A2.6 Inductively Coupled Plasma: Optical Emission Spectroscopy (ICP-OES)
Plasma is formed using argon that is subjected to an electrical spark while under the influence of an oscillating radio frequency radiation. The sample to be analysed is introduced to the plasma and the electrons in its atoms are excited to a high energy state.
As the electrons lose energy and fall to lower energy levels, they emit electromagnetic radiation (remember the hydrogen emission spectrum).
The electromagnetic radiation is passed through a diffraction grating or prism to separate the different frequencies (generate a spectrum).
More than one element can be detected at the same time.
Each element gives out its own characteristic frequency (or frequencies) which enable the element (or elements) in the sample to be identified. The intensity (brightness) of the emission(s) is proportional to the concentration(s) of the elements present in the sample. To measure an unknown concentration you firstly have to generate a calibration curve using samples of known concentrations. Then using the intensity of the radiation from the sample, the curve can be used to read off the concentration of the element(s) detected.

## Worked example 🖊

The calibration curve below shows results obtained from solutions containing lead up to a concentration of
$\textsf{4mg dm}^\textsf{-3}$
So if a solution of unknown lead concentration gave an intensity of 58, the lead concentration, from the graph, must be approximately
$\textsf{2.7mg dm}^\textsf{-3}$
.
Figure 2.3 The calibration curve of a concentration containing lead in a solution