What’s in an Oil Drop?
Table of Contents
The Chase Is On
Below is an expanded version of the applet from the previous page. Click the Excitiation radio button and watch again how those plates works. Then, to observe the role of the detector plates, click on Dectection radio button.
When an ion packet, such as Ion X and its friends, approach a detector plate, a stream of negatively charged electrons (equal in charge to the packet) travel through a second outside circuit to the detector’s electrode. This begins a game of cat and mouse between those electrons and the packet. No sooner do the electrons reach one electrode than the orbiting packet heads in the opposite direction. The electron current then takes a U-turn and skedaddles through the circuit to the electrode on the opposite plate. No sooner do they reach that electrode than the ions, continuing their orbit, circle back toward the opposite side, and the dogged electrons make another about face, hot on their trail. If they were Mounties, these electrons would be fired: They never get their man.
As the electrons chase the ions back and forth, a resistor in the outside circuit measures the voltage, which is an indirect measure of the ions circling inside the cell.
It’s important to remember that all the packets corresponding to all the masses in the sample are going through the same process at the same time. All this measuring takes place simultaneously, making FT-ICR an extremely efficient mass spectrometer.
Speaking of measuring: That step begins after the ion packets have reached their biggest orbits and the RF chirp, having done its job, is turned off. As the packets, now losing energy, spiral back down to their original orbits, the alternating current induced in the circuit running between the detector plates gradually subsides. The machine captures the decay of the orbits over that time, as you’ve seen by now in the applet. Feel free to click on the blue Reset button and watch this again.
This whole process, sweeping the entire range of ions in the sample, lasts about one second. It’s a blink of an eye, but at a quantum level, a lot has happened. Your average packet, in that single second, completes about 30 kilometers – 18.6 miles – worth of orbits. At that speed they leave the space shuttle, more than three times as slow in its own orbit, in the dust.
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