Trace detection of mercaptans in fuel

Significance of the Topic

Mercaptans, organic sulfur compounds with a thiol group, are present in crude oil and persist through distillation, posing corrosion risks and compromising fuel thermal stability. Elevated mercaptan levels can harm engine performance and increase pollutant emissions. Regulatory standards such as ASTM D1655 limit mercaptan content in jet fuel to 30 mg/L, highlighting the need for sensitive detection methods.

Objectives and Study Overview

This study evaluates Surface-Enhanced Raman Scattering (SERS) combined with a portable Raman spectrometer to detect trace mercaptans in fuel. The goal is to demonstrate rapid, low-waste analysis with enhanced sensitivity compared to conventional Raman techniques and established methods like potentiometric titration and chromatography.

Methodology and Instrumentation

Sample preparation involved serial dilution of a 2000 mg/L methyl mercaptan standard in paraffin oil. Five-microliter aliquots were applied to silver paper SERS (Ag P-SERS) substrates and allowed to equilibrate for five minutes. Spectral data were acquired using a handheld Raman spectrometer operating at 785 nm, 50 mW laser power, 1 s integration time, and ten averages for calibration or three averages for validation.

Instrumentation Used

• MIRA XTR handheld Raman spectrometer (785 nm laser)
• Silver paper SERS (Ag P-SERS) substrates
• Vision software for data acquisition and analysis

Results and Discussion

Conventional Raman measurements of the 2000 mg/L standard showed only toluene peaks, indicating insufficient sensitivity. SERS enhanced the Raman band at 675 cm-1 (S-C stretch), enabling detection at concentrations as low as 0.05 mg/L (50 ppb). Calibration over 0.00–1.00 mg/L yielded an R2 of 0.975, while validation samples achieved an R2 of 0.962 after bias and slope adjustment. A signal plateau above 1.00 mg/L suggests substrate saturation, indicating sample dilution may be required for higher mercaptan levels.

Benefits and Practical Applications

SERS combined with portable Raman instrumentation enables rapid, on-site quantification of trace mercaptans with minimal sample volume (<20 µL) and reduced chemical waste. The method requires limited operator training and improves laboratory throughput for fuel quality control and environmental monitoring.

Future Trends and Applications

Advancements may include the development of tailored SERS substrates for diverse sulfur species, integration into field-deployable sensors for continuous fuel monitoring, and implementation of machine learning algorithms for automated spectral interpretation and quantification.

Conclusion

The use of Ag P-SERS substrates in combination with a 785 nm handheld Raman spectrometer achieves trace-level detection of mercaptans down to 50 ppb, surpassing traditional analytical limits and providing a fast, safe, and cost-effective solution for fuel analysis.

References

1. Carroll JJ. Natural Gas Hydrates: A Guide for Engineers; Gulf Professional Publishing, 2003.
2. Shale Oil and Gas Handbook; 2016.
3. ASTM D1655 Standard Specification for Aviation Turbine Fuels; ASTM International, 2022.