## 3 Results and Discussion  
### Spectral and PC Analysis  
Figure 1 shows representative reflectance spectra of soil matrices used in this study without petroleum added. A KBr/10% _w/w_ petroleum standard is also included for reference. All soil spectra, namely, sea sand (sand), bentonite (bent), montmorillonite (mont), and red and brown soils, were acquired without significant difficulty, producing the distinctive signatures in 1000 to 1600 cm\({}^{-1}\) region. Sea sand samples do not provide any noticeable signal spectral reflectance in 1300 to 1600 cm\({}^{-1}\) region because the sand samples absorb much of the IR light in this region and a poor reflectance spectrum is produced. Therefore, sand spectra were scaled by a factor of 10 (dotted blue trace) and are included in Figure 1 to provide a profile of sea sand spectral details obtained by QCL from 1300 to 1600 cm\({}^{-1}\). Nevertheless, sand displays a sharp spectral contrast in the region from 1000-1300 cm\({}^{-1}\) with an intense signal at 1158 cm\({}^{-1}\) that is associated with Si-O vibrational stretching mode [57]. Spectral interpretations and band assignments in the fingerprint region (1000 to 1600 cm\({}^{-1}\)) for soil samples are challenging because of the various contributing soil signatures can overlap due to the presence of minerals or to interactions of organic chemicals [58; 59]. According to the report by  
Figure 1: Representative reflectance spectra of the various samples without petroleum contaminants: sea sand (sand), montmorillonite (mont), bentonite (bent), brown soil, red soil, and KBr/10%-Petrol reference sample. [PERSON] et al. 1998, most of the peaks below 1300 cm\({}^{-1}\) can be attributed to clay minerals. In the case of soils with dominant organic matter compositions, strong alkyl peaks (1449 cm\({}^{-1}\)) and strong broad bands due to carboxylic-protein-aromatic species (1750-1449 cm\({}^{-1}\)) can also be present. The standard reference spectrum of 10% petroleum in KBr shown in Figure 1 (solid orange trace) is where the most important petroleum signals are located. The weak signal at 1019 cm\({}^{-1}\) corresponds to twisted CH\({}_{2}\), the weak signal at 1030 cm\({}^{-1}\) is associated with C-C symmetric stretch. The medium-reflectance signal at 1156 cm\({}^{-1}\) is attributed to the combination band of CH\({}_{3}\) rocking + C-C stretching [60]. For a suitable identification of the signals, it is necessary to apply an ash spectral subtraction to the entire soil spectrum. Performing detection means that the soil to be analyzed does not require any form of physical separation, i.e., the soil remains in its natural way as much as is possible. For this reason, the detection of petroleum traces focused mainly on natural soils, such as red and brown soils, and the addition of synthetic sea sand produced one of the most challenging lands to analyze due to its ability to absorb infrared light. Nevertheless, in an attempt to get closer to in situ detection, this step was omitted. A PCA model was generated to classify the soil types. The scores plot of principal component 1 (PC 1) vs. PC 2 is shown in Figure 2. PC 1 accounts for 47.27% of the variation and can be used for differentiation between sand and clay. PC 1 also allowed the separation between clayey soil (e.g., mont, bent, and red soil) vs. sandy soil (e.g., brown soil and sand) can also be observed. The signal near 1020 cm\({}^{-1}\) in red soils was attributed to aluminosilicate lattice vibrations. Besides, the signature near 1100 cm\({}^{-1}\) noted for mont, bent, and red soils were assigned to the fundamental silicate stretching mode or to (O-Al-O) stretching deformations that produce similar intense reflectance bands. PC 2 can differentiate between the various types of clayish soils and indicates that red soils contain high proportions of bentonite.