The quality control of diesel exhaust fluids (DEF) is key to ensure the optimal catalytic performance and prevent damage to the exhaust system in diesel vehicles. The standard method to determine urea content is measuring the refractive index (ISO 22241-2:2019). The issue is that although this method is fast, it is not as accurate as other methods (e.g., HPLC). This application note demonstrates that the DS2500 Liquid Analyzer provides a fast solution with high accuracy for the determination of urea in DEF. With no sample preparation or chemicals needed, visible near infrared (Vis-NIR) spectroscopy allows for the analysis of diesel exhaust fluids in less than a minute.

DS2500 Liquid Analyzer and a diesel exhaust fluid sample filled in a disposable vial.
Figure 1. DS2500 Liquid Analyzer and a diesel exhaust fluid sample filled in a disposable vial.

Aqueous urea samples with different urea content from 0.5% to 40% (v/v) were measured in transmission mode with a DS2500 Liquid Analyzer over the full wavelength range (400–2500 nm). Reproducible spectrum acquisition was achieved using the built-in temperature control at 40 °C. For convenience, disposable vials with a path length of 2 mm were used, which made cleaning of the sample vessels unnecessary. The Metrohm software package Vision Air Complete was used for all data acquisition and prediction model development.

Table 1. Hardware and software equipment overview
Equipment Metrohm number
DS2500 Liquid Analyzer 2.929.0010
Disposable vials, 2 mm diameter, transmission 6.7492.000
Vision Air 2.0 Complete 6.6072.208

All 16 measured Vis-NIR spectra (Figure 2) were used to create a prediction model for quantification of the urea content. The quality of the prediction models was evaluated using correlation diagrams, which display a very high correlation between Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis.

Figure 2. Vis-NIR spectra of diesel exhaust fluids with varying urea content measured on a DS2500 Liquid Analyzer.
Figure 3. Correlation diagram for the prediction of urea content in diesel exhaust fluids using a DS2500 Liquid Analyzer.
Table 2. Figures of merit for the prediction of urea content in diesel exhaust fluids using a DS2500 Solid Analyzer.
Figures of merit Value
R2 0.999
Standard error of calibration 0.23%
Standard error of cross-validation 0.25%

This application note demonstrates the feasibility of the DS2500 Liquid Analyzer for the determination of urea content in diesel exhaust fluids. Vis-NIR spectroscopy enables a fast determination with high accuracy, and therefore represents a suitable alternative to the standard method. Additionally, it should be pointed out, that for further parameters such as density, similar NIR methods can be developed.

Table 3. Time to result for the urea content determination in aqueous urea solutions using HPLC
Parameter Method Time to result and workflow
Urea content HPLC ∼5 min (preparation) + ∼10 min (HPLC)
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