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This section explains troubleshooting when the mobile phase fails
to flow normally from the HPLC pump.
While potential causes may include an empty mobile phase
reservoir, pump flow rate setting errors, or pump system
malfunctions, the most likely cause is a check valve failure.
The check valve, also called a "non-return valve," controls the flow
direction of the mobile phase by turning the flow path ON and OFF,
ensuring it flows in only one direction. In HPLC, it typically
consists of a "ball valve" and a "valve seat" that the ball rests
against.
Although "serial pumps" exist, parallel pumps
are globally the standard. In parallel pumps, two pump heads operate
alternately, canceling out pulsations caused by the suction and
discharge of the plunger.
Each of the two pump heads in a parallel double-plunger pump has
an IN-side and OUT-side check valve. During normal flow, the ball
valve lifts off the valve seat, allowing the mobile phase to pass
through. Conversely, when the mobile phase attempts to flow
backward, the ball valve seals tightly against the valve seat,
preventing backflow. This mechanism ensures that the mobile phase
flows consistently in one direction, even under high pressure.
Each pump head typically has two check valves (IN and OUT), and
since most systems have two pump heads, the total number of check
valves is four. Identifying which specific check valve is
malfunctioning can be a highly challenging task.
The primary causes of check valve malfunctions in
HPLC systems are as follows:
-
Prolonged
Inactivity:
- When the HPLC system is left unused for
an extended period, the check valve and
valve seat may stick together and fail to
separate, resulting in a complete stoppage
of mobile phase flow.
-
Debris
Accumulation:
- Debris lodged between the check valve
and the valve seat prevents proper sealing,
causing backflow of the mobile phase.
If the flow rate is reduced, the issue may
not only involve the check valve but also air bubbles trapped inside
the cylinder of the pump head. The following methods can help
resolve the problem:
- Use the "Purge Button":
Perform a high-speed purge of the mobile phase using the purge
function.
- Create a Sudden Pressure Release: Attach an
old column or a restrictor tube, start flowing at a high rate,
and, once pressure builds up, quickly release the purge valve to
atmospheric pressure. The sudden pressure difference can expel
trapped air.
If there is an issue with the check valve, the
following steps can help determine which one is not
functioning properly:
-
Remove Both OUT-Side Check Valves:
- Detach the OUT-side check valves from
both pump heads.
-
Turn the Pump ON:
- Check if the mobile phase flows out from
the OUT-side openings.
-
No Flow from the OUT-Side Opening:
- If no mobile phase comes out from one of
the OUT-side openings, the IN-side check
valve on that side is faulty. Clean the
corresponding IN-side check valve.
-
Flow from the OUT-Side Opening:
- If mobile phase flows out from the
opening, the corresponding OUT-side check
valve may be faulty. Clean the corresponding
OUT-side check valve.
The following steps outline recommended methods
for cleaning check valves, distinguishing between
IN-side and OUT-side valves:
-
Initial
Cleaning:
- Place the problematic check valve (IN or
OUT) into a beaker filled with distilled
water.
- Subject the valve to ultrasonic cleaning
for approximately 10 minutes.
-
Alternative
Solvent Cleaning:
- If the issue persists, replace the
distilled water with ethanol
or 2-propanol (IPA) and
repeat the ultrasonic cleaning process.
-
Acid Cleaning:
- If the problem remains unresolved, fill
the beaker with approximately 1M
nitric acid solution (take
precautions to avoid skin contact) and
perform ultrasonic cleaning.
- Afterward, thoroughly clean the valve
again using distilled water in the
ultrasonic cleaner.
-
Manual
Disassembly and Cleaning:
- If the above steps do not resolve the
issue, carefully disassemble the check valve
unit and clean the ball valve and valve
seat. Be cautious, as reassembly may not
always be possible.
- If manual cleaning is not feasible or
successful, replace the check valve with a
new one.
Maintaining the performance of high-pressure pumps in HPLC is not
an easy task. While having maintenance performed by the manufacturer
under a service contract is a valid approach, chromatographers
should acquire the basic skills to perform simple maintenance tasks,
such as cleaning and maintaining check valves, on their own.
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