Common troubleshooting methods for ion chromatography!

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In the environmental monitoring and analysis laboratory, the ion chromatograph is used to determine anions in precipitation, groundwater, and clean surface water. During the few years since its operation began, the ion chromatograph has functioned well, but some problems have also arisen, such as increased system pressure, decreased pressure or no pressure at all, and prolonged or shortened retention times of the chromatographic peaks. To ensure the proper operation of the instrument, a thorough inspection should be carried out to identify the cause of the malfunction, and the issue should be resolved promptly so that the instrument can function properly. 1. Increase in system pressure: When starting up an ion chromatograph, it is necessary to set a high-pressure limit, which is usually around 15 MP. To protect the ion chromatograph, the system automatically shuts down when the system pressure reaches the high-pressure limit. Normally, when the system pressure exceeds 30% of the normal level, it can be considered that the system pressure is abnormal. The increase in pressure is related to the following situations: (1) The filter elements in the protection column and the filter membranes used for online filtration experience a gradual rise in pressure due to the accumulation of substances. At this point, the filter element and filter membrane should be replaced. (2) A blockage in a certain section of pipe causes a sudden increase in system pressure. Check section by section and replace it. (3) When the room temperature is low, such as below 10°C, the system pressure will increase. Try to keep the room temperature above 15°C. (4) When an organic solvent is mixed with water, the viscosity of the solution leads to changes in density, which in turn increases pressure. (5) When the flow rate exceeds 0.7 mL/min of normal pressure, the pressure also increases. 2. The system pressure drops, or it decreases when there is a leak in a system with no pressure. Carefully check whether all connections are tightened. Furthermore, when there are a large number of bubbles in the system’s flow path, cavitation is formed inside the pump; once the pump is started, no system pressure is indicated and no solution flows out. At this point, stop the pump first. Loosen the screws on the peristaltic pump, and use a 10 mL syringe to draw out any air bubbles inside the pump. This process can be repeated several times until all bubbles have been removed. After that, restart the pump. 3. An increase or decrease in retention time, that is, a change in the retention time of chromatographic peaks, can affect the qualitative and quantitative analysis of the components being tested. In chromatographic analysis, a stable retention time is essential for obtaining accurate and reliable results. The factors that affect the stability of retention time in ion chromatography include the following: (1) Certain parts of the instrument may leak, such as when connections are not tightened properly. (2) Bubbles in the system prevent the pump from delivering the washing liquid at the set flow rate. (3) The exchange capacity of the separation column decreases, resulting in a shortened retention time. (4) Effect of prolonged storage time of the sodium bicarbonate and sodium carbonate wash solutions on retention time. Solution: (1) Use stock solutions of sodium bicarbonate and sodium carbonate; replace the washing solution weekly with superwater having a conductivity of less than 0.5 Us/cm, as well as 3 mol/L dilute sulfuric acid solution. (2) All water used in the experiments was ultra-pure water with a conductivity of less than 0.5 μS/cm. Conclusion: Due to long-term use, pollutants have accumulated on the columns of the protection column as well as on the filter membranes used for online filtration, which has resulted in an increase in pressure in the ion chromatograph ； Leakage or bubbles entering the system will reduce the pressure ； The prolonged storage time of the sodium carbonate and sodium bicarbonate leachates affected the peak emergence time. Solution: Replace the filter elements and membranes, and clean the guard column and separation column of the ion chromatograph ； Tighten the joint or remove air bubbles ； Replace the mobile phase in a timely manner. Daily maintenance of ion chromatographs 1. Cleaning and maintenance of the chromatography column The chromatography column is one of the key components of an ion chromatograph; the separation of various ions in the sample takes place within this column. Therefore, the maintenance of the chromatography column is particularly important. Under no circumstances should the column collide, bend, or experience severe vibration. When the column is connected to the chromatograph, the valves or tubing must be thoroughly cleaned ； Pay attention to the degassing of the mobile phase ； Avoid using highly viscous solvents as the mobile phase ； The actual sample must be pre-treated before measurement, with strict control over the amount of sample introduced ； After completing the daily analysis work, it is necessary to clean the residual sample from the injection valve, and the column should also be cleaned using an appropriate solvent ； If the analysis column is not used for an extended period of time, the anion column should be rinsed thoroughly with pure water, and 3% H2BO3 (30 mL) or NaOH should be passed through it; thereafter, both ends of the chromatography column should be sealed and stored properly ； Columns that will not be used in the short term can be preserved by running water through them once a week. 2. Prevent damage caused by running the pump with no liquid inside. While the pump is in operation, it is necessary to constantly monitor the solution, being careful not to let the mobile phase in the bottle run out; it is strictly prohibited to draw the solution completely dry. Otherwise, the operation of the vacuum pump will cause wear on the plunger, and at the same time the zirconium rod of the seal ring will be worn out, ultimately leading to damage to the high-pressure pump and leakage. The filter head must always remain submerged at the bottom of the solution to prevent bubbles from being drawn in as it rebounds upward. The machine must be turned off when changing the solution. The eluent used in ion chromatographs must be filtered through a 0.22 μm filter membrane; other solvents and samples used as well must be filtered through such a membrane first, to prevent particles in them from clogging the column and reducing its efficiency ； For the analysis of unknown samples, they should first be diluted 100 times before sampling, and an appropriate dilution factor should be selected based on the results obtained ； The entire system must not contain bubbles, otherwise it will affect the separation efficiency. During pump operation, the flushing liquid should be added at appropriate intervals to prevent the solution from running out and damaging the pump ； Before and after testing the sample, the column should be repeatedly rinsed with flow to eliminate interference and ensure good column efficiency.