Optical Emission Spectroscopy, or OES, is a well trusted and widely used analytical technique used to determine the elemental composition of a broad range of metals.
The type of samples which can be tested using OES include samples from the melt in primary and secondary metal production, and in the metals processing industries, tubes, bolts, rods, wires, plates and many more.
The part of the electromagnetic spectrum which is used by OES includes the visible spectrum and part of the ultraviolet spectrum. In terms of wavelength, that’s from 130 nanometers up to around 800 nanometers.
OES can analyze a wide range of elements from Lithium to Uranium in solid metal examples covering a wide concentration range, giving very high accuracy, high precision and low detection limits.
The elements and concentrations that OES analyzers can determine depend on the material being tested and the type of analyzer used.
How does Optical Emission Spectroscopy work?
All OES analyzers contain three major components, the first is an electrical source to excite atoms within a metallic sample so that they emit characteristic light, or optical emission, lines – requires a small part of the sample to be heated to thousands of degrees Celsius. This is done using an electrical high voltage source in the spectrometer via an electrode. The difference in electrical potential between the sample and electrode produces an electrical discharge, this discharge passes through the sample, heating and vaporizing the material at the surface and exciting the atoms of the material, which then emits the element-characteristic emission lines.
Two forms of electrical discharge can be generated, either an arc which is an on/off event similar to a lightning strike, or a spark – a series of multi-discharge events where the voltage of the electrode is switched on and off. These two modes of operation are used depending on the element measured and the accuracy required.
The second component is an optical system. The light, the multiple optical emission lines from the vaporized sample known as a plasma pass into the spectrometer. A diffraction grading in the spectrometer separates the incoming light into element-specific wavelengths and a corresponding detector measures the intensity of light for each wavelength. The intensity measured is proportional to the concentration offset element in the sample.
The third component is a computer system. The computer system acquires the measured intensities and processes this data via a predefined calibration to produce elemental concentrations. The user interface ensures minimal operator intervention with results clearly displayed which can be printed or stored for future reference.
So how do we generate element-specific optical emission lines from a metallic sample?
When the energy of an electrical discharge interacts with an atom, some of the electrons in the atom’s outer shells are ejected. Outer-shell electrons are less tightly bound to the nucleus of the atom because they are further away from the nucleus and so require less input energy to be ejected. The ejected electrons create a vacancy making the atom unstable.
To restore stability, electrons from higher orbits further away from the nucleus drop down to fill the vacancy. The excess energy released as the electrons move between the two energy levels or shells is emitted in the form of element-specific light or optical emission.