GB1562321A - Pump - Google Patents

Description

(54) PUMP (71) We, IMPERIAL CHEMICAL INDUS TRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to pumps for pumping fluids and particularly to pistonless pumps for pumping fluids and to their use in chromatography.

Pistonless pumps for the pumping of fluids tend to leak if operated at high pressure; also they often comprise plastic bottles or bellows which, because they tend to be thick walled, offer resistance to collapse so that all the fluid being pumped cannot be expelled or can only be expelled with difficulty from the pump. Attempts to overcome these problems, especially in pumps for use in chromatography, have led to the development of so-called "gas displacement pumps". However, these pumps, which consist essentially of a hollow tube, often in the form of a helix, from which a liquid, e.g. an eluent, is expelled by admission of a gas under pressure, also have a number of disadvantages.For example, the gas at high pressure tends to dissolve in the eluent in the tube and then often forms bubbles in the eluent at the low pressure end, namely the detector, of a chromatographic apparatus of which the pump is a component. Also when expulsion of eluent from the pump into, for example the chromatography column is complete, gas tends to enter the column and so disturb the equilibrium of the apparatus.

Again, when the pressure in the pump is released any residual eluent in the tube, which may have been in excess of that required for the particular separation or may have been trapped in the coils of the helix, is often rejected from the pump into the atmosphere.

The present invention provides a pump for pulseless flow of fluids comprising a housing, provided with a plurality of ports, and a flexible container which is mounted in the housing and which is sealed thereto in a fluid tight manner to withstand a pressure of at least 50 psi and to prevent exchange of a fluid between the inside of the container and a space of variable volume defined by the container and the walls of the housing, at least one of the said ports providing fluid entry to and at least one of the said ports providing fluid exit from the inside of the container, at least one of the said ports providing fluid entry to and at least one of the said ports providing fluid exit from the said space, and the container having a shape and walls of such a thickness that it is variable in shape and volume substantially without resistance within the limits imposed by its construction so that substantially complete expulsion of a pumped fluid from the pump may be achieved by pressurising the pump with a working fluid, said variation in shape and volume of the container being effected primarily by lateral movement of the walls of the container relative to the walls of the housing.

In a preferred embodiment of the invention a first port in the housing provides fluid entry to and fluid exit from the inside of the container and a second port in the housing provides fluid entry to and fluid exit from the space of variable volume defined by the container and the walls of the housing.

Throughout this specificatin, where reference is made to "a port " or "the port" which allows flow of a fluid or fluids into and/or out of the container or into and/or out of the space defined by the container and the walls of the housing, such ex pressions are intended to Include the case where a plurality of ports allows such flow of a fluid or fluids.

In this specification, for convenience of description, we shall refer to "pumped fluid", by which we mean the fluid which is pumped by the pump, e.g. where the pump is employed as one component of a chromatographic apparatus the pumped fluid is the eluent, and to " working fluid", by which we mean the fluid which is used to pressurise the pump to expel the pumped fluid from the pump.

The pumped fluid may be a gas or mixture of gases or a liquid or mixture of liquids although typically it is a liquid or mixture of liquids. Typical liquids which may be pumped in a pump according to the invention include eluents for chromatography, e.g. for liquid/solid, liquid/liquid, ion exchange, and gel permeation/steric exclusion chromatography, fuels, e.g. for internal combustion engines or steam generators; and blood, e.g. in life support systems.

The working fluid may be a gas or mixture of gases or a liquid or mixture of liquids although conveniently it is a gas. Any suitable gas may be employed as the working fluid although compressed air or nitrogen are often the most convenient.

The invention will be further described by reference to the accompanying drawings which show, by way of example only, one embodiment of the invention: Figure 1 shows a longitudinal section of a pump according to the invention Figure 2 shows diagrammatic longitudinal and transverse representations of the pump shown in Figure 1 containing a pumped fluid, Figure 3 shows diagramatic longitudinal and transverse representations of the pump shown in Figure 1 when empty, Figure 4 is a longitudinal section of a detail of a modification of the pump shown in Figure 1.

