Mass Spectrometry: How to Weigh an Atom

Science in the Fast Lane

Got those neurons revved up? Excellent. Here’s what a traditional mass spectrometer looks like.

Mass spectrometers are really not so hard to understand; it’s just that some people seem to find them intrinsically less interesting than spaceships. Yet beneath their invisible-to-the-naked-eye microanalysis lies a drama every bit as compelling as a NASCAR race.

Let’s imagine you’ve nabbed a front-row seat at the Indianapolis 500. It’s been an exciting and exceptionally close race, and now three cars are in the last lap, careening toward the final curve.

Now, imagine that those 3,400-pound speed demons are atoms, and you’ll begin to understand what happens in a mass spectrometer.

The core of most MS machines is a magnet, placed outside a curving tube through which the sample passes. Its field disrupts (and in so doing, helps to measure) the steady stream of atoms passing by. Of course, race cars are subject to different laws of physics as they speed around a bend, but you get the idea – or you will in a minute. (These particles, by the way, actually zoom past the magnet at speeds Jeff Gordon could only dream of, measurable in nanoseconds.)

Before we see how this atomic race ends up, let’s shift into reverse and return to the starting line to get a clearer picture of what’s happening.

Mr. Gordon never won a race on foot. Likewise, any substance that’s going to make it from one end of an MS to the other needs a means to get there. That happens through vaporization. Turning a solid or liquid sample into a gas gives it the properties it will need to travel through the machine.

Now that we’ve got a means of getting from point A to point B, we’ll need some juice. To fuel the vaporized atoms, which are electrically neutral, the mass spectrometer turns them into ions. There are several ways of going about the ionization process. In this example, let’s bombard them with a stream of electrons, which in turn knock the electrons circling the specimen atoms out of their orbits. The loss of a negatively-charged electron leaves the atom with a positive charge, creating an ion that will respond to the field of the magnet it is about to encounter. (For a quick review of the parts of an atom, please click here.)

OK. So far we’ve got the driver (the atom), the car (the vaporization) and the gasoline (ionization). To burn rubber, we’ll also need to depress the accelerator. As it happens, the mass spectrometer features a chamber called the accelerator, in which electrical currents are used to give the ions a starting push. Each is given the same kinetic energy, making this a very fair race. This whole process, by the way, takes place inside a vacuum, eliminating potentially meddlesome air molecules that would otherwise interfere with the flow of the ions. After all, NASCAR drivers don’t need to contend with city traffic!

Now buckle your seat belts as we fast-forward in our contest, approaching the final curve. This is where things get really exciting.

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