Screw driven syringe pumps

Screw-driven syringe pumps consist of a large syringe in which the piston is moved by a motor-driven screw. They me pulse free, and the rate of delivery is easily varied. They suffer from lack of capacity and are inconvenient when solvents must be changed. 

The solvent is moved through the system by constant-flow or constant-pressure pumps which arc driven mechanically (screw-driven syringe or reciprocating) or by gas pressure with pneumatic amplifiers. For gradient elution Iwo pumps may be synchronised and programmed to provide a controlled, reproducible composition change. 

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. 

Two major types of pumps are used in LC llie screw-driven syringe type and the reciprocating pump. Reciprocating pumps are used in almost all modern commercial chromatographs. 

Syringe pumps driven by screw mechanisms were popular in the 1960s because of their inherent precision and pulseless flow characteristics. Their disadvantages are higher manufacturing costs and the problems associated with syringe refill cycles. Syringe pumps are currently used in specialized systems for microbore and capillary HPLC. 

A complete setup of a thin film balance is shown in Figure 7.7. The holder with the film is placed inside a sealed chamber, which contains the same solution as the film for equilibration at the bottom. The holder has been saturated with the solution of interest by immersing it before mounting for several hours. The free end of the capillary tube is exposed to a reference pressure, which in the simplest case is just ambient pressure. The capillary pressure in the film spanning the hole can be controlled by varying the pressure inside the sealed chamber. A simple way to do this with high precision is to coimect to the chamber a syringe pump that is driven by a micrometer screw and measure the chamber pressure with a pressure transducer. 

Delivery Pump. A constant-delivery pump permits the time axis of a strip-chart recorder to be used as the volume-of-titrant axis (with a simple conversion factor). Typically, a syringe driven by a synchronous motor (that drives a carriage or screw) is used, and solutions can be delivered at constant rates ranging down to a few microliters per minute. Because of their variable flow rates, the more common peristaltic pumps are not often used for thermometric titrations. 

---