In Figure 1 a housing is formed from a stainless steel tube (1) and two end components one of which is an Ermeto endcap (2). The other end component is formed from an Ermeto coupling (3) and a stainless steel mandrel (4) in which is mounted a stainless steel pipe (5) which projects through both ends of the mandrel and which defines a port (6). The inner end of the pipe (5) is formed with a head (7) and shoulder (8) on which is seated a rubber O-ring (9). The outer end of the pipe (5) is externally screw threaded to receive a wing nut (10). A flexible container (11) in the form of a tubular membrane of Melinex (RTM), 35p thick, sealed at one end with a screw clip (12), is fitted on the inner end of the pipe (5) to leave a space (13) of variable volume between the walls of the tube (1) and the tubular membrane (11).On tightening the wing nut (10) the O-ring (9) is deformed and the membrane (11) is clamped between the O-ring (9) and the coupling (3) to form a fluid tight seal with the housing and to seal the housing. The port (6) is provided at its inner end with a guard (14) which is formed from a stainless steel sinter (No. 30, ex Ugine Kuhl many), and which is supported on a perforated stainless steel backing plate (18). The port (6) is internally screw threaded at its outer end to receive a connector (15). The coupling (3) is provided with a port (16) and connector (17).

Where the pump ns used as a component of a liquid chromatographic apparatus, the connector (15) is typically connected via a 3-way tap to a reservoir for a pumped fluid, i.e. the eluent, and to a chromatography column, and the connector (17) is typically connected via a 3-way tap to a supply of working fluid, e.g. a nitrogen cylinder, and to the atmosphere. In operation eluent from the reservoir is allowed to flow through the connector (15) and port (6) into the tubular membrane (11) so that the tubular membrane expands to fill the space (13) as shown in Figure 2, expelling air through the port (16) and connector (17).

When the required amount of eluent has been admitted to the tubular membrane (11) the 3-way taps are adjusted so that the pump is connected to the chromatography column and to the nitrogen cylinder. Entry of nitrogen into the space (13) forces the tubular membrane (11) to collapse as shown in Figure 3, expelling all or a proportion of the eluent and forcing it through the chromatography column.

In Figure 4, a second O-ring (19) traps the tubular membrane (11) against the head (7), thus reducing the possibility of the pumped fluid in the tubular membrane contacting the O-ring (9).

In the mode of operation illustrated in the specific embodiment, the pumped fluid enters the flexible container, then the working fluid is admitted to the space between the container and the walls of the housing, to cause at least partial collapse of the container and so expelling at least a proportion of the pumped fluid therefrom. However the pump may be operated in the alternative mode of operation, i.e. the pumped fluid enters the space between the container and the walls of the housing, at least partially collapsing the container, the working fluid is then admitted to the container which expands and expels at least a proportion of the pumped fluid from the space between the container and the walls of the housing.

Where it is desired to expel all the pumped fluid from the space between the container and the walls of the housing the sizes of the container and the housing are chosen such that on expansion the container contacts substantially all of the internal surface of the housing.

The container is sealed to the housing in a fluid tight manner which prevents exchange of a fluid between the inside of the container and the space of variable volume defined by the container and the walls of the housing and which preferably reduces unwanted expulsion of a fluid from the pump, e.g. by seepage between components of the housing. For example, in the specific embodiment of the invention exchange of a fluid between the inside of the container and the space defined by the container and the walls of the housing is prevented and the possibility of unwanted expulsion of a fluid from the pump is reduced by clamping the container between an O-ring and a coupling to form a fluid tight seal.However we do not exclude the possibility that the container may be sealed to the housing in fluid tight manner which prevents exchange of a fluid between the inside of the container and the space defined by the container and the walls of the housing but does not affect the possibility of unwanted seepage of fluid from the housing occurring, e.g. the container may be sealed to the perimter of the tube and not be in clamping engagement with two components of the tube.

The container may be sealed to the housing by mechanical means or by adhesive bonding. Mechanical means, e.g. clamping the container between a resilient sealing member, e.g. an O-ring and a component of the housing, is preferred since this allows of ready replacement of the housing and avoids the possibility of contaminating the fluids with adhesive.

Where a resilient sealing member is employed preferably the pumped fluid does not contact it. Where such contact is possible, e.g. where the container contains the pumped fluid and is clamped between the housing and a resilient sealing member which is seated on a mandrel, it will be apparent to the skilled man that the possibility of contact may be reduced by the use of a suitably placed additional resilient member, e.g. an O-ring, which compresses the membrane on the mandrel.

Where the ports are positioned andjor have dimensions such that the container may be forced into them under pressure and hence destroyed or damaged, e.g. a container made of a thermoplastic may " creep " and distort, the ports are preferably provided with guards to obviate this.

