63 different isomerate samples with varying RON values were measured with the Metrohm DS2500 Liquid Analyzer (Figure 1) in transmission mode over the full wavelength range of 400–2500 nm. The built-in temperature controller ensured measurement stability with a constant sample temperature of 35 °C. For convenience, disposable vials with a pathlength of 8 mm were used which made a cleaning procedure obsolete. The Vision Air Complete software package from Metrohm was used for data acquisition and prediction model development.

The obtained Vis-NIR spectra (Figure 2) were used to create a prediction model for the quantification of research octane number in isomerate. The quality of the prediction models was evaluated using correlation diagrams based on the cross-validation algorithm. A value of R² > 0.98 displays a high correlation between the Vis-NIR prediction and the reference ASTM method. The respective figures of merit (FOM) display the expected precision during routine analysis (Figure 3).

This Application Note demonstrates the feasibility of NIR spectroscopy for the analysis of RON in isomerate samples. In comparison to the conventional method, the time to result (Table 2) is a major advantage of Vis-NIR spectroscopy. With NIRS, a single measurement is performed within one minute, while the CFR engine test is much longer.