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.