Determination of total fluorine, chlorine, and sulfur in liquefied petroleum gas using combustion-ion chromatography

Importance of the topic

Liquefied petroleum gas (LPG) and natural gas streams frequently contain trace halogenated organics and organosulfur species that affect product quality, downstream catalyst performance, and safety. Accurate, routine measurement of total fluorine, chlorine, bromine and sulfur—expressed as halide ions and sulfate after oxidative conversion—is therefore essential for process control, product specification and regulatory compliance. Combustion-ion chromatography (C-IC) provides a robust pathway to convert diverse organic contaminants to ionic species and quantify them by ion chromatography, enabling analysis of gaseous and low-volatility fuel matrices that are otherwise incompatible with direct aqueous IC methods.

Goals and study overview

This application study demonstrates a rapid, automated C-IC workflow for quantifying total fluorine, chlorine, bromine and sulfur in n-butane LPG. Key objectives were to: develop reproducible calibration approaches that minimize the need for multiple expensive gas standards; qualify method performance (linearity, detection limits, precision, accuracy); and showcase safe, practical operation of the Thermo Scientific Cindion C-IC system equipped with the Cindion LPG/Gas Module. Measurements targeted two concentration ranges representative of common additive levels (approx. 2–18 mg/kg and 15–135 mg/kg).

Methodology and analytical workflow

Principle of analysis:

• Small aliquots of LPG (25 µL loop increments) are delivered in the gas phase to a high-temperature oxidative combustion furnace (1000 °C) under argon and oxygen with added water vapor.
• Organic halogen and sulfur compounds are oxidized to hydrogen halides (HX) and sulfur oxides (SOx), which are absorbed into a small aqueous trapping solution containing hydrogen peroxide; sulfide/organosulfur species are converted to sulfate.
• The absorption solution is analyzed by suppressed-conductivity ion chromatography to quantify fluoride, chloride, bromide and sulfate; results are reported as mg/kg in the LPG sample.

Calibration strategy:

• Incremental combustions of a single LPG standard bottle are used to generate multiple calibration points by repeating loop fills (e.g., 1–9 cycles of a 25 µL loop), making standard preparation economical and conserving costly gas standards.
• Two calibration ranges were established: a low range (2–18 mg/kg) using a 2 mg/kg standard and a high range (15–135 mg/kg) using a 15 mg/kg standard; curve fits were typically quadratic with excellent r2 (>0.995).

Sample handling and blank control:

• Absorption solution: 2.0 mL of 100 mg/L H2O2 in ultrapure water (prepared from 30% SupraPur H2O2).
• Blank characterization is critical—trace fluoride and ~0.6 mg/kg chloride and sulfate were detected in the blank n-butane tank and were accounted for when calculating MDLs and reporting results.

Operational notes and safety:

• Secure grounding and safe venting of LPG tanks are required; the Cindion LPG/Gas Module includes safety venting and purge-out provisions.
• Autosampler boats for solids/liquids should be removed to avoid cross-contamination during LPG analysis.

Instrumentation used

The study employed a Thermo Scientific Cindion Combustion-Ion Chromatography system configured with the Cindion LPG/Gas Module and a Dionex Inuvion RFIC system. Major hardware and consumables included:

• Cindion Combustion/Absorption Module and Cindion LPG/Gas Module (with internal 25 µL loop)
• Dionex Inuvion IC with RFIC, NGES-A suppressor (2 mm), Eluent Generator Cartridge (EGC-500 KOH), CR-ATC trap column, Inuvion RFIC eluent degasser
• IonPac AG20 guard and AS20 analytical columns (2 mm i.d.)
• 100 µL IC injection loop; stainless tubing with CGA 510 fittings; UHP argon and oxygen supplies

Chromatographic conditions (summary):

• KOH gradient eluent (approx. 7–55 mM range over 20 min) at 0.38 mL/min, 35 °C column temperature
• Suppressed conductivity detection (recycle mode; ~52 mA, baseline ≈0.4 µS/cm)

Main results and discussion

Analytical performance:

• Linearity: Calibration curves (low and high ranges) produced coefficients of determination r2 >0.995; quadratic fits were commonly optimal in the 2–18 mg/kg range.
• Precision: Method reproducibility showed relative standard deviations (RSDs) generally <6% (reported RSDs for sample studies ranged from ~0.5% to 4.7%).
• Accuracy: Recoveries across analytes and concentration ranges were between ~81% and 99% (typical recoveries 88–99%).
• Method detection limits (MDLs): Estimated MDLs in the low-range analyses were approximately 0.49 mg/kg (F), 0.89 mg/kg (Cl), 0.75 mg/kg (Br) and 0.63 mg/kg (S), with blank corrections applied where appropriate.

Technique robustness and limitations:

• The incremental combustion (multiple loop fills into a single absorber) enables calibration creation from one gas bottle and reduces standard consumption, but accurate blank characterization and constant absorption time are essential to avoid bias.
• Sulfate peaks were not fully baseline resolved from nearby matrix peaks in some runs; automated integration was used but chromatographic separation optimization may further improve sulfate quantitation.

Benefits and practical applications

The Cindion C-IC approach delivers several practical advantages for LPG and gas analysis:

• Enables direct analysis of gaseous/liquid gas samples by converting organohalogens and organosulfurs to ionic species suitable for IC.
• Automated LPG handling, reproducible combustion and absorption steps reduce analyst time and variability.
• Incremental standard generation reduces the number of expensive gas standards required and lowers operating costs.
• Compatible with single-software control (Chromeleon CDS) simplifying instrument management and QA workflows.
• Method adheres to ASTM D7994, supporting regulatory and industrial acceptance.

Future trends and potential uses

Possible developments and extensions for combustion-ion chromatography in gas analysis include:

• Lowering MDLs through improved blank control, larger absorption volumes or enhanced trapping chemistries to meet stricter regulatory or product-purity requirements.
• Integrating C-IC with complementary detectors (e.g., mass spectrometry) for improved selectivity and speciation capability where identification of parent organohalogens is needed.
• Expanding validated matrices beyond LPG—natural gas blends, refinery streams and process gases—using similar incremental calibration and automation strategies.
• Continued miniaturization and more compact modules for field-deployable or near-line monitoring of fuel contamination.

Conclusion

This application demonstrates that the Thermo Scientific Cindion C-IC system with LPG/Gas Module provides a reliable, automated route for total halogen and sulfur determination in LPG. The method combines high-temperature oxidative conversion, efficient trapping in a peroxide absorber and suppressed-conductivity IC detection to deliver accurate, precise results with economy of standards and compliance to ASTM D7994. With MDLs at sub-mg/kg levels for most analytes, RSDs typically below 6% and recoveries near quantitative, C-IC is well-suited for routine QA/QC and process control tasks in petrochemical and gas-handling operations.

References

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