Guards, where they are employed, must be strong enough to withstand the pressure to which they are subjected in the pump while not unduly restricting the flow of fluid through the ports on which they are provided. Guards which may be employed include inter alia perforated metal plates, woven wire cloth, and porous members.

Perforated metal plates, e.g. or stainless steel, are often preferred. The wire for the woven cloth is preferably stainless steel.

The porous member may be of sintered metal, e.g. stainless steel, plastic or ceramic or any other suitable material providing it will withstand the pressure and environment of the pump; preferably the porous member is stainless steel. Where a woven wire cloth or a porous member is employed, it may be necessary to support them on a perforated backing plate. For example, we have found that where the container is made from a tube of polyethylene terephthalate 40v.

thick, for operation at 2000 p.s.i. the largest dimension of the meshes in the woven cloth or the perforations in the perforated plate should be less than 10001l. Typically guards employed with a polyethylene terephthalate tube 40ILL thick may be stainless steel plates having perforations lOOy.

in diameter, or woven wire cloth having meshes 75,a square, or sintered steel discs e.g. No. 30 ex Ugine Kuhlmann. The guard will be chosen in the light of the requirement of rate of fluid transfer through the guard and the pressure the guard has to withstand. Simple experiment will readily reveal a suitable guard for any particular application.

While the specific embodiment discloses a housing in the form of a tube of circular cross-section the invention is not restricted to a tube of circular cross-section. However, a tube of circular cross-section is preferred since, for any given crosssectional area, a circular cross-section will withstand the highest pressure and circular attachments such as valves, gaskets, and taps are often readily obtainable and usually convenient to thread. A straight tube is particularly preferred although we do not exclude the possibility that the tube may be distorted into a non-linear configuration, e.g. a helix. The tube may be a monolithic whole although preferably it comprises at least two separable components, e.g. a tubular component and end components.

Where the tube comprises two or more separable components, all the ports may be in one component; or the port or ports providing fluid entry to the container and the port or ports providing fluid exit from the container may be in one component and the port or ports providing fluid entry to the space and the port or ports providing fluid exit from the space may be in a second component; or all the entry ports may be in one component and all the exit ports may be in a second component; or any suitable arrangement of ports may be employed. Conveniently all the ports are in end components.

Whilst the end components can be specifically designed for use in the pump, it is convenient, where considerations of pressure allow, to construct the pump using standard proprietary fittings of the 'Positive Bite' (e.g. Ermeto) or swaged ferrule (e.g. Swagelok Registered Trade Mark) type.

A typical pump may, for example use 1" nominal bore Ermeto high pressure fittings for the end components. These components may be modified conveniently to provide ports for the pump. Pumps according to the invention comprising suitable 10 SWG stainless steel tubing and Ermeto fittings, used in accordance with the manufacturer's instructions, may be capable or operating at pressures up to 4000 p.s.i., although typical operating pressures are in the range 50 to 2500 p.s.i.

The internal diameter of the tube may typically be between 1 mm and 25 cm, and preferably it is between 5 mm and 5 cm.

The length of the tube may be between 1 cm and 100 cms, and typically it is between 5 cm and 50 cm, although we do not exclude the possibility that the dimensions of the tube may be outside these dimensions. The dimensions of the tube will be chosen with regard to the volume of pumped fluid required, the pressure to be employed and the size of attachments available.

Materials from which the tube may be made include metals such as steel, brass, or aluminium; glass; and plastics such as Perspex, Registered Trade Mark, polycarbonate, polysulphone, or polyacetal.

The material chosen for any particular application will depend on the pressure employed and on the fluid which will contact the tube. Stainless steel is preferred since it has been found to be convenient for a wide range of applications.

Preferably the flexible container is in the form of a tubular membrane and particularly preferably it has the same crosssectional shape as the housing in which it is mounted. More preferably the tubular membrane has approximately the same length and cross-sectional area as the housing in which it is mounted so that it may be supported by the housing when it is inflated. To facilitate formation of a fluid tight seal between the flexible tubular membrane and the housing it is particularly preferred that the diameter of the flexible tubular membrane is slightly less than the diameter nf the component to which it is to he sealed so that the membrane has to be stretched to be sealed to the housing and hence the possibility of forming creases in the membrane with the possibility of subsequent leakage of fluid is reduced.

