Portable light weight open tubular ion chromatograph for field determination of environmental anions

Measuring Nitrite and Nitrate in Rain and River Water Samples Using a Portable Ion Chromatograph in Step-gradient Mode and High Sensitivity Detection Flow Cell

With the increasing environmental pollution issues, there is a growing need for sensitive and real-time monitoring of pollutants. Nitrite and nitrate are common nutrients that are related to water quality. This study aims to enhance nitrate and nitrite detection capabilities using a portable ion chromatography-based nutrient analyzer, Aquamonitrix. By optimizing it for ultra-low detection limits (LODs), we address challenges in environmental water quality assessment in Tasmania, Australia. Using step-gradient mode with a stereolithography three-dimensional printed flow cell with a 5 cm optical path length, a 300 µL injection loop, and 60 mM KOH as eluent, LODs of 0.004 µg/mL for nitrite and 0.023 µg/mL for nitrate were achieved. Further improving to 0.008 µg/mL for nitrate with a 10 cm optical path length flow cell and 120 mM NaCl as eluent. A repeatability assessment over 84 automatic runs showed a relative standard deviation under 1.42% for peak area and 0.49% for retention time. The system demonstrated tolerance to salinity, handling up to 5 parts per thousand in artificial seawater. Comparative analysis of environmental samples revealed that nitrate levels in Tasmanian rainwater were five times lower than in Ireland. An average concentration of 2.08 µg/mL nitrate was found in Tamar River samples, aligning with local commercial lab data. Real-time, on-site analysis along the Derwent River detected an average nitrate concentration of 0.17 µg/mL. Validation against conventional standard ion chromatography showed no significant differences (p > 0.05), underscoring Aquamonitrix's robustness for field-based water quality monitoring. A 5-day deployment of Aquamonitrix further demonstrated the system's reliability under significant temperature fluctuations between day and night.

Cost-effective low-volume sample introduction system for open tubular capillary ion chromatography via a hybrid negative pressure approach

Open tubular capillary ion chromatography (OTIC) exhibits a minute sample/eluent volume separation with a low-pressure operation (<30 psi) and potentially develops for a portable analytical device. In this study, a novel and cost-effective low-volume sample-introduction system has been developed for the OTIC and applied for separating water-soluble small inorganic ions. This innovative design uses a hybrid negative pressure (HNP) system, considerably reducing sample consumption to ∼7 μL per injection while maintaining excellent repeatability (<4 % relative standard deviation) on a 50 μm bore capillary tube. Various sample-introduction parameters, such as moving time of the sample zone, injection pressure, and sample viscosity, were optimized for the separation of model anions (Cl-, Br-, NO2-, and NO3-) and cations (Na+, K+, Ca2+, and Mg2+) in a non-suppressed OTIC mode. The performance of this HNP-OTIC system was demonstrated through the analysis of water-soluble small inorganic ion determination in low-volume particulate matter extracted samples, yielding recovery percentages of 78-119 % for cation determination in PM10 samples. In addition, the determination of K+ in PM2.5 samples exhibited no significant difference from the results obtained using a commercial packed IC. The cost-effective and efficient OTIC system has the potential to become a promising alternative for atmospheric analysis, especially in tracking K+ as an ion tracer for PM2.5 source identification.

Application of a portable ion chromatograph for real-time field analysis of nitrite and nitrate in soils and soil pore waters

