Pumps, constant-pressure pneumatic

Constant pressure pumps. Constant pressure pumps (Figure 6.11) deliver solvent via a small headed piston which is driven by a pneumatic amplifier. A gas acts on the relatively large piston area of the pneumatic actuator. This is coupled directly to a small piston which pushes the eluant through the column. Pressure amplification is achieved in direct ratio to the piston areas and thus for low inlet pressures (approximately lOOpsi (690 kPa)) it is possible to obtain large outlet pressures (lOOOOpsi (69 MPa)). 

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. 

The other type of constant flow pump is a positive displacement syringe. It is pulseless but suffers from the limited volume it can deliver before refilling. The major type of constant pressure pump is a pneumatically driven syringe. 

Constant pressure pumps utilise pneumatics or hydraulics apply the pressure required to force the mobile phase through the column, either directly or indirectly. Two main designs of constant pressure pump exist the pressurised coil pump, and the pneumatic pressure intensifier type. The pressurised coil pump is now all but redundant, but as it represents the most simple means possible of pumping at high pressure through an HPLC column it is described briefly. 

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. 

There are three main designs of solvent delivery system (a) the reciprocating piston pump which may be of the single, dual or triple head design (b) the syringe or displacement design which delivers constant non-pulsating flow (c) the constant pressure or pneumatic pump. 

However, due to their mode of operation, pneumatic amplifier pumps have certain disadvantages. They are constant pressure rather than constant flow and therefore, as the elution volume is proportional to flow, fluctuations in the latter—due to, for example, partial column blockage or temperature change—can lead to poor precision and accuracy of analysis. The flow-rate is also dependent on solvent viscosity and coliunn back pressure. 

In the pneumatic pumping system, the pressure (and not the flow rate) is maintained constant as variations in chromatographic conditions occur. Thus, a change in mobile phase viscosity (e.g. gradient elution) or column back pressure will result in a change in flow rate for these types of pumps. The gas displacement pump in which a solvent is delivered to the column by gas pressure is an example of such a pneumatic pump. The gas displacement system is among the least expensive pumps available and is found in several low cost instruments. While the pump is nonpulsating and hence, produces low noise levels with the detectors in current use, its flow stability and reproducibility are only adequate. In addition, its upper pressure limit is only 2000 psi which may be too low in certain applications. 

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. 

A pneumatic pump feeds the printing paste and a sensor helps ensure that its level within the screen is kept fairly constant application of the paste is by adjustment of the squeegee relative to the counter-pressure roller. 

The regenerated extraction gas leaves the second regeneration column at its head and is cooled down in (WT3) to a temperature of approximately 20°C. Depending on the type of extraction solvent the buffer vessel (KP) contains liquid phase in equilibrium state with gas or merely gas of high density. In the last case a pressure controlled pneumatic pump feeds fresh solvent into the circular process. If a gas/liquid equilibrium is achieved in the buffer vessel the gas pressure remains constant until a minimal amount of liquid remains there. For this purpose two optical sensors are introduced into the buffer vessel registrating the minimum and maximum extraction liquid level If the level falls below minimum, fresh liquid extraction solvent is refilled. 

Manual or automatic operation. Pumps and compressors often exist in manual, electrically operated, or pneumatically operated versions. Whenever practical, manual instruments should be preferred for laboratory applications, especially when a high-pressure set-up is used close to its limit. This forces experimenters to closely control the system by requiring their constant attention and vigilance. 

In the DuPont 848 liquid chromatograph, a special Haskel mini-pump is used, derived from the Haskel Model M. The volume of the cylinder is small (about 2 ml). In the original Haskel Model M pump, the return of the piston is actuated by an air selector valve and a spring, so that the liquid pressure is not constant because the gas piston must compress the spring during its forward stroke. In the modified pump, the spring is replaced with a small counter-pressure. The main characteristics of commercial pneumatic amplifier pumps are summarized in Table I. 

DuPont system when the Model 833 precision flow controller is coupled with the gradient elution accessory immediately after the pneumatic pump on solvent A line (Fig. 7). The air pressure is adjusted so that the flow-rate is kept constant. Also in the DuPont device, solvents that differ widely in viscosity can be difficult to mix and the shape of the gradient can be altered. 

Figure 4.22 Schematic of a pumping system based on a pneumatic gas pressure amplifier with microfluidic flow control via feedback from a sensitive flowmeter. In this way the flow rate is maintained regardless of changes in system back pressure or mobile phase viscosity, and changes in flow rates can be established rapidly and accurately, (a) A gradient system in which the mobile phase composition is controlled via flow rates of both mobile phase solvents, (b) A gradient system in which both back pressures and flow rates are monitored volume flow rates = k. (P(- -P ) and Ug = kg.(Pc - Pg) where k and kg are calibration constants, (c) A demonstration of the precision and accuracy with which controlled flow rates can be changed rapidly at total flow rates in the nL.min range, suitable for packed capillary HPLC. Reproduced from company literature (Eksigent 2005, 2006) with permission from Eksigent LLC.

As mentioned previously, the main function of the sample introduction system is to generate a fine aerosol of the sample. It achieves this with a nebulizer and a spray chamber. The sample is normally pumped at about 1 mL/min via a peristaltic or syringe pump into the nebulizer. A peristaltic pump is a small pump with lots of minirollers that all rotate at the same speed. The constant motion and pressure of the rollers on the pump tubing feeds the sample through to the nebulizer. A syringe pump delivers the sample via a pneumatic piston, which is typically 2-3 times faster... 

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