Materials from which the tubular membrane may be made include natural and synthetic polymers such as cellulose, nylon, polyethylene terephthalate, polypropylene, polycarbonate, woven or non-woven fabrics, natural or synthetic rubbers e.g. crosslinked natural rubber, neoprene or polyurethanes. Where the tubular membrane is made of a plastics material it may be reinforced with fibrous or particulate fillers.

Typical values of membrane thickness are from ly to 150,a. A convenient thickness for tubular membranes of polyethylene terephthalate is between 12,a and 65p, and 30p to 50,u is more convenient.

Where a tubular membrane made of a textile or textiles is employed it may be necessary to treat the textile or textiles with an appropriate sealant to make the membrane suitably impermeable to the fluids employed. The permeability of tubular membranes may- be reduced by coating them with a coating which is less permeable than the materials of which they are formed to the fluids which they may contact. Such a coating may be, for example polyvinyiidene chloride or copolymers thereof, e.g. polyacrylonitrile; cellulose nitrate; or metals, e.g. aluminium copper, silver, or gold. The coating, where it is employed, may be in contact with the pumped fluid and/or the working fluid.

Preferably the coating does not dissolve in either of the fluids although we do not exclude the possibility that in some instances it may dissolve. The coating may be applied by wellknown techniques such as deposition or lamination. The thickness of the coating may range from 5mP to 20,', typical thickness of coating are vacuum evaporated aluminium 10my, aluminium foil 10,', polyvinylidene chloride/polyacrylonitrile copolymer 5y.

Choice of suitable materials and thicknesses for the flexible tubular membrane and coating thereof will present no problem to the skilled man and will be made with regard to the internal pressure to which the pump may be subjected, compatibility of the flexible tubular membrane and the coating thereof with the fluids and, where the working fluid is a gas, the permeability of the flexible tubular membrane and coating thereof to the gas. Preferably polyethylene terephthalate film is employed since it has a low permeability to a wide range of gases, in particular oxygen and nitrogen, and is compatible with a wide range of liquids, and polyethylene terephthalate film coated with a layer of polyvinylidene chloride/ polyacrylonitrile co-polymer is particularly preferred.

Where the port or ports providing entry to and exit from the tubular membrane are disposed at one end of the tubular membrane, the other end of the tubular membrane is necessarily closed. Closure may be mechanical e.g. by means of a screw-clamp; adhesive-bonding e.g. by use of a solvent-based or melt adhesive or adhesive tape; a heat seal e.g. by bead sealing or ultrasonic sealing; or the tubular membrane may have been formed with one end closed. The membrane may be coated with a coating to facilitate heat sealing e.g.

where the membrane is polyethylene terephthalate it may be coated with polyethylene, polypropylene or related polymers.

The capacity of the pump may be altered by employing alternative tube components and/or an alternative tubular membrane of appropriate size. Preferably however, where the size of the tube allows, the capacity of the pump is altered by fitting a flexible tubular membrane of different size.

Pumps according to the invention may be operated in any attitude although they are preferably operated in a horizontal position to minimise the possibility of pressure fluctuations in the system due to the hydrostatic pressure of the fluids.

Where pumps according to the invention are used in chromatography, the elution may be isocratic or gradient. Pumps according to the invention may conveniently be used for analytical and preparative chromatography since if only a proportion of the eluent is used the residual amount is not ejected into the atmosphere on release of pressure. Furthermore they have a further advantage over commercially avail ablc pulseless pumps in that they do not have to be filled completely with eluent, the pump need only be charged with as much eluent as is required for an elution. The pumps are also less wasteful of eluent and of time in changing from one eluent to another.

Where a pump according to the invention is employed in liquid chromatography to force eluent through a chromatography column the higher operating pressure of the pump affords the opportunity to use smaller particles to pack the chromatography column and/or to achieve rapid flow of eluent through the chromatography column.

The rate of elution will depend inter alia on the pressure employed, the size of the particles in the column and the scale of the operation. Cholce of the optimum conditions for any one particular application will present no problem to the skilled man.

Typical rates of elution obtained with a pump according to the invention are; for analytical work : 2 ml/ min; for preparative work : 20 ml/min.

The invention is illustrated by the following Example.

ExnsnpZe A 2-5,u litre sample of a .1-lGo solution of a mixture of toluene, phenanthrene, nitrobenene and 2,4-dinitrotoluene in a 1% acetonitrile/hexane solution was injected onto a 10 cm long 4 mm column of Sphere sorb ASY (ex Phase Separations Limited).

