Emission spectrometry techniques are classified by the energy source for exciting the electrons. Several divergent energy sources are used. Many emission methods share a low background noise from light of the wavelength being measured. Sensitivity is enhanced by this low background noise. This is a diagram of a typical detector for emission.

The light emitted is first dispersed according to wavelength using a grating. The intensity of light at carefully selected wavelengths is then measured. The light detector may be a solid state CCD similar to those common in digital cameras and video cameras or similar photodiode array which measures several wavelengths simultaneously. Alternately, the light detector may be a more traditional photomultiplier that measures the light at one wavelength which is selected by a slit. The grating or slit moves to change the wavelength observed.

Spark Emission

The energy source in this technique is a high energy electric spark. The spark produces a vapor of excited atoms which emit atomic lines.

A low noise level permits sensitive simultaneous detection of many wavelengths with a photodiode array or CCD. The process is usually carried out in a vacuum to eliminate both chemical and spectral interferences. The excited atoms are easily oxidized if air is present, reducing the concentrations detected. Air also absorbs at some UV wavelengths which can present a spectral interference.

This technique is frequently employed for solid samples like steel or other metal mixtures.

Fluorescence

The energy source in this technique is a high energy light source, either UV or X-ray. The high energy light excites electrons in molecules. A lower energy light is emitted and measured.

Inductively coupled plasma - ICP

The energy source in this technique is a plasma of hot argon produced with an magnetic coil. This emission technique does have a high noise level from the hot plasma. The plasma produces excited atoms and ions which emit atomic/ion lines. A fine aerosol is pumped into the torch at a constant flow rate (typically 1-2 mL/minute). The aerosol is swept into the magnetic field with argon gas. A spark initiates the flame and production of plasma, but after initiation the plasma is sustained by the energy from the magnetic field. The turbulent plasma area is not used for measurement because of the high noise level. The light emitted above the coils is used instead.

Commercial instruments use solutions of samples for analysis. Aqueous solutions are preferred.

A comparison between atomic absorption and inductively coupled plasma helps the user select the proper technique for specific analysis.

Varian maintains a website with many applications papers available as small pdf files. An ICP-AES analysis of a NIST sample is presented as an example of using NIST samples for verification of a technique.