Quadrupole Mass Spectrometry (QMS) FOR BEGINNERS
• Quadrupole Mass Spectrometer
Mass spectrometry is something we have talked about several times on this channel. In a nutshell, mass spectrometers consist of:
1. An ion source
2. A mass analyzer
3. A detector
My videos on electrospray ionization and electron ionization are two examples of ionization. In my video on MALDI-TOF I briefly explained the ionization method MALDI as well as the Time-Of-Flight mass analyzer. I will leave a link to my playlist containing all my videos relevant to mass spectrometry by the end of this one.
However, today we are taking a closer look at another mass analyzer used in mass spectrometry, namely the quadrupole ion analyzer. Broadly speaking, the QMS consists of 4 parallel rods that have fixed DC and alternating RF potentials applied to them. Ions produced in the source of the instrument are then focused and passed along the middle of the quadrupole rods. QMS separates ions based on the stability of their flight trajectories through this oscillating electric field in the quadrupole. As you can tell, there is quite a lot to unpack here so let us dive into what this look like in action:
1. First, as always with mass spectrometry, the sample has to be ionized. In the case of quadrupole mass spectrometry, this is usually done by electron bombardment. (If you want to know more about how that works I have a video linked by the end of this one) In a nutshell, the sample gets shot repeatedly by electrons, in turn also knocking off some of its electrons making it ionized.
2. Then the ionized sample enters the QMS, where the 2 electrical fields caused by the 4 parallel rods start affecting it. What is vital to understanding how this is done is remembering that the degree to which these two fields interact with the ions, is dependent on their size. Smaller ions are affected more. This can be likened to a small boat and a large ship. A small boat can easily maneuver as soon as the rudder is turned while a large ship takes a lot more time to turn.
For the X-direction, smaller ions will stay in phase with the RF drive, gain energy from the field and oscillate with increasingly large amplitude until they encounter one of the rods and are discharged. Larger ions on the other hand will not be effected as much, staying their course without encountering any of the rods. Therefore the X-direction is a **high-pass mass filter**, i.e. only high masses will be transmitted to the other end of the quadrupole without striking the X-electrodes.
On the other hand, in the Y-direction, heavy ions will be unstable because of the defocusing effect of the DC component, but some lighter ions will be stabilized by the AC component if its magnitude is such as to correct the trajectory whenever its amplitude tends to increase. Thus the Y-direction is a low-pass mass filter, i.e. only low masses will be transmitted to the other end of the quadrupole without striking the Y electrodes.
Did, you find that confusing? Don’t worry! So did I when I first read about this! The main point is that when the net charge interacting with the sample is mainly positive, the larger ions are stable and the smaller ions are unstable. On the other hand, when the net charge interacting with the sample is manly negative, the smaller ions are more stable and the larger ions are more unstable. If you want to better understand that I honestly recommend you check out [**ShortChemistry**](https://www.youtube.com/channel/UCBwi... video on it. I found that his explanation was the best one available. I will link it in the description!
Anyway, by selecting a suitable ratio of RF-to-DC, the two directions together give a mass filter which is capable of resolving individual atomic masses.
This is why QMS is so useful. By simply adjusting the RF/DC-ratio we can create a convenient filter for a particular mass. Simultaneously by varying the amplitude of DC and RF voltages, the entire spectrum can be scanned.
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