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The most commonly used techniques for the determination of trace concentrations of elements in samples are based on atomic emission spectrometry (AES). To dissociate sample molecules into free atoms, thermal sources such as flames, furnaces and electrical discharges are used.
More recently, other types of electrical discharges, namely plasmas have been used as atomization/ excitation sources for AES. These techniques include inductively coupled plasma (ICP) and direct coupled plasma (DCP).
Plasma sources offers several advantages compared with flame and electrothermal methods. The advantages are that it is a multi-element technique and it has wide range. Current plasma sources (DCP) provide a much easier method of handling liquid and gaseous samples. Spectra for dozens of elements can be recorded at the same time which is important when the sample is very small. Plasma sources also permit determination of non-metals such as chlorine, bromine, iodine and sulfur.
Inductively coupled plasma
An inductively coupled plasma can be generated by directing the energy of a radio frequency generator into a suitable gas, usually ICP argon. Other plasma gases used are Helium and Nitrogen. It is important that the plasma gas is pure since contaminants in the gas might quench the torch.
Coupling is achieved by generating a magnetic field by passing a high frequency electric current through a cooled induction coil. This inductor generates a rapidly oscillating magnetic field oriented in the vertical plane of the coil. Ionization of the flowing argon is initiated by a spark from a Tesla coil. The resulting ions and their associated electrons from the Tesla coil then interact with the fluctuating magnetic field. This generates enough energy to ionize more argon atoms by collision excitation. The electrons generated in the magnetic field are accelerated perpendicularly to the torch. At high speeds, cations and electrons, known as eddy current, will collide with argon atoms to produce further ionization which causes a significant temperature raise. Within 2 ms, a steady state is created with a high electron density. A plasma is created in the top of the torch. The temperature within the plasma ranges from 6,000-10,000 K. A long, well-defined tail emerges from the top of the high temperature plasma on the top of the torch. This torch is the spectroscopic source. It contains all the analyte atoms and ions that have been excited by the heat of the plasma.
The success of ICP leans on its capability to analyze a large amount of samples in a short period with very good detection limits for most elements.
ICPs used in the market today are often connected to different detection systems, such as ICP mass spectrometry and ICP atomic emission spectrometry.
Direct coupled plasma
A direct-current plasma (DCP) is created by an electrical discharge between two electrodes. A plasma support gas, commonly ICP argon, is necessary. Samples can be deposited on one of the electrodes, or if conducting can make up one electrode. Insulating solid samples are placed near the discharge so that ionized gas atoms sputter the sample into the gas phase where the analyte atoms are excited. This sputtering process is often referred to as glow-discharge excitation.
A nebulizer converts the sample to an aerosol that is introduced into the excitation area of the plasma.
The plasma jet source is made of three electrodes formed like a tripod. In each arm there is a graphite anode and at the inverted base, a tungsten cathode is located. A high-velocity inert gas, usually ICP argon, produces a high temperature plasma and separates the excitation region from the analytical observation zone. The excitation area is situated in the crook of the tripod and it has a temperature of 6,000 K. To increase the current density and thus the plasma temperature it is necessary to squeeze the plasma in order to decrease the current cross section. This is accomplished by cooling the edges of the plasma with a high-velocity inert gas.
The analyzer is either a mono- or polychromator.
A photomultiplier converts radiant energy to measurable signals.out
Atomic Emission Spectroscopy
Inductively coupled plasma - atomic emission spectroscopy is a type of emission spectroscopy that uses the inductively coupled plasma to produce excited atoms and ions to emit electromagnetic radiation at wavelengths characteristic of a particular element. The intensity of this emission is indicative of the concentration of the element within the sample.
Examples of ICP-AES applications include the determination of small quantities of metalic compounds in wine, arsenic in food, trace elements in soil and trace elements bound to proteins. It has also be referred to as inductively coupled plasma optical emission spectrometry (ICP-OES), where it is widely used in minerals processing to provide the data on grades of various ore streams for the construction of mass balances.
This analytical method is also referred to as Inductively Coupled Plasma Atomic Emission Spectrophotometry and Inductively Coupled Plasma - Atomic Emission Spectrometer.