Mass spectrometry (III)

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

Mass spectrum

First of all, let's walk through these confusing terms.

Mass spectrometry is the technique or process to separate ions depending on their masses.

A mass spectrometer is an equipment that allows us to perform a mass spectrometry analysis. It is usually connected to a computer which comes with specific softwares to collect, sort, analyze, and show the result of a mass spectrometry analysis.

The result from the mass spectrometry analysis will be shown in a mass spectrum.

We could simply put "mass spectrometry is a technique where we use a mass spectrometer to generate a mass spectrum". Clear?

So what does a mass spectrum look like?


A mass spectrum

We have already understood the abundance and how to determine it using the mass spectrometry. But what does the m/z mean?

mass-charge ratio (m/z)

The full name of m/z is "mass-charge ratio", which is the ratio between the mass and the charge of the particle being detected..

In the deflection step, the particles would be deflected by the magnetic field to different extends. This is because the force that the particles experience is depending on the charge on these particles, while the extend of deflection is determined by their masses.

The higher the charge of the particle, the greater the force applied on it by the magnetic field, and consequently a greater deflection.

On the other hand, the heavier the particle is, the less the deflection will be as the force could not move it much.

Therefore, we must consider both the mass and the charge of the particles instead of any one factor alone. By applying some physics theory and calculation, we reach the conclusion that we can simply use the mass-charge ratio to determine the deflection. That's to say, particles with different mass-charge ratio, instead of different mass or charge alone, have different deflection.

So our mass spectrum could be read as how many particles with a specific mass-charge ratio are being detected.

But in our simplified scenario, we could assume all the ions we generated have the same charge of \( +1 \) (as we discussed in this post). Therefore, mass-charge ratio will simply give us the mass of the ions (\( m/z = m \)).

Now we could say that the x-axis tells us the mass, or rather relative mass of the ions of different isotopes, in short, atomic mass.

Relative atomic mass from mass spectrum

Let's take a look at the mass spectrum of \( \ce{B} \) atoms.


We could then deduce the two isotopes of boron being \( \ce{^10B} \) and \( \ce{^11B} \), and their abundances are 20% and 80% respectively. We could now calculate the relative atomic mass of boron using

\[ 10 \times 20\% + 11 \times 80\% = 10.8 \]

Please do take note of the particles producing the peaks. The peaks will represent only ions but not atoms. Neutral atoms will not be accelerated by the electrical field, and hence will not enter the magnetic field nor deflected by it. They can not be detected either. Only ions, or to be more specific, cations could be accelerated, deflected, and then detected. Therefore, you'll notice the peaks are being identified as \( \ce{^10B+} \) and \( \ce{^11B+} \) cations.

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