Similarities 
of Atomic Absorption Spectroscopy (AAS), Atomic Emission Spectroscopy (AES) & Atomic Fluorescence Spectroscopy (AFS):
  • The sample is vaporized (2000-6000K)
  • Atomic concentration is determined by measuring at a characteristic wavelength
  • High Sensitivity
  • Able to perform simultaneous multi-element analyses
  • Analyte concentration (ppm-ppb)
  • Precision (1-2%)
  • Expensive
  • Widely available
 
Atomic Absorption Spectroscopy (AAS)

Suitable for both Qualitative & Quantitative (usually)
Qualitative: limited to the element of interest
Quantitative: can quantity trace element in the sample (concentration of the analyte in a sample)

Quantitative determination of chemical engineering elements employing the absorption of radiation (light) by free atoms in the gaseous state.
 
Principle Operation
i) The solution is put into flame, sample element converted into atomic vapor (mostly remain in the ground state)
ii) Light source struck onto vaporized atom 
iii) The ground-state atom absorbs radiation of a particular wavelength (light source)
iv) The detector detects absorption of a sample by the light source

Modified Beer’s Law:
A=k.c
* no way to know the path length
*molar coefficient of absorption not calculated

The concentration of atomic vapor is directly proportional to the concentration of the analyte in the solution.

Calibration curve – can find the concentration of an unknown solution

Instrumentation
 
1) Light Source: Hollow-cathode Lamp (HCL)
  • The emitted light will be absorbed by the test element
  • Hollow cylinder cathode must be coated with element same as analyte (e.g. interested in Sodium, the cathode must be coated with Sodium)
  • Drawback: Different lamp is required for each element
2) Atomizer: Burner/ Thermal Device
  • The burner can go up to 6000K
 
  • Furnace is used when we have a solid sample
  • Can only attain up to 3000K
  • Flameless atomizer
  • Uses premix burner (fuel, oxidant & sample are mixed before introducing into flame)
  • Degree of atomization is temperature-dependent
  • Different Fuel and Oxidant is used to attain different temperature range

3) Monochromator

Enables the elimination of a large part of the stray light due to the hollow cathode lamp, and the selection of the most intense spectral line in order to obtain a better sensitivity

Interference
Changes the signal while analyte concentration remains unchanged
  1. Spectral
    -unwanted signals overlapping analyte solution
     
  2. Chemical
    -chemical reactions decreasing the concentration of analyte atoms
     
  3. Ionization
    -ionization of analyte atoms within the flame decreases the concentration of neutral atoms

Background Correction
To reduce interference (unwanted signal) & avoid significant error
  1. Beam Chopping (correct beam for flame emission)
    - Blocks lamp using a rotating chopper

     
  2. Deuterium Lamp
    - Light from HCL is absorbed by analyte & absorbed scattered by background
    - Light from D2 is absorbed and scattered only by background.
    - Disadvantage:  Complex optical assembly
     
  1. Smith-Hieftje (pulsed HCL)
    - run the HCL at low current and then at high current
    - Difference between the output = absorbance due to the analyte
    - Disadvantage:  High-current pulse reduce lamp life
 
  1. Zeeman Effect
    - Shifting of energy levels of atoms and molecules in a magnetic field
    - Disadvantage:  Expensive
 
 
Exercise
Next, we have done an exercise through Mentimeter.
 

Reflection 

Atomic spectroscopy is definitely a new topic for me. After this lecture, I am able to understand the principle, instrumentation and mechanism of Atomic Absorption Spectroscopy. Not only that, I learn how to find the concentration/ absorbance of an unknown from the calibration curve with informative sample.
The tutorial session this week is used to discuss our Test 1 answers. After the discussion, I am able to understand the mistakes that I have done during the test. However, I still have some doubts with the questions because we, students have a different approach on the question.
We have tried to discuss with the tutor during the past tutorial, but unfortunately we didn't have the same view on the questions.

For question 1, the question stated that "find the amount if the indicated element/compound (in moles) in (a)8.75g of B2O3 & (b) 167.2mg of Na2B4O7.10H2O.

So as from the students' perspective, we can choose to calculate the element or the whole compound. But our tutor say he will give 3/4marks for those who calculate for compound but not element. We don't understand why.

For question 6, the question asked us to check whether 22.09 is a valid measurement at 95% confidence level. Those who used the Grubbs test or q-test to solve the question has their marks got deducted. I think that the question is not giving a clear instruction on which method should be used.
 

Test 1 Question & Answer

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