Quantification of biodiesel and adulteration with vegetable oils in diesel/biodiesel blends using portable near-infrared spectrometer

• Portable NIR spectrometer was used to quantify biodiesel and vegetable oil in diesel.
• Performance of the portable NIR spectrometer was compared with the benchtop one.
• PLS and MLR regression models with variable selection were developed.
• Portable NIR spectrometer is feasible for monitoring quality of diesel blends.

The performance of a portable near infrared spectrophotometer to determine biodiesel content in diesel/biodiesel blends, as well as to quantify adulteration by vegetable oil, was evaluated. The NIR transmittance/absorbance spectra of 118 samples in the range 950–1650 nm were acquired using an ultra-compact spectrophotometer (MicroNIR), and also by using a Fourier transform benchtop instrument (FT-NIR) from 800 to 2500 nm. In a preliminary step, partial least squares (PLS) models aimed at the determination of biodiesel quality parameters were developed by using full spectral information. Thereafter, the variables showing significant regression coefficients were selected and the PLS models were re-processed. Successive Projections Algorithm (SPA) was used to screen for the most orthogonal variables to be used in a model constructed by using Multiple Linear Regression (MLR). For determining the contents of both biodiesel and vegetable oil in diesel/biodiesel blends, using MicroNIR and FT-NIR, the results indicated MLR-SPA as the best modelling strategy. RMSEP values of 0.22% v/v (MicroNIR) and 0.13% v/v (FT-NIR) for biodiesel and 0.34% v/v (MicroNIR) and 0.22% (FT-NIR) for vegetable oil contents were obtained. LOD values of 0.31% (MicroNIR) and 0.22% (FT-NIR) for biodiesel and 0.40% (MicroNIR) and 0.34% (FT-NIR) for vegetable oil determination were achieved. The results revealed comparable performances between the portable and the benchtop instruments, demonstrating the feasibility of monitoring in situ the quality characteristics of diesel/biodiesel blends according to biodiesel content and for detection of adulteration with vegetable oils by using the portable spectrophotometer.