Equimolar Response to ASTM D5623 Relevant Sulfur Compounds

Significance of the Topic

The accurate measurement of sulfur compounds in light petroleum liquids is critical for environmental compliance, process control, and prevention of catalyst poisoning during refining. Equimolar detection simplifies quantitation across a range of sulfur species, supporting both regulatory testing and quality assurance.

Objectives and Study Overview

This study evaluates an analytical method conforming to ASTM D5623 using an Agilent 8890 gas chromatograph coupled with an 8355 sulfur chemiluminescence detector (SCD). Key goals include achieving equimolar response across 19 representative sulfur compounds, demonstrating linearity over four orders of magnitude, and assessing the benefit of a tandem flame ionization detector (FID) for solvent elution monitoring.

Methodology and Instrumentation

A series of GC/SCD and GC/FID–SCD systems were configured identically to assess reproducibility and equimolar behavior. Sample preparation involved a 50 ppm mixture of 19 sulfur standards in isooctane, injected at 1 µL via an Agilent 7693A autosampler. The chromatographic method used an Agilent DB-Sulfur SCD column (40 m × 0.32 mm, 0.75 µm) with helium carrier at 3.5 mL/min, oven ramp from 40 °C to 170 °C, and an inlet split ratio optimized at 100:1 to minimize solvent peak distortion.

Main Results and Discussion

• Solvent Elution and Coelution: Tandem FID/SCD traces identified isooctane and residual toluene peaks. Increasing the inlet split ratio narrowed solvent peaks and reduced overlap with target analytes (e.g., diethyl sulfide, 1-butanethiol, 3-methylthiophene).
  
• Area Reproducibility: All 19 compounds yielded area %RSD values below 5% across three detector configurations (two standalone SCDs and one FID/SCD), meeting ASTM requirements.
  
• Linearity: Calibration for tert-butyl disulfide spanned 0.5–10 000 pg S/µL, producing correlation coefficients (R2) > 0.9999 on both SCD units.
  
• Equimolar Response: Relative molar sensitivity (RMS) calculations showed average responses proportional to the number of sulfur atoms in each molecule. Monosulfur species averaged RMS ≈ 1, disulfides ≈ 2, and trisulfide ≈ 3, with RSDs below 5% for most analytes.
  

Benefits and Practical Applications

The described GC/SCD approach delivers:

• Robust equimolar quantitation of diverse sulfur species without complex calibration factors.
  
• High sensitivity for trace-level and bulk sulfur analysis in petroleum matrices.
  
• Improved chromatographic performance through solvent peak monitoring with tandem FID.
  
• Reduced maintenance due to low-bleed DB-Sulfur column selection.
  

Future Trends and Potential Applications

Future developments may include coupling sulfur chemiluminescence detection with mass spectrometry for structural elucidation, automation of sample preparation for high-throughput screening, expansion into biofuel and alternative energy streams, and real-time online monitoring in refining processes to support continuous quality control.

Conclusion

The Agilent 8355 SCD paired with the 8890 GC meets ASTM D5623 performance criteria, delivering linear, reproducible, and equimolar detection of 19 sulfur compounds in light petroleum liquids. The addition of an in-line FID enhances solvent peak tracking, ensuring reliable peak shape and minimizing coelution artifacts.

Reference

1. Adlard, E.R., ed. Chromatography in the Petroleum Industry. Journal of Chromatography Library Series, Vol. 56, Elsevier Science B.V., 1995.
   
2. ASTM International. Standard Test Method for Sulfur Compounds in Light Petroleum Liquids by Gas Chromatography and Sulfur Selective Detection. ASTM D5623-24, West Conshohocken, PA, 2024.