Quadrupole Ion Trap Mass Spectrometry or QIT MS is a mass analyzer designed to trap ions and then eject them sequentially while analyzing them. This is done by placing 1 ring of AC current (RF) around it so that the ion stays in the middle as well as 2 static (DC) plates forcing the ion to stay in the middle due to repulsion. In addition there is a source and a detector.
The main idea is to create a hyperbolic field which keeps the ions in place, the mathematics are beyond the scope of this video but can be described with Mathieu’s equation and visualized using an a-q stability diagram so just Google those concepts if you want to understand the mathematics behind this device better.
Now, in the beginning all ions inside of the QIT mass analyzer are stable within this hyperbolic field. However, in order to detect them they have to be pushed out sequentially by their mass-to-charge ratio. By increasing the RF voltage, heavier mass ion orbits become more stabilized and the light mass ions become less stabilized, and this can be utilized in order to push the ions out of the machine.
But what if we want to analyze the smallest ion in a sample in more detail. How can we get rid of these larger ions first? By matching this RF supplemental voltage frequency to all of the ions we are NOT interested in, we can empty the trap of all of the ions which we are not interested in. **W**e can apply an RF pulsation tuned to a frequency that matches the frequency at which ions of a particular mass-to-charge ratio oscillate. This causes a sort of resonance frequency, meaning that those ions increase their movement patterns until they are pushed out of the trap.
Something that perhaps makes more intuitive sense is an opera singer, singing at the exact pitch matching the natural frequency of a fine wine glass, causing it to resonate until the point of breaking. You may also imagine that you are pushing someone on a swing and each time you match your push to the velocity of the swing, further increasing its energy.
This selective ejection is especially useful in Tandem MS since we can use this mass analyzer to easily target ions of a specific size.
