Quantitative Analysis by LIBS

The quantitative aspects of LIBS even for laboratory applications may be considered its Achilles heel, first due to the complex nature of the laser-sample interaction processes, which depends upon both the laser characteristics and the sample material properties, and second due to the plasma-particle interaction processes, which are space and time dependent. All those parameters influence strongly on plasma conditions and, consequently, on emission lines intensities. Consequently it may sound correct, that LIBS is a mix of great potential and severe limitation, that one of the major problems that precludes its more quantitative use is a lack of reproducibility of spectra at a given wavelength on a shot to shot basis, and that the sensitivity remains modest, precision mediocre and matrix dependence strong (Hahn and Omenetto 2012). 

Many models and techniques have been developed in order to overcome such problems. Usually, calibration curves are needed for the quantitative determination of the elements. They emerge from a number of reference samples with known elemental composition, but a limit is set to the applicability of the method, because a similar composition for the unknown sample is also required. This can be good for laboratory measurements, where reference and unknown samples can be embedded in the same matrix, but is a severe restriction for field experiments. Thus, it was concluded that field experiments are limited to a semi-quantitative analysis in case of highly variable or unknown rock compositions. 

It sounds very pessimistic but LIBS has been shown to be capable of performing quantitative analysis at a level considered adequate for the specific analytical task investigated. Therefore, the problem is not that LIBS cannot perform quantitative analysis, but rather how to make LIBS become as accurate and reliable as the other spectroscopic methods. It will be demonstrated, that in many industrial online applications LIBS is capable to give the quantitative analytical data which are accurate enough for process control tasks (Gaft et al. 2014a). 

Nevertheless, strong limitations are also typical for LIBS as an analytical technique. The major drawback of LIBS in industrial quantitative applications is related to the stability of the spectroscopic plasma signals, which undergo considerable fluctuations that originate from the very nature of the method. The following factors are the mostly important in real fife scenarios. 

Real industrial application usually means that the uncontrolled mixture of several minerals present. The meaning is that a significantly different signal response is observed at a given spectroscopic transition of an element present at the same concentration level in several different samples. Such different response can be attributed to differences in the laser-sample interaction, resulting from changes in the ablarirm mechanism and efficiency. Thus the nature of the sample has a 

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It has to be noticed here that quantitative elemental analysis by pLlBS is still on progress for the moment. The images in Fig. 9.2 are only normalized intensity maps of multi elements. The intensity variation of element in the crystalline phase and in the residual glass is due to the increase of image contrast, which is less likely linked to inhomogeneous element distribution of in each phase. To further improve the ability of quantitative analysis by LIBS, more experiments are required, including calibration of element concentrations and determination of analyzed sample... 

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