Determination of Existent and Potential Sulfate and Total Inorganic Chloride in Denatured Ethanol by Direct Injection Using an RFIC System

Significance of the Topic

Denatured ethanol is widely used as a renewable fuel or blending agent to reduce reliance on fossil fuels.
Trace levels of inorganic chloride and sulfate can cause corrosion in fuel systems and must be monitored to comply with ASTM specifications.
Ion chromatography (IC) offers sensitive separation and quantitation of these anions, but direct injection of ethanol can lead to baseline drift when using chemical suppressors.

Study Objectives and Overview

This work aimed to develop a reagent-free IC method using recycle-mode electrolytic suppression for direct analysis of total inorganic chloride, existent sulfate, and potential sulfate in denatured ethanol.
The method addresses baseline instability of previous approaches and eliminates the need for chemical regenerants.

Instrumentation Used

• Dionex ICS-2100 IC system with pump, vacuum degasser, eluent generator, and conductivity detector
• AS-AP autosampler with 250 µL sample syringe
• Dionex AERS 500 Anion Electrolytically Regenerated Suppressor (4 mm) operated in recycle mode
• IonPac AG22 guard column (4 × 50 mm) and AS22 analytical column (4 × 250 mm)
• EGC III K2CO3 eluent cartridge with EPM III pH modifier
• Chromeleon 7.2 chromatography data system software

Methodology

Eluent was generated in-line as 4.5 mM potassium carbonate/1.4 mM potassium bicarbonate at 1.2 mL/min.
Samples (25 µL injection) were analyzed at 30 °C with a 14 min runtime.
Total inorganic chloride and existent sulfate were measured by direct injection of denatured ethanol.
Potential sulfate was determined after treating samples with 1.5 % (v/v) hydrogen peroxide to oxidize sulfite and other species.
Calibration standards were prepared by diluting 1 000 mg/L chloride and sulfate solutions to cover 0.3–50 mg/L (chloride) and 0.3–20 mg/L (sulfate).

Main Results and Discussion

• Baseline Stability: No drift observed over >400 consecutive injections using the AERS 500 suppressor in recycle mode.
• Chromatographic Performance: AS22 column achieved complete separation of common anions in under 14 min.
• Calibration Linearity: r² = 0.9983 for chloride (0.3–50 mg/L) and r² = 0.9993 for sulfate (0.3–20 mg/L).
• Sensitivity: LODs of 13 µg/L for chloride and 45 µg/L for sulfate; LOQs of 43 µg/L and 151 µg/L, respectively.
• Precision: Retention time RSD <0.2 %; peak area RSD 0.3–5.2 % for spiked ethanol samples.

Benefits and Practical Applications

• Reagent-free suppression avoids handling and disposal of corrosive acids.
• Simplified workflow using only deionized water for eluent generation.
• High throughput and ruggedness support routine QA/QC in biofuel production.
• Fully compliant with ASTM D4806-13a and D7319-13 requirements for chloride and sulfate limits.

Future Trends and Potential Applications

• Integration of IC with mass spectrometry for enhanced selectivity of inorganic and organic anions.
• Development of portable IC systems for on-site monitoring of fuel quality.
• Automation of sample treatment steps, including peroxide oxidation, for streamlined analysis.
• Extension of the method to other biofuels and alternative energy carriers to ensure compliance and performance.

Conclusion

The developed RFIC method using the AERS 500 suppressor in recycle mode delivers stable baselines, excellent sensitivity, and precise quantitation of chloride and sulfate in denatured ethanol.
This approach eliminates the drawbacks of chemical suppressors, reduces operational complexity, and meets stringent regulatory limits.

References

1. ASTM International. D4806-13a Standard Specification for Denatured Fuel Ethanol for Blending with Gasolines for Use as Automotive Spark-Ignition Engine Fuel; West Conshohocken, PA.
2. ASTM International. D7319-13 Standard Test Method for Determination of Existent and Potential Sulfate and Inorganic Chloride in Fuel Ethanol and Butanol by Direct Injection Suppressed Ion Chromatography; West Conshohocken, PA.
3. Dionex. Application Note 175: Determination of Sulfate and Chloride in Ethanol by Ion Chromatography; Sunnyvale, CA, 2006.
4. Dionex. Application Update 161: Determination of Sulfate and Chloride in Ethanol Using Ion Chromatography; Sunnyvale, CA, 2008.
5. Dionex. Application Note 290: Determination of Total and Potential Sulfate and Total Chloride in Ethanol According to ASTM Method D7319; Sunnyvale, CA, 2012.
6. Thermo Scientific. Application Note 1052: Determination of Chloride and Sulfate in Gasoline-Denatured Ethanol; Sunnyvale, CA, 2013.
7. Karu, N. Suppressed Ion Chromatography of Organic Acids with Universal Detection. PhD Dissertation; University of Tasmania, 2012.