2.4. Spectral acquisitions
Soil FTIR-PAS spectra were measured using an FTIR spectrometer (Nicolet 6700, ThermoFisher, USA), equipped with an MTEC model 300 PA accessory. A nitrogen purge was continuously done in the optical system to eliminate the interferences from CO2 and water vapor. Approximately 5 g soil samples were placed in the photoacoustic cell and the dry helium was then purged into the cell for 10 s. The spectrum of the reference black carbon was used as background to correct the soil spectra. Each spectrum between the wavenumbers of 4000 and 650 cm−1 with a resolution of 8 cm−1 was finally recorded by averaging 32 successive scans under moving mirror velocity of 0.32 cm s−1.
Soil FTIR-ATR spectra were acquired using an attenuated total reflectance infrared spectrophotometer (Agilent 4300, Agilent Technologies Inc., USA). Approximately 2 g soil samples were placed on the FTIR-ATR crystal and then were compressed for measuring. Each spectrum was finally recorded by averaging 64 successive scans between the wavenumbers of 4000 and 650 cm−1 with a resolution of 0.466 cm−1. A background spectrum of air was measured to correct the soil spectra before sample screening.