Optical Emission Spectroscopy (OES)

Introduction

Optical Emission Spectroscopy is founded on the fact that each element gives forth light at a wavelength under stimulation. In OES, the sample atoms are excited by an energy source that can be an electric arc, spark, or plasma to liberate photons. The photons at these wavelengths are separated with a spectrometer and are measured with the aim of ascertaining the elemental composition of the sample. OES is highly appreciated due to fast analysis, little or no sample preparation required, and quantification of the concentration of both major components and trace impurities. It is used in steel manufacturing and foundry industries, and aerospace, automotive, and environmental applications. 

Principle and Methodology

In OES, high-energy sources excite atoms in the sample to an elevated energy level. As these atoms regain their ground state, they produce light of wavelengths characteristic of each of the elements. The window is then read by a spectrometer of the emitted light into its component wavelengths and the intensity of each wavelength at a detector, quantifying how much of each element there is. The procedure would require the sample to be placed on a discharge stand and the excitation source started (spark, arc, or plasma), followed by collecting the emission spectrum and data processing using calibration curves to determine the correct value of concentration.

Instrumentation

The OES system is usually comprised of a few important components that cooperate to realize accurate element analysis. They are excited by the excitation source, which is a photoexcitation source, sometimes a high-voltage spark or arc, which excites the atoms in the sample. The light is fed into an optical system, which usually contains a diffraction grating that breaks the light into its separate wavelengths. It is this dispersed light that is detected using light-sensitive devices such as photomultiplier tubes (PMTs) or charge-coupled devices (CCDs) that convert this light signal to an electrical signal. Such signals are analyzed by a computer system that determines and measures the elements based on calibration information and libraries of spectra.

Strengths

OES has a fast, multi-element performing analysis, which is accurate and repeatable. It can identify a broad variety of components, which include light components like carbon, nitrogen, and sulfur in metals. It has limited sample preparation requirements, can accommodate solid metallic samples in their raw form, and is continuous with immediate real-time results that apply to in-process quality control. It is especially useful in the identification of alloys, confirmation in terms of industry standards, and trace detection of impurities.

Limitations

OES is quite sensitive and applicable to a wide variety of samples, but tends to work best with solids, namely conductive samples, such as metals. Non-metallic materials, which are not given special preparation, are less ideal targets of the technique. Some light elements may have varied sensitivity, and precision needs to be on proper calibration and maintenance. Its use may also be hampered in field cases where access to a clean, preferred, flat sample surface may be inadequate. Moreover, the initial cost of equipment and its continuous maintenance can be rather high in comparison to less complex methods of analysis.