Shortcomings of LIBS

Plasma analysis is known to be subject to spectroscopic disturbances including plasma line instability, self-absorption, line broadening, slit problems and grating imperfections. 

One of the shortcomings of LIBS, particularly in relation to quantitative elemental analysis, arises from the instability of the laser-induced plasma emission resulting from laser intensity fluctuations (1-5%) the amount of scattered light present depends on local matrix effects and on physical and chemical properties of the target material. The most common way of compensating for signal fluctuations in LIBS is by calculating the ratio of the spectral peak intensity to that of a reference intensity. However, this internal calibration method provides relative rather than absolute concentrations. 

One of the major drawbacks of the LIBS technique is the poor reproducibility of shot-to-shot spectra. This problem arises from various sources including laser instability variations in lens-to-lens distance optic coating or destruction matrix effects (e.g. of the sample composition, degree of homogeneity, colour, moisture content) and the atmospheric conditions. 

The lack of homogeneity of the sample is a major source of variance. The laser samples an average area of 1 mm% which is not sampled uniformly as different parts are exposed to different temperatures. A routine measurement typically involves about 100 laser pulses, so the examined area is in the region of 1 cm-. Inhomogeneities influence laser absorption, the mass ablated and the plasma conditions. On the other hand, there can also be substantial inhomogeneities in the standard reference samples that will pose problems with analyses. These problems become even more severe if the laser is focused more tightly in order to sample smaller amounts of material. 

One other intrinsic problem of LIBS stems from optical instability as a result of the distance of the surface from the lens varying by effect of the usual roughness of the surface. As a result, the location of the spark relative to the collecting fibre also varies. In addition, the fraction of reflected light changes from pulse to pulse owing to differences in impact angle on the microscopic level. Optical instability causes fluctuations in the ablation process, the plasma profile and the inspected volume of plasma. 

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