Screw driven syringe

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. 

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. 

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. 

In general autotitrators that work with a preset endpoint lend themselves to application as pH-stats. All that is necessary is to record the volume of titrant added as a function of time. If a motor-driven syringe is used, this can be combined with a helical potentiometer and a strip-chart recorder to provide a volume-time curve a manual version would include a digital register ganged to the motor-driven screw of the syringe. The digitizer can be read as a function of time and provides the necessary data for analysis of the kinetics of hydrolysis reaction. 

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. 

An entire titration can be performed automatically by titrators equipped with microcomputers and analog-to-digital converters and using dedicated software. The most widely used apparatus for automatic reagent addition consists of a calibrated syringe that is activated by a motor-driven micrometer screw. The volume is determined from the number of turns the screw makes during the titration. 

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. 

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. 

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