Study Of The Local Thermodynamic Equilibrium State Of The Plasma In Laser Induced Breakdown Spectrum (LIBS)

The technology of the laser induced plasma spectroscopy can be dated back to the invention of the laser in1960s. Along with development of understanding of the characteristic and evolution behavior of the plasma, Laser Induced Breakdown Spectrum has been more widely applied in many fields. Not only can it be used to detect metal elements, it can also be used to detect the nonmetal elements. Compared to other element detection methods such as optical emission spectrometry, X-ray fluorescence spectrograph, inductively coupled plasma atomic emission spectrometry, the LIBS technology is more adapted to real-time, online, and rapid detections. Until now the LIBS technology has been successfully applied to detect elements in alloys, coal and to determine ingredients in Mars.The principle of the LIBS technology is based on spectral analysis of the plasma emission induced by high-energy laser pulses. Elements in the sample to be detected can be analyzed qualitatively and quantitatively through this method. Qualitative analysis is quite convenient and it can be realized by recognizing characteristic spectral lines emitted by elements of interest contained in the detected objects. However, it is It is very conveniently to qualitative detection whether the detected objects contain a special element relying on the spectrum. If the spectra include the characteristic line of the element it manifest that the detected objects contain this element. However, it is very difficult to determine the element concentrations due to the complexity of plasma formation of plasma and the matrix effects of the detected objects. In general, the formation of the plasma is dependent on the external experimental conditions and can be divided into three stages; breakdown, expansion and confinement. And only when the plasma stays in the confinement stage and reaches the Local Thermodynamic Equilibrium (LTE) state, the spectra of the plasma can be used to quantitatively determine the concentrations of some elements. Until now, the most common standard used to determine whether the plasma is in the LTE state is the McWhirter criterion, but the recent research shows that it is only a sufficient condition, in order to ensure the establishment of the plasma in the LTE condition another condition must be added. In order to deepen the understanding of the evolution of plasma LTE state, we should take the various physical and chemical reactions within the plasma into consideration. In generally, accurately determining the concentrations of the elements is the most difficult problem faced by all plasma spectrum detection technology, until now there are many methods tackling the matrix effects such as internal method, external method and Calibration-Free (C-F) method, among these methods, the method which is most suitable to the changing detection environment is the C-F method. However, it is not enough to detect the analyte which containing various elements based on analysis of a single line of one element. Therefore, there are many proposals devoting to distinguish the detected object based on the spectra. And the most popular method is the Partial Least Squares (PLS) which has been used to classify biochemical bacteria and minerals.