Mass Spectrometry

Chemistry: Cambridge International A Level, topic 1.1.3

Analysis to Identify Molecules

A mass spectrometer is a piece of equipment used to help identify molecules. Unknown molecules are passed through the spectrometer and a mass spectrum is produced.

Spectrum produced

When the sample has passed through the mass spectrometer, a spectrum is produced by the spectrometer.
On this spectrum:
The x-axis is mass/charge ratio.
The y-axis is % abundance.

Main peak

The spectrum produces lots of peaks, but the most important is the molecular ion peak.
This is the peak of the greatest mass/charge ratio.
This represents the mass/charge value of the molecule we are analyzing.

Isotopes

Smaller peaks will cluster around the molecular ion peak.
These are from the same molecules but with different isotopes in them.
The isotopic molecules have different masses and so different mass/charge ratio values.

Fragmentation

Any smaller and significantly lighter peaks in the spectrum are because of fragmentation.
The molecule can fragment in the spectrometer.

Analysis to Calculate

Once we have the mass spectrum, we can calculate the relative atomic mass.

Relative atomic mass

Relative atomic mass is the average weighted mass of an atom relative to carbon-12.
The key word to look at here is "average".
This is because its value is calculated taking into account all of its isotopes and their relative abundances.

Using the spectrum

We can use the spectrum to view all the different isotopes and their relative abundance.
Isotopic mass is along the x-axis.
Isotopic abundance is along the y-axis.

Calculation

We can then calculate the relative atomic mass since we have all of the isotopic masses and their relative abundances.
It is easiest to show how to carry out the calculation with an example - see the next slide.

Example - boron

This is the mass spectrum of a sample of elemental boron.
Boron has two isotopes, ¹⁰B and ¹¹B.
You can see from the spectrum that approximately 20% of the boron is ¹⁰B and 80% is ¹¹B.
You can use this to work out the relative atomic mass:
80% × 11 + 20% × 10 = 10.8

Test yourself on Mass Spectrometry

What do clustered small peaks around the molecular ion peak represent?

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