Analysis of Thiophene in Benzene: Comparison of FPD(S) and SCD Analyses

Significance of the Topic

Trace levels of sulfur compounds in hydrocarbons pose significant challenges to quality control and environmental monitoring due to their toxicity and regulatory limits.
This work focuses on identifying the optimal detector for analyzing thiophene in benzene, a representative sulfur‐containing analyte, to ensure accurate quantitation at low concentrations.

Objectives and Study Overview

The primary goal was to compare three gas chromatography detectors—Flame Ionization Detector (FID), Flame Photometric Detector in sulfur mode (FPD(S)), and Sulfur Chemiluminescence Detector (SCD)—for sensitivity, selectivity, linearity, and repeatability when measuring thiophene in benzene at concentrations from 0.05 to 10 ppm (v/v).

Methodology and Instrumentation

A split‐injection GC method was configured with a SH-Rtx-WAX capillary column (30 m × 0.32 mm I.D., 1 μm film thickness) and helium as carrier gas (37.7 cm/s linear velocity).
Injection volume was 1 µL at 200 °C with a split ratio of 1:15.
SCD operated under negative pressure with H₂, N₂, O₂, and O₃ gases; FID/FPD(S) used atmospheric pressure with H₂ and air.

Instrumentation

• Main unit: Nexis™ GC-2030 coupled with AOC-20i auto injector
• Column: SH-Rtx™-WAX, 30 m × 0.32 mm I.D., 1 µm
• Carrier gas: He (linear velocity 37.7 cm/s)
• Detector gases: FID-2030 (H₂ 32 mL/min, air 200 mL/min, He makeup 24 mL/min); FPD-2030 (H₂ 40 mL/min, air 60 mL/min); SCD-2030 (H₂ 100 mL/min, N₂ 10 mL/min, O₂ 12 mL/min, O₃ 25 mL/min)

Main Results and Discussion

Chromatograms revealed that benzene produced strong FID responses but was suppressed or distorted in FPD due to quenching by coeluting hydrocarbons; SCD showed negligible benzene response, reflecting its high sulfur selectivity.

• S/N comparison at 0.1 ppm: SCD (13.6) vs FPD (7.2); at 10 ppm: FPD (6085) vs SCD (1305).
• Calibration: FPD exhibited a log-log linear response over 0.05–1 ppm and 0.5–10 ppm (R²>0.99) but limited dynamic range (~10³); SCD delivered direct linearity across 0.05–10 ppm (R²≈0.997).
• Repeatability: RSD values for SCD (0.21–5.37 %) outperformed FPD (1.78–7.21 %), particularly at low concentrations.

Benefits and Practical Applications

SCD’s superior selectivity and broad linear range make it ideal for routine trace sulfur analysis in complex matrices without interference from major hydrocarbon components.
FPD offers competitive sensitivity at higher concentrations and can detect phosphorus or tin when equipped with alternative filters, supporting flexible analytical requirements in petrochemical quality control.

Future Trends and Applications

Emerging detector designs integrating multi‐element chemiluminescence and enhanced optical filters may further improve selectivity and sensitivity.
Advances in GC–MS coupling and data‐driven deconvolution algorithms will enable simultaneous multi‐analyte screening with minimal sample preparation.
Automation and AI-assisted method optimization promise faster method development and robust performance in industrial laboratories.

Conclusion

Comparative evaluation demonstrates that while both FPD(S) and SCD can quantify thiophene at trace levels with acceptable repeatability, SCD outperforms FPD in selectivity, linearity, and low-level detection.
Detector choice should align with concentration range, matrix complexity, and target analyte profile to achieve optimal analytical results.