheels, which would require nothing more than an ordinary kitchen or lab scale. Finally, imagine you wanted to weigh a single atom scraped from the surface of the lug nut. How would you measure it? Even the most sensitive laboratory balance wouldn't register the weight of something so small.
This was the situation confronting chemists at the beginning of the 20th century. Thanks to John Dalton's atomic theory, they knew that matter was made of atoms and that atoms of one element were the same. But what did an atom look like, and how much did it weigh? In 1897, J.J. Thomson discovered the electron by studying the behavior of cathode rays, the stream of negatively charged particles originating at the cathode, or negative electrode, in a gas-filled vacuum tube. A year later, Willy Wien began working with "positive rays" -- the stream of positively charged particles that emanate from the anode and move toward the cathode. Wien observed that a magnetic field could deflect positive rays. Then, in 1907, Thomson began deflecting positive rays with both electric and magnetic fields. He discovered that he could determine the particles' mass by measuring how far they were deflected.
In 1919, Francis Aston improved on Thomson's methods and apparatus, leading to the first mass spectrometer -- a machine that literally weighs atoms and molecules. Aston used his spectrometer to study hundreds of naturally occurring isotopes. Today, chemists still use the mass spectrometer to measure the molecular weights of elements, isotopes and compounds. But they also use it to identify the chemicals in a sample, determine how much of each chemical is present in a sample and analyze the structure of complex molecules.
Next, we'll take a closer look at what's going on inside a mass spectrometer.