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LC Pumps

Dec 1, 2008
By: John W. Dolan
LCGC North America

John W. Dolan

Here we are, more than 40 years after the initial exploration of high performance liquid chromatography (HPLC). Many things have changed, some innovations have come and gone, but the reciprocating-piston pump still remains as a key component of the LC system. We've tried pneumatically driven pumps, piston-diaphragm pumps, syringe pumps, and some other ideas, but none have proven to be superior to the reciprocating-piston pump. Yes, some changes have been made, but the basic operation remains the same. This month's "LC Troubleshooting" installment will take a look at the design of these durable pumps and also examine some of the potential weaknesses and how to overcome them.

The Basic Design

Figure 1: Single-piston reciprocating pump: (a) Pump in intake, or fill cycle; (b) pump in delivery cycle; and (c) partial section of piston showing cut-away portion of piston seal. See text for discussion.

The basic design and operation of the reciprocating-piston pump is illustrated by the single-piston pump shown in Figure 1. The key components are a piston, pump seal, pump head, and a couple of check valves. The piston, usually made of sapphire, is driven back and forth in the pump head by a rotating motor. Various means have been conceived to convert the rotary motion of the motor into the bidirectional movement of the piston. Most commonly, this is done by a cam pressing against one end of the piston to push it into the pump head and a spring to push the piston back out. The pump seal is shown in the inset in Figure 1c. It is a polymer ring that fits around the piston, and a small lip forms a liquid-tight seal against the piston with the aid of a spring and liquid pressure. A pair of ruby check valves with sapphire seats are mounted on the top and bottom of the pump head. The check valves control the direction of flow through the pump. On the intake stroke (Figure 1a), the piston is withdrawn, which creates a low pressure area inside the pump head. This allows the outlet check valve to close and the inlet check valve to open, so that mobile phase flows in to fill the pump head. On the delivery stroke (Figure 1b), the piston moves into the pump head, and the inlet check valve is closed as the pressure increases. When the pressure inside the pump head exceeds the pressure in the column, the outlet check valve opens and mobile phase flows to the column. When all is working well, this simple pump design is quite reliable.

The weak points in the design are the check valves and the pump seal. The operation of the check valves and some design improvements were discussed in the June 2008 installment of "LC Troubleshooting" (1). Contamination of the check valves can cause them to leak, and under the right circumstances, acetonitrile in the mobile phase can cause the inlet check valve to stick closed. The pump seal forms a seal against the moving piston, and although the sapphire piston is very smooth, the seal eventually will wear out. This wear is accelerated if buffer is allowed to sit in an unused pump, because the liquid behind the seal evaporates and leaves an abrasive layer of buffer crystals. These abrade the seal when the pump is restarted, and can shorten the seal life. Generally the pump seals will last 6–12 months under normal operation if buffers are rinsed from the system before shutdown.

Bubbles of air trapped in the pump head will cause the pump to under-deliver mobile phase. Bubbles can result when mobile phases are not degassed sufficiently. Fortunately, most of today's LC systems incorporate an inline vacuum degasser to degas the mobile phase automatically. So if the pump is purged of bubbles when it is started, bubble problems in the pump are not a common problem today — they were the bane of the chromatographer in years past.

The original reciprocating-piston pumps were crude by today's standards. I remember using the Milton–Roy Minipump, one of the standard pumps in the late 1960s and early 1970s, particularly for laboratory-built systems that were common in graduate school laboratories. The flow rate was controlled by adjusting a stop that limited the distance the piston could move. With a single-piston design, the pump spent half the time filling and half the time delivering mobile phase. This resulted in pulses of pressure and flow that were reflected in a fluctuating baseline and short column lifetimes of the then-poorly-packed columns. Huge pulse dampeners, sometimes 50–100 mL in volume, were required to reduce the pulses to an acceptable level. Refinements in the shape of the driving cam and the use of stepper-driven motors helped to minimize pulses with single-piston pumps, but as other aspects of the LC system were improved, single-piston pumps were not sufficiently pulse-free to be acceptable.

Enter the Dual-Piston Pump

Figure 2: Two-piston pump designs: (a) Dual-piston pump with left piston in fill cycle and right piston in delivery cycle and (b) accumulator-piston pump with top piston in delivery cycle and bottom piston in fill cycle. Arrows show direction of motion of pistons; pressure and flow profiles are shown at right. See text for discussion.

The simple cure for the pulsating nature of the single-piston pump was to operate two such pumps so that when one piston was filling, the other was delivering solvent. This is shown in the diagram in Figure 2a, where the left-hand piston is at the end of its fill stroke and the right-hand piston is at the end of its delivery stroke. The solvent delivered from both pump heads is combined into a single flow-stream. Because one piston is always in the delivery cycle, solvent flows continuously to the column and pulses are minimized or eliminated. In practice, dual-piston pumps are configured so that the pistons are mounted side-by-side in parallel. The dual-piston pump was developed early in the history of HPLC and remains as one of the two most popular designs in use today.

Accumulator-Piston Pump

A later development in pump design is the accumulator-piston pump shown in Figure 2b. This also is called a tandem-piston pump. In this design, the two pistons deliver solvent at two different flow rates. For example, if the pump of Figure 2b is set to deliver 1 mL/min of solvent to the column, the top piston (shown at the end of the delivery stroke) pumps at 1 mL/min. Meanwhile, the bottom piston fills at 2 mL/min. Next, the top piston fills at 1 mL/min. The 2 mL/min delivered by the bottom piston is split so that 1 mL/min serves to fill the top piston and the other 1 mL/min flows directly to the column. In this manner, solvent always flows to the column at 1 mL/min.