Understanding HPLC Pumps (Part II): Practical Maintenance
Part I:
Understanding HPLC Pumps: Common Misconceptions
Common HPLC Pump Terminology
Most HPLC pumps are short-stroke reciprocating piston pumps. The plunger generates flow through check valves, housed within the pump head (dark gray).
A stepper motor drives a cam that moves the plunger back and forth. During each stroke, liquid is displaced. The asymmetry check valve is closed if pressure is higher on the top; open if pressure is higher on the bottom.
Key Components
- Plunger and Piston Rod: Move reciprocally to create pressure and deliver the mobile phase precisely.
- Plunger Seal: Provides tight sealing between plunger and pump body to prevent leakage or air ingress.
- Pulsation: Refers to cyclic pressure fluctuations during mobile phase delivery. Also known as pressure fluctuation.
- High- vs. Low-Pressure Mixing: High-pressure mixing happens after the pump, while low-pressure mixing happens before the pump. The gradient delay volume in low-pressure systems is usually 2–4× larger.
- Serial vs. Parallel Pumping:
A serial pump has a main and an auxiliary head, with the stroke volume of the main head approximately twice that of the auxiliary head. The main head discharges while the auxiliary head refills, drawing in half the fluid while the other half enters the system directly. Then, the main head draws and the auxiliary head advances, discharging all stored fluid into the system. Alternating the actions, they effectively minimize pulsation.
In parallel pumps, two plungers of equal stroke volume operate 180° out of phase, delivering alternating strokes that smooth out the flow.
- Check Valves:
HPLC check valves act as one-way gates, allowing flow in a single direction to maintain stability. A valve consists of a valve ball, valve seat, core housing, sealing gasket or end cap, and valve body.
Check valves can be active or passive. Active valves are programmable and actively controllable, generally electromagnetically controlled; passive valves are pressure-driven by external forces.
| Comparison Aspect | Active Valve | Passive Valve |
|---|---|---|
| Complexity | More complex; includes controller units (e.g. electromagnets or springs) | Simpler structure |
| Operation | Actively controlled (e.g. electronically) | Relying on external forces (e.g. pressure or gravity) |
| Maintenance | Requires disassembly before cleaning | Often ultrasonically cleaned as a whole |
| Performance | Stable; less likely pressure loss due to medium properties | May experience leakage (e.g. polymer film due to acetonitrile) |
Common Pump Failures and Troubleshooting
Common pump malfunctions include abnormal pressure, unstable flow, and liquid leakage. This table includes solutions to common malfunction symptoms and their possible causes.
| Symptom | Possible Cause | Solution |
|---|---|---|
| Pressure Fluctuation | Air bubbles in system | Purge to remove bubbles Use online degasser or helium sparging |
| Blocked inlet line | Clean solvent filters (for glass filters, 30% nitric acid soaking may be used) Flush the line from solvent vial to pump head with 60°C water & IPA, including degasser and proportioning valve. | |
| Contaminated or damaged valves | Ultrasonic to clean active valve spools, inlet and outlet check valve (note: valve body may come apart) Replace the check valve if cleaning does little help. | |
| Active valve damage | Replace valve | |
| Worn seals | Replace seals | |
| Elevated pressure | System flow path blockage | Sequentially disconnect each component in the flow path (analytical column, inline filter, guard column, etc., see the flow chart below) and observe pressure changes to locate the blocked section. Then clean or replace it. |
| Retention time drift | Unstable mobile phase | Re-prepare the mobile phase. |
| System mobile phase leakage | Check if system lines and column connections are tightened; reinstall if necessary. | |
| Unstable system flow | Examine system flow path and pressure. See "Pressure Fluctuation" above for solutions to unstable pressure. | |
| Proportioning valve malfunction | Flush proportioning valve with 60°C water & IPA; replace if it does not work | |
| Unstable column temperature | Use column oven | |
| Column bleeding | Replace the column |
Preventive Maintenance Recommendations
Solvent & Mobile Phase Management
- Prepare buffer (e.g. PBS) mobile phases fresh when involved; flush the pump with ultrapure water before using buffers; after use, rinse the system thoroughly with ultrapure water (>30 mins), followed by methanol to prevent salt crystallization.
Regular Maintenance Schedule
A recommended maintenance interval is summarized in the table below.
| Task | Interval | Notes |
|---|---|---|
| Replace plunger seals | 1 year | Most frequent preventive task. Prevents leaks, pressure fluctuation, and inaccurate flow rates. Specific interval depends on usage time, pressure, and the mobile phase (buffer salts accelerate wear). |
| Clean inlet/outlet check valves ultrasonically | Every 1–3 months | Primary troubleshooting item when pressure fluctuates or is too low. Performance can be restored after ultrasonic cleaning; if ineffective, replacement is required. |
| Clean/replace inline filters | Every 6 months | Prevents particle damage to pump heads, seals, and check valves. Prevents abnormal pressure and inaccurate flow. |
| Clean/replace solvent filter heads | Every 6 months | Prevents suction issues, unstable pressure, and bubble formation. Dilute nitric acid may be used as needed. |
| Inspect/replace purge valve | 1 year | Aging seals cause leakage or ineffective purging. |
| Clean pump heads and flow path | Every 6 months | Always flush with ultrapure water after using high-water-content mobile phases and store in an organic phase to prevent buffer salt precipitations. |
| Lubricate drive mechanism | Every 2–3 years | Complicated operation. Contact expert service engineer to perform the action. |
| Replace fuses | Every 2–3 years | Keep spares for emergencies |
| Inspect/replace proportioning valve | 1 year | Inaccurate proportions or improperly sealed channels can cause retention time drift and pressure fluctuations. |
Final Advice
Don’t forget — the user manual left by your service engineer is a real "treasure trove." Take time to read it and revisit it often. Good operating and maintenance habits ensure your HPLC pump remains your most reliable teammate in scientific research.