Reciprocating pumps, HPLC

Compared to syringe type or reciprocating pumps, pneumatic amplifier pumps are very cheap. They tend to be rather difficult to dismantle for repairs, and some types are very noisy in operation. Because they do not provide a constant flow of mobile phase, they are not used much in analytical hplc. They can, however, operate at high pressures and flow rates and so are used mainly for packing columns, where high pressures are needed and variations in the flow rate through the column do not matter. 

Pump HPLC reciprocating pump (Waters) with a Rheodyne switching valve. 

SFC chromatographs represent hybrids between GC and HPLC instruments (Fig. 6.4). In order to deliver the supercritical fluid, syringe pumps or reciprocal pumps are used and maintained above the critical temperature using a cryostat regulated at around 0 "C. In instances where an organic modifier is used, a tandem pump is employed which has two chambers, one for the critical fluid and one for the modifier. The liquid then passes through a coil maintained above the critical temperature so that it is converted into a supercritical fluid. Stainless steel packed columns like those used in HPLC (1 to 4 mm in diameter) or fused silica capillary columns like those used in capillary GC (2 to 20 m in length, internal diameters as low as 50 pm and stationary phase film thickness of at least 1 pm) are used in SFC. 

The bulk of the SFE experiments performed to date were executed with systems typically consisting of a syringe or reciprocating pump, a high-pressure containing sample vessel comprised of HPLC column hardware, and a fused silica capillary restrictor. Extraction vessel temperatures of 40-80°C were usually accomplished using a converted oven or with the use of a thermostatted tube heater (2,3). Instrument manufacturers now offer a variety of commercially available SFE systems that vary in design, operation, features, ease of operation, and limitations. 

Reciprocating piston pump The reciprocating piston pump is a con" uous-flow pump similar to an HPLC pump. Three major differen of a reciprocating pump from an liquid chromatographic (LC) pum are the addition of a pump cooling system, the requirement of a pu dampener, and the greater minimization of postpump interface vol... 

Figure 4.3 shows the schematic of a reciprocating pump mechanism used in most HPLC pumps. Here, a motorized cam drives a piston to deliver solvent through a set of check valves. A microprocessor coordinates the piston speed with other components. Since only the inward piston stroke delivers the liquid, a pulse dampener is used to reduce flow fluctuations. All components in the fluidic path are made from inert materials (e.g., stainless steel pump heads, ruby balls and sapphire seats in check valves, sapphire pistons, and fluorocarbon pump seals). 

Figure 19-17. A dual piston two-cam reciprocating pump. (Courtesy - R. Macrae, HPLC in Food Analysis, Academic Press, London, 1982)...

The most commonly used pump for HPLC is the reciprocating pump. This has a small cylindrical piston chamber that is alternately filled with mobile phase and emptied via back-and-forth movement of the piston. This produces a pulsed flow that must be damped. Reciprocating pumps have a number of advantages. They have a small internal volume, are capable of high output pressures, and they can readily be used for gradient elution. They provide constant flow rates, independent of solvent viscosity or column backpressure. Other pumps used are motor-driven syringe pumps and pneumatic (constant-pressure) pumps. 

An HPLC-like setup with two reciprocating pumps designed to provide amixed mobile phase with a packed analytical column placed in an oven followed by a detector, where the pressure and flow rates can be independently controlled. 

Reducing the non-shot Category III noise sources of the fixed wavelength detector will require attention to Improved thermomechan-Ical detector design to reduce noise due to thermal variations Imposed on the mobile phase/flow cell and on the sample and reference photodiodes, and continued reduction In HPLC reciprocating pump flow pulsations. 

As is quite evident from interpreting the ehromatograms in Fig. 4.50, RP-HPLC columns have a finite lifetime It is good practice for the analyst to keep a record of N (as calculated using the above equation) versus either time or number of injected samples in an attempt to continuously monitor column performance. Because HPLC reciprocating pumps maintain constant flow rate, a continuous observation of the back-pressure or pressure buildup at the front on the HPLC column is an important parameter to monitor. Making sure that there are no leaks in an operating HPLC is also very important. 

Figure 13.6 Operation of piston and check valve reciprocating pump (a) suction stroke and (b) exhaust stroke. (From Katz, E. et al., eds., Handbook of HPLC, Marcel Dekker, Inc., New York, 1998. With permission.)...

There are multiple types of pumps utilized for HPLC however, this discussion will focus on reciprocating pumps since most commercial instrumentation is equipped with this type of pump. Reciprocating pumps are widely used, because they have many qualities desirable for HPLC, including the ability to generate high pressures and reproducible gradients. This type of pump uses a piston to... 

The technique uses both HPLC- and GC-like instrumentation. Carbon dioxide is the most commonly used supercritical fluid with a critical temperature of 31°C and critical pressure of 72.86 atm. However, other fluids such as nitrous oxide, sulphur hexafluoride and xenon have been used. The supercritical fluid is pumped through the system by using either a syringe or a reciprocating pump, which is chilled to ensure that the fluid is maintained in the liquid form. The sample for analysis is dissolved in a low-boiling solvent (methylene chloride) and is injected into the system via an HPLC high-pressure injection valve. Separation can be affected by use of either LC-like packed columns... 

Figure 1. Schematic diagram of the flow injection manifold used for the determination of iron (III). Potassium iodide, 8%, and 0.25M hydrochloricid acid were used as reagents, and deinized distilled water as carrier. Pi = peristaltic pump P2 = HPLC reciprocating pump INJ = injection valve (lOO-pl loop) LI = 50 cm mixing coil L2 = 20 cm reaction coil, D = detector set at 360 nm INT-PR = integrator and printer W = waste. All flow lines were made from 0.5mm i.d. PTFE tubing.

Liquid pressurizing systems can be classified into four types, namely (i) those which have no true pump—the pneumatic pumps (ii) those with reciprocating piston pumps, (iii) those with syringe type pumps, and finally (iv) those with a combination of the first and second systems, namely pneumatic amplifier pumps. Of these various types the first two (cheaper ones) will be discussed here because they are the more popular and because the syringe type pumps have been discontinued by most HPLC pump manufacturers in favour of the modern reciprocating pump. 

Table 1. List ot modern HPLC dual piston reciprocating pump operating specifications.

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