Instrumental Methods of Analysis (Triple)AQA GCSE Chemistry: Revision notes
Section 1
What Are Instrumental Methods of Analysis?
Instrumental methods use machines rather than simple chemical tests to analyse substances. Compared with traditional chemical tests, instrumental methods are:
- Accurate
- Sensitive
- Rapid
One key instrumental method on the specification is flame emission spectroscopy, which is used to analyse metal ions in solution.
If a question mentions accuracy, sensitivity or speed of analysis, it's almost certainly asking about instrumental methods.
Section 2
How Does Flame Emission Spectroscopy Work?
In flame emission spectroscopy, a sample is placed into a flame and the light it emits is analysed. Each metal ion produces light of characteristic wavelengths, which appear as lines on a spectrum.
By comparing the result against reference data (given in chart or tabular form), the identity and concentration of metal ions in a solution can be determined precisely — far more precisely than judging a flame colour by eye.
A lab compares the spectrum from an unknown solution against a chart of known spectra to identify which metal ions are present and in what concentration.
Section 3
How Do Instrumental Methods Compare with Chemical Tests?
Traditional chemical tests can also identify metal ions, but they rely on the observer's judgement:
- A flame test identifies metal ions by eye from a characteristic flame colour (e.g. lithium crimson, sodium yellow, potassium lilac) — but colours can be masked in a mixture
- Adding sodium hydroxide solution identifies metal ions from a coloured precipitate (e.g. copper blue, iron(II) green, iron(III) brown)
Both chemical tests work well for single, known ions, but instrumental methods are more accurate, sensitive and rapid, especially for mixtures or very small quantities.
Don't say instrumental methods are only 'quicker' — the specification wants accurate, sensitive AND rapid as the three advantages.
Section 4
Chromatography as an Analytical Technique
Paper chromatography separates mixtures based on how substances distribute themselves between a stationary phase (the paper) and a mobile phase (the solvent).
Interpreting a chromatogram allows an Rf value to be calculated for each substance:
Rf = distance moved by substance divided by distance moved by solvent
Chromatography can help distinguish pure from impure substances: a pure compound produces a single spot in all solvents, whereas an impure substance or mixture produces more than one spot.
Must Know
- Instrumental methods are accurate, sensitive and rapid compared with chemical tests
- Flame emission spectroscopy analyses the light produced by a sample in a flame to identify and measure metal ions, using reference data for comparison
- Traditional chemical tests (flame tests, sodium hydroxide precipitates) rely on judging colours by eye and can be masked in mixtures
- Chromatography separates mixtures using a stationary and mobile phase; Rf = distance moved by substance divided by distance moved by solvent
- A pure compound gives a single spot in chromatography in every solvent used
That's the notes covered.
Carry on to the next subtopic.