Although the “FT” comes before the “ICR” in the instrument’s name, the ICR part actually happens first. We’ll get to Fourier transform soon enough.
The analyzer cell, surrounded by a multi-ton superconducting magnet, is the heart of an FT-ICR MS.
Some cells are square or rectangular, some are cylindrical, as is this one. But they all pretty much have the same parts and work the same way.
Let’s take an ion’s eye view of what happens. Meet Ion X, a mystery molecule from a sample of crude oil we’re testing. It’s one of hundreds of thousands, perhaps even a million individual ions in our sample. It’s easy for Ion X to feel lost in the crowd, but in fact it has a special kinship with some of the other ions in the sample – those representing the same type of molecule, those of the same molecular mass. Imagine them as members of the same team, their molecular weights (for the moment unknown to us) emblazoned on their jerseys. Let’s also give these ions a team color: red.
When they are ushered into the cell from the ion guide, the ions are drawn toward the magnetic field running lengthwise through the center of the cell. They then begin to circle it, perpendicular to the field, in tiny orbits, as the ions are doing above. Ion X is one of those little red guys – a bit hard to detect in the cluster of orbiting molecules.
Though the orbit radius is about the same for all of these divergent ions, the speed at which they travel is not; that is determined by each ion’s mass. That means, of course, that all the ions on the same “team” orbit at the same speed – what’s known as their cyclotron frequency. As you’d guess, the lighter ions are fleeter of foot (if you can imagine ions in Nikes) than the heavier ones – and therefore have higher cyclotron frequencies. That is ultimately how this machine will differentiate the molecules.
Another fact about these frequencies: They increase as the power of the cyclotron’s magnet increases. And when the frequencies increase, so does the difference between any two ICR frequencies, which means it’s easier to tell the different types of ions apart. This is what accounts for the resolving power discussed earlier. This is also why scientists try to develop FT-ICRs with ever more powerful magnets, like the 21 T system now under development at the MagLab.
Even though Ion X is traveling at the same speed as all its like molecules, it shows no inclination toward fraternizing with them. This group has no coherence, no esprit de corps. Rather, its members are randomly distributed throughout the analyzer cell, mixed up among all the other ions.
This is poor teamwork, and no way to do science. Luckily, FT-ICR MS is well suited to remedy this situation.
Our ICR MS is equipped with two plates, occupying opposing walls of the cell, called detector plates. They’re the gray top and bottom plates in our diagram.
As you might expect, their job is to detect the ions inside the cell. They do this via electrodes, one on each plate, hooked up to an electric circuit that runs between them, outside the cell. When the ions in the chamber come close enough to the electrodes, they induce a flow of negatively-charged electrons in that circuit, and in so doing get measured (more on that soon enough).
That’s how it’s supposed to work anyway. But for the moment, as you can see, the ions are not cooperating.
The dilemma is two-fold.
First, the ions are all mixed up, distributed across the cell like a herd of cattle across the prairie: If you can’t round ‘em up, you can’t count ‘em. And even if you could count them that way, spread out across the prairie (so to speak), the exercise would prove fruitless. That’s because the ions are in different phases of their orbits, as our illustration shows: While one approaches the top detector plate, another nears the bottom plate. So any electrons attracted to the top detector plate would be cancelled out by what’s happening on the opposite side of the cell, leaving nothing to measure. The ions are working against each other: rivals rather than teammates.
Second, even if all the like ions were traveling in sync, they still would not be detected: in their little 0.1-millimeter orbits, they’re too far from the detector plates to get noticed.
Elegantly, FT-ICR disposes of both of these pesky problems with a single solution: the excitation plates, which are the brown plates on the either side of the analyzer cell.
Classroom Outreach
What’s in an Oil Drop?