Accurate and Precise Automated Dilution and In-line Conductivity Measurement Using the AS-AP Autosampler Prior to Analysis by Ion Chromatography

Importance of the Topic

The accurate and precise dilution of samples combined with real-time conductivity measurement is critical in ion chromatography workflows. High-ionic strength samples, such as fracking wastewater or concentrated brines, frequently exceed column capacity or calibration ranges. Automating dilution decisions and sample handling improves throughput, reduces manual errors, and ensures regulatory compliance in environmental, industrial, and quality-control laboratories.

Objectives and Study Overview

This study demonstrates the integration of the Thermo Scientific Dionex AS-AP Autosampler with the Sample Conductivity and pH Accessory for automated dilution decisions based on in-line conductivity measurements. The goals were to validate dilution accuracy, assess reproducibility of conductivity readings, compare mixing approaches, and establish best practices for reliable pre-injection handling prior to analysis by suppressed-conductivity ion chromatography.

Methodology and Instrumentation

The workflow uses suppressed conductivity detection on a RFIC system. Samples pass through the AS-AP Sample Conductivity and pH Accessory, where conductivity is measured against user-defined limits (0–1500 µS/cm). If readings exceed the cutoff, a conditional program in the Chromeleon CDS triggers an automated dilution routine (vial-to-vial or partial-loop injection). Various draw/dispense speeds, syringe volumes, and mixing methods (carousel shaking or draw-and-dispense) were evaluated across multiple dilution factors (10× to 1000×). Precision (%RSD) and accuracy were quantified by mass measurements and chloride peak area comparisons.

Instrumentation Used

• Thermo Scientific Dionex ICS-2100 RFIC system with RFIC-ER suppressor
• Dionex AS-AP Autosampler with Sample Conductivity and pH Accessory
• Chromeleon CDS version 6.8 or 7.1
• Dionex EGC III KOH eluent generator cartridge
• Dionex IonPac AG18 guard and AS18 analytical columns
• Dionex CR-ATC anion trap column and ASRS 300/ERS 500 suppressor
• Analytical balance (±0.001 mg) and 18 MΩ·cm degassed water

Main Results and Discussion

• Conductivity measurements at 2 g/L and 0.2 g/L chloride showed excellent reproducibility (%RSD < 0.06).
• Dispensing accuracy of the 1 mL syringe was > 99% for volumes from 70 µL up to several mL; accuracy dropped to ~93% at 5 µL.
• Mixing by carousel shake in 10 mL vials yielded improved precision and accuracy as final volume decreased; optimal at 2 mL final volume (%RSD ~0.8%, accuracy ~98%).
• Draw-and-dispense mixing in 10 mL vials improved results with 10 mixing iterations (accuracy ~103%, %RSD ~6%).
• For 1.5 mL vials, two draw-and-dispense iterations at 70% vial fill achieved %RSD < 1% and accuracy > 97% for 10× and 100× dilutions.
• Positioning dilution vials in peripheral rack rows increased mixing efficiency due to higher centrifugal forces.

Benefits and Practical Applications

• Eliminates manual conductivity checks and manual pipetting errors.
• Automatically adapts injection volumes to maintain analyte concentrations within calibration ranges.
• Saves reagents, reduces column overload, and shortens turnaround times.
• Supports regulated methods in environmental monitoring, industrial QC, and research.

Future Trends and Applications

Emerging directions include tighter integration of autosampler logic with chromatographic data systems, AI-driven predictive dilutions, multi-step microfluidic dilution workflows for ultra-low volume samples, and expansion of in-line sensor arrays (e.g., UV, pH, conductivity) for comprehensive sample characterization prior to injection.

Conclusion

The combination of the Dionex AS-AP Autosampler, Sample Conductivity and pH Accessory, and Chromeleon CDS enables precise, reliable, and fully automated sample dilutions based on in-line conductivity thresholds. Proper selection of syringe speeds, mixing methods, and vial configurations ensures high accuracy and reproducibility, enhancing the robustness of ion chromatography analyses.

Reference

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