(**a**) R2 of linear calibration curves at 3.8 J/cm2.

(**b**) R2 of linear calibration curves at 6.4 J/cm2.

**Figure 9.** Results of 5-fold cross-validation of Ca data for PLSR (2 components).

By comparing the linear relationship between the laser energy of 3.8 J/cm<sup>2</sup> and 6.4 J/cm2, increasing the output energy was not determined to result in a higher number of spectral lines with good linearity. The appropriate laser energy should be selected according to the experimental results.

As shown in Table 1, the mixing ratio of the two Ca ion compounds in samples #2-1 through #2-6 was different, but the linear relationship between the concentration and the intensity was not affected. Increasing the number of compounds containing the same cation did not influence the concentration relationship in the LIBS analysis of Ca.

In the laser-induced plasma generation process, the target material and the ions and atoms excited by the gas molecules in the air together constituted a component in the laser-plasma. The pulsed laser ablated the surface of the sample, and the energy was sufficient to cause the surface temperature to rise, melt, and evaporate to cause ion bond rupture and the further formation of laser-plasma. Both the Ca ions in CaCO3 and CaSO4 were ionized after reaching laser energy sufficient to form an ionic state

of Ca. When the ionized Ca acquired enough energy, it transitioned from the ground state to various excited states and then rapidly transitioned back to form atomic emission lines and ion lines. If the concentration of the ionic state was the same, then the atomic emission spectrum was not a ffected. This disregarded the compound type or mixture of the original state, which was similar to results from Na analysis.
