Topic 5

Question 1

Ionisation

Answer 1

The gas in the lamp is ionised using the anode and cathode

Ne—> Ne+ + e-

Question 2

Sputtering

Answer 2

The positively charged ions collide with negatively charged cathode, dislodging metal atoms

M(s) + Ne+ —> M(g) + Ne0

Question 3

Excitation

Answer 3

Collision of sputtered atoms with gas atoms or electrons excite metal atoms to higher energy states

M(g) + Ne+ —> M*(g) + Ne0

Question 4

Emission

Answer 4

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)

Question 5

Atomisation

Answer 5

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

Question 6

Pneumatic nebulizer

Answer 6

• Liquid is sucked through capillary tube
• Sample converted into a fine mist
• Fine aerosol mist is aspirated into a mixing chamber

Question 7

Mixing chamber

Answer 7

• 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

Question 8

Important types of flame

Answer 8

Fuel: Acetylene Oxidant: Air Temp.: 2100-2400

Fuel: Acetylene Oxidant: Nitrous oxide Temp.: 2600-2800

Question 9

Air acetylene flame

Answer 9

Absorbs at <200 nm
possible chemical interferences

Question 10

Nitrous oxide acetylene

Answer 10

May cause ionisation interference
able to overcome chemical interferences in air acetylene flame

For elements that are difficult to atomise: Al,Si,B,Ba

Question 11

Monochromator

Answer 11

Used to isolate a single wavelength from the line spectrum emitted by hollow cathode lamp

Question 12

Functions of monochromator

Answer 12

• allow monitoring one wavelength in the spectrum of analyte
• minimise the effects of light emission from the flame

Question 13

Functions of Photomultiplier tube - detector

Answer 13

• convert light signal into electrical signal
• electrical signal is then converted to absorbance

Question 14

Sample treatment

Answer 14

Digestion
- used for analysis of inorganic analyte
- involves dissolving solids in a solvent with externally supplied energy

Question 15

Methods of Digestion

Answer 15

Wet ashing
Dry ashing
Microwave-assisted digestion

Question 16

Wet ashing

Answer 16

• use of strong acids to destroy organic matter
• only inorganic elements remain in the acid solution

Question 17

Dry ashing

Answer 17

• 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

Question 18

Microwave-assisted digestion

Answer 18

Decomposition of organic substances using microwave radiation at 80atm and 250 degrees

Question 19

Interferences

Answer 19

Matrix
Chemical
Ionisation

Question 20

Matrix interferences

Answer 20

Causes either lower or higher analyte signals

Is cause by varying physical characteristics of sample and standard ( viscosity and surface tension)

Question 21

Minimising matrix interference

Answer 21

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

Question 22

Chemical Interferences

Answer 22

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

Question 23

Minimising chemical interferences

Answer 23

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

Question 24

Ionisation interference

Answer 24

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

Question 25

Minimising ionisation interferences

Answer 25

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)

Question 26

Flame atomisation

Answer 26

Consists of:
1) Pneumatic nebulizer
2) Mixing chamber