Time Of Flight Mass Spectrometry Explained For Beginners (TOF MS)

Описание: Time-of-flight is one possible mass analyzer, frequently utilized in mass spectrometry. Just like any other mass analyzer its purpose is to separate the different particles of a sample based in their mass-to-charge ratio. In a time-of-flight mass analyzer this is achieved by accelerating all particles at the same electric potential and then measuring the time it takes them to reach the detector. From this, the individual masses of the particles can be calculated. Let us take a closer look at how this is done:

1. First, before the sample even can enter the mass analyzer it has to be ionized. This can happen in several ways but one common method to use together with time-of-flight is MALDI.
2. Once the sample has been ionized, it enters the mass analyzer. The aforementioned ionization ensures that the sample is charged and as a result its sub components can be accelerated by a high-voltage current. This is where the magic of this technique happens. Since all components are accelerated by the same amount of current, their resulting speed will only be dependent on size. This is because their kinetic energy will be the same as their electric potential energy (Ek = qV). Kinetic energy on the other hand is half of the mass of the particle times its velocity squared. In other words the velocity is inversely proportional to the size of the particle. In even simpler terms, small particle means fast particle and big particle means slow particle.
3. So, once the particles have been accelerated, they fly through the “drift” region of the mass analyzer and hit the detector on the other side. Since the velocity is just the distance traveled divided by the time taken to travel it, finding the velocity of each particle comes down to rudimentary physics. Furthermore, all of this is handled automatically by the device but if you want to know how it does it, there you have it! The detector records the number of events within a specific time period and uses the information of calibration standards to determine what the sample consists of.

One thing that tends to kinda suck with just regular time-of-flight mass analyzers is that the resolution tends to be kinda mediocre. To remedy this one can combine it with other types of mass analyzers to create a so called tandem mass spectrometer. Another solution more unique to time-of-flight mass analyzers is to add a reflectron TOF. This doubles the distance traveled by the particles since an ion mirror is added at the end of the drift region, causing the ions to travel back and forth again. This increase in the distance traveled results in an improved resolution.