Pump maintenance is an important part in the liquid chromatography system module. Actually, many problems in LC systems can be traced back to the pump. Normally, the pump of the liquid chromatography system will provide continuous and stable liquid flow at hundreds of bar pressure. In a series of normal operation of liquid chromatography system, the system pressure should not change sharply, and the smaller pressure fluctuation is a normal phenomenon. Most of the advanced LC systems on the market have the function of monitoring the pressure fluctuation of the pump. Although pressure fluctuations differ with system and method, sharp changes or deviations from expected values are clear signs of pump problems. Another sign that pump problems may produce is abnormal signals, such as detection of baseline noise rise or significant deviation of retention time, which means the pump should be checked.
When back pressure has a steady increase, it usually indicates that the module component is contaminated. The most possible reason is that the pump is introduced into the LC system for particle contamination. Particles may also cause frequent failures of degassing machines or blockage of columns. Usually the mobile phase is the most common cause, but once the pressure starts to rise, eliminating blockage becomes a problem with priority.
Tracing the root of the back pressure rise should start from downstream of the detector. Because the column is usually the greatest contributor to system back pressure, it is always a challenge to identify whether the root cause is columns or other components in the system. When checking the source of the increase of back pressure, the column should be taken and replaced by two channels, or replaced by “limited capillary “(the capillary provides a certain back pressure, which can achieve reliable troubleshooting). It is suggested that the experimenter should know the back pressure range of his instrument system under normal use, which will help to find the problem quickly and trace the source. Disconnect the pipeline from the components that move steadily upstream towards the degasser until the back pressure drops. The part that causes the pressure to return to normal when disconnected is the root cause.
During the maintenance of the pump, the system pressure should be paid special attention to. Monitoring system pressure is a useful means of diagnosis, pressure anomalies are often accompanied by: pump failure, bubble in the system, leakage or blockage, baseline noise, abnormal retention time. And the pump structure maintenance, mainly involves the following three aspects:
• Replacement of plunger rods and septa;
• Replacement of vent valve spool;
• Clean and replace inlet one-way valve and outlet ball valve.
Pump problems can usually be attributed to two common causes: bubbles in the pump, and pump septa or pump valves. During the use of LC, special attention should be paid to the degassing of mobile phase (some modern LC systems have on-line degassing function of mobile phase). Bubbles in pipes or components can usually be flushed with high flow organic solvents. When washing system and pipeline, it is best to use high flow rate acetonitrile (if necessary, use “high flow rate acetonitrile-high viscosity IPA– high flow rate acetonitrile” step to wash the bubble in the system. Do not exceed the system pressure), and note that the column should be and install two-way valve. In multi-line system, flushing all pipelines at the same time can solve the problem of bubble residue.
Replacement of plunger rod and septa
If the system problem still exists after the bubble is eliminated, the septa or valve can be checked for wear or damage. Septa contamination or valve malfunction is usually manifested as abnormal pressure fluctuations that affect the baseline, significant increase or decrease in system pressure, or drift of retention time. Buffer salts, harsh solvent conditions, extreme pH can all challenge septa tolerance. If you run the system under this extreme condition for a long time, you may need to replace septa and valves more frequently. Although some disassemble is required, replacing septa or valves on most systems is a relatively simple and fast task, so the experimenter can do disassemble himself. Many LC instrument manufacturers have provided pump maintenance kits containing all possible replacement components such as pipes, wrenches, nuts, etc., as well as detailed operating instructions. Following is an example of a common Agilent LC system pump module diagram (other modules may differ slightly):
Traditional LC system pump module
As above picture shows, the red indicator is a septum, which should be checked and replaced every month (the septum is made of different materials, so be sure to use solvent compatible materials). Inspection and maintenance of plunger rod assemblies should be carried out every year. It is recommended to make regular replacement plans in advance rather than solve it until the problem occurs, especially in QA/QC environments where the instrument is frequently used and maintaining the shortest system downtime is required.
Change vent valve core
The vent valve (flushing valve) is a useful tool for flushing and pouring pumps. Also take Agilent commonly used LC pump module as an example. This LC system is equipped with an emptying valve and a PTFE filter element (also known as a spool, filter white head) that protects the rest of the system from particle contamination. As time goes by, the PTFE filter is prone to pollute or clog, which means the filter should be replaced. Filter element replacement is easy to operate, see the figure below:
Replacement of vent valve core
Like pump septa, regular replacement of filter elements is of great significance. The vent valve has a septa, please fix it to the sealing position with a plastic cap. Minor leaks may occur here. When the flushing valve is dismantled two or three times, pay attention to replacing the septa.
As mentioned above, the retention time drift may be due to pump problems, and if the pump is running normally (or the current pump setting is consistent with the current solvent type), the problem may be on the proportional valve. Proportional valve, also known as gradient valve or solvent selection valve, will degrade over time and need to be replaced. Like the degassing machine, a water flushing system is required after the use of a mobile phase containing buffer salts, which can extend the lifetime of the valve.
Cleaning and replacement of inlet and outlet ball valves
Take Agilent LC pump module as an example, the most important maintenance step of the inlet one-way valve is to replace the spool, as picture shown below:
The maintenance of the outlet ball valve is usually carried out by dismantlement and cleaning:
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