WO1987001616A1

WO1987001616A1 - Multi-cavity washing apparatus

Info

Publication number: WO1987001616A1
Application number: PCT/GB1986/000552
Authority: WO
Inventors: Richard Harry Mark Freeman; Abram David Yeudall
Original assignee: Flow Laboratories Limited
Priority date: 1985-09-16
Filing date: 1986-09-16
Publication date: 1987-03-26
Also published as: EP0235236B1; GB8522872D0; JPS63501313A; EP0235236A1

Abstract

Apparatus for washing microtitration plates comprises a carriage (16) which runs beneath a liquid dispensing head (46). Rinsing fluid supply nozzles (48) and suction nozzles (50) project downwardly from the head (46). The nozzles can be lowered into a row of wells in the plate (14) by movement of the head (46) about a pivot (55). The carriage (16) containing the plate (14) is moved under the nozzles to align a row of wells beneath them in order to perform a washing operation. The washing process comprises filling the wells with rinsing fluid from the supply nozzles (48) and evacuating the fluid by means of the suction nozzles (50). The raising and lowering of the dispensing head and the movement of the carriage are effected by stepper motors (36 and 57) controlled by a preprogrammed microprocessor. The microprocessor offers several different washing cycles from which the user can select one suitable for his requirements. In one cycle the carriage is caused to move to and fro whilst the suction nozzle is evacuating the wells so that the side walls of the well are brought up to the nozzle enabling the fluid that tends to collect in the corners of the wells to be removed. In another cycle the suction nozzle is positioned just above the level of the upper surface of the microtitration plate. Rinsing fluid is supplied to the well whilst suction is supplied to the well. The well is overfilled without the rinsing fluid overflowing into adjacent wells because of the section nozzle. In this way the wells can be washed to the top. The supply of rinsing fluid to the wells is metered by timing the period for which the supply valve (84) is opened. To ensure that the flow through the valve is at a uniform rate, the output of the pump (80) is smoothed by passing it through a length of flexible walled tubing (82) and an adjustable restrictor valve (91).

Description

MULTI-CAVITY WASHING APPARATUS

This invention relates to washing of small cavities, for example, arrays of wells in a microtitration plate.

In diagnostic and epidemological investigations it is a common practice to test a number of biochemical reactions collectively in the wells of a microtitration plate. At various stages in such tests it is necessary to wash out the wells. For example in enzyme immunoassay (E.I.A.) it is necessary to wash out each well between each step to remove unabsorbed or unreacted substances.

Manually operated single row washers are currently available which wash the wells of microtitration plates one row at a time as part of the EIA technique. The wash process is a careful rinsing of each individual well of the microplate by evacuating the fluid in the well, refilling it with a controlled volume of rinse fluid, allowing a soak period, washing the well again and repeating this process three or more times. On each occasion that the well is refilled the remaining unbound contents are diluted by approximately 100:1, thus for a three wash cycle' the unbound contents are diluted by 10 :1. It is important that the wash is carefully controlled so that none of the attached components are stripped from the well. Also there should be no overfilling leading to cross-contamination overflowing between wells.

An example of such a manually operated single row washer is the Titertek Handiwash manufactured by Flow Laboratories Limited.

One problem that arises with the known well washing apparatus is that unevacuate fluid tends to congregate in the bottom corners of the wells.

According to the present invention in a first aspect there is provided a method for evacuating fluid from wells in a microtitration plate including the steps of lowering suction nozzles into wells in the plate, applying suction to the nozzles, and moving the microtitration plate horizontally relative to the suction nozzles so that the suction nozzles traverse the bottoms of the wells. By causing the suction nozzles to traverse the bottoms of the wells the fluid in the corners of the wells is more effectively removed.

Another problem of known well washing apparatus is that wells can become cross-contaminated by fluid on the outside of the suction nozzles being carried across from one well to the next.

