Free and Total Glycerol in B100 Biodiesel by Gas Chromatography According to Methods EN 14105 and ASTM® D6584

Significance of the Topic

The rapid expansion of biodiesel as an alternative to fossil diesel has placed renewed emphasis on rigorous quality control. Measurement of free and total glycerol content is critical because residual glycerol and glyceride intermediates indicate incomplete transesterification or inadequate washing steps. Ensuring these metrics remain within established limits protects engine performance, reduces deposit formation, and complies with international standards.

Objectives and Overview

The primary objective of this study is to demonstrate a robust gas chromatography–flame ionization detection (GC–FID) method for quantifying free and bound (mono-, di-, and triglyceride) glycerol in B100 biodiesel according to EN 14105 and ASTM D6584 protocols. The workflow encompasses sample derivatization, chromatographic separation, calibration, and quantitative reporting to distinguish fully reacted, washed biodiesel from incomplete or unwashed samples.

Methodology and Sample Preparation

• Derivatization: Samples and standards are silylated with MSTFA to convert polar glycerol and glycerides into thermally stable trimethylsilyl derivatives.
• Internal Standards: Butanetriol is used for free glycerol, and tricaprin for glyceride groups, following each method’s specifications.
• Calibration: EN 14105 employs a four-point calibration; ASTM D6584 uses a five-point calibration. Linearity exceeds R² > 0.99 for all analytes.
• Data Processing: TotalChrom CDS automates calibration curve fitting and performs weight-percent calculations to generate pass/fail reports.

Used Instrumentation

• Gas Chromatograph: PerkinElmer Clarus GC with programmable on-column injector.
• Column: Metal biodiesel capillary column (Elite-Biodiesel M, 14 m × 0.53 mm i.d., 0.16 µm film) with integrated guard segment.
• Detector: Flame Ionization Detector (FID) at 380 °C.
• Carrier Gas: Helium at 3 mL/min.

Main Results and Discussion

• Calibration Performance: All analytes (glycerol, mono-, di-, triglycerides) exhibited linear FID response (R² ≥ 0.9945), confirming reliable quantitation across method ranges.
• Complete Transesterification Sample: Chromatogram of washed soy biodiesel showed no detectable glycerol or glycerides, indicating full conversion and effective washing.
• Incomplete Transesterification Sample: Unwashed, used vegetable-oil biodiesel displayed pronounced peaks for free glycerol, mono-, di-, and triglycerides, reflecting operational shortcomings in reaction completion and cleanup.
• Process Improvement: Incomplete samples suggest the need for optimized methanol removal and enhanced water-washing protocols to meet ASTM D6751 and EN 14214 quality criteria.

Benefits and Practical Application of the Method

• Compliance: Aligns with both European and ASTM fuel quality standards for biodiesel.
• Efficiency: Simplified sample work-up and standardized derivatization reduce analyst time and minimize errors.
• Reliability: On-column injection and robust metal columns deliver consistent retention times and minimize leak risks at high temperatures.
• Reporting: Automated CDS workflows generate clear, weight-percent reports for decision-making in production and QC laboratories.

Future Trends and Opportunities

• Enhanced Detection: Integration of mass spectrometry could allow simultaneous monitoring of minor impurities and trace contaminants.
• Automation and Miniaturization: Further development of autosampler derivatization and micro-column technology may accelerate throughput and reduce solvent use.
• Green Chemistry: Exploration of alternative, less-toxic derivatization reagents and solvent-free workflows to reduce environmental impact.
• Industry Integration: Real-time online GC-FID monitoring within continuous biodiesel production lines for rapid process control.

Conclusion

This application note demonstrates that GC–FID analysis, following EN 14105 and ASTM D6584, provides accurate and reproducible quantification of free and total glycerol in biodiesel. The combination of MSTFA derivatization, on-column injection, and robust chromatography delivers clear pass/fail results, supporting compliance with fuel standards and enabling actionable insights into production efficiency.

References

• EN 14105: Fatty acid methyl ester (FAME) determination in biodiesel by GC-FID.
• ASTM D6584: Measurement of free glycerin and glycerides in biodiesel by GC-FID.