Test method for low level detection of biodiesel in diesel using the Agilent 5500t FTIR spectrometer

Importance of the Topic

Detecting low-level biodiesel (FAME) contamination in diesel fuel is essential for users who require strict fuel composition control. Although diesel blends up to 5% biodiesel meet ASTM D975 standards, the exact biodiesel content is not disclosed, which can lead to performance, regulatory, and warranty concerns.

Objectives and Study Overview

This application note presents a method developed by Agilent Technologies for quantifying biodiesel in diesel fuel at trace levels (0.025–20% v/v). It combines the sensitive transmission sampling interface defined in EN 14078 with the multivariate calibration algorithm and sample set from ASTM D7371 to improve detection limits and accuracy over existing approaches.

Instrumentation

• Agilent 5500t portable FTIR spectrometer series
• Patented transmission sampling interface with 100 µm pathlength and rotating cell windows
• Microlab software for automated calibration and result selection

Methodology

• Sample introduction: a single drop of diesel sample is placed between two windows; rotation locks the 100 µm pathlength cell.
• Cleaning: windows are wiped clean when opened, ensuring reproducibility (<0.2 µm variation).
• Calibration models: three partial least squares (PLS) models covering 0.025–1%, 1–10%, and 10–20% biodiesel in diesel are built, using mean centering, baseline correction, and thickness compensation.
• Spectral regions: ester carbonyl stretches (1950–1720 cm⁻¹, 1846–1758 cm⁻¹, 1738–1719 cm⁻¹) and ester C–O bands (1327–1119 cm⁻¹).
• Quantitation: PLS multivariate analysis provides improved baseline correction and interference compensation compared to univariate Beer's law calibration.

Main Results and Discussion

• Cross-validation results: all models achieved R² > 0.999 and standard errors of cross-validation (SECV) well below target levels (0.0016–0.04%).
• External validation: separate sample sets yielded average relative errors of 1.37% (0.025–1%), 0.06% (1–10%), and 0.57% (10–20%).
• Round robin test: in a blind study across six laboratories, the Agilent 5500t FTIR method showed the lowest total average relative error (2.1% for 2–20% range) and the best performance at low concentrations (1.1% relative error).
• Comparison: the combined EN/ASTM approach outperformed standalone EN 14078 transmission and ASTM D7371 ATR-based methods in sensitivity and ease of use.

Benefits and Practical Applications

• High sensitivity allows detection of biodiesel down to 0.025% v/v, supporting QA/QC, regulatory compliance, and warranty protection.
• Portable 5500t FTIR systems enable on-site testing in fuel distribution, storage terminals, and field operations.
• Automated calibration selection via Microlab software reduces operator training and potential errors.

Future Trends and Potential Applications

• Integration with remote monitoring and networked quality management systems for real-time fuel tracking.
• Advances in chemometric algorithms for even lower detection limits and interference reduction.
• Development of standardized portable kits for rapid field screening of fuel blends, including biodiesel from various feedstocks.
• Expanding the methodology to detect other contaminants or biofuel components in diesel and gasoline matrices.

Conclusion

By merging the EN 14078 transmission cell design with the ASTM D7371 multivariate calibration approach on the Agilent 5500t FTIR platform, this method delivers rapid, accurate, and user-friendly quantitation of biodiesel in diesel fuel from trace levels to 20%. Its superior precision and low detection limit make it valuable for stakeholders requiring tight control over fuel composition.

Note: No explicit literature references were provided in the source document.