Trace detection of mercaptans in fuel

Importance of the topic

Mercaptans are sulfur-containing organic compounds that occur naturally in crude oil and persist through distillation. Even at trace levels, they can corrode fuel systems, impair thermal stability, reduce engine performance, and increase emissions. Regulatory standards, such as ASTM D1655, limit mercaptan content in aviation fuel to 30 mg/L. Conventional analytical methods for mercaptans (titration, UV fluorescence, GC, HPLC) deliver accurate results but involve complex sample handling, lengthy protocols, specialized personnel, and chemical waste.

Objectives and study overview

This application note investigates the use of Surface-Enhanced Raman Scattering (SERS) for rapid, sensitive detection of methyl mercaptan in fuel matrices. The goals are to establish detection limits well below standard Raman spectroscopy, develop and validate a quantitative calibration model for trace mercaptan concentrations (down to 50 ppb), and demonstrate applicability in a portable, easy-to-use format suitable for quality control.

Used Instrumentation

• MIRA XTR handheld Raman spectrometer (785 nm excitation, 50 mW).
• Vision software for data acquisition and calibration.
• Silver paper SERS substrates (Ag P-SERS).

Methodology

Methyl mercaptan stock solution (2000 mg/L in toluene) was serially diluted in paraffin oil to concentrations between 0.00 and 1.00 mg/L (ppm). Five-microliter aliquots were deposited on Ag P-SERS substrates and allowed to adsorb for five minutes. Raman spectra were acquired on the MIRA XTR at full laser power, 1 s integration, with 10 averages for calibration and 3 averages for validation samples. Peak intensity at 675 cm⁻¹ (S–C stretching) was used for quantification.

Key results and discussion

Standard Raman measurements of the stock solution showed only solvent peaks, confirming no mercaptan detection at 2000 mg/L. In contrast, SERS analysis revealed a clear band at 675 cm⁻¹ down to 100 ppm and as low as 0.05 ppm (50 ppb). A calibration curve covering 0.00–1.00 mg/L achieved an R² of 0.975 and PRESS of 0.063, demonstrating sensitivity and linearity in the trace range. Validation samples yielded an R² of 0.962 and PRESS of 0.053 after bias and slope correction, confirming quantitative reliability. A signal plateau above 1 ppm suggests substrate surface saturation and indicates the need for sample dilution at higher concentrations.

Benefits and practical applications

• Detection limits an order of magnitude below regulatory thresholds.
• Minimal sample volume (<20 µL) and simplified preparation.
• Rapid analysis (<10 minutes from sample to result).
• Portable instrumentation suitable for field or lab use.
• Reduced chemical waste and operator training requirements.

Future trends and possibilities

Advancements may include novel nanoparticle substrates to further enhance sensitivity and selectivity, integration of machine learning for automated spectral interpretation, and inline or at-line SERS monitoring in fuel processing. Expansion to other sulfur or trace contaminants could broaden the technique’s industrial applicability.

Conclusion

SERS using MIRA XTR and Ag P-SERS substrates enables fast, cost-effective, and ultrasensitive detection of mercaptans in fuel well below conventional detection limits. This approach offers a practical alternative for routine quality control and environmental monitoring, aligning with regulatory requirements and operational efficiency goals.

Reference

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