A Comparison of Sulfur Selective Detectors for Low Level Analysis in Gaseous Streams

Significance of the Topic

In hydrocarbon processing and environmental monitoring, quantifying trace sulfur compounds at low parts-per-billion levels is critical to prevent catalyst deactivation, comply with tightening fuel regulations, safeguard equipment integrity, and control odors in natural gas and food-grade gases.

Objectives and Overview of the Study

This study evaluates four sulfur-selective GC detectors—Flame Photometric Detector (FPD), Pulsed Flame Photometric Detector (PFPD), Sulfur Chemiluminescent Detector (SCD), and Atomic Emission Detector (AED)—for single-digit ppb quantitation of eight sulfur compounds in gaseous streams. A dynamic blending system generates low-level calibration mixtures on demand. Selection criteria include sensitivity, selectivity, stability, ease of use, and dynamic range.

Methodology and Instrumentation

• GC System: Agilent 6890 with low-volume volatiles interface, 6-port Hastelloy C valve, Silcosteel tubing and sample loops (1.0 mL for max sensitivity).
• Dynamic Blending: Auxiliary EPC delivers diluent (He, CO₂, ethylene, or propylene) to dilute high-level cylinder standards to target ppb concentration.
• Column: DB-1 methyl silicone, 105 m × 0.53 mm × 5 µm; typical oven program from –20 °C to 255 °C.
• Detector Conditions: Tailored flows and temperatures optimized for max sensitivity. Key settings included hydrogen-rich flames for FPD/PFPD and specific cavity temperatures for AED.
• Calibration: Eight-component sulfur mix (H₂S, COS, CS₂, methyl/ethyl/t-butyl mercaptans, dimethyl sulfide, tetrahydrothiophene) plus SO₂.

Main Results and Discussion

• Detection Limits: FPD ~50 ppb, PFPD and SCD ~5–10 ppb, AED ~5 ppb, reflecting supplier MDLs (20, 1, 0.5, and 2 pg S/s, respectively).
• Selectivity and Coelution: COS coelutes with propylene on DB-1; alternative GC phases (e.g., GasPro) may be required.
• Dynamic Range: All detectors achieve ≥10³ span; SCD and AED extend to 10⁵–10⁶ for sulfur.
• Stability and Quenching: FPD/PFPD quenching demands complete chromatography; SCD can drift over days and may coke; AED offers stable, quench-free, equimolar responses and multi-element capability.
• Practical Examples: Chromatograms of 25 ppb sulfur in CO₂, ethylene, and propylene demonstrate each detector’s response under realistic matrices.

Benefits and Practical Applications

• FPD: Low cost, easy to use, suitable for >100 ppb in simple matrices.
• PFPD: Enhanced sensitivity (5–7× FPD), equimolar sulfur response, but limited column flow and tuning required.
• SCD: No quenching, high selectivity, wide dynamic range, best for experienced users.
• AED: Broad element detection (C, H, N, O, S, etc.), equimolar, quench-free, single-run multi-analyte profiling, ideal for complex petrochemical streams.

Future Trends and Applications

• Integration of multi-element detectors with AI-driven tuning for real-time, on-line monitoring.
• Development of inert, low-adsorption GC phases to simplify trace sulfur analysis in complex matrices.
• Enhanced dynamic blending and microfluidic calibration devices for improved ppb-level standards.
• Stricter environmental regulations driving demand for ever-lower detection limits and automated workflows.

Conclusion

Selecting the optimal sulfur detector requires balancing sensitivity, matrix complexity, dynamic range, and ease of use. FPD offers cost-effective performance for moderate-level analyses, while PFPD and SCD extend sensitivity in more demanding applications. The AED delivers the greatest versatility and multi-element capability for comprehensive trace and major species profiling.

Reference

1. Agilent FlowCalc 2.0 software, Agilent Technologies.
2. M. J. Szelewski, Empirical Formula Determinations and Compound-Independent Calibration Using a GC-AED System, Agilent Application Note 228-382, 1997.