Mass spectrometry (II)

Published: 22 Jun, 2020 | Last modified: 14 Oct, 2020

Mass spectrometry


After vaporization and ionization, we now have a lot of boron ions in hand. Next, we'll start to make them move.

Using an electrical field, we will be able to attract positively charged ions towards the negative side of the electrical field. The electrical field will exert a constant force on the boron cations and accelerate them in one direction. In this manner, we could generate a beam of moving boron ions.

Up to this stage, we're still unable to separate boron ions of different isotopes. This is because usually electrical fields are quite powerful and they will accelerate the ions in such a way that the small difference between masses of different isotopes is quite insignificant. All boron ions have having same speed of travelling. Therefore, we need another step to separate the ions.


This is where magic happens. You'll see after this step, ions of different isotopes will be separated. Now let's take a closer look at what's happening here.

As we said earlier, charged particles not only can be pushed by electrical fields, they could also be affected by magnetic fields. This time, ions of different isotopes are able to show significantly different behaviors in a magnetic field.

We shall now pass the beam of boron ions into a magnetic field.


Deflection in mass spectrometry

This diagram illustrates the deflection of the ions when they are passed between the two poles of a magnet. The magnetic field will push the ions in a direction perpendicular to the direction of their movement. This pushing force (named as Lorentz force in physics) is depending on the charge of the ion, speed of the ion, and the strength of the magnetic field.

In a mass spectrometer, all boron ions are ionized to carry the same charge (\( +1 \)). As a result, when we pass them into the same magnetic field with the same speed, they will experience the same force.

However, not all boron ions could be pushed to the same extend by that force. Image you're trying to push a bicycle as compared to a car. You may easily push the bicycle for 1 kilometer, but for the car, not that easy.

The same thing applies here. The heavier the ions are, the less they could be pushed by the Lorentz force. Why they have different mass? Because they are ions of different isotopes with different mass number (or number of neutrons).

So, we have separated the ions of different isotopes.

As you may see from the diagram above, \( \ce{^10B+} \) is lighter hence being pushed more (more deflection), while \( \ce{^11B+} \) is heavier with less deflection.


The last stage of mass spectrometry would be simple then. We simply detect the electrical signal when the ions hit our detector. The positive charge of the ions would interfere with the semiconductor chip of the detector, and hence alter its conductivity. By monitoring the conductivity of the chip, we could know when an ion reaches the detector.

The rest would simply be math. We are going to count how many signals are detected at each position. For example, we detected 80 signals at the lower detector (the black line at the end of the red line in the diagram), and 20 signals at the upper detector (the black line at the end of the green line in the diagram). This tells us we have in total 100 boron ions, and 80 of them are \( \ce{^10B+} \) while 20 of them are \( \ce{^11B+} \). So we say \( \ce{^10B} \) has an abundance of 80% while \( \ce{^11B} \) being 20%.

In reality

Of course, in reality a mass spectrometer will be more complicated and it is capable of generating more sophisticated information. Meanwhile, it will usually be coupled with other analyzing techniques to a better result. Nevertheless, the same principles described here still apply. No matter how advanced a mass spectrometer is, it is still utilizing magnetic field to separate cations depending on their mass differences.


A mass spectrometer (credit: Carnegie Mellon University)

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