Reducing Carbonate Interference in Anion Determinations with the Carbonate Removal Device (CRD)

Importance of the Topic

The accurate determination of inorganic and organic anions by ion chromatography (IC) is critical in fields such as semiconductor manufacturing, environmental monitoring, drinking water quality, and the food and beverage industry. Carbonate, formed by dissolved CO₂, often coelutes or interferes with target anions when using hydroxide or tetraborate eluents, leading to poor reproducibility and compromised quantification. Eliminating carbonate interference directly in the IC flow path improves sensitivity, reliability, and throughput by avoiding time-consuming sample pretreatment steps.

Objectives and Study Overview

This study evaluates the Dionex CRD 200 Carbonate Removal Device for three representative applications:

• Trace anions and organic acids at low µg/L levels in ultrapure water (AS15 column, gradient, 1 mL injection)
• Single-digit µg/L perchlorate in drinking water with high mg/L chloride, sulfate, and carbonate (AS16 isocratic method)
• Anion profiling in carbonated mineral water (AS18 column, gradient) without prior degassing

The goals are to demonstrate the installation, theory of operation, and performance improvements offered by in-line carbonate removal before conductivity detection.

Methodology and Instrumentation

Key components and conditions:

• Ion chromatography system: RFIC platform (e.g., ICS-2500/3000) with eluent generator (EG50/EG cartridge)
• Carbonate Removal Device: Dionex CRD 200 (2 mm microbore or 4 mm standard bore) installed downstream of the ASRS ULTRA II suppressor
• Suppressor: Dionex ASRS ULTRA II in recycle or external water mode
• Columns: IonPac AS15 (3 × 150 mm), AS16 (4 × 250 mm), or AS18 (2 × 250 mm) with corresponding AG guard columns
• Autosampler: AS50 or AS autosampler with partial (1100 µL) or full (5 µL) loop injection strategies
• Eluents: Electrolytically generated KOH gradients or isocratic solutions (7–60 mM, 22–40 mM, or 65 mM) at 0.25–1.2 mL/min, 30 °C

Main Results and Discussion

1. Ultrapure water (AS15, gradient, 1 mL injection): The CRD 200 removed 85–86% of the carbonate peak, reducing its tailing to reveal low µg/L adipate, sulfate, oxalate and other anions. Retention times increased by ~0.08–0.15 min and plate counts remained satisfactory, enabling ng/L–µg/L quantification without blank contamination.

2. Perchlorate in drinking water (AS16, isocratic, 1 mL injection): The CRD 200 eliminated most carbonate from 100 mg/L and 500 mg/L spikes, achieving >86% apparent removal. Perchlorate peak efficiency and retention time were maintained or slightly improved, and recoveries of 5–25 µg/L spikes in natural water were within 104–112% without the device and 105–108% with it.

3. Carbonated mineral water (AS18, gradient, 5 µL injection): Direct five-fold dilution still leaves high carbonate. The CRD 200 removed 92–99% of the carbonate peak, unmasking low-level fluoride, bromide, chlorate, nitrite, nitrate, and phosphate. Retention times shifted by ~0.2–0.3 min and efficiencies decreased modestly (60–160 plates), acceptable for routine QC.

Benefits and Practical Applications

The in-line CRD 200 device:

• Eliminates sample degassing or offline carbonate removal, reducing handling steps and contamination risk
• Improves resolution and quantification of target anions eluting near carbonate
• Enables direct analysis of challenging matrices, from ultrapure water to carbonated beverages
• Maintains system pressure and compatibility with RFIC, suppressor, and columns without hardware modifications

Future Trends and Opportunities

As IC evolves toward higher throughput and lower detection limits, integrated flow-through sample cleanup like the CRD 200 will become more critical. Potential developments include:

• Membrane materials optimized for wider pH ranges and faster CO₂ transport
• Adaptation to borate or mixed-eluent systems for specialty analyses
• Miniaturized devices for microbore or capillary IC platforms
• Online coupling with mass spectrometry for trace contaminants

Conclusion

The Dionex CRD 200 Carbonate Removal Device effectively removes sample carbonate in-line for hydroxide-based IC, improving the detection and quantification of anions in ultrapure water, drinking water, and carbonated beverages. Installation is straightforward, and performance gains—85–99% peak removal—outweigh minor retention time shifts, enabling robust, contamination-free analyses without offline pretreatment.

References

• Dionex Technical Note 48, LPN 01127 (2001)
• Application Updates AU 142, AU 148; AN 154 (2001–2004)
• Product Manuals: EG50, CR-TC, ASRS ULTRA II, AS15/16/18 Columns, AS50/AS Autosamplers, CRD 200 (2002–2005)