Simultaneous determination of quality parameters of biodiesel/diesel blends using HATR-FTIR spectra and PLS, iPLS or siPLS regressions

Partial least-squares (PLS), interval partial least squares (iPLS) and synergy partial least squares (siPLS) regressions were used to simultaneous determination of quality parameters of biodiesel/diesel blends. Biodiesel amount, specific gravity, sulfur content and flash point were evaluated using spectroscopic data in the mid-infrared region obtained with a horizontal attenuated total reflectance (HATR) accessory. Eighty-five binary blends were prepared using biodiesel and two types of diesel, in concentrations from 0.2 to 30% (v/v). Fifty-seven samples were used as a calibration set, whereas 28 samples were used as an external validation set. All samples were characterized using the appropriated standard methods. The specific gravity values at 20 °C were in the range of 848.2-866.2 kg/m³. Flash point values lay between 47.0 and 79.5 °C. Sulfur content values varied from 312 to 1351 mg/kg. Raw spectra of the samples were corrected by multiplicative scatter correction (MSC) and were pre-processed using a mean-centered procedure. Algorithms iPLS and siPLS were able to select the most adequate spectral region for each property studied. For all the properties studied, the siPLS algorithm produced better models than the full-spectrum PLS, selecting the most important bands. The quantification of biodiesel was performed using two spectral regions between 650-1909 cm⁻¹ and 2746-3165 cm⁻¹, and an excellent correlation coefficient of R² = 0.9996 was obtained. The specific gravity was determined from the spectral region from 650 to 1070 cm⁻¹, which yielded a very good correlation coefficient of R² = 0.9987. The sulfur content was evaluated from the spectral regions of 1070-1491 cm⁻¹ and 2746-3165 cm⁻¹. A very good correlation coefficient of R² = 0.9995 was obtained, regardless of whether the samples were formulated with metropolitan or countryside diesel. Finally, the flash point was determined from the spectral region between 756 and 968 cm⁻¹ and a very good correlation coefficient of R² = 0.9982 was obtained.