Analytical Instrumentation Analytics

The Ion and the Triple Quad: A Tale of Wanderers, Disturbing Interactions, and Hopefully, Oblivion

I often hear people talk about the triple quad. They fawn over it and are scared of it simultaneously. And with good reason. For they know that mass spectrometry bequeaths awesome power. It just comes at a price.

A quadrupole refers to the mass analyzing device inside of a mass spectrometer. It is constructed of four perfectly parallel, cylindrical or hyperbolic rods. An ion, let’s say positively-charged to keep things upbeat, is formed by the ion source, which generates ions typically via specific chemical or strong electrical means.

Regardless of the specifics of how the ion got there, it’s now there, and a series of lenses serve to focus the ion towards one of the four singular poles, of which there are two positively and two negatively charged. Electricity can drive opposites together, and the positive ion finds solace in one of the two negatively charged poles.

Image Credit: Shimadzu

Like a marionette, the ion can be prevented from discharging onto, and thus succumbing to, one of the negative poles. The spectrometrist can play puppeteer, omnipotently altering the world as is known by the ion, by adjusting the acceleration of the ion in the x, y directions, which relate the position of the ion from the center of the four poles. And with the right oscillations to these positions, the ion can be ferried through the quadrupole, levitating its way into oblivion once it collides with the detector. Ions that crash and burn onto the poles along their journey aren’t detected, and thus, are not witnessed by the analytical chemist.

Like humans, ions have differences. Some are heavier than others. Some can be highly charged. Those types are less controllable along the highway to detection. The lightweight ions can spiral out of control in their unstable trajectories. So, parallel to achieving long-term existence with other humans, a compromise is needed between the loss of heavy and light ions due to their lack of focus and defined direction, respectively.

There’s a shaman inside the quadrupole-equipped mass spectrometer, called the ion guide, which can also be a quadrupole, hexapole, or octapole. Given the propensity for ions to be wanderers, the ion guide serves to transport all ions effectively and efficiently on their migration to their demise in detection. Vagrant or resident gas molecules can careen into ions, displacing them from their destiny. Thus, it’s important to focus these back onto their course as well.

What happens when you link quadrupoles (or combos of the others) in centipedal fashion? The end of days?

Image Credit: Wikipedia

Image Credit: Waters Corporation

Nothing quite so dramatic, but the ion can be macheted into representative fragments, like hacking a puzzle into its pieces. So, you can first analyze the ion, then cause it to collide with an inert gas, which energizes the ion through transference of energy, like an ionic kiss. The kiss is so distracting to the ion that it is torn asunder. These divisive interactions take place in the 2nd quadrupole, known as the collision cell. The 3rd quadrupole, essentially, picks up the pieces, literally, by analyzing the fragments of the ion that once was.

Like Jodorowsky’s El Topo, the story of the triple quad has it all: the journey, distracting encounters, and some sense of cognitive peace after finding answers to what’s been sought.

Header Image Credit: Comsol

About the author

Jason S. Lupoi, Ph.D.

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