An infrared spectroscopic test for total petroleum hydrocarbon (TPH) contamination in soils

The application of near- and mid-infrared (NIR and MIR) spectroscopy as a rapid screening tool for TPH concentrations in contaminated soils is presented. MIR-DRIFT (diffuse reflectance infrared Fouriertransform) spectroscopy, in particular, promises to be a revolutionary new technique for the in-situ analysis of soils at contaminated sites. Infrared is sensitive to alkyl vibrational frequencies, allowing the use of partial least squares (PLS) to be used for quantification of TPH. This work showed that neat whole soils could be used, spectra can be acquired rapidly, and MIR TPH spectra could be separated from those of natural soil organic matter. PLS regression analyses were carried out in three stages; spiking diesel and crude oil (as TPH) into reference minerals, spiking into reference soils, and actual TPH concentrations in contaminated soils. Results of PLS cross-validation for the spiked minerals showed that prediction errors (RMSECV) with the MIR DRIFT were approximately 2000-4000 mg/kg for a TPH range of 0-100,000 mg/kg, but slightly higher for the NIR. (4500-8000 mg/kg). RMSECV values for the reference soils were approximately 1500-2500 mg/kg for a 0-25,000 mg/kg TPH range. Tests with actual contaminated soils identified specific peaks in the MIR that were characteristic of TPH, and showed that predictions using these peaks resulted in RMSECV of approximately 4,500 mg/kg for a 0-60,000 mg/kg TPH range and thus suitable for use as a screening tool.