12.6 Instrumental Analysis

A free GCSE Chemistry lesson for AQA (8462) from OBA School: why chemists use instruments, flame emission spectroscopy, and finding concentrations from calibration data. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Give the advantages and disadvantages of instrumental methods compared with chemical tests
• Explain why metal ions give out light in a flame, in terms of electrons and energy levels
• Describe how flame emission spectroscopy is used to identify metal ions
• Identify metal ions in a sample by comparing its spectrum with reference spectra
• Use calibration data to find the concentration of metal ions in a solution

Key words

• Instrumental method: An analysis carried out by a machine (instrument), e.g. flame emission spectroscopy or gas chromatography.
• Accurate: Close to the true value.
• Sensitive: Able to detect very small amounts of a substance.
• Spectroscope: An instrument that splits light into its different wavelengths.
• Line spectrum: A pattern of bright lines at particular wavelengths. Each metal ion has its own pattern.
• Reference spectrum: The known spectrum of a pure substance, used for comparison.

Why use instruments?

• accurate – results are close to the true value; Disadvantages equipment is expensive to buy and run
• sensitive – can detect tiny amounts of a substance; Disadvantages needs trained people to use it and interpret results
• rapid – quick, and many samples can be run automatically; Disadvantages results must be compared with reference data from known samples
• can analyse mixtures and measure concentrations; Disadvantages usually needs electricity and a laboratory
• needs only a very small sample; Disadvantages cannot be carried out in the field as easily as a simple chemical test

Instrumental methods are used, for example, to check the levels of metals in drinking water, to monitor blood in hospitals and to test steel in industry.

Flame emission spectroscopy

Flame emission spectroscopy (FES) is an instrumental version of the flame test.

• A solution of the sample is sprayed into a flame. The metal ions give out light.
• The light is passed through a spectroscope, which splits it into a line spectrum.
• Each metal ion gives a different, characteristic set of lines. The spectrum is compared with reference spectra to identify the ions.
• The intensity (brightness) of the lines depends on the concentration of the ions, so FES can also measure how much is present.

Why metal ions give out light

The heat of the flame gives energy to the electrons in the metal ions. An electron can absorb this energy and jump up to a higher energy level (shell): it is excited. It soon falls back to a lower energy level and gives out the energy as light.

• The size of each energy jump fixes the wavelength (colour) of the light given out.
• Each metal ion has its own arrangement of electrons and energy levels, so it gives its own set of wavelengths: a characteristic line spectrum, like a fingerprint.
• This is also why flame tests work: the mixture of wavelengths we see is the flame colour.

Reading spectra

Worked example: identifying ions in a mixture

The unknown sample has lines at 589 nm, 610 nm and 671 nm.
589 nm matches sodium. 610 nm and 671 nm both match lithium. Every line is accounted for.

Answer: the sample contains sodium ions and lithium ions

Exam tip: A mixture shows all the lines of every metal ion in it. Check that every line of a reference spectrum is present before you say that ion is in the sample. A flame test on this mixture would just look yellow, because sodium masks lithium.

Measuring concentration

Solutions of known concentration (standards) are run first. A graph of reading against concentration (a calibration graph) is drawn. The reading for the unknown is then used to read off its concentration.

Worked example: tiny amounts in standard form (Higher tier)

A 50 cm³ sample of river water contains 5.6 × 10⁻⁷ g of cadmium ions (Ar of Cd = 112), detected by flame emission spectroscopy.
moles = mass ÷ Ar = 5.6 × 10⁻⁷ ÷ 112 = 5.0 × 10⁻⁹ mol
volume = 50 ÷ 1000 = 0.050 dm³, so concentration = 5.0 × 10⁻⁹ ÷ 0.050 = 1.0 × 10⁻⁷ mol/dm³

Answer: 1.0 × 10⁻⁷ mol/dm³ – far too little to detect with a chemical test

Worked example: reading a calibration graph

The unknown gives a reading of 39. The line goes up by 15 for every 2 mg/dm³ (7.5 per mg/dm³).
concentration = 39 ÷ 7.5 = 5.2 mg/dm³

Answer: 5.2 mg/dm³

Common mistakes

• Writing just “instruments are more accurate”. Give the reason, e.g. “sensitive: can detect very small amounts” or “rapid: gives results quickly”.
• Thinking the flame or spectroscope makes the colours. The metal ions give out the light (when excited electrons fall back to lower energy levels); the spectroscope just separates it.
• Saying an ion is present when only one of its lines matches. All of its lines must be there.
• Forgetting to scale up a concentration when the sample was diluted before testing.

Revision questions

Try these, then check your answers below each one.

Question 1. Which is an advantage of instrumental methods of analysis? (Choose from: They are sensitive and can detect tiny amounts, They are cheaper than chemical tests, They do not need reference data, They can be used without any training.)

Answer: They are sensitive and can detect tiny amounts

Working: Instrumental methods are sensitive, accurate and rapid, but expensive and need trained users and reference data.

Question 2. True or false? Flame emission spectroscopy can identify the different metal ions in a mixture.

Answer: True

Working: Each metal ion has a characteristic line spectrum, so all the ions in a mixture can be identified.

Question 3. In flame emission spectroscopy, what does the spectroscope produce from the light given out? (Choose from: A line spectrum, A chromatogram, A precipitate, A calibration graph.)

Answer: A line spectrum

Working: The spectroscope splits the light into a line spectrum.

Related lessons

• Previous lesson: 12.5 Tests for Negative Ions
• Next lesson: 12.7 Required Practicals: Chemical Analysis
• In this chapter: 12.1 Pure Substances and Mixtures
• In this chapter: 12.2 Analysing Chromatograms
• In this chapter: 12.3 Testing for Gases

Continue in the full course

This lesson is section 12.6 (AGCCH1206) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.