Atomic Emission Spectroscopy 427 PHC Atomic Emission Spectroscopy

Introduction Atomic emission spectroscopy (AES) is based upon emission of electromagnetic radiation by atoms.

AES is an analytical method that uses the intensity of light emitted from atoms excited by a proper source at a particular wavelength to determine the quantity of an element in a sample. The wavelength of the atomic spectral line gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element.

AES uses quantitative measurement of the 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 an atomization sources. 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 all atoms in a sample are excited simultaneously, they can be detected simultaneously, and this is the major advantage of AES compared to AAS.

Schematic diagram of AES

AES Principle As in AAS, 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.

Since the atomic emission lines are very narrow, a high-resolution polychromator is needed to selectively monitor each emission line.

Excitation sources: Inductively coupled plasma source. Direct current plasma source. Arc and spark sources. Flame emission source.

Inductively coupled plasma atomic emission spectroscopy ICP-AES

ICP-AEC

ICP-AES is one of the most common techniques for elemental analysis. A plasma is an electrical conducting gaseous mixture containing a significant concentration of cations and electron. It is used to produce excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. All kinds of dissolved samples can be analyzed, varying from solutions containing high salt concentrations to diluted acids.

Advantages of ICP-AES: Excellent limit of detection. Linear dynamic range. Multi-element capability. Stable and reproducible signal.

Disadvantages: Spectral interferences (many emission lines). Cost and operating expense. samples typically must be in solution.

Principle: A plasma source is used to dissociate the sample into its constituent atoms or ions, exciting them to a higher energy level. They return to their ground state by emitting photons of a characteristic wavelength depending on the element present. This light is recorded by an optical spectrometer.

Diagram of ICP-AES

Instrumentation: The ICP-AES is composed of two parts: The ICP torch. The optical spectrometer. The ICP torch consists of 3 concentric quartz glass tubes. The output or "work" coil of the radio frequency (RF) generator surrounds part of this quartz torch. Argon gas is typically used to create the plasma. When the torch is turned on, an intense electromagnetic field is created within the coil by the high power RF signal flowing in the coil. This RF signal is created by the RF generator.

The plasma torch

The argon gas flowing through the torch is ignited with a Tesla unit that creates a brief discharge arc through the argon flow to initiate the ionization process. Once the plasma is "ignited", the Tesla unit is turned off. The argon gas is ionized in the intense electromagnetic field and flows in a particular rotationally symmetrical pattern towards the magnetic field of the RF coil.

A stable, high temperature plasma of about 7000 K is then generated as the result of the inelastic collisions created between the neutral argon atoms and the charged particles. A pump delivers an aqueous or organic sample into a nebulizer where it is atomized and introduced directly inside the plasma. The sample immediately collides with the electrons and other charged ions in the plasma and is broken down into charged ions. The various molecules break up into their respective atoms which then lose electrons and recombine repeatedly in the plasma, giving off radiation at the characteristic wavelengths of the elements involved.

Plasma Torch

One or two transfer lenses are then used to focus the emitted light on a diffraction grating where it is separated into its component wavelengths in the optical spectrometer. After the light is separated into its different wavelengths (colors), the light intensity is measured with a photomultiplier tube. The intensity of each line is then compared to previously measured intensities of known concentrations of the elements, and their concentrations are then computed by extrapolation along the calibration lines.

Optical Spectrometer