Mass Spectrometry: How to Weigh an Atom
Table of Contents
Two Better than One
Ready? Good!
MS machines come in all shapes, sizes and configurations. The simple mass spectrometer we just described is a single sector MS – 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.
PHYSICS FACTOID: Ions come in two flavors. Cations (positive) and anions (negative). The word "ion" comes from the Greek word "to go" or "goer." According to this etymology, anions go "up," and cations go "down."
Dual sector MS doesn’t use electron bombardment to ionize the sample (which, as just described, gives all the ions the same speed). Instead, it uses other methods (one of which is described in the following section) that result in ions of the same mass traveling at different velocities.
These different speeds give the ions different trajectories: Some will end up in the detector, many won’t. That’s no good! To get them all in the detector (and accurately counted), they must travel the same speed. So after ions are analyzed by mass (with the magnet), they must be analyzed by speed (or kinetic energy) with the electrostatic field as well. This is how it works.
As you can see in the interactive Java applet below, the ions enter the first (magnetic) analyzer, where they are sorted by mass before a select group is waved ahead 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.
Fortunately, this dispersion is corrected as the ions pass smack dab 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, naturally, 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, triumphantly, into the detector.
If you’ve still got NASCAR on the brain, think of it this way: The electric field is a hill, sloping from positive down to negative. As the “race cars” approach it, the faster cars shoot up and down the slope more rapidly than the slower cars. But as you can see from the applet, the slower cars travel a shorter distance from point A to point B. Given the shorter distance and smaller effect from the positive plate, they catch up with the fast ones by the time they get to the slit. Ladies and gentlemen, it’s a dead heat!
You already have two approaches to MS under your belt. Pretty impressive! Now you’re ready to examine the novel type of MS that Humayun uses to try to solve comet conundrums.
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