Mass Spectrometry Analysis

Mass spectrometry is a technique used to work out:

• The relative isotopic abundance.
• Atomic and molecular mass.
• The structure of a compound.

• 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.

• 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.

• 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.

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

• 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.

• 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.

• 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.

• 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