Determinations of Inorganic Anions and Organic Acids in Beverages Using Suppressed Conductivity and Charge Detection

Significance of the Topic

Accurate profiling of inorganic anions and organic acids in beverages is critical for quality control, regulatory compliance, and consumer safety. Organic acids contribute to flavor, acidity, and stability of juices and soft drinks. However, weakly ionized species often produce poor signals in conventional suppressed conductivity detection, limiting analytical sensitivity and accuracy.

Objectives and Study Overview

This study evaluates a dual-detector configuration combining suppressed conductivity (CD) and charge detection (QD) in series for capillary ion chromatography (IC). The aim is to demonstrate improved detection sensitivity, linearity, and peak purity assessment for inorganic anions and organic acids in fruit juices.

Methodology and Instrumentation Used

Samples of commercial fruit juices were diluted, filtered (0.2 µm), and analyzed on a Thermo Scientific Dionex ICS-4000 capillary HPIC system. Separation employed a Dionex IonPac AS11-HC-4 µm capillary column (0.4 mm ID) with a potassium hydroxide gradient generated by an EGC-KOH cartridge and CR-ATC trap column. Detectors in series:

• Suppressed conductivity detector (Thermo Scientific ACES 300 suppressor, recycle mode).
• QD Charge Detector (Dionex QDC 300 cell, 6 V, recycle mode).

Chromeleon CDS software managed chromatography and data processing.

Main Results and Discussion

High-efficiency separations were achieved at system pressures exceeding 3500 psi. Key findings:

• QD detection yielded significantly higher responses for weakly ionized organic acids (e.g., formate, acetate) compared to CD, enhancing sensitivity.
• Linearity improved for multi-charged species (R² > 0.999) under QD, whereas CD showed reduced slope for weak acids.
• Dual detection clarified ambiguous quantitation by matching conductivity and charge signals to verify peak identity and purity.
• QD revealed additional minor peaks in orange juice not observed by CD, enabling detection of trace organic acids and co-eluting species.

Benefits and Practical Applications of the Method

The combined CD‐QD approach offers:

• Enhanced sensitivity for weakly ionized organic acids in beverage matrices.
• More reliable quantitation through improved linearity and proportional charge response.
• Peak purity assessment by comparing detector signals for identification of co-eluting or multi-charged analytes.
• Broader analyte coverage, detecting previously unobserved minor constituents.

Future Trends and Potential Applications

Advances in capillary IC are likely to focus on higher-pressure systems, miniaturization, and integration with mass spectrometry for structural confirmation. Further development of charge detection may extend to other weakly ionized or neutrally charged compounds. Real-time monitoring and automated peak-purity algorithms will enhance throughput in QA/QC laboratories.

Conclusion

The dual CD‐QD detection on a high-pressure capillary IC platform significantly improves the analysis of inorganic anions and organic acids in beverages. This approach increases sensitivity, linearity, and confidence in peak identification, making it a valuable tool for food and beverage quality control and research applications.

Reference

• Thermo Fisher Scientific. High-Pressure Ion Chromatography Charge Detection Webinar. Sunnyvale, CA, 2012.
• Thermo Fisher Scientific. Analysis of Carbohydrates & Organic Acids Using Capillary IC Methods Webinar. Sunnyvale, CA, 2013.
• Thermo Fisher Scientific. White Paper 70585: What is Charge Detection?. Sunnyvale, CA, 2013.