## 3 Experimental Results and Discussion
The other lines were assigned as follows: (**a**) 228.408 nm Fe(I), 228.733 nm Fe(I), 228.998 nm Ni(I), 229.119 nm Fe(I), 229.268 nm Sb(I); (**b**) 405.067 nm Fe(I), 405.486 nm Fe(I), 406.026 nm Ti(I), 406.359 nm Fe(I), 406.508 nm Mn(I); (**c**) 402.962 nm Fe(I), 403.076 nm Mn(I), 403.449 nm Mn(I), 403.573 nm Mn(I). Figure 5: Calibration curves and measurements of pristine soil samples of (**a**) As(I) at 228.812 nm, (**b**) Pb(I) at 405.78 nm, and (**c**) Mn(I) at 403.3 nm. The points shown as black squares were used for the linear fit, while the red squares belong to the calibration measurements but were excluded from the linear fit due to the effect of saturation, typical in LIBS measurements for high analyte concentrations. The points shown as green triangles correspond to measurements of pristine soil samples. The linear range was 50–587 mg/kg for As, 14–214 mg/kg for Pb, and 188–488 mg/kg for Mn. The statistical parameters for regression are shown in Table 1.  
Figure 3: Characteristic LIBS spectrum of soil samples in the 200–800 nm wavelength range. At low analyte concentrations, calibration curves usually exhibit a linear dependence. However, they start to deviate from linear dependence and experience saturation [29; 30]. The points that deviate from linearity are usually excluded from the linear fit. This effect was especially noticeable in Mn calibration measurements because we had to use relatively high concentrations, in thousands of mg/kg, due to the high abundance of Mn in soil. To assess the analytical capability of our LIBS system, we explored the linearity of calibration curves with analyte concentration and determined the limit of detection using expression (1). The detection limit (LoD) was calculated as 29.5 mg/kg for lead (LoDPb = 29.5 mg/kg), 95.5 mg/kg for arsenic (LoDAs = 95.5 mg/kg), and 327 mg/kg for manganese (LoDMn = 327 mg/kg). We have found that under an argon atmosphere, arsenic and lead signals were enhanced approximately threefold compared to the ambient atmosphere. This significant (approximately threefold) enhancement enabled the limit of detection (LoD) of the LIBS system to reach the double-digit range for both As and Pb. Consequently, this improvement allowed us to accurately measure the concentrations of As and Pb in practically all soil samples collected from the affected area. This capability is particularly valuable for comprehensive environmental assessments, ensuring that even low concentrations of these elements can be detected and quantified with relatively good precision. Table 2 shows the permissible amounts of As, Pb, and Mn in soil, as established by the United States Environmental Protection Agency (EPA). As we can see, the detection limit of our LIBS system is reaching the safety level due to the improvement of the collection system and additional purging with argon during measurement. We used soil that visually consisted of sand, clay, and loam to create samples for calibration curves. However, some samples presumably contained a large amount of coal, considering the place of origin close to the coke factories; these samples produced a signal about twofold weaker, likely due to a considerably larger plasma plume, which was visually observable. This larger plasma plume particularly affected the coupling efficiency of the detection system due to its size. However, normalizing the spectra by the integral signal of the plasma over the entire spectral range substantially mitigated this matrix effect, as seen in the Mn detection case.