A water company routinely monitors its treated drinking water for traces of dissolved substances. Scientists use instrumental methods of analysis rather than traditional chemical tests to check the water quality. Explain why instrumental methods are preferred for this purpose, and describe one instrumental technique that could be used to identify the substances present in the water sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Water companies are required by law to ensure drinking water is safe and free from harmful dissolved substances. Even tiny amounts of certain substances can make water unsafe to drink.
Model answer (5 marks)
Instrumental methods are preferred because they are far more sensitive than traditional chemical tests, allowing detection of trace amounts of harmful substances that would otherwise be missed. They are also faster, giving results in minutes rather than hours or days. Additionally, instrumental techniques can analyse very small sample volumes, minimising waste and cost.
One suitable technique is gas chromatography (GC). In GC the water sample is first extracted and the dissolved substances are carried by an inert gas through a column containing a stationary phase. Each compound interacts differently with the column, so they travel at different speeds and elute at distinct times – the retention times. By comparing these retention times with those of known standards, the identity of each substance can be determined.
One suitable technique is gas chromatography (GC). In GC the water sample is first extracted and the dissolved substances are carried by an inert gas through a column containing a stationary phase. Each compound interacts differently with the column, so they travel at different speeds and elute at distinct times – the retention times. By comparing these retention times with those of known standards, the identity of each substance can be determined.
Examiner tips
- Mention sensitivity, speed and small sample size first; then name GC and explain retention time basis for identification
Common mistakes
- Failing to state the key advantages (sensitivity, speed, small sample); confusing GC with other techniques; not explaining how retention time leads to identification
Mark scheme (5 marks)
- Instrumental methods are more sensitive than traditional chemical tests
- Instrumental methods are faster/quicker than traditional chemical tests
- Instrumental methods can analyse very small samples
- Named instrumental technique — gas chromatography (GC) or mass spectrometry (MS) or gas chromatography–mass spectrometry (GC–MS)
- Correct description of how the named technique works or what information it gives — e.g. gas chromatography separates the substances in the mixture / the time taken to travel through the column (retention time) identifies each substance; OR mass spectrometry identifies substances by their mass (relative molecular mass / mass-to-charge ratio)
Key terms in this question
Related
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