Analysis of Impurities in Model Biodiesel using UltraPerformance Convergence Chromatography (UPC2)

Importance of the Topic

Quality assessment of biodiesel is critical for environmental compliance and engine performance. Residual glycerol and acylglycerols can impair fuel stability and emissions, so rapid, accurate analysis methods are essential for producers and quality laboratories.

Objectives and Study Overview

The aim was to apply UltraPerformance Convergence Chromatography (UPC 2) to a model biodiesel blend and demonstrate fast, derivatization-free separation and quantification of glycerol, mono-, di-, and triacylglycerols at trace levels.

Methodology and Instrumentation

• Sample: 5 % model biodiesel (C16, C18, C18:1, C18:2, C18:3 FAEEs) spiked with glycerol and acylglycerol standards.
• Column: ACQUITY UPC 2 HSS C18 SB, 3.0 × 150 mm, 1.8 µm.
• Mobile phase: supercritical CO₂ with acetonitrile/methanol (90:10), gradient from 98:2 to 80:20 over 18 minutes.
• Pressure and temperature: ABPR at 1500 psi, column at 25 °C.
• Flow: 1 mL/min CO₂ plus make-up IPA at 0.2 mL/min.
• Detectors: ELSD (cooling nebulizer, drift tube 50 °C, 40 psi) and PDA (210 nm, ref 400–500 nm).

Main Results and Discussion

• Baseline separation of all analytes in under 15 minutes, including free glycerol and TAG, DAG, MAG species.
• Excellent reproducibility: area RSD 1–4 %, retention time RSD < 0.1 %.
  
• Detection limits: 0.05 % w/w glycerol visible with S/N ~ 10:1 at 2 µg on-column and ~ 40:1 at 4 µg on-column.
• Calibration across 0.2–5.0 % spiking levels showed clear peak resolution and quantitation capability down to low ppm levels.

Benefits and Practical Applications

• No pre-column derivatization or extensive sample prep reduces analysis time and artifact risk.
• Orthogonal selectivity compared to reversed-phase LC and GC, enabling analysis of thermally labile esters.
• High throughput screening of biodiesel batches for QA/QC and regulatory compliance.

Future Trends and Potential Applications

• Coupling UPC 2 with high-resolution mass spectrometry for detailed lipid profiling.
• Development of novel stationary phases to target specific contaminants.
• Green analytics using CO₂ as mobile phase to reduce organic solvent consumption.
• Extension to analysis of other renewable fuels, plant oils, and complex lipid matrices.

Conclusion

UltraPerformance Convergence Chromatography with ELSD provides a fast, sensitive, and robust approach for quantifying glycerol and acylglycerol impurities in biodiesel. The method meets stringent detection requirements, simplifies sample preparation, and completes separations in less than 15 minutes.

Used Instrumentation

• ACQUITY UPC 2 System
• ACQUITY UPC 2 HSS C18 SB Column (3.0 × 150 mm, 1.8 µm)
• ELSD detector and PDA detector

Reference

1. Rodriguez-Guerrero J, Filho R, Rosa P. Production of Biodiesel from Castor Oil using Sub and Supercritical Ethanol. J Supercrit Fluids. 2013;83:124.
2. Zhou W, Gao B, Zhang X, Xu Y, Shi H, Yu L. Chemical Profiling of Triacylglycerols and Diacylglycerols in Cow Milk Fat by Ultra-Performance Convergence Chromatography Combined with Q-TOF MS. Food Chem. 2014;143:199.