## 3 Experimental Results and Discussion  
The quantitative analysis of arsenic (As), manganese (Mn), and lead (Pb) was achieved by constructing a calibration curve for each element and plotting the emission line intensity against the concentration determined by ICP-MS. The specific emission lines used in these experiments were As at 228.812 nm, Mn at 403.30 nm, and Pb at 405.78 nm. Identifying an appropriate internal standard for the soils analyzed was challenging given the high element content, a wide range of concentrations, and variability in physical and chemical properties such as texture, particle size heterogeneity, organic matter content, and thermal properties. Therefore, the chosen methodology for calibration curve construction involved normalizing the spectral intensity to the overall (integral) plasma emission signal in the 200-850 nm spectral range. This approach is acceptable by the consistent experimental conditions across all experiments and is further validated by the good linearity observed in the calibration curves. The calibration linearity was confirmed by a correlation coefficient higher than 0.96 for Mn and higher than 0.98 for As and Pb. The limit of detection was calculated using the following equation [25; 26; 27; 28]:  
\[LoD=3.3\frac{\sqrt{\sigma_{BG}^{2}+\sigma_{a}^{2}}}{b}, \tag{1}\]  
where \(\sigma_{BG}\) is the standard deviation of the sample's signal with zero concentration (in our experiments, we measured this deviation as a deviation of the background outside emission lines); \(\sigma_{a}\) is the deviation of intercept in the calibration curve (\(y=a+bx\)). At the initial creation of laser-induced plasma, a broad spectrum of continuum emission is observed due to Bremsstrahlung radiation. This phenomenon can mask the analyte emission lines until the plasma expands and its temperature decreases. Thus, selecting the correct detector gate delay and gate width is essential to acquire optimal line emissions while minimizing the continuum radiation. Through our experiments, we have found that the optimal parameters for manganese (Mn) detection were a 1.5 \(\upmu\)s delay time, a 5 \(\upmu\)s gate width, and an accumulation of 100 pulses. In the case of arsenic (As) and lead (Pb), different settings were required due to the distinct parameters associated with plasma formation in an argon atmosphere. Our findings indicated that the following parameters were optimal for As and Pb detection: a 2 \(\upmu\)s delay time, a 30 \(\upmu\)s gate width, an accumulation of 10 pulses, and 100 averages to enhance the signal-to-noise ratio. For Mn, the LIBS spectra were recorded in an ambient atmosphere, while for Pb and As, the spectra were recorded in an argon atmosphere at ambient pressure. Initially, the LIBS system chamber, with a volume of 100 L, was purged with argon at a rate of 13 L/min for 30 min before measurements commenced. Following this, LIBS spectra were measured under continuous argon purging at the same rate to maintain consistency and accuracy in our observations. Figures 3 and 4 present the plasma emission spectra generated by the fundamental harmonics of an Nd:YAG laser in soil samples. The spectra show emission lines of As(I) at 228.812 nm, Pb(I) at 405.78 nm, and Mn(I) at 403.3 nm. Figure 5 displays the calibration curves and measurements for pristine soil samples of As(I) at 228.812 nm, Pb(I) at 405.78 nm, and Mn(I) at 403.3 nm. The points represented by black squares were used for the linear fit. In contrast, although part of the calibration measurements, the red squares were excluded from the linear fit due to saturation effects common in LIBS measurements at high analyte concentrations. The points depicted as green triangles correspond to the measurements of pristine soil samples. Figure 4: Emission spectra of plasma produced with fundamental harmonics of Nd:YAG laser in soil samples, showing emission lines of (**a**) As(I) at 228.812 nm, (**b**) Pb(I) at 405.78 nm, and (**c**) Mn(I) at 403.3 nm.