The Guide to the Five Key Technical Points of Ion Chromatography

01 Principle of Ion Chromatograph

Ion chromatography separates ionic substances using ion-exchange resin with a low exchange capacity as the stationary phase and continuously detects changes in the conductivity of the eluents with a conductivity detector.

02 Types of Ion Chromatography

Ion chromatography can be divided into three types: ion-exchange chromatography, ion-exclusion chromatography, and ion-pair chromatography.

03 Basic Components of Ion Chromatograph

An ion chromatograph mainly consists of the eluent system, detection system, chromatographic pump system, injection system, flow path system, separation system, chemical suppression system, and data processing system.

Figure 1: Explanation contact angle.Image from Linseis

04 How to Operate More Standardly?

The workflow of an ion chromatograph is generally similar across different models. So, how can it be operated more standardly?

1. Perform the necessary checks on the eluent system. Open the argon gas cylinder valve, and adjust the pressure regulator to indicate 0.2-0.3 MPa. Open the air source device of the eluent system and adjust the pressure regulator to show 3-6 psi.

2. Sequentially turn on the power switches of the main unit, computer, printer, and other devices to power up the equipment.

3. After powering up the system and control system, enter the operation interface and system operation panel to prepare and manage pre-operation tasks.

4. Turn on the pump. If the chromatograph has not been used for a long time or after replacing the eluent, open the PRIME valve on the pump head to release air before starting the pump. After the pump pressure stabilizes, turn on the suppressor.

5. Before entering the chromatographic column, introduce the sample through the sampler, where the mobile phase carries the sample into the column. In the column, the components are separated and flow sequentially with the mobile phase to the detector.

6. The signals detected by the detector are sent to the data system. After completing the sample operation using the interface, select the detection standards and proceed to data processing, including recording, processing, printing, or saving the collected data.

7. Shutdown: The shutdown process depends on the sample being tested. For anions and cations, turn off the suppressor current first, then turn off the pump, and finally shut down the main unit.

05 Common Problems and Solutions of Ion Chromatographs

1. Common Issues with Conductivity Detectors

   • The most common issue with conductivity detectors is contamination of the detection cell.

   • Cause: Contaminants mainly come from samples that have not been adequately pretreated, such as those with high concentrations or complex sample matrices.

2. Common Issues with the Analytical Pump

   • Problem: Increased baseline noise and deteriorated chromatographic peak shapes (e.g., erratic peaks).

   • Solution: Common issues with the analytical pump include air bubbles in the pump and liquid leakage.

3. Common Issues with Suppressors and Solutions
   Suppressors play a crucial role in ion chromatographs, and their performance significantly affects analytical results. The most common issue with suppressors is leakage, which leads to a decrease in peak area (sensitivity loss) and an increase in background conductivity.

   • (1) Decreased Peak Area

     Causes: The main causes of decreased peak area include membrane dehydration, suppressor leakage, restricted solution flow, and membrane contamination. If the suppressor has not been used for an extended period, the membrane may dehydrate. To activate the suppressor, inject a small amount of 0.2 mol/L sulfuric acid solution into the anion suppressor using a syringe in the opposite direction of the eluent flow. Inject a small amount of purified water into the regenerant inlet and let the suppressor sit for more than half an hour. Contaminated metal ions in the suppressor can be cleaned with sodium oxalate.

   • (2) High Background Conductivity

     During chemically suppressed conductivity detection, high background conductivity indicates an issue with the suppressor. Most problems are caused by improper operation, such as blockage in the eluent or regenerant flow path, no solution flow in the system leading to elevated background conductivity, or the current setting of the electric suppressor being too low. If the membrane is contaminated, its exchange capacity decreases, causing background conductivity to rise. A malfunctioning suppressor will show a continuous increase in background conductivity, indicating the need to replace it.

   • (3) Leakage

     The main cause of suppressor leakage is insufficient membrane hydration. Therefore, a suppressor that has not been used for an extended period should be allowed to hydrate before use. Additionally, ensure that the regenerant outlet is unobstructed, as high back pressure can also cause suppressor leakage. Improper storage of the suppressor, causing the membrane to shrink or crack, can also lead to leakage.

4. How to Eliminate Air Bubbles in the Flow Path between the Mobile Phase and Pump?

   Solution: First, unscrew the plastic flow path fitting connected to the pump. Use a syringe filled with deionized water to inject water into the flow path tubing connected to the pump, eliminating the air bubbles. Then, elevate the mobile phase bottle (usually a deionized water bottle) and reconnect the flow path fitting to the pump. Start the pump, open the pump’s vent valve, and expel any remaining air bubbles. Once the eluent flows evenly, tighten the pump vent valve. (Note: During this procedure, the entire flow path should be disconnected from the chromatographic column.)

5. What Are the Symptoms of a Clogged Check Valve, and How to Fix It?

   If the check valve is clogged with microorganisms, the pump will fail to draw in liquid. The most obvious symptom is no liquid flow in the waste line or no liquid flowing when the pump is started, or the flow is very slow.

   If the check valve is clogged, it needs to be cleaned. The cleaning method is as follows:

   First, unscrew the flow path fitting and fitting 1, then unscrew fitting 2 on the left side, and use tweezers to remove the two check valves (note that the check valves have a direction; the small ring on the check valve indicates the inlet side). Place them in a 50 ml beaker, add enough anhydrous ethanol to cover the check valves, and place them in an ultrasonic cleaner for 30 minutes. Then, add a 1:1 ratio of 10% HNO₃ (diluted with anhydrous ethanol) and clean for 5 minutes. Rinse the check valves thoroughly with deionized water and reinstall them into the pump. (Note: Do not overtighten the fittings to avoid damaging the threads.)