Atomic absorption spectroscopy AAS

Question 1

AAS- electronic transitions

Answer 1

• The electrons of the atoms (ions) in the flame can be promoted to higher orbitals by absorbing a set of energy (light)
• This amount of energy is specific to a particular electron transition in each particular element (ion)
• The absorbance is proportional to the conc. Of the element in the sample being measured

Question 2

AAS- the instrument

Answer 2

look at ppt for diagram
- the temperature of the flame is not high enough to promote all atoms (ions) to their excited states

• this is achieved by the light supplied by the hallow cathode
• cathode light is then absurd by the atoms and the absorption spectrum is recorded, generally at a single specific wavelength for a particular ion

Question 3

AAS instrument - the light source

Answer 3

• the source of light is a lamp whose cathode is composed of the element being measured
• each element requires a different lamp
• the lamp is housed inside the lamp compartment of the instrument

Question 4

AAS instrument - the sample compartment

Answer 4

• One of the most common means of introducing the sample into the flame is by preparing a solution of the sample in a suitable solvent (most frequently water)
  1) Aspirator tube sucks the sample into the flame in the sample compartment
  2) Sample is vaporized in the flame
• The sample holder is really the flame since it is in the flame that the atoms absorb radiation for the source

Question 5

AAS instrument- flame specifications

Answer 5

• The flame is arranged such that it is laterally long and not deep. The height of the flame must also be controlled by controlling the flow of the fuel mixture.
• A beam of light is focused through this flame at its longest axis (the lateral axis) onto a detector past the flame.

Question 6

AAS- quantitative analysis

Answer 6

• Quantitative analysis can be achieved by measuring the absorbance of a series of solutions of known concentration.
• A calibration curve and the equation for the line can be used to determine an unknown concentration based on its absorbance.

Question 7

AAS - applications

Answer 7

• Analysis of the total mercury in human tissues and blood

- The method developed in this study allowed the determination of Hg poisoning levels

Question 8

limitations of flame ionisation sources

Answer 8

• Flame techniques can only detect one element at a time.
• The optimum excitation condition can vary widely from element to element (temperatures, excitation wavelengths…etc).
• These techniques are not useful to analyse compounds which are highly resistant to decomposition by heat (i.e. refractive compounds as boron. phosphorus, tungsten, uranium, zirconium).