A pump according to the invention, having a stainless steel tube 40 cm long, 2.5 cm internal diameter and an inflatable tubular membrane of Melinex (RTM) was filled with eluent, 180 ml of a 1% acetonitrile/ hexane solution. Nitrogen at 200 psi was used to pump the eluent through the column at a flow rate of 2 ml/min. The eluent emmitted from the column was passed through a Cecil UV spectrometer to detect the separated components of the mixture.

WHAT WE CLAIM IS: 1. A pump, for pulseless flow of fluids comprising a housing, provided with a plurality of ports, and a flexible container which is mounted in the housing and which is sealed thereto in a fluid tight manner to withstand a pressure of at least 50 psi and to prevent exchange of a fluid between the inside of the container and a space of variable volume defined by the container and the walls of the housing, at least one of the said ports providing fluid entry to and at least one of the said ports providing fluid exit from the inside of the container, at least one of the said ports providing fluid entry to and at least one of the said ports providing fluid exit from the said space, and the container having a shape and walls of such a thickness that it is variable in shape and volume substantially without resistance within the limits imposed by its construction so that substantially complete expulsion of a pumped fluid from the pump may be achieved by pressurising the pump with a working fluid, said variation in shape and volume of the container being effected primarily by lateral movement of the walls of the container relative to the walls of the housing.

2. A pump as claimed in claim 1 wherein a first port in the housing provides fluid entry to and fluid exit from the inside of the container and a second port in the housing provides fluid entry to and fluid exit from the space of variable volume defined by the container and the walls of the housing.

3. A pump as claimed in claim 1 or 2 wherein at least one of the ports is provided with a guard.

4. A pump as claimed in claim 3 wherein the guard is a perforated metal plate.

5. A pump as claimed in claim 3 wherein the guard is a sintered material.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (28)

**WARNING** start of CLMS field may overlap end of DESC **. Where the port or ports providing entry to and exit from the tubular membrane are disposed at one end of the tubular membrane, the other end of the tubular membrane is necessarily closed. Closure may be mechanical e.g. by means of a screw-clamp; adhesive-bonding e.g. by use of a solvent-based or melt adhesive or adhesive tape; a heat seal e.g. by bead sealing or ultrasonic sealing; or the tubular membrane may have been formed with one end closed. The membrane may be coated with a coating to facilitate heat sealing e.g. where the membrane is polyethylene terephthalate it may be coated with polyethylene, polypropylene or related polymers. The capacity of the pump may be altered by employing alternative tube components and/or an alternative tubular membrane of appropriate size. Preferably however, where the size of the tube allows, the capacity of the pump is altered by fitting a flexible tubular membrane of different size. Pumps according to the invention may be operated in any attitude although they are preferably operated in a horizontal position to minimise the possibility of pressure fluctuations in the system due to the hydrostatic pressure of the fluids. Where pumps according to the invention are used in chromatography, the elution may be isocratic or gradient. Pumps according to the invention may conveniently be used for analytical and preparative chromatography since if only a proportion of the eluent is used the residual amount is not ejected into the atmosphere on release of pressure. Furthermore they have a further advantage over commercially avail ablc pulseless pumps in that they do not have to be filled completely with eluent, the pump need only be charged with as much eluent as is required for an elution. The pumps are also less wasteful of eluent and of time in changing from one eluent to another. Where a pump according to the invention is employed in liquid chromatography to force eluent through a chromatography column the higher operating pressure of the pump affords the opportunity to use smaller particles to pack the chromatography column and/or to achieve rapid flow of eluent through the chromatography column. The rate of elution will depend inter alia on the pressure employed, the size of the particles in the column and the scale of the operation. Cholce of the optimum conditions for any one particular application will present no problem to the skilled man. Typical rates of elution obtained with a pump according to the invention are; for analytical work : 2 ml/ min; for preparative work : 20 ml/min. The invention is illustrated by the following Example. ExnsnpZe A 2-5,u litre sample of a .1-lGo solution of a mixture of toluene, phenanthrene, nitrobenene and 2,4-dinitrotoluene in a 1% acetonitrile/hexane solution was injected onto a 10 cm long 4 mm column of Sphere sorb ASY (ex Phase Separations Limited). A pump according to the invention, having a stainless steel tube 40 cm long, 2.5 cm internal diameter and an inflatable tubular membrane of Melinex (RTM) was filled with eluent, 180 ml of a 1% acetonitrile/ hexane solution. Nitrogen at 200 psi was used to pump the eluent through the column at a flow rate of 2 ml/min. The eluent emmitted from the column was passed through a Cecil UV spectrometer to detect the separated components of the mixture. WHAT WE CLAIM IS:

