Instrumental analysis and flame photometry

Chemists often use machines rather than test tubes to identify substances. A flame photometer can identify metal ions and measure how much is there.

Practise this lesson free (6 questions)

Why use instruments?

Compared with simple chemical tests, instrumental methods are:
more accurate (closer to the true value);
more sensitive (detect very small amounts);
faster, and can be automated;
able to work with very small samples.
But the equipment is expensive and needs trained people to run it.

Flame photometry

A solution is sprayed into a flame. Metal ions give out light at particular wavelengths.
The pattern of wavelengths (a spectrum) identifies the metal ion, even in a mixture where flame test colours would hide each other.
The intensity (brightness) of the light shows the concentration of the ion.

Using a calibration curve

Measure the reading for solutions of known concentrations and plot a calibration graph. Then measure the unknown and read its concentration from the graph.
Concentration of sodium ions (mg/dm3)Reading
00
520
1040
1560
2080
Worked example

Using the calibration table, a sample gives a reading of 50. What is its sodium ion concentration?

  1. The readings go up by 20 for every 5 mg/dm3.
  2. So 1 mg/dm3 gives a reading of 4.
  3. 50 ÷ 4 = 12.5 mg/dm3

Answer: 12.5 mg/dm3

Key idea

Instrumental methods are accurate, sensitive, fast and need only small samples. In flame photometry, the wavelengths of light emitted identify a metal ion and the intensity shows its concentration, using a calibration curve.

Key words
Ion
An atom or group of atoms with a charge, because it has lost or gained electrons.
Concentration
The amount of solute in a given volume of solution, in g dm^{-3} or mol dm^{-3}.
Solution
A mixture formed when a solute dissolves in a solvent.
Mixture
Two or more substances not chemically combined. They can be separated by physical methods.

All GCSE and International GCSE Chemistry key terms and definitions →

Quick check

Try two quick questions

See if it has sunk in. Answers are marked straight away.

Extra practice

2 more questions

Try each one, then tap to check.

  1. A calibration shows that each 1 mg/dm3 of sodium ions gives a reading of 5. A sample gives a reading of 32. What is its sodium ion concentration in mg/dm3?

    Show the answer

    Answer: 6.4 mg/dm3

    32 ÷ 5 = 6.4 mg/dm3

  2. Which is an advantage of instrumental methods?

    1. They are fast and accurate
    2. They are always cheap
    3. They need no electricity
    4. They need very large samples
    Show the answer

    Answer: They are fast and accurate

    Instruments are sensitive, accurate and rapid.

Check you have got it

Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.

Practise this lesson free (6 questions)All GCSE and International GCSE Chemistry lessons

Video for this lesson coming soon. Subscribe on YouTube

Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

What do exam command words like explain and evaluate mean? →

Related lessons