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From 2014.igem.org

Background / Inductively coupled plasma optical emission spectrometry (ICP-OES)

Atoms can be excited by thermal energy. Thereby valence electrons are lifted from ground state to a higher discrete state. The occupation of the different energy levels depends on the temperature and can be described by Boltzmann equation.

Since the excited state is no steady state, excited electrons move back to ground level by emitting the excess energy. The emitted energy is characteristical for the excited element. Through energy absorption atoms can either be excited or ionized. That’s the reason why optical emission spectrometry shows atomic lines as well as ionic lines. Though the emission lines are element specific, they can be disrupted by other elements. By choosing undisturbed emission lines, consideration of several lines per element and adjustment of the standards to the sample matrix or the use of a standard addition method, mistakes in determinations can be avoided.

The basic setting of an optical emission spectrometer with inductively coupled plasma is shown in picture 1. The dissolved sample is feed into the dispersion system by a pump and then nebulized into an aerosol.

Pic. 1: Schema of the inductively coupled plasma optical emission spectrometry (ICP-OES).

This in turn is carried to the atomizing unit, the plasma torch, by a carrier gas. The plasma torch is rinsed with argon in a constant flow rate. The upper part of the torch is wrapped with a water-cooled induction coil, which is supplied by a high frequency generator.
[…] Ohmic heating is part of ICP-OES. Thereby temperature rises up to 10 000 K and the amount of excited atoms is dramatically increased, so that a detection is possible to a low μg/kg range. Energy transferring hits with the plasma particles lead to an atomization, ionization or excitation of the probe aerosol. With the help of a spectral device and mono- or polychromators the emitted energy is fragmented into the separate wave lengths (λ). The photons of the wave lengths are detected via charge-coupled devices (CCD) and charge injection devices (CID) and quantifiable via a proportional relationship between concentration of the analyte and measured intensity.

Source: Jan Thieleke, Institute of Inorganic Chemistry (Hanover); translated by Lisa Amelung