∫ I learned from teaching undergraduates: Mass Spectrometry Edition

During TA training at Stony Brook this week, one of the things they had us do was perform one of the lab experiments taught in the freshman lab courses and give a presentation on it as though we were explaining it to our students. I was assigned a lab about measuring the charge-to-mass ratio of the electron, which was a pretty slick lab in and of itself. But along the way I also learned something new about how mass spectrometry works, and I learned that phasers might just be electron beams fired in a fluorescent gas. But first things first.


I confess, before this week, "mass spectrometry"  didn't mean much to me. I knew, vaguely, that it was a technique used to determine the chemical composition of an unknown substance, but beyond that I had really no idea how it worked. I just knew that it was one of those nice "technobabble"  words that the writers of shows like "CSI" love to toss about whenever they need the boys at the lab to come up with a new clue.

Well, it turns out it's quite simple. Basically, they take their unknown sample, and they break it apart into little tiny bits (really tiny-- I mean like individual atoms and molecules). Then, they ionize these little bits, usually by blasting them with a beam of particles. This knocks some of the electrons out of the atoms and molecules, so that what's leftover is a bunch of ions. Think of this as a microscopic version of rubbing a balloon on your hair-- it takes something that started out electrically neutral and leaves it crackling with electric charge. Finally, now that they have all of the bits that made up the original sample floating around someplace, they measure the charge-to-mass ratio of the ions; that is, they find out how much electric charge each one carries per unit mass. Nowadays, we know the charge to mass ratios of all the common chemical compounds, and so once you know the charge-mass ratio of the various constituents, you know what chemicals were in your sample to begin with.

The cool part, of course, is actually measuring that ratio. Now, I don't know exactly how professional mass spectrometry is done, but it can't be too different from exactly what I did in this freshman lab. The idea is quite clever: it turns out that when you have a charged particle, there are two things you can do to make it move: you can put it in an electric field, which will make it speed up or slow down, and you can put it in a magnetic field, which will make it curve about. Well when measuring the charge-to-mass ratio of something, you use BOTH types of fields. First you expose it to an electric field that makes it want to move in a straight line, and then you apply a magnetic field that makes it want to turn in a circle. The heavier the particle is, the more it wants to keep going straight, because it has more momentum. On the other hand, the more electrical charge it has, the more it feels the magnetic field and the more it wants to turn.

Well, the magnetic field is always going to win, so the charged particle will end up moving in a circle. But just how big that circle is depends on both the charge and the mass, since the two effects are "at war with each other"  as described above. In fact, if you do the math, it turns out to depend on the ratio of the two, which is just what you wanted.

So this brings me to the pretty picture at the top of the post. What you're seeing is a beam of electrons being fired upwards through a hole in the center of a large metal plate. Underneath the plate, an electric field is used to accelerate the particles and make them want to travel straight upwards, but above the plate, a pair of Helmholtz coils are used to create a large, uniform magnetic field that bends the electron beam to the right in a perfect circle (quiz question for physics buffs: Can you tell me what direction the magnetic field must be pointing? Remember that this is an electron beam). The metal plate is marked with grooves so that it can act like a ruler, so to find the charge-mass ratio of the electron you just have to look at where the beam hits the plate to determine the diameter of the circle, and then do a tiny bit of arithmetic.

Incidentally, the coolest part about doing this experiment in the lab is that you can actually see the electron beam. Now, I suspect that the people in the CSI labs have a smarter way to determine the size of the circle than just "looking at a ruler,"  but for a freshman lab its much more important to have things that look a little bit like the phaser beams from Star Trek. The trick is the glass bulb that you can see in the picture. It's filled with some sort of gas that fluoresces in the presence of electrons-- basically the atoms of the gas absorb a few of the electrons from the beam and then convert them into visible photons. I don't know what gas is being used in the picture; it didn't say in the lab manual. I know Helium can fluoresce like that, but I think it's more likely to appear green.

Seriously though, it looks just like a phaser beam.

So there you go: Next time the guy on "Bones"  says "We ran a mass spectrometry test on the sample and confirmed that it was pure cane sugar. Our killer must work at the molasses plant," then you'll know just what they're talking about.