Constant pressure pumps

The types of pumps used for HPLC can be divided into two categories constant-pressure pumps (e.g. the inexpensive gas-displacement pump) and the constant-volume type (e.g. the reciprocating and syringe pumps). The most commonly used pumps in HPLC are the single- or multi-head reciprocating type. The former delivers the flow as a series of pulses which must be damped... 

Unfortunately, neither the computer nor the potentiometric recorder measures the primary variable, volume of mobile phase, but does measure the secondary variable, time. This places stringent demands on the LC pump as the necessary accurate and proportional relationship between time and volume flow depends on a constant flow rate. Thus, peak area measurements should never be made unless a good quality pump is used to control the mobile phase flow rate. Furthermore, the pump must be a constant flow pump and not a constant pressure pump. 

The second type of constant pressure pumping system is the... 

The function of the pump in hplc is to pass mobile phase through the column at high pressure and at a controlled flow rate. One class of pump (constant pressure pump) does this by applying a constant pressure to the mobile phase the flow rate through the column is determined by the flow resistance of the column and any other restrictions between the pump and the detector outlet. Another type (constant flow pump), generates a given flow of liquid, so that the pressure developed depends on the flow resistance. 

The earliest form of constant pressure pump in hplc (the coil pump) used pressurised gas from a cylinder to drive mobile phase from a holding coil through the column. This type of pump was used in some of the older hplc instruments, but is now only of historical interest. If you want to know any more about it, there are details in most textbooks. 

Importantly, in a constant-pressure pump the flow rate will change if the flow resistance changes. Whereas in the constant flow pumps the changes in flow resistance are compensated duly by a change of pressure. Therefore, it is always advisable to use constant flow pump in HPLC determinations. 

Fig. 9. Apparatus for sluijy packing of columns. After the properly fittul column tube is attached to the bottom of the reservoir, both are filled up with the slurry ofthe micropartic-ulate stationary phase. Thereafter a displacement liquid is pumped into the reservoir by the constant pressure pump, e.g., Haskel Model DST>I00, which is driven by preuurized air. Upon displacement, the slurry from the reservoir is filtered over the porous etal frit at the bottom of the column tubing which becomes densely packed with the partic s. By intermit-tently operating the liquid shut-off valve between the pump and the reservoirpressure waves can te generated in order to flirther compact the column packing. Reprinted from Bakalyar et at. U05) with permission from Spectra-Physics.

Equipment. A LDC (Laboratory Data Control) Constametric III pump was used together with a Rheodyne 7120 20 yl loop injection valve and two LDC Spectromonitor III, variable wavelength UV-detectors. A Stanstead constant pressure pump was used for packing the columns. 

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. 

Long-term noise can be erroneously considered as late-eluting peaks, while drift affects the obtained data when the instrument is running over a long time period. Flow sensitivity affects flow rate and gradient programs using constant pressure 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... 

The mobile phase is pressed by gas continuously at constant pressure into the column by constant pressure pumps. The big advantages of this type of pump are really the constant pressure and the continuous pumping of the mobile phase. 

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. 

L. Fossey and F. F. Cantwell, Characterization of Solvent Extraction/Flow Injection Analysis with Constant Pressure Pumping and Determination of Procyclidine Hydrochloride in Tablets. Anal. Chem., 54 (1982) 1693. 

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)). 

In addition, the chamber has to be refilled periodically with solvent, thus resulting in an interruption of the analysis. The design of such pumps also makes it difficult to change solvent compositions. However, though they are not ideally suited for analytical LC constant pressure pumps are still conunonly used for slurry packing of columns due to their ability to deliver high flows and pressures. 

Basically, two types of pumps exist Constant-pressure pumps and constant-flow pumps. Pressure-constant pumps are often used to pack HPLC columns. 

The suspension is pressed into the empty column. which is fitted with a frit on its far side, by means of a constant-pressure pump (pressure filtration). The usual packing pressure for a standard analytical column (250x4 mm i.d.) is between 40 and 60 MPa. In this case. l.7-2.0g of stationary phase has to be suspended in ca. 30-50mL of suspension fluid. Detailed discussion of column packing techniques is given in [73], [19], [76]. 

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