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.