Real-time monitoring of nitrite and nitrate is crucial for maintaining soil health and promoting plant growth. In this study, a portable ion-chromatograph (IC, Aquamonitrix) analyser, coupled with a field-applicable ultrasonic-assisted extraction method, was utilised for in-field determination of nitrate and nitrite in soils. This is the first application of this type of analyser to soil nutrients. On-site analysis of soil from a local sports field showed 94.8 ± 4.3 μg g-1 nitrate, with LODs of 32.0 μg g-1 for nitrate and 5.4 μg g-1 for nitrite. The results were in close agreement with those obtained using a conventional lab-based IC. Relative standard deviations (%RSDs) for soil analysis using Aquamonitrix were consistently below 10%. The obtained average recoveries of samples spiked with nitrite were 100% and 104% for the portable IC and conventional IC, respectively. Furthermore, to assess the suitability of portable IC for samples with high organic matter content, various natural organic fertilisers were extracted and analysed. The results showed 16.2 ± 0.7 μg g-1 nitrite and 28.5 ± 5.6 μg g-1 nitrate in sheep manure samples with LODs of 2.0 μg g-1 for nitrite and 12.0 μg g-1 for nitrate. The portable IC system was further demonstrated via real-time on-site analysis of soil pore-water acquired using a portable battery-based ceramic pore-water sampler. A continuous increase in nitrate concentration over time was observed (from 80 to 148 μg mL-1) in the soil pore-water in a vegetable garden four days after heavy rain. Unlike conventionally sampled natural waters, 7-day storage of the studied pore water samples revealed no changes in nitrate concentrations. An average of 558 ± 51 μg mL-1 nitrate was detected in the soil pore-water samples analysed on a spinach farm, immediately after irrigation.

On-column capillary suppressor for open tubular ion chromatography

BACKGROUND

Suppressors with different dead volumes are required to match different suppressed ion chromatography systems. Especially for suppressed open tubular ion chromatography (SOTIC), the dead volume is a critical parameter. Both connection tubes between open tubular (OT) columns and suppressors and the dead volumes of the suppressors should be as short/small as possible to minimize peak dispersion. Suppressors with different dead volumes are required to match the various suppressed ion chromatography systems that operate at low flow rates 20-200 nL/min.

RESULTS

We describe three designs of on-column capillary suppressors for SOTIC: (A) on-column electrodialytic suppressor prepared by making small cracks on the cycloolefin polymer (COP) capillary at targeted locations, (B) on-column electrodialytic suppressor built on a polyether ether ketone (PEEK) capillary by removing the wall materials at target locations, (C) on-column chemical suppressor based on a single cut on a PEEK capillary at a targeted location a single cut on a PEEK capillary at a targeted location. The on-column electrodialytic suppressors work in two different modes with suppression voltage applied in co-current and counter-current direction to the eluent flow. Because of very narrow column inner diameter (i.d.), up to several hundred volts were required to suppress the hydroxide eluent, but it was found the there was a >90% loss of analytes in the suppressor accompanied with a high noise level after on-column electrodialytic suppression. Theoretical analysis reveals that high suppression voltage significantly affects the retention of specific analytes by electromigration. Further analysis indicated that the electrodialytic on-column suppressor in co-current mode would behave totally different from traditional suppressors. The on-column chemical suppression, with minimum dead volume of 0.27 nL, provides fairly well suppression of low hydroxide eluent without analyte loss in the suppressor. In design C, an efficiency of 47000 ± 1800 plates/m for Cl-, corresponding to a peak volume of 17.9 ± 0.7 nL, was obtained when separating five anion mixture (0.5 mM each) in the 25 μm i.d. AS18 latex coated PEEK OT column with an injection of 7.3 nL. Theoretical calculation revealed that a column efficiency loss of ≤3% would result in a cylindrical chemical suppression channel and thus it is taken as the acceptable dispersion contribution originating from the on-column chemical suppressor.

SIGNIFICANCE

Different on-column suppressors have been designed on OT columns with i.d.s less than 30 μm. Two electrodialytic on-column suppressor designs with eluent flow parallel to the direction of electric field were proposed and tested. The eluent flow rate, analytes' retention behavior, resistance of suppression channel, current-voltage relationship, and working principles in both co-current and counter-current were experimentally investigated and comprehensively discussed. It was found that although the on-column electrodialytic suppressions (Design A and B) are not feasible in practice, the electrodialytic on-column suppressor on co-current mode has a potential of being used as an enriching capillary column for analyte ions. Design C provides fairly well chemical suppression. Theoretical calculation indicates that the loss of column efficiency can be controlled within 3%.