2.3. Spectral measurements
Soil samples were pressed flat against the diamond for improved contact, and a blank reference was used before each sample was scanned. ATR spectra were detected using a handheld attenuated total reflectance infrared Fourier transform spectrometer (Trudefender FT, Thermo Scientific, Waltham, MA, USA). The spectral scanning region was 4000 to 600 cm−1, with a spectral resolution of 4 cm−1. The background noise was subtracted from each scan to correct for atmospheric and instrumental effects.
LIBS spectroscopy was performed using a MobiLIB system (IVEA, Orsay, Essonne, France) with a fourth-harmonic Nd: YAG laser operating at 266 nm, with a 5-ns pulse energy of 16 mJ per pulse, and a high-resolution CCD (Sony ILX511B, Tokyo, Japan) spectrometer (Mechelle 5000 Echelle, Andor Technology, Belfast, Northern Ireland) ranging from 200 to 975 nm. Samples were pressed into pellets with a 1 cm diameter and 0.2 cm height. The beam diameter at the focal point was 20 μm. The shot points were 4 × 4 matrices with 4 layers of shots at each point (Fig. 2). The ablation spot after each shot was approximately 150 μm. The distance between two horizontal points was 0.5 mm. As the study was focused on spectra at the micro-level, and the intensity of LIBS spectra can be affected by the shots, we only obtained three shots at the vertical level.