The demand for accurate, efficient, and cost-effective water quality monitoring necessitates miniaturization of benchtop chromatography systems. Inadequate access to safe drinking water poses a significant global health concern, with certain anions like fluoride and nitrate linked to chronic diseases through long-term exposure. Chloride and sulfate also impact water aesthetics. Existing on-site test kits provide only qualitative, single-element results, while laboratory-based ion chromatography (IC) offers quantitative, simultaneous determination but suffers from bulkiness. While chip-based gas and liquid chromatography have advanced, chip-IC development lags behind. Previous attempts have relied on external pumps, valves, and detectors, limiting miniaturization. This research addresses the challenges of developing a compact, sensitive, integrated chip-IC system, focusing on the integration of a micropump (deemed non-essential due to the limited band broadening), the use of a packed column with commercially available resins to improve reproducibility, the integration of on-chip microvalves to improve fluid control, and a highly sensitive on-chip conductivity detector to enhance the detection of anions without suppression.

Existing literature highlights advancements in chip-based GC and LC, but chip-IC development remains a challenge. Studies have explored packed silicon micro-channels with external components or PMMA chips with external pumps and detectors. While integrated micropumps can reduce system size, developing an integrated and sensitive on-chip conductivity detector remains crucial. Different column configurations (open-tubular, monolithic, pillar array) have been explored, but packing with commercially available resins offers better reproducibility. Challenges include effective frit structures and robust bonding techniques for high-pressure resistance, especially given limitations of materials like PDMS. While on-chip sample injection methods exist, integrated microvalves are needed for pressure resistance and improved bonding strength. Ultrasensitive detectors like mass spectrometry and LIF are used in chip-LC, but conductivity detection (CD) is preferred for IC due to its universality. Nonsuppressed CD is more suitable for chip-IC due to the complexity of integrating on-chip suppressors, though contact mode CD can suffer from baseline noise.

This study developed a compact chip-IC system using micromilling and laser bonding techniques. A PMMA substrate was micromilled to create a channel network with cavities for integrating commercial sintered frits, valve membranes, and electrodes. Laser bonding ensured robust sealing at both homogeneous and heterogeneous interfaces, withstanding high working pressures. The five-electrode conductivity detector improved signal-to-noise ratio in the nonsuppressed mode. The system's fabrication involved micromilling to create single-layered chips, assembly with frits, valve membranes, and electrodes, and laser bonding for sealing. The efficacy of the bonding was evaluated through tensile testing and burst pressure measurements. The column was packed using a slurry method, and the packing quality was assessed via the dimensionless flow resistance (φ), which indicated favorable results. Anion standard solutions and tap water samples were analyzed using the chip-IC system, with results compared to a benchtop IC system. The five-electrode conductivity detector employs two reference electrodes, two detection electrodes, and a ground electrode. A sinusoidal current is applied to the reference electrodes, and the potential drop across the detection electrodes is maintained constant by adjusting the current. This results in a voltage variation across a sampling resistor that is proportional to the conductance, eliminating problems associated with Faradaic impedance found in traditional contact conductivity detectors. The system uses screw-driven valves, though micromotors or solenoid valves could be incorporated for automation. Data acquisition and processing were performed using a custom circuit and software.

The fabricated chip-IC system demonstrated successful separation of common anions. The five-electrode conductivity detector showed good linearity (R² = 0.996) for SO₄²⁻ in the range of 5–100 mg L⁻¹, with an LOD of 0.6 mg L⁻¹. The RSDs of retention time and peak height were low (0.2–0.5% and 0.6–2.4%, respectively). While fluoride and chloride peaks overlapped slightly under the tested conditions, this could be resolved via post-processing. Varying flow rates impacted resolution and analysis time, with a 25 µL min⁻¹ flow rate achieving separation within 8 minutes. The number of theoretical plates (N) varied with flow rate, indicating good separation efficiency. Analysis of tap water using the chip-IC system showed results comparable to those of a commercial benchtop IC system, with relative deviations of quantified concentrations less than 10%. The laser bonding technique yielded a high average normal strength (74.5 MPa) and burst pressure (10.6 MPa), exceeding values reported in previous studies, ensuring effective sealing and resistance to high pressure.

The developed chip-IC system successfully addresses the need for a compact, sensitive, and integrated platform for anion detection in drinking water. The high bonding strength achieved through the laser bonding technique and the improved sensitivity offered by the five-electrode conductivity detector are crucial advancements. The use of commercially available resins simplifies fabrication and enhances reproducibility. While minor peak overlap was observed for fluoride and chloride, post-processing techniques can resolve this issue, and optimization of buffer conditions and resin selection could eliminate this overlap. The comparable results to a commercial IC system validate the accuracy and reliability of this miniaturized platform. The relatively small size and lightweight nature make it suitable for field-deployable applications and decentralized monitoring.

This work demonstrates a compact and sensitive chip-IC system for detecting multiple anions in drinking water. The integrated design, robust fabrication process, and enhanced detection capabilities make it a promising tool for water quality monitoring. Future work may focus on integrating a micropump for complete portability, further optimizing separation conditions to improve resolution, and exploring the application of this platform to other environmental samples.

The current system uses screw-driven valves which are not fully automated, although the design allows for future integration of automated valves like micromotors or solenoid valves. The overlap of fluoride and chloride peaks under the present conditions, although resolvable by software, suggests future optimization of the chromatographic conditions is needed. The study focused on four common anions and expanding to additional ions may require further method development and optimization. The number of theoretical plates obtained for some ions were lower than reported in other studies, potentially related to the effects of the water dip in the nonsuppressed mode.