FT-ICR is a type of mass spectrometry particularly suited to identifying heavy molecules. This tutorial shows how this technique measures the weight of ionized molecules (determined by the type and number of atoms in them) using a powerful magnetic field and radio frequency pulses.
Above is a schematic of an FT-ICR analyzer cell, or Penning trap. Inside are ionized molecules; the charge allows them to respond to the magnetic field running through the center of the circular cell (the field is going into the Web page). For the sake of simplicity, three types of molecules are shown, each color representing a different molecular weight. The red are heaviest, the purple are lighter and the blue are lightest.
These ions circle the magnetic field in tiny orbits. Their speeds vary by mass: the lighter ions are faster than the heavier ones. All the ions, though, are orbiting in a jumble, mixed together. The instrument will separate them by weight using radio frequency pulses emitted through electrodes in the brown Excitation Plates. Click on the Excitation radio button to watch. Slow down the tutorial using the Applet Speed slider to examine this process more closely if you want.
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, an ion will respond to only one – the one that corresponds to its particular cyclotron frequency, which is a function of its mass. The chirps start at a low frequency then increase, so the heavier ions respond first.
The molecules absorb this RF energy, using it to increase their orbits around the magnetic field. In their larger orbits, each group coalesces into a packet. The chirps continue as the packets follow a spiral-like path, reaching a maximum radius close to the Detector Plates. Click on the Detector radio button to see what happens next.
When an ion packet approaches a detector plate, a stream of negatively charged electrons (equal in charge to that 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. (In this interactive animation, the electrons carry the color of the ions to which they correspond).
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.
This measuring process begins after the ion packets reach their maximum orbits and the RF chirp 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 this data, and a computer then does some fancy math (that's the Fourier Transform part, not depicted here) that results is a graph depicting the mass-to-charge ratio of the ions in the sample, which essentially identifies what molecules are in the sample. Feel free to click on the blue Reset button and watch this again.