Analytical techniques

• AES


  • analysis of light emitted by atoms (atomic emission spectroscopy)
• AAS


  light absorbed by atoms (atomic absorption spectroscopy)
• What is AAS

  
• Simplified diagram of AAS equipment

  
• Hollow cathode lamp

  
• Lens and monochromator

  
• How AAS works


  1. Substance is vaporised in a burner. This changes the substances into atomic vapour

  2. Light from cathode lamp passes through the vaporised sample

  o Only the element being tested for will absorb the light (as this light has a specific wavelength and hence a specific energy)

  o Other elements present can't absorb the light as it possess different energy levels (and hence different amounts of energy)

  3. Light passes through the sample and focused through a slit

  4. Enters the monochromator (selects only the one wavelength of light to be analysed)

  5. Detector then measures the intensity of the light which is displayed as a number

  • The number is a measure of the amount of light NOT absorbed (not concentration)

  • A.k.a. Absorbance value
• mass spectrometry


  · Identifies the amount and type of chemicals present in a sample

  · Can also identify different isotopes present in an element

  · Not based on light

  · Based on mass and charge of an atom
• Mass Spectrometer


  Relative isotopic masses of elements and their isotopic abundances are determined by using amass spectrometer.•separatesthe individual isotopes in a sample of the element• Determines themassof each isotope, relative to the carbon-12 standard• calculates therelative abundancesof the isotopes in the sample
• Parts of MS

  
• atomisation


  • The sample is vaporised first before being injected into the machine
• Ionisation


  • Atom/molecule becomes ionised

  • The atom/molecule is bombarded by a stream of electrons. If there is enough energy then one electron from the atom/molecule is knocked off (even for those that don't normally form positive ions

  • Most atoms/molecules will form charge of +1 as it is difficult to remove a further electron

  • Completed in a vacuum (no air present)

  - Ions can move freely without hitting air molecules
• Acceleration


  · Ions are all accelerated so they all have the same amount of kinetic energy

  · This stage is also in a vacuum so that air molecules don't collide with the ions
• Deflection


  • Magnetic field used to deflect ions by mass (m) and charge (z)

  • The lighter the atoms the more deflection

  • The more charged the particle, more deflection

  - E.g. +2 charged particle is deflected more than +1 charged particle
• Detection


  · Electrical detection of number of particles
• Mass Charge ratio (m/z)

  • Consider the following atoms:

  • Atom A: m = 28, z = +1, m/z = 28

  • Atom B; m = 56, z = +2, m/z = 28

  • Therefore atom A and B are deflected the same amount as m/z ratio is the same

  
  

  • But assume most atoms are +1, deflection will be mainly due to mass of ion

  - This is because it is very hard to remove another electron to become +2

  • Remember: if m = 28, z = +1, m/z = 28

  • Therefore Mass of ion = mass of neutral atom = m/z ratio