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Coating thickness in LIBS is mostly related to the number of laser shots penetrating through the coating, reaching the core of the tablet. At each laser shot (which indicates the depth of the coating), elemental composition can be determined, thereby giving the user a measure of the coating thickness qualitatively & quantitatively.
In this figure, the shot # is directly to the depth at which the laser is probing the sample. From the chart, one can see that the intensity of the Calcium line is very low on the outside of the tablet (coating) and increases as one gets closer to the core - which is to be expected as Calcium is a part of the core of the tablet. Simultaneously, the intensity of the titanium line, which is a part of titanium oxide in the coating of the tablet, decreases as the laser gets closer to the core.
SpectrolabSystems.net