Mass Spectrometry

Chemistry: AQA A Level, topic 1.1.3

Workings of a Mass Spectrometer

The mass spectrometer is split into four stages: ionisation, acceleration, ion drift and detection.

The first step is ionisation of the sample. The main techniques are:
Electrospray ionisation.
Electron impact ionisation.
Electrospray ionisation is a gentler technique and prevents fragmentation.
It’s typically used for polymers and biological materials like DNA.
The sample is dissolved in a solvent and a high voltage is applied.
The high voltage rips a proton off the solvent and attaches it to the sample molecules.
The sample molecules are now positively charged ions.

In electron impact ionisation, the sample is first vaporised and then hit with electrons from an electron gun.
The electrons knock off electrons from the molecule
The molecules are now positively charged ions.
This method often causes the sample to fragment.

Molecules are accelerated to all have the same kinetic energy.
From standard equations, the kinetic energy is equal to half of the mass multiplied by the velocity squared:
Kinetic energy = $\frac{1}{2}$ mv²
All the molecules have the same kinetic energy, so the speed is dependent on the mass of the molecule.
Lighter particles move faster and are detected before heavier particles.

The time of flight is given by:
Time of flight = $\frac{distance}{velocity}$
This leads to an equation for the time travelled that depends on mass:
Time = distance ÷ $\sqrt\frac{{2KE}}{m}$
Lighter ions take less time as the time is dependent on the square root of the mass.

The ions hit a negatively charged plate.
This causes a current and the size of this current gives a measure of the number of molecules hitting the plate.
This gives the abundance of the molecule.

Once a sample has passed through the mass spectrometer, we can analyse the data to identify the molecule.

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.

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

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