Professor Went: Atomic Absorption Spectroscopy

Question 1

What is the effect of temperature on the number of excited atoms compared to ground state electrons?

Answer 1

• The fraction of atoms in the excited state is very temperature dependant.
• Ground-state population is virtually constant.
• Emission measures the excited state.
• Atomic absorption measures the ground state and is more populated and thus has lower detection limits.

Question 2

What is atomic absorption spectroscopy, and what is it’s main drawback?

Answer 2

• Until recently, this was the most widely used atomic spectral method, but has been superseded by the atomic emission technique.
• The main drawback is the need for a different lamp for each element.

Question 3

What is the process of atomic absorption spectroscopy?

Answer 3

• A hollow cathode lamp atomises the sample into ions.
• The ions are then irradiated by absorption of light, which then pass through a monochromator in order to separate the element-specific radiation from the background radiation.
• This element-specific radiation is then measured by a detector.

Question 4

What is typical of an atomic absorption spectrum?

Answer 4

• The spectrum has extremely narrow intrinsic spectral lines.
• Line broadening occurs due to:
  
  • Uncertainty principle
  • Pressure effects - collisions with other atoms cause changes in ground state energy
  • Electric and magnetic field effects.

Question 5

Why is a line source used rather than a continuous source?

Answer 5

• With a monochromator band from a continuous source, the relative amount of incident light absorbed is much smaller.
• This gives less sensitivity and a curved calibration curve.

Question 6

What is typical of a hollow cathode lamp?

Answer 6

• They consist of a hollow cathode and an anode, separated by an insulating disc.
• A quartz or glass window allows the light to pass through.

Question 7

What is the role of the hollow cathode lamp?

Answer 7

1. The electric potential ionises rare gas atoms accelerates them into the cathode where they sputter metal atoms into the gas phase.
2. Collisions with gas atoms or electrons excite the metal atoms.
3. If they have enough energy, gaseous cations dislodge metal ions from the cathode and produce an atomic cloud.
4. A portion of the atoms are in an excited state, and emit their characteristic radiation.

Question 8

What is typical of an electrodeless discharge lamp?

Answer 8

• Greater intensities but not so reliable.
• Lamp is energised by intense field of radiofrequency or microwave radiation.
• Ionisation of Ar gives ions which are accelerated by high frequency of field.
• Sputtering occurs, where the gaseous cations dislodge some of the metal atoms from the cathode and produce an atomic cloud.
• A portion of these atoms are in an excited state