Evaluation of diesel exhaust fluid (DEF) using near-infrared spectroscopy and multivariate calibration

Diesel engines are responsible for high NOx emission rates in the atmosphere. The urea-selective catalytic reduction (SCR) system is one of the most promising techniques for NOx emission control. The SCR system uses a urea solution known as Diesel Exhaust Fluid (DEF) that contains 32.5% (v/v) of urea dissolved in deionized water. The quality control of the urea concentration in DEF, which directly affects the efficiency of the SCR system, follows the ISO 22241-2 protocol. A simple, practical and efficient methodology to evaluate DEF requirements that uses near-infrared (NIR) spectroscopy and multivariate calibration is presented in our work. For this, DEF commercial samples and mixtures containing urea and deionized water were analyzed in a Fourier Transform Near Infrared Spectrometer (FT-NIR) and in a MicroNIR handheld spectrometer. The urea concentration in the commercial samples and mixtures was quantified by refractive index, according to ISO 22241-2. Partial Least Squares (PLS) models were developed to predict urea content. Variables selection techniques, such as Interval Partial Least Squares (iPLS) and Jack-knife, were tested. The handheld NIR instrument performed well and provided PLS models with RMSEP values statistically equivalent to the PLS models developed in the benchtop instrument. All the RMSEP values obtained were lower than the reproducibility of the ISO protocol. A feasible calibration transfer approach between the benchtop and the handheld instrument was demonstrated. The described methodology, aimed at in-field use for surveillance and DEF quality control purposes, represents a potential alternative to determine the urea concentration in DEF.