IC: Sensitivity and Accuracy in Water Purity Analysis
Using ion chromatography to measure inorganic ions in drinking water
What do you think about when you go to the kitchen sink to get a glass of water to drink? Probably nothing much, beyond “I’m thirsty.” Incredibly, this nonchalance is a luxury that is less than 50 years old. The fact that we can be nonchalant is due in part to the reliability of ion chromatography (IC) as an analytical technique to measure, among other things, inorganic ions in drinking water.
Measurement of inorganic anions in drinking water
There is a long list of inorganic anions that can be detected in drinking water using IC, including chloride, fluoride, sulfate, nitrate, and nitrite. Excess environmental fluoride can cause fluorosis of teeth and bones; water high in sulfate acquires an unpalatable rotten-egg flavor and smell; and chloride ions can react in solution with particulates to generate oxyhalides. Additionally, chlorination and ozonation can generate dangerous trihalomethanes and bromates. Nitrate and nitrite are particularly dangerous for infants susceptible to hematological defects. The maximum containment level (MCL) standards are unique for each anion, with milligram-per-liter levels acceptable for some comparatively innocuous ones, and MCLs in the low microgram-per-liter range for putative carcinogens. Innovation in column and instrument design is driven in part by progressively tighter environmental and health standards. As MCLs decrease, greater sensitivity in measurement is required.
A standard IC instrument consists stereotypically of guard, separator, and analytical columns, a suppressor device, and a conductivity detector cell linked to a mass spectrometric readout. The choice of column material and particle size, and of eluent composition, contributes to accuracy and reproducibility, while a suppressor supplies additional sensitivity. For anion detection, a suppressor commonly uses a high-capacity cation exchange column to replace eluent cations with hydronium. This serves to neutralize a carbonate or hydroxide eluent and impart acidity to analyte ions, giving them artificially high conductivity and therefore greater signal-to-noise ratios.
A balance between accuracy and sensitivity
Ion quantification with IC is sometimes a tricky balance between accuracy and sensitivity. The latter can be optimized with appropriate column choice and use of suppressors, while the former is often dependent on proper sample dilution and filtration, and general good housekeeping where instrument and reagent use and maintenance are concerned.
Suppressed conductivity measurements for bromate in trace amounts can be artificially amplified and inconsistent, whereas for a common ion such as chloride, suppression can force measurements outside of linear ranges established by instrument calibration.
A reliable strategy is to select elution reagents and column types to optimize the expected analyte retention time, favoring preliminary elution of unwanted anions preparatory to obtaining the desired one. For instance, detection of fluoride, chloride, or sulfate ions can each be accomplished with sodium carbonate eluents and separation column particle sizes in the four-to-10-micron range. However, the peak elution range of fluoride notoriously coincides with many extraneous ions that are shed from the column, resulting in interference problems. Chloride ions are often present even in deionized reagent water certified with 18-megaohm resistance, providing another source of interference. Moreover, reagent water often contains high levels of total organic carbon (TOC), which can elute in carbonate streams and alter conductivity, resulting in broad and confounding MS peaks. Finally, trace cations can cause precipitation of hydroxides in basic solutions and in water before it is equilibrated to neutral pH.
A primary solution to interference or contamination is to be assiduous about preparation of reagents and standards. For measurements to be reproducible, standard curves for analytes of known concentration are required prior to sample testing. Therefore, users must maintain highly pure reagent water, which can be vacuum-filtered and degassed to eliminate contaminating ions and TOC. Additionally, one can implement acidic gradients in which a slight decrease in pH from standards to analytes will tighten detection peaks; nitric acid is often a good substitute for water in the mobile phase. Contemporary instrumentation often maintains inline ultrafiltration systems that aid in sample preparation by removing excess particulates that can interfere with analyte quantification. Similarly, proper sample dilution can be programmed into workflows to ensure that resulting detection levels remain within a linear range compared to standards. In this way, one can take maximum advantage of a venerable and reliable platform for ion quantification.
Clean water for people and industry
The Safe Drinking Water Act and the Clean Water Act precipitated a system of regulations governing impurities in drinking, surface, and ground water, and capping allowable discharges in wastewater. Cations and inorganic anions comprise many of these impurities, and each has intrinsic conductivity in solution. IC can leverage this conductivity to partition ions of interest across exchange columns, allowing ions to be specifically eluted and quantified using pH changes and other indicators of varied conductance. In the 1980s, the Environmental Protection Agency (EPA) introduced the IC-based Method 300.0 to measure inorganic anions and disinfection byproducts in drinking water, and in 1997 updated it to Method 300.1, which still applies today. The EPA publishes and enforces analogous methods for determination of cation levels, impurities in ground and surface water, and constituents of effluent wastewater.
The intent is to protect human consumers from toxicity; however, industrial processes such as oil and gas refinement, paper production, polymer, and microchip manufacture all require vast quantities of ultrapure water or steam. Given the large volumes, and attendant issues of cost and sustainability, industrial process water must often be repeatedly recycled through facilities before being discarded as waste. Source water can contain scaling ions such as silica, calcium, and magnesium; and corrosive anions including chloride, fluoride, and sulfate. Chemical deionization before use, and filtration and decontamination preparatory to recycling or discharge, can reveal and introduce indicators and inhibitors of corrosion, which must be removed at alternating points in production cycles. Wastes, improperly handled, can contribute to some of the most devastating and inter-generational environmental hazards experienced by our society. Mitigation is dependent on changes in policy, but these can be enforced and improved most effectively by starting with accurate and sensitive measurement via ion chromatography.