topic 5 atomic spectro

Question 1

working principle of AAS (2)

Answer 1

when the right frequency of energy is absorbed by an atom, it is promoted from ground to excited state

using Beer’s law, absorbance is directly proportional to concentration of unknown

Question 2

working principle of AES (2)

Answer 2

atom in excited state eventually drops back to ground state and releases the energy absorbed previously in the form of light/heat

using Beer’s law, emission intensity is directly proportional to concentration of unknown

Question 3

process of atomic spectroscopy (5)

Answer 3

1. aspiration
   analyte in solution introduced as fine spray
2. nebulisation
   analyte is turned into fine mist
3. atomisation
   solvent evaporates, salt dissociates to form free atoms
4. excitation
   atoms are promoted from ground to excited state
5. emission (only for AES)
   excited atoms return to ground state and emit light

Question 4

main components of AES and AAS spectrometer (6) and their functions

Answer 4

1. sample delivery system
   introduces sample
2. atomiser (flame)
   supplies energy to turn sample into excited state
3. monochromator
   select light of specific frequency/wavelength of interest to be absorbed by element
4. detector
   measures change in intensity of light to be absorbed by analyte before converting it into signal using photomultiplier
5. read out device
   displays result
6. radiation source (hollow cathode lamp, HCL) (only in AAS)
   emits radiation of a specific wavelength to be absorbed by element

Question 5

what burner is used in atomic spectrometer (2)

Answer 5

Lundegardh burner

oxidant + fuel = flame

Question 6

types of flames used in atomic spectrometer (3) and characteristics

Answer 6

1. air-acetylene flame
   - most widely used
   - temp: 2100-2400 deg C
2. nitrous oxide-acetylene flame
   - temp: 2600-2800 deg C
   - very useful for elements that form heat-stable oxides in air-acetylene flame
   - but may cause ionisation of alkali metals
3. air-hydrogen and argon-hydrogen flame
   - temp: 2000-2100 deg C
   - lower noise level when analysing arsenic and selenium compared to air-acetylene flame

Question 7

AAS vs AES (4)

Answer 7

• AAS consumes a photon and brings an atom from ground to excited state; AES generates a photon and brings an atom from excited state back to ground state
• in AAS, radiation measured is absorbance; in AES, radiation measured is emission intensity
• source for excitation for AAS is HCL; source for excitation for AES is flame
• AAS has fewer interferences than AES

Question 8

why is qualitative analysis less commonly carried out using GC? (3 key points)

Answer 8

• not a conclusive test
• only provides rough estimation
• especially when there are multiple cations present in sample

Question 9

types of calibration methods for GC following Beer-Lambert law (2)

Answer 9

1. general/external calibration method

2. standard addition calibration method

Question 10

compare the two types of calibration methods for GC (6)

Answer 10

• general/external is more commonly used
• general/external is more convenient when a large number of similar samples are to be analysed
• standard addition is more complicated and takes longer as each calibration curve can only determine concentration of one unknown
• standard addition requires larger sample volume
• standard addition is useful when the analyte is present in a complicated matrix with no ideal blank available
• standard addition helps to overcome matrix effects

Question 11

process of general/external calibration for GC (4)

Answer 11

1. prepare series of standard solutions and analyse
2. plot graph of analytical signal vs concentration of standard
3. analyse unknown and record signal
4. use graph to find concentration of unknown

Question 12

process of standard addition calibration for GC using GRAPHICAL APPROACH (4)

Answer 12

1. equal volumes of sample solution is added to each volumetric flask
2. standard solution is added to each flask in increasing volume
3. flask is topped up to the mark with solvent and homogenised
4. signal vs concentration curve is obtained and concentration of unknown is obtained by extrapolating the curve on the concentration axis

Question 13

process of standard addition calibration for GC using EQUATION APPROACH (4)

Answer 13

1. equal volumes of sample are added to unknown and standard volumetric flask
2. fixed volume of standard added to standard flask
3. both flasks topped up to the mark with solvent and homogenised
4. obtain unknown concentration using equation below:
   x/(x + a)= Rx/R1

where 
Rx: abs of unknown
R1: abs of unknown + spiked standard
x: unknown concentration
a: spiked standard concentration