Pneumatic amplifier pumps

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. 

Figure 5.3 Different types of pusqps used in high pressure liquid chronatography. A, gas displacesent punp B, syringe punp C, pneumatic amplifier pump D, reciprocating punp.

The operating principle of the pneumatic amplifier pump is shown in Fig. 2.2b. 

In order to obtain very low flow rates without pulsation (e.g. 1 pl/min), pumps based on the principle of high volume motorised syringes are used. The piston, activated by a pneumatic amplifier, moves at a constant linear velocity. These pumps are still in widespread use. 

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. 

Pneumatic amplifier pump This pump is composed of two cylind that are different in piston cross-sectional area. The piston cros sectional area ratio between the two cylinders equals the pressure ampll fication factor from the low-pressure cylinder to the high-pressure i inder, and also equals the flow rate attenuation factor from th high-flow-rate cylinder to the low-low-rate (high-pressure) cylinder, practice, an area ratio of 5 10 is recommended for reasons such safety, reliability of ultrahigh-pressure seals and connectors, fluid com pressibility, and high-pressure cylinder volume. 

Pneumatic amplifier pumps are used for column packing and for applications where a pressure of over 500 bar is needed. With this design, a relatively low gas pressure activates the larger cross-sectional area of a piston which at its other end is in contact with the eluent over a much smaller area. The force is the same on both sides of the piston but the pressure is higher on the smaller area, according to the ratio of the two areas. The pressure amplification can be as high as 70-fold this means that a gas pressure of 10 bar can generate an eluent pressure of 700 bar. 

This overcomes the major disadvantage of pneumatic amplifier type pumps, and makes syringe pumps ideal for reproducibility of retention time data. However, the major problem encountered with the syringe pump is the design of a suitable refill mechanism. However, it is possible to use two pumps in concert giving limited gradient elution capabilities also the finite volume of the eluant chamber (250-500 ml) is not such a disadvantage with narrow bore columns with their small flow-rate requirements. 

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)). 

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. 

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. 

Pneumatic pumps typically use a pressurized gas as the driving force to force the liquid through the column. The gas pressure can be either applied directly to the liquid (gas displacement pumps) or amplified so that the liquid pressure is greater than the gas pressure (pneumatic amplifier pumps). 

A piston moving in a cylinder can also be used to separate the liquid from the gas, as in the Morgan pump, but generally such a design is used for pneumatic amplifiers see the next section). 

A pneumatic amplifier pump uses the Pascal principle to amplify the pressure of a gas. It consists of two rigidly connected pistons with different surface areas, the piston with a large surface area (s ) being on the gas side and that with a small surface area (S ) on the liquid side. If is the gas pressure and P. the liquid pressure, then... 

Fig. 1. Diagram of a pneumatic amplifier pump, a = gas pressure controller, b, c = valves, d = gas piston, e = liquid piston, f = piston seal, g = solvent Chamber, h = column check valve, i = to column, j = reservoir check valve, k = reservoir.

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. 

These pumps, especially the direct gas displacement pumps, are the cheapest chromatographic pumps. With pneumatic amplifier pumps, it is possible to obtain high pressures (400 or 500 atm) at low cost. No electric power but only a gas supply and a pressure controller are required. 

It is impossible, however, to carry out column recycle chromatography even with pneumatic amplifier pumps because the compounds partially separated would be re-mixed in the hydraulic chamber. Here recycle operation must be understood... 

Basically, hydraulic amplifier pumps are similar to pneumatic amplifier pumps as they contain a low-pressure, high-surface-area piston rigidly connected to a high-pressure, low-surface-area piston which deliver the solvent to the column. 

The ACS (Applied Chromatography Systems) Model 750 gradient elution digital programmer can be used with two pneumatic amplifier pumps, which are set at the... 

Type of pump Dual head reciprocating Reciprocating Pneumatic amplifier Pneumatic amplifier Dual head reciprocating... 

Type of pump dual piston dual bead reciprocating reciprocating diaphragm reciprocating pneumatic amplifier pneumatic amplifier syringe dual head reciprocating... 

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