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In recent years, there has been a significant push to reduce the environmental impacts of fuels through improvements to fuel quality. This requires engines to be more efficient, along with increasing the octane content of fuel so higher compression engines can be utilized. The determination of key quality parameters of gasoline, namely research octane number (RON, ASTM D2699-19), motor octane number (MON, ASTM D2700-19), anti knock index (AKI), aromatic content (ASTM D5769-15), and density, conventionally requires several different analytical methods, which are laborious and need trained personnel. This application note demonstrates that the XDS RapidLiquid Analyzer, operating in the visible and near-infrared spectral region (Vis- NIR), provides a cost-efficient and fast solution for the multiparameter analysis of gasoline.

XDS RapidLiquid Analyzer and 8 mm disposable vial filled with a gasoline sample.
Figure 1. XDS RapidLiquid Analyzer and 8 mm disposable vial filled with a gasoline sample.

Gasoline samples were measured with the XDS RapidLiquid Analyzer (RLA) in transmission mode over the full wavelength range (400–2500 nm). Reproducible spectrum acquisition was achieved using the built-in temperature controlled sample holder. For convenience, disposable vials with a path length of 8 mm were used, which made a cleaning procedure unnecessary. The Metrohm software package Vision Air Complete was used for data acquisition and prediction model development.

Table 1. Hardware and software equipment overview
Equipment Metrohm number
XDS RapidLiquid Analyzer 2.921.1410
Disposable vials, 8 mm diameter, transmission 6.7402.000
Vision Air 2.0 Complete 6.6072.208

The obtained Vis-NIR spectra (Figure 2) were used to create prediction models for the determination of several key fuel parameters. The quality of the prediction models was evaluated using correlation diagrams, which display a correlation between the 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. This selection of gasoline Vis-NIR spectra was obtained using a XDS RapidLiquid Analyzer and 8 mm disposable vials. For display reasons a spectra offset was applied.

Result RON value

Figure 3. Correlation diagram for the prediction of the RON value in gasoline using a XDS RapidLiquid Analyzer. The reference lab values were determined according to CFR engine tests under controlled conditions.
Table 2. Figures of merit for the prediction of the RON value in gasoline using a XDS RapidLiquid Analyzer.
Figures of merit Value
R2 0.989
Standard error of calibration 0.26
Standard error of cross-validation 0.29

Result MON value

Figure 4. Correlation diagram for the prediction of the MON value in gasoline using a XDS RapidLiquid Analyzer. The reference lab values were determined according to CFR engine tests under controlled conditions.
Table 3. Figures of merit for the prediction of the MON value in gasoline using a XDS RapidLiquid Analyzer.
Figures of merit Value
R2 0.889
Standard error of calibration 0.50
Standard error of cross-validation 0.53

Result aromatics content

Figure 5. Correlation diagram for the prediction of the aromatics content in gasoline using a XDS RapidLiquid Analyzer. The lab values were determined with gas chromatography/mass spectrometry techniques.
Table 4. Figures of merit for the prediction of the aromatics content in gasoline using a XDS RapidLiquid Analyzer.
Figures of merit Value
R2 0.974
Standard error of calibration 0.97 vol%
Standard error of cross-validation 1.07 vol%

Result density

Figure 6. Correlation diagram for the prediction of gasoline density using a XDS RapidLiquid Analyzer. The lab values were determined using a density meter.
Table 5. Figures of merit for the prediction of gasoline density using a XDS RapidLiquid Analyzer.
Figures of merit Value
R2 0.973
Standard error of calibration 0.0021 kg/L
Standard error of cross-validation 0.0023 kg/L

Result AKI value

Figure 7. Correlation diagram for the prediction of AKI value in gasoline using a XDS RapidLiquid Analyzer. The reference lab values were determined according to CFR engine tests under controlled conditions.
Table 6. Figures of merit for the prediction of the AKI value in gasoline using a XDS RapidLiquid Analyzer.
Figures of merit Value
R2 0.945
Standard error of calibration 0.45
Standard error of cross-validation 0.46

This application note shows the feasibility of NIR spectroscopy for the analysis of RON, MON, AKI, aromatic content, and density. In comparison to wet chemical methods (Table 7), the time to result is a major advantage of NIR spectroscopy, since a single measurement is performed within one minute.

Table 7. Time to result with conventional testing methods
Parameter Method Time to result
RON CFR engine test ∼30 minutes per sample
MON CFR engine test ∼30 minutes per sample
AKI CFR engine test ∼30 minutes per sample
Aromatic content Gas Chromatography ∼45 minutes per sample

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