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
  • accuracy
  • sensitivity (they can detect very small amounts of a substance in a small amount of sample)

Flame emission spectroscopy

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

  1. Identify metal ions in a sample.
  2. Determine the concentration of metal ions in dilute solutions.

Identifying metal ions

In the flame emission spectroscope, 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.

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 spectroscope 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.An example of a calibration curve

Worked example

A solution containing sodium ions gives a reading of four 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 dm-3.

Question

A solution containing sodium ions gives a reading of nine 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 dm-3.

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