Ion Chromatography

1. Ion Chromatography What is ion chromatography? Projects using ion chromatography Figure 1 illustrates the process of ion chromatography. An aqueous solution, called a mobile phase or eluent is used to carry the water sample into and through a column containing a positively or negatively charged resin (stationary phase). The mobile phase is of the same charge as the ions of interest, but is less efficiently retained on the resin. Initially, therefore, the target ions are retained on the resin whilst the mobile phase continues through the column. However, the target ions can be displaced by increasing the concentration of the mobile phase. The ions of interest are then detected as they leave the column, typically by conductivity or UV/Visible light absorbance. • Chromatography is the process of separating different components from a solution or a gas. Chromatographic techniques work by passing the solution (or gas) of interest over a ‘stationary phase’ - a substance with which the solution interacts. The stationary phase interacts more strongly with some components in the solution than others. This results in components passing through the stationary phase at different rates, effectively separating them for analysis. • Ion chromatography is the process of separating ions (positively or negatively charged atoms or molecules) from a solution using a stationary phase that contains oppositely charged ions. There are two main types of ion chromatography: • Anion exchange chromatography is commonly used in environmental sciences to measure negatively charged anions such as nitrate, nitrite, phosphate and sulphate. • Cation exchange chromatography is commonly used to measure positively charged anions such as ammonia, sodium or potassium. We use ion chromatography primarily to investigate water quality. Recent third year IGS projects have included analysis of nutrient levels in bathing pools on Hampstead Heath affected by toxic algae blooms, water quality in Rutland Water Reservoir and the River Mole. Examples of research at QMUL Fotis Sgouridis is investigating nutrient fluxes in a restored floodplain along the River Cole. He is monitoring ground water concentrations of anions, including chloride, sulphate and nitrate, along a transect extending away from the river. Boreholes at 40 m intervals provide a cross section of ground water nutrient levels at the site (figure 3). . Stage 1. Water sample and eluent are injected into column - - - - - - - - - - - - - - + + + + + + + - - - + + + + - - + + + - Stage 2. Anions in the sample interact with cation exchange resin in the column. The ‘blue’ anion interacts more strongly with the column and passes through relatively slowly. + + + - - - + + + + - + + + - - - - + + + + + + + - + + + + + - + + + - - + + + + - + + + + + + + + + + + + + + Figure 3. Anions in ground water on the restored floodplain of the River Cole (Fotis Sgouridis). + + + + + + + + - • Other recent projects include: • Analysis of nitrate, phosphate, sulphate, chloride and fluoride in storm flows from streams on Exmoor in collaboration with researchers from the University of Plymouth. • Analysis of nitrate levels in water samples from a potentially contaminated village water supply in China for More4 News. • Analysis of sulphate in sediment extracts as part of a sequential extraction procedure to identify binding sites on estuarine sediments. - - - - - - Stage 3. Anions leave the column. At a rate determined by the chemical properties of the anions, the resin and the eluent Figure 1. Dionex ICS 2500 Ion Chromatograph in the Physical Geography laboratories The Dionex ICS 2500 can be used for both anion and cation exchange chromatography. Figure 2. Ion chromatography.