Plasma (Plasma) term was first proposed by Langmuir in 1929, now generally refers to the degree of ionization of more than 0.1% of the gas is ionized, the gas not only contains neutral atoms and molecules, but also contains a large number of electrons and ions, and the concentration of electrons and positive ions is in a state of equilibrium, as a whole, it is in the neutral. From a broad sense like the flame and the high temperature part of the arc, spark discharge, the sun and the ionosphere on the surface of the star are plasma.
Plasma can be divided into two categories according to temperature: high-temperature plasma and low-temperature plasma. When the temperature is as high as 106-108K, all the atoms and molecules of the gas are completely dissociated and ionized, which is called a high-temperature plasma; when the temperature is lower than 105K, the gas is partially ionized, which is called a low-temperature plasma.
In practical applications, low-temperature plasma is divided into hot plasma and cold plasma. When the gas pressure in 1.013X105Pa (equivalent to 1 atmospheric pressure) or so, the particle density is larger, the electron concentration is high, the average free range is small, the collision between the electron and the heavy particles are frequent, the electron from the electric field to obtain the kinetic energy is quickly transferred to the heavy particles, so that a variety of particles (electrons, positive ions, atoms, molecules) of the thermal movement of the energy tends to converge, the entire gas to enter or reach a thermodynamic equilibrium state, at this time, gas Temperature and electron temperature is basically equal, the temperature is about thousands of degrees to tens of thousands of degrees, this plasma is called thermal plasma. For example, direct current plasma flame (DCP) and inductively coupled plasma torch (ICP), etc. are thermal plasma, if the discharge gas pressure is lower, the electron concentration is smaller, the electrons and heavy particles collision opportunities are less, the kinetic energy of electrons from the electric field is not easy to produce an exchange of heavy particles, the kinetic energy between them is a large difference in the average kinetic energy of the electrons up to dozens of e-volts, and the gas temperature is lower, so the Plasma in non-thermodynamic equilibrium system, called cold plasma, such as Grim glow discharge, hollow cathode lamp discharge.
In the spectral analysis of the so-called plasma light source, usually refers to the appearance of a class of discharge light source similar to the flame. Three types are most commonly used at present: i.e., inductively coupled plasma torch (ICP), direct current plasma flame (DCP) and microwave inductive plasma torch (MIP). For MIP, although it allows micro-injection, small gas consumption, low power, and easy determination of non-metals, it has poor detection limits for most metals, serious inter-elemental interference, and requires helium, so it is mainly used as a detector for chromatographic analysis.
ICP and DCP, two types of plasma light source with better analytical performance, have been applied to atomic emission spectrometer.
The pioneers of the inductively coupled plasma atomic emission spectroscopy (ICP-AES) technique were Greenfiald and Fasel, who published their results in 1964. The technique made real progress after the seventies, and in 1974 the first commercial inductively coupled plasma atomic emission spectrometer was developed by Leeman in the United States.
The main advantages of the ICP source are:
1)? Low detection limit: 1ug/L for many elements
2)? Wide dynamic range of measurement: 5-6 orders of magnitude
3)? Good accuracy
4)? Small matrix effect: ICP is a high-temperature excitation light source with 6000-7000K, the sample is chemically processed, the analysis of the standard series is very easy to formulate with the sample solution in the acidity, matrix composition, total salinity and other properties of the solution is very similar. At the same time, the high energy density of the light source, special excitation environment - channel effect and excitation mechanism, so that the ICP light source has the outstanding advantages of small matrix effect.
5)? High precision: RSD~0.5%
6)? Short exposure time: generally only 10-30 seconds
7)? Atomic emission spectrometry has the characteristics of simultaneous analysis of multiple elements and other analytical methods for individual determination of each element compared to the efficiency of the economy, technology and other aspects have great characteristics. This is one of the reasons why ICP atomic emission spectrometry has made great progress.