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It has been 25 years since ICP optical emission spectrophotometers (ICP-OES) began to be widely used, and is now one of the most versatile methods of inorganic analysis. Its features are often compared to atomic absorption spectrophotometers. Compared to atomic absorption spectrophotometers, in which the excitation temperature of air-acetylene flame measures 2000 to 3000 K, the excitation temperature of argon ICP is 5000 to 7000 K, which efficiently excites many elements. Also, using inert gas (argon) makes oxides and nitrides harder to be generated.
ICP, abbreviation for Inductively Coupled Plasma, is one method of optical emission spectrometry. When plasma energy is given to an analysis sample from outside, the component elements (atoms) are excited. When the excited atoms return to low energy position, emission rays (spectrum rays) are released and the emission rays that correspond to the photon wavelength are measured. The element type is determined based on the position of the photon rays, and the content of each element is determined based on the rays' intensity.
To generate plasma, first, argon gas is supplied to torch coil, and high frequency electric current is applied to the work coil at the tip of the torch tube. Using the electromagnetic field created in the torch tube by the high frequency current, argon gas is ionized and plasma is generated. This plasma has high electron density and temperature (10000K) and this energy is used in the excitation-emission of the sample. Solution samples are introduced into the plasma in an atomized state through the narrow tube in the center of the torch tube.
The following features of ICP-OES distinguish it from atomic absorption spectrophotometers used for similar purposes.
The majority of the above features are derived from the structure and characteristics of the light source plasma.
Equipment for ICP optical emission spectrometry consists of a light source unit, a spectrophotometer, a detector and a data processing unit. There are several types of equipment based on differences in the Spectrophotometer and the detector. The most common type is shown in Figure 1.
A spectrophotometer with a Czerny-Turner monochrometor, and a detector with a photomultiplier is most common for this type. With this equipment, programmed wavelength of the spectrophotometer is consecutively varied to measure multiple elements. This causes rather long measuring time, however, with its high resolution spectrophotometers, it is favorable for measurement of high-matrix samples.
Figure 1: Sequential Type ICP-OES
This type typically uses an echelle cross disperser in spectrophotometers and semi-conductor detector such as CCD for the detector. Echelle cross disperser disperses light of measurable wavelength range two-dimensionally by combining prism and echelle diffraction grating. Combination of echelle cross disperser and a CCD detector enables multi-element measurement at any wavelength. The most notable feature of this equipment is the high-speed measurement, providing information on all 72 measurable elements in measurements of 1 to 2 minutes normally.
Figure 2: Simultaneous ICP-OES
One major application field of ICP optical emission spectrometry is material analysis. The below example is an analysis of a steel sample.
Chart1: Analysis result of high tensile structural steel series
|Element||Certified value||Measured value||Certified value||Measured value||Certified value||Measured value|
Bastnasite is a mineral of lanthanide rare earth type. There are many issues with analyzing rare earth with atomic absorption spectrophotometers, such as difficulty to obtain a light source lamp and difficulty with atomization due to oxides easily generated from rare earths, but these problems are eliminated for the most part with ICP-OES, which is widely used in this field. One major difficulty with analyzing rare earths is that they tend to have a high number of emission rays and has a high possibility of interference by nearby rays on the analysis wavelength.
Chart 2: Analysis result of Bastnasite
Human hair has attracted attention because it is thought to contain a person's health history on some level and is thought to act as an excretory organ for heavy metal in the body. However, there are problems because there are few usable samples and knowledge about multiple elements is required. With simultaneous analysis equipment, we can collect useful information with a small amount of sample.
ICP optical emission spectrometry is now highly rated as a multipurpose analysis technique and there are over 2,000 units of ICP-OES in use in Japan. It is well regarded as an environmental measurement technique, along with atomic absorption spectrometry and ICP mass spectrometry, and its use is expected to expand even further in the future.