Principle
Ion chromatography is a high-performance liquid chromatography technique specifically designed to separate and detect ionic substances in aqueous solutions. Its core principle for determining anions in water is based on the combination of ion exchange separation and conductivity detection. The analysis process typically consists of three main stages: sample injection, column separation, and detector detection.
First, the water sample is introduced into the system through an autosampler and enters the anion exchange separation column with the eluate, an alkaline solution such as the sodium carbonate-sodium bicarbonate buffer system. The separation column is filled with a surface-functionalized polymer or silica matrix with anion-exchange groups such as quaternary ammonium groups bonded to its surface. Different anions (e.g., F⁻, Cl⁻, NO₂⁻, Br⁻, NO₃⁻, PO₄³⁻, SO₄²⁻) pass through the column at different rates due to different ion exchange equilibrium constants (affinity) between them and the stationary phase, resulting in separation. The separation process can be described by the selectivity coefficient K, the simplified formula of which can be expressed as:
K = [concentration of ions on the stationary phase] / [concentration of ions in the mobile phase]
The separated anions flow out of the separation column and enter the suppressor. Suppressors are key components of ion chromatography, and their function is to convert high-conductivity eluents (e.g., Na₂CO₃/NaHCO₃) into weak acids with low conductivity (e.g., H₂CO₃) through ion exchange reactions, while converting sample anions into corresponding high-conductivity acids (e.g., HCl, HNO₃). This process significantly reduces the background conductance and amplifies the conductance signal of the target ion, greatly improving detection sensitivity.
Finally, the treated solution flows into the conductivity detector. The detector measures the change in the conductivity of the solution, and its output signal is proportional to the concentration of anions in the solution. The data system records chromatograms of conductivity over time and quantitatively analyzes them by retention time qualitatively and by comparing peak area or peak height with standard curves.
Technical advantages
This method is widely used in the field of water quality analysis, mainly due to its outstanding technical features.
This method has excellent multi-component simultaneous analysis capabilities. A single injection can continuously measure a variety of common inorganic anions and some small molecule organic acids in about 20 minutes, with high analytical throughput and significant improvement in work efficiency.
Sensitivity and detection limits performed well. Thanks to chemical suppression technology and high-sensitivity conductance detectors, the detection limit of most common anions can reach micrograms per liter (μg/L), which can meet the monitoring needs of trace components in various water quality standards.
The method is selective and has strong anti-interference ability. Based on the separation of ion exchange mechanism, it can effectively distinguish ions with similar properties (such as NO₂⁻ and NO₃⁻), and most neutral molecules or organic matter in the sample matrix do not interfere with the determination.
The method has a high degree of automation, from injection, separation, detection to data processing, which can be automatically completed by the instrument, which is easy to operate and has good repeatability, which is conducive to standardized operation and quality control in the laboratory.
It has a wide range of applications. In addition to environmental water samples such as drinking water, surface water, and groundwater, this method is also suitable for the determination of anion content in various water bodies such as industrial water, wastewater, and precipitation, providing a reliable tool for environmental monitoring, industrial process control, and scientific research.
Typical Anion Analysis Performance Examples
| Analyze ions | Typical Detection Limit (μg/L) |
| Fluoride ions (F⁻) | 1-5 |
| Chloride Ions (Cl⁻) | 2-10 |
| Nitrite (NO₂⁻) | 5-15 |
| Bromine (Br⁻) | 5-20 |
| Nitrate (NO₃⁻) | 5-15 |
| Phosphate (PO₄³⁻) | 10-30 |
| Sulfate (SO₄²⁻) | 5-20 |