Anion Analysis

Sample preparation may not be necessary for simple sample matrices such as drinking water, because selection of analytical columns and methods is sufficient to eliminate matrix interference. For high-matrix samples, such as wastewater, matrix elimination after filtration is an important step to prepare samples. For soil samples, it may be necessary to extract the analytes from the matrix, thus removing the majority of the interfering anions.

Most often, matrix elimination removes interfering ions based on those ions having different chemical properties from the target analyte species. Solid phase extraction (SPE) is the most frequently used technique to remove interferences. For example, dissolved hydrophobic compounds in water can be removed using reversed phase trap columns with C18-based resin. Metal cations, such as iron (Fe³⁺⁾ can precipitate on an analytical column causing clogging and shorten column life. Using specific inline or offline SPE techniques, sample matrix species such as chloride, sulfate, and transition metals, can be efficiently removed.

Modern anion chromatography systems provide fast and accurate anion analysis. The systems contain the following major components:

Autosamplers: Autosamplers are designed for automatic sample loading and rinsing between samples to achieve reliable and reproducible results. Through a series of liquid handling steps programmed for chromatography applications, autosamplers are easy to use and have cost-efficient benefits.

Autosamplers introduce the samples to the ion chromatography system for sample analysis and are capable of online pH monitoring, online conductivity measurement, automatic dilution, even the online generation of standard curves. These capabilities provide many benefits. For example, prior to injection, samples with an out-of-range conductivity or pH indicate column overloading, which can automatically trigger an autodilution step. This prevents column fouling, reduces reagent waste, and eliminates bad, unusable data.

Pumps: A high-quality chromatography pump is critical to deliver pressurized samples and mobile phase to the column. Different types of pumps (single-piston or dual-piston) can be used for both isocratic and gradient elution procedures. Ion chromatography pumps can employ different flow rates and use metal-free materials, such as PEEK, to eliminate any possible metal contamination. Metal contamination can clog the column, interfere with suppressor performance, and foul the electrochemical detector.

Eluent: Modern IC techniques offer the option to automatically produce eluent (eluent generation), thereby reducing errors and variability caused by manual eluent preparation. Two common types of eluents for anion chromatography, hydroxide and carbonate, can be conveniently generated using an eluent generator cartridge. The development of electrolytic eluent generation allows the user to just add water to the system, and eluent is generated automatically and online. The use of concentrated solutions for eluent dilution is less efficient and problematic as they absorb carbon dioxide from the atmosphere. Carbon dioxide results in poor chromatography and always results in hydroxide eluents that are diluted online. Real eluent generation relies on online, electrolytically generated eluent, not dilution.

Columns: The chromatography column is the heart of the IC analysis. There are two types of IC columns for anion analysis: carbonate optimized columns and hydroxide selective columns. Carbonate optimized columns are suited for isocratic separation of anions in simple matrices using carbonate or carbonate/bicarbonate eluents. Hydroxide selective columns are suited for both isocratic and gradient separations using hydroxide eluents, and typically provide higher sensitivity than carbonate optimized columns.

Advances in column technologies have provided a variety of anion exchange columns with resins of different chemical properties. Columns with different chemical properties are created for the selectivity of different analytes. The following parameters can be considered for choosing a column for separation:

• Capacity: High capacity columns can efficiently eliminate matrix anions by preventing the interference of trace target anions with high ionic strength matrix anions, thereby eliminating the need for sample preparation. High capacity columns are preferred when a high ratio of matrix to analyte ion is expected, but require longer run times thus slowing down throughput. For simple samples, a low capacity column can be used to deliver a faster separation.
• Inner diameter: Columns with smaller inner diameter, such as microbore (1–2 mm) and capillary (<1 mm) columns, provide the advantage of high mass sensitivity, an important factor when a limited amount of sample is available. Smaller diameter columns also consume less eluent compared to standard-bore columns.
• Particle size: Traditionally, IC columns have used 7–9 µm resin particles. However, recent development of smaller 4 µm particles offers more benefits for separation. When a high pressure ion chromatography system is available, smaller particle size resin provides better separation efficiencies; thus shorter columns can be used to reduce run times compared to larger particles.

Suppressor: Introduced in 1975, suppressors offer IC the advantage of reducing the conductivity background while increasing the analyte conductivity. Selection of a particular suppressor is based on the eluent, analyte, and matrix concentration. It also depends on whether organic solvents are used. The use of an electrolytically regenerated suppressor eliminates the need to prepare and deliver the reagents required for the regeneration of the suppressor. Outdated suppressors require the addition of toxic reagents, while electrolytic suppressors are simply plug-and-play devices.

Detector: Traditional UV detectors are not generally good for anion analysis because most analytes of interest separated by ion exchange chromatography lack a chromophore. Thus conductivity detection in suppressed conductivity mode is now the main detector used for anion analysis, and has the suppressor advantages mentioned above. When suppressed conductivity is combined with postcolumn derivatization that uses Vis/UV detection, the detection limit is further reduced. Postcolumn derivatization has been successfully used for trace contaminant anion analysis, such as for bromate and chromium.

With reagent-free ion chromatography (RFIC) and high pressure ion chromatography, even trace levels of anions can be measured accurately in 10–30 minutes. The analyst can choose from different instrumentation depending on his/her specific needs, including RFIC and high pressure systems for fast performance without sacrificing sensitivity.

One important component of meeting regulatory compliance for anion analysis is data analysis and reporting. Very often, a lab has to transfer the data to a spreadsheet manually for data calculation and assessment. As a result, it is prone to human error, and it is difficult to track the data versions. A powerful software information system that is able to integrate with informatics systems, such as LIMS for total workflow management, is necessary for instrument control, automation, and data processing.