1.1.3

# Mass Spectrometry

Test yourself

## Workings of a Mass Spectrometer

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

### 1) Ionisation

• 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.

### 1) Ionisation cont.

• 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.

### 2) Acceleration

• 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.

### 3) Ion drift

• 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.

### 4) Detection

• 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.

## Analysis to Identify Molecules

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

### 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