Application Update 161 1 Application Update 161 Determination of Sulfate and Chloride in Ethanol Using Ion Chromatography

Importance of the Topic

Monitoring sulfate and chloride levels in fuel-grade ethanol is critical to prevent corrosion in engine components and protect downstream catalysts. Compliance with ASTM D4806 ensures fuel quality, making reliable analytical methods essential for both regulatory and industrial laboratories.

Objectives and Study Overview

This application update introduces an improved ion chromatography (IC) protocol for simultaneous quantification of sulfate and chloride in ethanol samples denatured with gasoline. The method addresses two main challenges of previous approaches: baseline drift during electrolytic suppression and instability of high-level sulfate peaks.

Instrumentation Used

• Dionex ICS-3000/ICS-2000/ICS-1500/ICS-1000/ICS-90 IC system
• AS Autosampler with TAC-ULP1 trace anion concentrator
• IonPac AS22 analytical column with AG22 guard
• Anion Self-Regenerating Suppressor (ASRS ULTRA II) in recycle mode
• Chromeleon Chromatography Management Software, version 6.8

Methodology

Samples are prepared by diluting ethanol (including 5% unleaded gasoline denaturant) with deionized water in volumetric flasks and filtering through 0.2 μm IC-grade filters. Calibration standards (1–20 mg/L) are prepared daily in the same matrix. Key operating conditions:

• Eluent: 4.5 mM sodium carbonate / 1.4 mM sodium bicarbonate
• Flow rate: 1.2 mL/min, column temperature: 30 °C
• Injection volume: 200 μL
• Suppressor current: 31 mA, background conductance ~20 μS
• Run time: 15 minutes per injection

Main Results and Discussion

• Separation: Complete resolution of chloride and sulfate within 15 minutes without chemical regenerant, even in gasoline-spiked matrix.
• Sensitivity: Eightfold improvement over previous AS14A-based method, enabling accurate detection at 1 mg/L.
• Stability: Sulfate peak area variation remained below 5% over 99 consecutive injections (≈24 hours) at 20 mg/L, compared to a 20% drop in earlier methods within 2 hours.
• Baseline quality: Flat, reproducible baseline achieved via ASRS ULTRA II in recycle mode, supporting precise integration of low-level chloride peaks.

Benefits and Practical Applications

• Robust compliance testing for fuel ethanol under ASTM D4806.
• Enhanced operational safety: elimination of corrosive acid regenerants.
• Simplified workflow: direct injection with autosampler concentrator reduces manual steps.
• Compatibility with automated eluent generation (RFIC) for high-throughput labs.

Future Trends and Possibilities

• Trace-level analysis: adapting preconcentration for sub-µg/L anion determinations.
• Green chromatography: reducing solvent use and waste through miniaturization.
• Integrated fuel testing: coupling IC with automated sampling and reporting for streamlined certification.
• Extended analyte panels: expanding to other anionic impurities relevant to biofuel quality.

Conclusion

The presented IC method offers a sensitive, robust, and user-friendly solution for determining sulfate and chloride in gasoline-denatured ethanol. By leveraging autosampler preconcentration and recyclable suppression, it overcomes baseline and stability limitations of prior approaches and facilitates reliable compliance testing.