Atomic Emission Spectroscopy (AES, OES)

Introduction

Atomic emission spectroscopy (AES or OES [optical emission spectroscopy]) uses quantitative measurement of the optical emission from excited atoms to determine analyte concentration. Analyte atoms in solution are aspirated into the excitation region where they are desolvated, vaporized, and atomized by a flame, discharge, or plasma. These high-temperature atomization sources provide sufficient energy to promote the atoms into high energy levels. The atoms decay back to lower levels by emitting light. Since the transitions are between distinct atomic energy levels, the emission lines in the spectra are narrow. The spectra of samples containing many elements can be very congested, and spectral separation of nearby atomic transitions requires a high-resolution spectrometer. Since all atoms in a sample are excited simultaneously, they can be detected simultaneously using a polychromator with multiple detectors. This ability to simultaneously measure multiple elements is a major advantage of AES compared to atomic-absorption (AA) spectroscopy.

Schematic of an AES experiment

Instrumentation

As in AA spectroscopy, the sample must be converted to free atoms, usually in a high-temperature excitation source. Liquid samples are nebulized and carried into the excitation source by a flowing gas. Solid samples can be introduced into the source by a slurry or by laser ablation of the solid sample in a gas stream. Solids can also be directly vaporized and excited by a spark between electrodes or by a laser pulse. The excitation source must desolvate, atomize, and excite the analyte atoms. A variety of excitation sources are described in separate documents:

• Direct-current plasma (DCP)
• Flame
• Inductively-coupled plasma (ICP)
• Laser-induced breakdown (LIBS)
• Laser-induced plasma
• Microwave-induced plasma (MIP)
• Spark or arc

Picture of an inductively-coupled plasma atomic emission spectrometer