Soil sample processing and High-Throughput Mid-Infrared (MIR) spectroscopy analysis

This Standard Operating Procedure establishes a rigorous laboratory protocol for high-throughput diffuse reflectance mid-infrared Fourier transform spectroscopy (DRIFTS, 4000–400 cm⁻¹) of soils, enabling accurate chemometric prediction of key soil properties.

The workflow begins with ultra-fine mechanical comminution using a FRITSCH PULVERISETTE 14 variable-speed rotor mill fitted with a <0.2 mm (200 µm) sieve ring. The mill is operated at 6,000 rpm for coarse-textured soils and at 8,000–10,000 rpm for fine-textured soils, using approximately 6 g of soil for 1 minute. This procedure minimizes particle-size effects and specular reflection artifacts. Strict solvent cleaning and vacuum decontamination are performed between samples to prevent cross-contamination.

The powdered samples (~15–20 mg) are loaded into Bruker 96-well aluminum microplates in analytical quadruplicate, providing replicate spectra for reliable spectral outlier screening and robust chemometric averaging. The prepared plates are then equilibrated overnight in a desiccator to stabilize moisture-related absorption bands.

Instrumental analysis is conducted using a Bruker INVENIO S / TENSOR 27 FT-IR spectrometer coupled to an automated HTS-XT microplate module. The instrument is operated following an orderly power-up sequence, including 2–3 hours of optical and thermal stabilization, cryogenic cooling of the photovoltaic MCT detector with liquid nitrogen to 77 K (−196 °C), and automated instrument validation using the OPUS Validation Program (OVP).

Routine spectral acquisition is performed through Bruker OPUS/LAB. Before scanning, the optical check signal amplitude is verified to be within 15,000–30,000 counts, the 96-well sample coordinate map is configured, and pre-calibrated multivariate QUANT chemometric methods are loaded. The motorized HTS-XT carrier drawer is then used for automated sequential spectral acquisition, followed by standardized data processing and archiving.