Syringe-type pumps

From the last shown elution method, it is clear that the pump is a very important accessory for high-pressure liquid chromatography. There are three main types of pumps used in modern chromatographs membrane pumps reciprocating piston pumps syringe type pumps constant pressure pumps... 

Depending on the principle of operation, we shall distinguish four categories of pumps pneumatic pumps, syringe-type pumps, reciprocating pumps and hydraulic amplifier pumps. The principles of operation of these four types are different and will be described briefly. 

At the time of the solvent methanol experiments a metering pump was used. In some experiments the pulsating action of the pump can be disturbing, so a high-pressure syringe-type pump can be used. Since mass flow controllers are available now, the combination of a gas-pressurized feed tank on an electronic scale for liquid level indication and a mass flow controller seems to be a good choice. Both the feed tank and separator can be heated or cooled. In the case of the solvent methanol experiments. 

A central component of the LC instrument is the pump, for its characteristics can strongly influence an analysis (e.g., flow reproducibility). The three principal types of systems— pneumatic, syringe type and reciprocating piston—have been reviewed in the literature (18-20). 

The syringe type pump consists of mechanically driven piston emptying a large volume chamber 250 ml). In some... 

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. 

Two types of constant flow pump have been used in hplc. Fig. 2.2c shows a syringe type pump. 

Pump characteristic Simple single- head Single- head smooth pulse Simple dual-head Dual-head, compressibility-corrected, smooth pulse Dual-head, closed loop flow control Triple head low-volume Syringe- type Hydraulic amplifier Simple Amplifier Amplifier with flow control... 

The stopped-flow method uses syringe-type pumps, (a), to feed the components, A and B, through a mixing cell, (c), into the reaction cell, (d), which can be an optical cell (Fig. 3.3-5). The pumps, mixing cell, and reactor are well thermostatted. The flow is stopped when the syringe, (e), is loaded and operates a switch, (f), to start the monitoring device. The change in concentration is detected either by spectroscopy or conductivity measurement. 

Figure 3.3-5. Stopped flow apparatus [19]. a, Syringe type pump b, thermostat c, mixing cell d, reaction cell e, stop syringe f, switch g, photo multiplier h, monochromatic filter i, lamp j, controller k, transducer 1, computer.

In the syringe-type pump the liquid is enclosed in a cylinder. A piston moves at a constant speed to push the liquid. Eluent compressibility induces time-consuming flow equilibrium. Nevertheless, the flow from a syringe pump is pulse free. For micro LC, flow rates of 50 yuL/min are utilized in spite of the drawback of column pressurization. With very low flow rates (in the nanoliter range) the use of pumps is tedious, and split-flow techniques are required. 

The vast majority of pumps currently used in analytical HPLC are of the constant flow variety. With this type of pump, changes in the chromatographic system, for instance those leading to variations in the back pressure experienced by the pump, are compensated for, and the flow rate remains constant this is an important factor in most analytical applications. Two major types of constant flow pump exist the reciprocating piston design and the syringe type. 

An infusion control device (ICD) is a device that maintains a constant infusion rate in a gravity flow system (controller) or via a positive pressure pump. A positive pressure pump is a device that provides mechanical pressure (2-12 psi) to overcome the resistance to flow in the vessels. The types of positive pressure pumps are categorized according to how they deliver the solution and their degree of precision in the flow rate. Positive pressure pumps include peristaltic pumps, cassette pumps, syringe pumps, non-electiic or disposable pumps, and patient-controlled analgesic... 

Although several high-pressure pumps have been used in SFC, the syringe-type pump has been the preferred to deliver CO2 into the system. This choice is made due to the absence of pulses of syringe pumps and the possibility of flow rate and pressure control. 

Syringe type pumps are usually used for chromatography with capillary columns because this pump can deliver a finite volume of mobile phase before it is refilled. These pumps have a volume of hydraulic chamber between 250 to 500 ml. 

Syringe-type pumps are also known as screw-driven syringe-type pumps, constant displacement pumps, constant-drive piston pumps, single displacement pumps, single-stroke displacement pumps and positive displacement type pumps. 

Fig, 2. Syringe-type pumps, a = stepping motor, b = gear box or sprockets roller speed reducer assembly, c = screw, d = plunger piston, e = piston seal, f = to column. 

The Instrumentation Specialities Co. (I5CD) Model 314 pump is one of the simplest syringe-type pumps. A piston follower indicates the volume of solvent remaining in the chamber. The scheme of the hydraulic circuit is shown in Fig. 

The Varian Model 8500 pump (Fig. 5) is the most sophisticated syringe-type pump. There are two gas solenoid valves, the first of which pressurizes the solvent container to re-fill the chamber and the second actuates a pneumatic valve downstream of the pressure transducer on the column tubing. The operating controls are selected automatically by depressing push-button switches on the front panel of the pump controller. These switches control the opening of the... 

Syringe-type pumps are usually equipped with safety device to prevent damage to the mechanical parts in the event of an excess pressure. 

Syringe-type pumps deliver a pulseless flow, thus offering better baseline stability and a lower detection limit than pulsating pumps. They are generally silent, reliable and easy to operate. 

Independent of the drive mechanism used, reciprocating pumps can be distinguished from syringe-type pumps by the high piston frequency and by the small volume of solvent delivered during each piston cycle (one or a few tenths of a millilitre). 

Gradients generated with two syringe-type pumps 57... 

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