A hospital laboratory tests 500 blood samples each day for lithium ions. The concentration of lithium ions in each sample is about 1 mg/dm31\,\mathrm{mg/dm^{3}}1mg/dm3. The laboratory can run one instrument or test for 8 hours each day.
| Method | Lowest concentration detected (mg/dm3\mathrm{mg/dm^{3}}mg/dm3) | Time to test one sample (min) |
|---|---|---|
| Flame test by eye | 50 | 2 |
| Flame emission spectroscopy | 0.01 | 0.5 |
| Mass spectrometry | 0.001 | 10 |
Which method should the laboratory use, and why?
Flame test by eye, because it does not need an expensive instrument.
Flame emission spectroscopy, because it can detect 1 mg/dm31\,\mathrm{mg/dm^{3}}1mg/dm3 and can test all the samples in one day.
Mass spectrometry, because it can detect the lowest concentration of lithium ions.
Mass spectrometry, because it is the only method that can detect 1 mg/dm31\,\mathrm{mg/dm^{3}}1mg/dm3.
52 exam-style questions on OCR GCSE Chemistry 4.2 Identifying the products of chemical reactions, covering 4.2.1 Tests to identify gases, 4.2.2 Tests for aqueous ions and flame tests, and 4.2.3 Instrumental methods of analysis. Each one has a worked solution and a mark scheme showing where the marks go.