Quantification of Ethanol in Complex Oil Samples: A Comparison of Different Headspace Methods and an Automated Direct Injection Procedure

Importance of the Topic

The accurate measurement of ethanol in engine oils and blow-by gas condensates has become critical as ethanol blends in fuels rise globally. Variations in ethanol concentration can affect engine performance, lubricant longevity, and emission profiles. Reliable quantification methods ensure quality control in automotive, petrochemical, and environmental laboratories.

Objectives and Overview

This study compared four headspace and direct injection techniques for ethanol determination in complex oil matrices. The goal was to identify methods that combine accuracy, reproducibility, and minimal sample handling. Four procedures were evaluated in two consecutive round-robin tests:

• Procedure 1: Manual standard addition with automated headspace GC/FID
• Procedure 2: Matrix adjustment using decahydronaphthalene
• Procedure 3: Full Evaporation Technique (FET) headspace GC/FID
• Procedure 4: Automated direct injection into μ-vials with thermal desorption

Methodology and Instrumentation

Procedures 1 and 2 required manual aliquoting and mixing, leading to significant handling-induced variability. Procedures 3 and 4 simplified sample introduction:

• Procedure 3: Aliquots (5–100 µL) placed in 20 mL vials, heated to drive complete volatility, and analyzed by static headspace GC/FID.
• Procedure 4: Direct autosampler injection of 1 µL (oil) or 0.1 µL (condensate) into μ-vial inserts in a Thermal Desorption Unit (TDU), followed by transfer to a cooled PTV injector and GC/FID analysis.

Used Instrumentation

• Agilent 7890N GC with FID
• GERSTEL MultiPurpose Sampler (MPS) with Headspace Option
• GERSTEL Thermal Desorption Unit (TDU) and μ-vial inserts
• GERSTEL Cooled Injection System (CIS 4) PTV inlet
• 30 m AT-Wax GC columns

Main Results and Discussion

In the second round robin, both FET headspace (Procedure 3) and TDU μ-vial (Procedure 4) yielded reproducible ethanol measurements. Procedure 3 achieved deviations below ±10% for oil samples but larger errors for condensates. Procedure 4 consistently delivered <10% deviation in oil and up to 15% in condensates, benefiting from full automation and reduced manual handling.

Benefits and Practical Applications

Procedure 4 offers the greatest efficiency by eliminating manual preparation steps, minimizing analyte loss, and protecting the GC column from high-boiling residues. It is ideal for high-throughput laboratories monitoring ethanol in fuels and lubricants.

Future Trends and Opportunities

Ongoing studies will include used engine oils, extended analyte scopes such as ethylene glycol and methanol, and variable water contents in condensates. Further standardization through a third round robin aims to refine method robustness and expand normative guidelines.

Conclusion

Both the Full Evaporation Technique headspace and the automated TDU μ-vial injection methods provide accurate ethanol quantification in complex oil matrices. The TDU μ-vial approach stands out for automation, minimal sample handling, and consistent performance.

References

• [1] Agilent Application Note 5989-0959 EN
• [2] GERSTEL Application Note 4/2006 “Elimination of Non-Volatile Sample Matrix Components…”
• [3] M. Markelov, J.P. Guzowski Jr., Analytica Chimica Acta 276 (1993) 235–245
• [4] M. Markelov, O.A. Bershevits, Analytica Chimica Acta 432 (2001) 213–227