1. A pump, for pulseless flow of fluids comprising a housing, provided with a plurality of ports, and a flexible container which is mounted in the housing and which is sealed thereto in a fluid tight manner to withstand a pressure of at least 50 psi and to prevent exchange of a fluid between the inside of the container and a space of variable volume defined by the container and the walls of the housing, at least one of the said ports providing fluid entry to and at least one of the said ports providing fluid exit from the inside of the container, at least one of the said ports providing fluid entry to and at least one of the said ports providing fluid exit from the said space, and the container having a shape and walls of such a thickness that it is variable in shape and volume substantially without resistance within the limits imposed by its construction so that substantially complete expulsion of a pumped fluid from the pump may be achieved by pressurising the pump with a working fluid, said variation in shape and volume of the container being effected primarily by lateral movement of the walls of the container relative to the walls of the housing.

2. A pump as claimed in claim 1 wherein a first port in the housing provides fluid entry to and fluid exit from the inside of the container and a second port in the housing provides fluid entry to and fluid exit from the space of variable volume defined by the container and the walls of the housing.

3. A pump as claimed in claim 1 or 2 wherein at least one of the ports is provided with a guard.

4. A pump as claimed in claim 3 wherein the guard is a perforated metal plate.

5. A pump as claimed in claim 3 wherein the guard is a sintered material.

6. A pump as claimed in claim 3

wherein the guard is a woven wire cloth.

7. A pump as claimed in claim 5 or 6 wherein the guard is supported on a backing plate.

8. A pump as claimed in any one of the preceding claims wherein the flexible container is sealed to the housing a manner which at least reduces the possibility of unwanted expulsion of fluid from the pump occurring.

9. A pump as claimed in any one of the preceding claims wherein the housing is in the form of a tuee.

10. A pump as claimed in claim 9 wherein the tube has a circular cross-section.

11. A pump as claimed in claim 9 or 10 wherein the tube is straight.

12. A pump as claimed in any one of claims 9 to 11 wherein the tube comprises a plurality of components.

13. A pump as claimed in claim 12 wherein the ports are in an end component of the tube.

14. A pump as claimed in any one of the preceding claims wherein the flexible container is in the form of a tubular membrane closed at one end.

15. A pump as claimed in claim 14 wherein the thickness of the tubular membrane is between 1,' and 150,a.

16. A pump as claimed in claim 14 or 15 wherein the flexible tubular membrane is formed from a natural or synthetic polymer.

17. A pump as claimed in claim 16 wherein the polymer is polyethylene terephthalate.

18. A pump as claimed in claim 17 wherein the polyethylene terephthalate is coated with a layer of polyvinylidene chloridejpolyacrylonitrile co-polymer.

19. A pump as claimed in any one of claims 14 to 18 wherein the tubular membrane has to be stretched to make a fluid tight seal with the tube or component thereof.

20. A pump as claimed in claim 19 wherein the sealing engagement is effected by mehanical means.

21. A pump as claimed in claim 20 wherein the tubular membrane is trapped between a resilient sealing member and the wall of the tube, the resilient sealing member being mounted on a mandrel.

22. A pump as claimed in claim 21 wherein the resilient sealing member is an O-ring.

23. A pump as claimed in claim 21 or 22 wherein a second resilient sealing member compresses the tubular membrane on the mandrel so that contact of the fluid in the space of variable volume with the first resilient member is reduced or eliminated.

24. A chromatographic apparatus comprising a pump as claimed in any one of claims 1 to 23, connected to a reservoir for eluent, a supply of working fluid, and a chromatography column.

25. A method of passing through a chromatography column comprising the steps of admitting the eluent to the cintainer of a pump as claimed in any one of claims 1 to 23 and expelling the eluent from the container by admitting a gas to the space defined by the container and the walls of the housing.

26. A method of passing eluent through a chromatography column as claimed in claim 25 wherein the gas is air.

27. A pump as substantially described and illustrated with reference to any one of the accompanying drawings.

28. A method of liquid chromatography substantially as described and with particular reference to the Example.