Determination of glycerine, methanol, mono-, di- and triacylglycerides

Importance of Biodiesel Quality Analysis

Biodiesel derived from plant oils such as rapeseed offers a sustainable alternative to fossil fuels by recycling atmospheric CO2, lowering sulphur and soot emissions, and enhancing engine lubrication. Ensuring consistent fuel properties and meeting regulatory requirements is critical for environmental safety, engine protection, and widespread market adoption.

Study Objectives and Overview

This study aimed to develop and validate gas chromatographic methods for the quantification of methanol, free and total glycerine, and mono-, di-, and triacylglycerides in fatty acid methyl esters (FAME). Adherence to emerging European standards (DIN EN 14103, 14105, 14108, 14110) and compatibility with industrial biodiesel production processes were key drivers.

Methodology and Instrumentation

The analytical workflow comprised two complementary GC approaches:

• Methanol Determination: Headspace GC analysis with isopropanol as internal standard, using a headspace autosampler; quantification via FID.
• Glycerine and Glyceride Profiling: Silylation of glycerine and glycerides with MSTFA in pyridine, followed by cool on-column injection into a 10 m × 0.32 mm ID × 0.10 µm film 5% diphenylpolysiloxane column (up to 400 °C), with hydrogen carrier gas and FID detection. Calibration employed 1,2,4-butantriol for glycerine and tricaprine for glyceride groups.

Used Instrumentation

• Gas chromatograph with cool on-column injector and FID
• Headspace sampler coupled to GC‐FID
• 10 m × 0.32 mm ID × 0.10 µm diphenylpolysiloxane capillary column
• MSTFA derivatization kit and pyridine
• Internal standards: 1,2,4-butantriol and tricaprine

Key Findings and Discussion

Chromatograms of an ideal biodiesel sample showed clear resolution of glycerine, methanol, monoacylglycerides, diglycerides, and triglycerides with retention times matching standards. Integration windows were defined for each glyceride group, using an "integration-off" function to exclude interfering peaks. Typical results for rapeseed biodiesel included:

• Methanol content ≈ 0.02 % (w/w)
• Free glycerine ≤ 0.02 % and total glycerine ≤ 0.25 % (w/w)
• Monoacylglycerides, diacylglycerides, and triacylglycerides within prescribed limits

Reproducible quantification enabled compliance with EURO-III exhaust and fuel quality standards, demonstrating robustness across different feedstocks.

Benefits and Practical Applications

The validated methods support:

• Routine quality control in biodiesel production plants
• Conformance testing to DIN EN and Euronorm specifications
• Optimization of transesterification and refining processes
• Assurance of engine compatibility and environmental performance

Future Trends and Opportunities

Advances in biodiesel analysis may include:

• Online and at-line GC monitoring for real-time process control
• Miniaturized or portable chromatographic systems
• Coupling with mass spectrometry for structural isomer identification
• Automated data interpretation using machine learning for peak deconvolution
• Expansion to novel feedstocks and biodiesel blends (e.g., ethanol-derived esters)

Conclusion

Gas chromatographic techniques with headspace sampling and derivatization reliably quantify key impurities and glyceride profiles in biodiesel. These protocols ensure regulatory compliance, protect engine integrity, and support sustainable fuel production.

Reference

• DIN EN 14103: Determination of ester and linoleic acid methyl ester content in FAME
• DIN EN 14105: Determination of free and total glycerine and mono-, di- and triglyceride content in FAME
• DIN EN 14108: Determination of methanol content in FAME
• DIN EN 14110: Determination of methanol content in FAME