What’s in an Oil Drop?
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Beam Us Up!
These excitation plates are like cheerleaders, rallying our unmotivated team members into action. The interactive Java tutorial below demonstrates how these plates work (we'll discuss the subsequent stages of the process on the next page).
Both of these brown excitation plates has an electrode, part of a circuit external to the cell. Through the circuit a series of oscillating radio frequency pulses (called a chirp) is sent to the excitation plates. Of the range of frequencies emitted over the course of a chirp, Ion X will respond to only one – the one that corresponds to its particular cyclotron frequency. The chirps start at a low frequency then increase, so the heavier ions will respond first.
The upshot of this is that Ion X and its like molecules are suddenly seized by a sense of kinship, of team spirit. They absorb this extra “cheerleading” energy from the RF, using it to increase the size of their orbits around the magnetic field. In their new orbits, they discover each other – their team members, all the other ions of the same weight, which have answered the same clarion call. That’s how like ions coalesce into a team huddle (scientists call these packets), orbiting in sync. Click on the Excitation Plates On radio button in the applet above and you’ll see how this works. (You can adjust how fast the ions orbit with the Applet Speed slider.)
Gaining coherence is half the battle for these ions. The other half is getting close enough to the detector plates to get noticed. That’s accomplished, also, with the help of the oscillating RF energy boosts, which, as they continue to pulse, provide the oomph for the ion packet to gradually increase its orbit, following a spiral-like path until it reaches its maximum radius of about a centimeter or so – close enough to the detector electrodes to get a reaction. By the time you’ve finished reading this paragraph, the ion packets in the applet will have finished this upward spiral.
(When doing these measurements, scientists generally calibrate the chirp in such a way that the ions’ final orbit – the parking orbit, as it’s called – remains shy of the actual cell wall, so the ions never crash into it).
The work of the excitation plates is now done, and the detector plates take over.
Next Page The Chase Is On
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