Environmental Water Applications Notebook

Significance of the Topic
Protecting high‐efficiency chlor‐alkali membranes from scale and poisoning requires monitoring trace calcium and magnesium in 30 % NaCl brine to levels below 50 µg/L total hardness. Conventional spectroscopic or wet chemical methods are labor‐intensive, costly, and often lack the convenience of a single run for both divalent and monovalent cations. An automated, capillary IC approach offers rapid, sensitive, and cost‐effective analysis to extend membrane life and improve process control.

Objectives and Study Overview
This study demonstrates a robust IC method for quantifying low µg/L levels of calcium and magnesium in saturated brine. Key goals are to eliminate the overwhelming sodium matrix, concentrate the target divalent ions, and achieve reliable quantitation in a single chromatographic run.

Methodology and Instrumentation
• Sample preconditioning: Adjust brine to pH 11.5 with 500 mM NaOH to maximize MetPac CC‐1 selectivity for Ca2+ and Mg2+.
• Sample handling: Inject 100 µL of brine through a precleaned TMC‐1 trap to remove Ca/Mg carryover in rinse reagents.
• Concentrator column: MetPac CC‐1 (5 × 23 mm) retains Ca2+ and Mg2+ while allowing Na+ to pass.
• Matrix removal: Rinse MetPac with 1 mM HCl at 2 mL/min for 20 min to eliminate Na+ and other matrix ions.
• Separation: Switch MetPac in-line with IonPac CG12A (5 × 50 mm) guard and CS12A (5 × 250 mm) analytical columns. Gradient elution with 20 mM methanesulfonic acid at 1.0 mL/min resolves Ca and Mg in 30 min.
• Detection: Suppressed conductivity (CSRS ULTRA, 4 mm, recycle mode, 100 mA).

Main Results and Discussion
• Linearity: Calcium and magnesium are linear from 5 to 200 µg/L (r2 > 0.999)
• MDLs: ~0.2 µg/L for both cations (n=7 replicates)
• Precision: <1 % RSD for retention time and <1 % RSD for peak area in low µg/L standards
• Recovery: 5 µg/L spikes in brine recover Ca and Mg at 100 ± 5 %
• Sodium interference: 20 min HCl rinse successfully removes a 10 000:1 Na:Mg ratio without biasing Ca and Mg peaks
• Ruggedness: Continuous operation with electrolytic MSA eluent generation (EG40) and self‐regenerating suppressor reduces downtime and reagent use

Benefits and Practical Applications
• Single‐run analysis of Ca2+ and Mg2+ in high‐strength brine simplifies lab workflow and reduces operator error
• High capacity MetPac CC‐1 concentrator column extends the method detection range to trace levels in a large Na+ matrix
• Automated eluent generation and suppression minimize maintenance, eliminate bulk reagent storage, and lower waste disposal costs
• Enhanced sensitivity enables tighter process control in chlor‐alkali and other brine‐based industries

Future Trends and Possible Applications
• Integration of two‐dimensional concentrator technology to monitor other trace metals in brines
• Coupling with mass spectrometric detection for speciation of metal complexes in challenging matrices
• Development of field‐deployable capillary IC systems for on‐site brine quality monitoring
• Expansion to continuous, on‐line process analyzers for real‐time feedback control in industrial desalination and chlorine production plants

Conclusion
The described IC method using a MetPac CC‐1 concentrator column, IonPac CS12A separation column, and electrolytic MSA eluent generation provides a rapid, sensitive, and reliable tool for trace calcium and magnesium determination in high‐strength brine matrices. Its ruggedness, low detection limits, and fully automated operation meet the demanding requirements of chlor‐alkali and desalination process control.

Reference
1. J.W. Clarke et al., J. Chromatogr. Sci. 23 (1985) 510–517.