State Ranking Guide to AAS (Atomic Absorption Spectrum) Questions
This falls under the topic of ‘Monitoring and Management’
Post written by Joanna Huang (1oth in the state Chemistry 2016). See all articles first and personally get in touch with our state rankers here
This topic is very content heavy, and hence contains a lot of memory work. Whereas the ‘Acidic Environment’ was quite calculation heavy with pH and titration questions, in this topic, there are a lot of long 4 to 7 marker theory based questions.
Section 3 of this module is all about testing for the chemical composition of different substances. It covers:
- Solubility rules and the flowcharts used for testing anions and cations. Refer to the flowchart in the Jacaranda Textbook to help with the memory load
- Flame tests (selected cations only). You need to remember the colour of the flame when selected metal cations are sprayed into the flame. The metals are: Calcium, strontium, barium, copper, sodium
- Atomic Absorption Spectroscopy (AAS)
- How it works
- Being able to plot a graph given a table of data, or do simple calculations given graph of Absorbance vs. Concentration
- Areas of application
- It’s role in helping us understand the importance of trace elements
- It’s use in pollution control
- Determining the sulfate content of lawn fertiliser. This is a practical work study with a discussion on validity, accuracy and reliability (with calculations as well)
In this post, I’ll be focusing on how you should answer Atomic Absorption Spectroscopy (AAS) questions of Section 3 of this module.
Question 1
Describe the principle of atomic absorption spectroscopy and its application in environmental monitoring. Include a diagram in your answer. (5 marks)
The diagram drawn is the diagram of how AAS works (easily found in textbooks). Make sure it is labelled and include a brief description of how it works.
Points to cover:
- The labelled diagram (see the textbook or the 2009 HSC sample answers)
- The principle of AAS
- Rough description of how the AAS apparatus works i.e. from the lamp right to the plotting of the graph
- It’s application in environmental monitoring
- Advantages and Disadvantages of AAS and relate to its use in environmental monitoring
Sample response
AAS relies on the principle that when an electron absorbs a discrete amount of energy, it is able to become excited and jump into a higher energy level. When it falls from an excited energy level to it’s ground state, it releases this energy (to become more stabilised).
The lamp containing the metal to be analysed is heated up to direct a unique wavelength through the apparatus and into the photomultiplier and finally the detector so an initial intensity reading can be recorded. Then the sample is atomised through the aspirator. When the ray hits the target ions, these ions absorb some of the photons and re-emit them in all directions, causing a reduction in intensity. By comparing the difference in intensity, and hence the fraction of light absorbed of that particular wavelength, the apparatus calculates an absorbance reading. The absorbance reading is then compared to samples of previously known absorbance (i.e. the calibration curve), so that the concentration of that element in the sample can be determined.
Because AAS is extremely sensitive to ppm levels, it is used to measure micronutrients in agriculture. If soil is lacking in trace elements such as copper and zinc, plants exhibit deficiency diseases. AAS allows scientists and farmers to maintain healthy pastureland and crops. AAS can also measure the concentration of heavy metals in waterways to very accurate concentrations, and pollution/bioaccumulation in living organisms (such as mercury concentrations in fish).
Because a different lamp is needed for each element, it is not as convenient when multiple elements reside in one sample. However AAS is still widely used in environmental monitoring as it is sensitive, accurate, and fast.
[be sure to also include the diagram]
Question 2
Assess the impact of atomic absorption spectroscopy (AAS) on the scientific understanding of the effects of trace elements (4 marks).
First of all, make sure you understand what a trace element is.
Dangerous heavy metals like lead are NOT trace elements. So do NOT talk about lead!
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Points to cover:
- What is a trace element?
- What has AAS helped us achieve?
- What is the scientific understanding of trace elements?
- Judgement. Has AAS revolutionised scientific understanding?
Sample response
AAS has allowed the detection of metal ion concentrations in very low concentrations. This has led to the discovery that living things require trace elements at very low levels to maintain their biochemical functions.
For example, we now know that small trace amounts of copper are required for the production of enzymes involved in biochemical reactions, and for the formation of haemoglobin. Manganese is also required for blood clotting as well as carbohydrate metabolism.
Before the introduction of AAS, other commonly used methods (like volumetric or gravimetric analysis), were not sensitive enough to detect the low concentrations of these elements. Hence scientists were NOT able to link people’s symptoms (of being unwell) with the cause (lack of trace elements in the body).
Now with AAS, a reliable, accurate and sensitive method of analysis, chemists are able to conduct analyses rapidly and in multiple batches, allowing scientists know the role of different trace elements in biochemical functions.
Hence AAS has had a great impact upon scientific understanding of trace elements, providing immense health benefits to the global community.
To finish off:
A lot of longer (6–7) marker questions come from either the Haber Process (Section 2), AAS (section 3) the Ozone layer and UV radiation (Section 4) and Water quality (section 5). So be ready to continually add to your memory bank.
People also tend to ask me if they should prepare some answers to common questions.
It’s not a bad idea to organise your thoughts beforehand. This is just so that if the question pops up, you are able to answer it in a logical and concise manner in the minimum amount of time. It’s always nice to have a ‘go to’ bulk of info that you can copy down, but be sure you don’t get carried away. Actually answer the question required.
In other words, just as you shouldn’t hammer down a pre-memorised English essay word for word, you shouldn’t do that for Chemistry either.
Remember to always include equations wherever/whenever they fit, even if it doesn’t specifically say so in the question.
Post written by Joanna Huang (10th in the state Chemistry 2016)
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