Ion Chromatography

Purpose: Ion chromatography is used to separate organic or inorganic charged substances. The stationary phases used are based on typical ion exchange resins, while the pumps and detectors are high performance/high sensitivity instruments more typical of high performance liquid chromatography (HPLC). The principal detection problem in ion chromatography is the large background ion concentration from the eluant. In the Dionex instrument, the eluant stream from the separation column is passed through an ion exchanger which neutralizes the eluant while leaving the analytes in charged form, thus greatly increasing the detection sensitivity and allowing separation and analysis at low analyte concentrations. As in any form of chromatography, considerations of retention, band broadening, and resolution are also important. This experiment introduces the use of ion chromatography for the qualitative and quantitative analysis of anions in water.

References: H. Small, Ion Chromatography, Plenum Press, 1989; J.G. Tarter, Ion Chromatography, Marcel Dekker, 1987.

Equipment: Dionex Ion Chromatograph, chart recorder or Hewlett-Packard chromatography integrator, Dionex IonPac AS4A Separator Column (an anion exchange resin), water sampling bottle, volumetric glassware, compressed Argon for Eluant and Regenerant, compressed air for driving chromatograph injector.

Reagents:

• Chromatographic Solutions. Eluant: 1.8 mM Na₂CO₃/1.7 mM NaHCO₃ buffer; Regenerant: 25 mN H₂SO₄
• Standards. ~10-20 mL of each of the following anion standards: 20 µM F^-, 30 µM Cl^-, 100 µM NO₃^-, 150 µM HPO₄^2-, 150 µM SO₄^2-
• ~10 mL of one sample containing all of the above standards.

Sample: Cache Valley well or tap water, and/or snow melt

Procedure

Consult the Ion Chromatograph manual for general operating instructions. Make sure that you have enough eluant and regenerant available to complete the experiment. Hook up either a chart recorder or the chromatographic integrator to the recorder output. The Regenerant and Eluant reservoirs are pressurized with argon. The Regenerant flow rate is adjusted by changing the Argon regulator pressure; start with a pressure of 10 p.s.i. (which will also be the eluant pressure; however, the eluant flow rate is controlled by the pump, and the pressure magnitude on the eluant is not critical). The injector is driven by compressed air; use a regulator pressure of 80-100 p.s.i. For each of the samples run, record the retention time(s) and peak area(s) (if a chart recorder is used, determine peak areas via cut-and-weigh).

• Eluant Flow Rate: 2.0 mL/min. Detector Scale: 10 µS
• Regenerant Flow Rate: 3-5 mL/min. (adjust as necessary)

Qualitative Analysis

1. Run each of the individual standards.
2. Run the mixture of standards.
3. Identify the ions in the mixture.
4. Run the water sample.
5. Identify any ions present in the sample.

Quantitative Analysis

1. Choose one of the ions present in the well water sample and prepare 5-6 standards at concentrations in the range of (bracketing) the ion concentration in the sample.
2. Prepare a standard curve of peak area vs. concentration to determine the concentration of the unknown.

Report

1. Report the identity of any ions in the well water sample.
2. Report the concentration of the chosen ion in the sample.
3. Comment on the effect of other ions on the retention times of ions in the mixture.
4. What is the role of the regenerant in the experiment? Why is it necessary to have a continuous flow of it? How would the chromatogram be affected without regenerant?
5. Describe the factors that affect resolution in ion chromatography.
6. Give an example of the separation of organic compounds by ion chromatography. Specify the eluant and regenerant.