Getting Started in HPLC

Section 2B. HPLC Pumps

   
In classical open-column chromatography, the mobile phase flows through the bed of column packing because of gravity. This works (barely) with large-particle-size packings (particle diameters larger than 50-100 microns or so), but separation times can be extremely long. In order to reduce separation time and allow the use of smaller particle size packings (10 microns and below), we must force the liquid mobile phase through the column under pressure. This is the function of the pump (also called the "solvent delivery system"): to maintain a constant flow of mobile phase through the HPLC regardless of the pressure (back pressure) caused by the flow resistance of the packed column.


 

Most commercial HPLC pumps are based on a reciprocating piston design, as shown here. A motor-driven cam pulls the piston back and forth in the pump head. A flexible seal around the periphery of the piston prevents leakage of mobile phase out the back of the pump. Check valves mounted in the head open and close in response to small changes in pressure to maintain a one-way flow of solvent.

The pump cylinder with its check valves is often accessible from the outside to allow easier servicing of the check valves and replacement of the pump seals. This part of the pump is called the pump head.


Cross-sectional diagram of a simple single-piston reciprocating pump


Typical pump head assembly.


   
The diagram below shows how pump flow varies with time:

During the delivery stroke, flow increases from zero up to a maximum, then decreases back to zero. During the intake stroke, flow is zero. The pressure inside the pump changes in the same way as flow -- going from zero to a maximum value, then staying at zero during the intake stroke.

Single-piston reciprocating pump operation.


   
The kind of flow shown above, where flow rate changes during the pumping cycle, causes pressure pulses. Pulses are undesirable for several reasons:
  • they cause detection problems
  • they prevent good quantitative analysis
  • they lead to early column failure.

Most of the differences in pumps from different manufacturers are modifications to give more uniform flow. One approach is to keep the single-piston design, but to vary the shape of the cam and /or the speed of the motor. This leads to a change in the flow as shown at right. The shape of the cam leads to a flatter flow curve at the middle of the delivery stroke. In addition, the motor speeds up during the intake stroke and slows down during the delivery stroke. Some pulsation still remains, however, and these pumps often use some form of "pulse dampening" to further reduce the flow fluctuations.


Shaped (non-circular) cam to minimize pressure pulses.


Output flow from a shaped-cam pump.


   
Another common approach combines the output flow from two heads operating 180 degrees out of phase, such that the intake stroke from one head coincides with the delivery stroke from the second head. This means that while one cylinder is filling the pump cylinder, the second cylinder is delivering mobile phase. Then, when the second refilling, the first cylinder delivers. We can combine these two flows by feeding each pump output into a tee that connects with the HPLC system. Now the combined flow of both heads delivers a much smoother, less pulsing flow to the LC system. The inlet line from the reservoir likewise is fed to a tee that branches to feed both cylinders of the pump. Pump pulsations can be reduced further by special cam shapes, by varying the speed of the pump motor, and by the use of pulse dampers.


 
Another approach to reducing pump pulsations while keeping the pump design fairly simple is the Tandem Piston Pump. A large and a small piston / cylinder unit are combined to provide continuous flow of mobile phase from the pump. While the large piston fills, the small piston delivers. When the small piston fills, the large piston delivers enough mobile phase to both fill the small piston and provide a net flow of mobile phase to the LC system. Notice that only three check valves are required for this pump versus four check valves for a conventional two-piston pump.


A tandem-piston pump provides a continous output flow.


   
Most LC pumps (even those with multiple heads) have some pulsation in the flow from the pump outlet. This is often smoothed out by the use of a Pulse Damper. These devices typically consist of a coil of flexible stainless-steel tubing. When the pressure rises (as a result of flow pulsation), the tubing stretches and its volume increases so that the extra flow from the pump at this moment is accommodated by the extra volume of the pulse damper. When the pressure decreases, the tubing volume decreases, with the extra solvent going to make up for the flow deficit. In effect, the pulse damper acts as a hydraulic "shock absorber" to smooth out pulsating flow.


 
   
 

 


2000, LC Resources Inc. All rights reserved.
Last revised: April 02, 2001.

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