Instrumental methods of analysis

Instrumental methods of analysis rely on machines. There are several different types of instrumental analysis. Some are suitable for detecting and identifying elements, while others are better suited to compounds.

Compared to simple laboratory tests, instrumental methods of analysis may give improved:

  • speed (they are quick)
  • accuracy (they reliably identify elements and compounds)
  • sensitivity (they can detect a substance in a very small amount of sample)

An example - the flame photometer

The flame photometer is a scientific instrument based on flame testing. Data from a flame photometer can be used to:

Identifying metal ions

In the flame photometer, the coloured light from a vaporised sample can be split to produce an emission spectrum. The different lines in an emission spectrum look like a coloured barcode. Each metal ion produces a unique emission spectrum.

An emission spectrum for mercury.An emission spectrum for mercury

The metal present in a sample is identified by comparing its spectrum with reference spectra. These are emission spectra from known metal ions. If two spectra match, they must be from the same metal ion.

Determining concentrations

A reading is taken from the flame photometer for different concentrations of a metal ion in solution. These readings are used to plot a calibration curve.

An example of a calibration curve from a graph of flame photometer reading and concentration of sodium ions.

Example

A solution containing sodium ions gives a reading of 4 units on the flame photometer. Use the calibration curve above to determine the concentration of sodium ions in this solution.

The concentration is 0.02 g/dm3.

Question

A solution containing sodium ions gives a reading of 9 units on the flame photometer. Use the calibration curve above to determine the concentration of sodium ions in this solution.

The concentration is 0.045 g/dm3.

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