Analysis of Dibenzyl Disulfide and Total Sulfur in Insulating Oil Using SCD

Significance of the Topic

Sulfur-containing additives in transformer insulating oil can accelerate sulfide corrosion of metal components, compromising equipment reliability. Dibenzyl disulfide (DBDS) is a common anti-corrosion additive that requires precise quantification. Gas chromatography coupled with a sulfur chemiluminescence detector (GC-SCD) offers high sensitivity and selectivity for trace sulfur analysis, meeting industry standards such as IEC 62697-1.

Study Objectives and Overview

This application note aims to demonstrate:

• Quantitative analysis of DBDS in insulating oil at trace levels.
• Determination of total sulfur content using the equimolar response of an SCD.
• Comparison of separated and unseparated sample injection approaches for total sulfur measurement.

Methodology and Instrumentation

Samples of standard DBDS and DPDS (internal standard) were prepared in toluene and spiked into mineral oil at concentrations from 0.1 to 100 mg/kg. Insulating oil samples were similarly spiked with DPDS. Two analytical workflows were implemented:

1. Separated analysis of DBDS and other sulfur species using a capillary column (SH-Rtx-5, 30 m×0.32 mm, 0.25 µm) with a temperature program from 90 °C to 275 °C.
2. Unseparated total sulfur analysis by direct transfer through a deactivated fused silica tube to the SCD.

Used Instrumentation

The following system configuration was employed:

• Gas chromatograph: Nexis GC-2030
• Detector: SCD-2030 (sulfur chemiluminescence)
• Injection: Split mode (1:5 for DBDS, 1:50 for total sulfur), injector temperatures 275 °C and 280 °C
• Carrier gas: Helium, constant linear velocity or pressure control
• Detector gases: H₂, N₂, O₂, O₃ at optimized flow rates

Main Results and Discussion

• Calibration linearity for DBDS was excellent (R² = 0.9999), with detection down to 0.1 mg/kg.
• Quantified DBDS in five commercial insulating oils ranged from 0.1 to 92.2 mg/kg.
• Total sulfur results from the separated method (summing individual peaks via equimolar response) matched those from direct unseparated injection, confirming SCD uniform sensitivity across sulfur species.
• Direct injection simplified sample handling and reduced analysis time without compromising accuracy.

Benefits and Practical Applications

GC-SCD provides:

• High selectivity for sulfur compounds, eliminating matrix interference.
• Trace-level detection capability for preventive maintenance and quality control.
• Flexible workflows for targeted analyte quantification and bulk sulfur determination.

Future Trends and Potential Uses

Advancements may include:

• Automation and on-line monitoring of transformer oil quality.
• Extension to alternative insulating fluids and emerging sulfur chemistries.
• Integration with data analytics for predictive corrosion management.

Conclusion

GC-SCD analysis under IEC 62697-1 conditions delivers reliable quantification of DBDS and total sulfur in transformer oils. The equimolar response of the SCD streamlines total sulfur measurement, while direct injection offers a rapid screening tool. These methods support rigorous quality assurance and preventative maintenance in power systems.

Reference

• IEC 62697-1: Methods for measurement of dibenzyl disulfide in insulating oils
• Shimadzu Nexis GC-2030 and SCD-2030 operating manuals