3.1.1.2 - Mass number and isotopes

Relative atomic mass

Average mass of atom of element on scale where 1 atom of carbon-12 = 12

Relative isotopic mass

Mass of atom of isotope of element on scale where 1 atom of carbon-12 = 12

Relative molecular mass

Average mass of molecule on scale where 1 atom of carbon-12 = 12

Stages of TOF mass spectrometer

1. Ionisation

2. Acceleration

3. Ion drift

4. Detection

Electrospray ionisation (mass spectrometry)

• Sample dissolved + pushed through nozzle at high pressure

• High voltage applied → each particle gains H+ ion

• Sample turned into gas made of +ve ions

Electron impact ionisation (mass spectrometry)

• Sample vaporised → ‘electron gun’ fires high energy electrons at it

• 1 e- knocked off each particle → become 1+ ions

Acceleration (mass spectrometry)

• +vely charged ions accelerated by electric field so they all have same KE

  → lighter ions move faster than heavier ions

Ion Drift (mass spectrometry)

• Ions enter region with no electric field, so drift through it
  → lighter ions drift faster than heavier ions

Detection (mass spectrometry)

• Lighter ions travel faster in drift region → reach detector quicker than heavier ions

• Detector detects charged particles + mass spectrum produced

Mass Spectrum

y-axis - abundance of ions

x-axis - mass/charge ratio

Mᵣ with electron impact ionisation

1 electron knocked off each particle to make 1+ ions
→ mass/charge ratio of peaks are same as Mᵣ of isotope

Mᵣ with electrospray ionisation

H+ ion added to each particle to form 1+ ions
→ mass/charge ratio of peaks are 1 greater than Mᵣ of each isotope

How to calculate Mᵣ from Mass Spectrum

1. For each peak, read % relative isotopic abundance (y-axis) and relative isotopic mass (x-axis)
   Multiply them to get total mass for each isotope

2. Add up the totals

3. Divide by 100

Example Mᵣ calculation Mass Spectrum

1. 79×24 = 1896; 10×25 = 250; 11×26 = 286

2. 1896 + 250 + 286 = 2432

3. 2432/100 = 24.3 (3 s.f.)

Identifying Elements with Mass Spectrometry

Elements with diff isotopes produce more than 1 line in mass spectrum because isotopes have diff masses

Identifying Molecules with Mass Spectrometry

1. Molecular ion, M+, is formed in mass spectrometer when 1 e- is removed from molecule

2. This gives a peak in spectrum with mass/charge ratio equal to Mᵣ of molecule

3. Can help to identify unknown compound