Topic 5 Flashcards
Ionisation
The gas in the lamp is ionised using the anode and cathode
Ne—> Ne+ + e-
Sputtering
The positively charged ions collide with negatively charged cathode, dislodging metal atoms
M(s) + Ne+ —> M(g) + Ne0
Excitation
Collision of sputtered atoms with gas atoms or electrons excite metal atoms to higher energy states
M(g) + Ne+ —> M*(g) + Ne0
Emission
The excited atoms relax to ground state, giving up their energy as photons of visible/ultraviolet radiation. Decay to lower energy by emission of light
M*(g) —> M(g) + hv (photons)
Atomisation
Process of converting an analyte in solid, liquid or solution form to a free gaseous atom
Methods of atomisation:
1) Flame atomisation
2) Electrothermal atomisation
Pneumatic nebulizer
- Liquid is sucked through capillary tube
- Sample converted into a fine mist
- Fine aerosol mist is aspirated into a mixing chamber
Mixing chamber
- Aerosol mist mixes with combustion gases in the mixing chamber
- At the flame, there is combustion using fuel and oxidant
- Heat from burner vaporizes the aerosol particles, forming free atoms
Important types of flame
Fuel: Acetylene Oxidant: Air Temp.: 2100-2400
Fuel: Acetylene Oxidant: Nitrous oxide Temp.: 2600-2800
Air acetylene flame
Absorbs at <200 nm
possible chemical interferences
Nitrous oxide acetylene
May cause ionisation interference
able to overcome chemical interferences in air acetylene flame
For elements that are difficult to atomise: Al,Si,B,Ba
Monochromator
Used to isolate a single wavelength from the line spectrum emitted by hollow cathode lamp
Functions of monochromator
- allow monitoring one wavelength in the spectrum of analyte
- minimise the effects of light emission from the flame
Functions of Photomultiplier tube - detector
- convert light signal into electrical signal
- electrical signal is then converted to absorbance
Sample treatment
Digestion
- used for analysis of inorganic analyte
- involves dissolving solids in a solvent with externally supplied energy
Methods of Digestion
Wet ashing
Dry ashing
Microwave-assisted digestion
Wet ashing
- use of strong acids to destroy organic matter
- only inorganic elements remain in the acid solution
Dry ashing
- heating using an electrical furnace
- destroys organic compounds by oxidation, leaving inorganic compounds in the ash
- dissolve the ash in an appropriate acid to for a solution for further analysis
Microwave-assisted digestion
Decomposition of organic substances using microwave radiation at 80atm and 250 degrees
Interferences
Matrix
Chemical
Ionisation
Matrix interferences
Causes either lower or higher analyte signals
Is cause by varying physical characteristics of sample and standard ( viscosity and surface tension)
Minimising matrix interference
1) Match the matrix components in the blank, standard and sample
2) Allow solutions to be same temp. before analysis
3) Apply standard additions method
Chemical Interferences
Causes number of atoms capable of absorbing light in the flame to be lower, causing lower absorbance
Caused by formation of stable chemical compounds within hight-temp environment
Minimising chemical interferences
1) Higher flame temperature
2) Using releasing agents to react with the interferent
3) Using protective agents (EDTA) to react with analyte to form a volatile compound
Ionisation interference
Causes less ground state atoms for radiation absorption, causing lower absorbance
Caused by high flame temperature producing enough energy to cause the removal of an electron from the atoms, forming ions
Minimising ionisation interferences
1) Add excess easily ionisable elements (K,Na,Rb,Cs) to the blank, standards and sample to suppress ionisation of analyte
2) Use a cooler flame (chemical interference may still occur)
Flame atomisation
Consists of:
1) Pneumatic nebulizer
2) Mixing chamber