According to the present invention in a second aspect there is provided apparatus for removing fluid from reservoirs comprising a support for the reservoirs, suction nozzles for removing fluid from the reservoirs on the support, a suction means connected to the suction nozzles, means for lowering the suction nozzle towards the reservoirs, and control means for applying suction to the suction nozzles before the nozzles are lowered to the fluid in the reservoirs and for lowering the nozzles sufficiently slowly that the nozzles do not penetrate the surface of fluid in the reservoirs as the fluid is drawn up into the suction nozzles. By causing the suction to be applied to the nozzles before they are lowered to the fluid in the reservoirs the wetting of the outside of the nozzles with the fluid in reservoirs can be avoided and this cross- contamination is also avoided. In one application the reservoirs are wells in a microtitration plaste.

In a preferred form, the apparatus includes filling nozzles and means for supplying rinsing fluid to the filling nozzles, the control means being adapted to position the suction nozzles just above the level of the upper surface of the microtitration plate and to actuate the supply means to supply rinsing fluid through the filling nozzles to the wells of the microtitration plate whilst suction is being applied to the suction nozzles in the lowered position whereby the wells are filled to their rims with rinsing fluid without overflowing.

In some washing cycles it is desirable .accurately to control the quantity of rinsing fluid dispensed into the wells. This can be achieved by controlling the time for which a metering valve is open provided the rinsing fluid is delivered at a constant rate. Unfortunately the output of pumps is usually pulsed so that the flow rate through the metering valve is varied.

According to the present invention in a third aspect there is provided apparatus for supplying liquid comprising a pump, a length of tubing of flexible resilient material connected to the output of the pump, and a restrictor valve connected to the downstream end of the tubing to receive the output of the pump through the tubing, the walls of the tubing being so elastic as to dampen pulses in the output of the pump.

A specific embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a view from above of a titration plate washing apparatus incorporating the present invention;

Figure 2 is a section on the line II-II of Figure 1;

Figure 3 is a view from the underside of the apparatus of Figure 1 with the base cut away;

Figure 4 is a view of a spiral cam plate;

Figure 5 is a side elevation along the line V-V of Figure 1 with parts omitted for clarity;

Figure 6 is a plan view of the apparatus of Figure 1 with the cover removed and some parts omitted for clarity;

Figure 7 is a schematic diagram of a supply pump;

Figures 8 to 13 are sectional views of various embodiments of a nozzle feed arrangement.

Figure 14 is a fragmentary view through the head and well illustrating bi-direction movement of the carriage during evacuation of the well; and

Figure 15 is a fragmentary view through the head and well illustrating a cycle for washing the wells to the rim.

Referring to the drawings, apparatus for washing microtitration plates comprises a carriage 16 which runs beneath a liquid dispensing head 46. Rinsing fluid supply nozzles 48 and suction nozzles 50 project downwardly from the head 46. The nozzles can be lowered into a row of wells in the plate 14 by movement of the head 46 about a pivot 55 (see Fig. 5). The carriage 16 containing the plate 14 is moved under the nozzles to align a row of wells beneath them in order to perform a washing operation. The washing process comprises filling the wells with rinsing fluid from the supply nozzles 48 and evacuating the fluid by means of the suction nozzles 50. The raising and lowering of the dispensing head and the movement of the carriage are effected by stepper motors 36 and 57 _ n -

controlled by a preprogrammed microprocessor. The microprocessor offers several different washing cycles from which the user can select one suitable for his requirements.

Referring to Figures 1, 2 and 3, a housing 9 consists of a base portion 10 connected with a front wall 5, a rear wall 6, and side walls 7 and 8, thus providing a rigid supporting structure on which the well-washing apparatus is constructed. A cover 11 rests on the front, rear and side walls 5, 6 and 8 and extends across the left hand half of the apparatus as viewed in Fig. 2

A microtitration plate 14, shown cut away for the sake of clarity, is held in position on a carriage 16. The microtitration plate 14 for use with this apparatus is the common 8 x 12 rows type of clear transparent moulded plastics material. Usually each well is formed with a flat bottom such that the extent of a reaction in a well can be measured optically according to the amount of light passed by the reacted substance in each well.

The carriage 16 is rectangular with a raised lip 15 around its edge to locate the plate and an upper surface 17 which is shaped as a shallow funnel sloping down towards a central drain hole 13 to collect spillage from the plate. The carriage 16 travels from front to back on a linear track to one side of the housing. The track comprises a rail 18 and a groove 19 fixed on the housing. The rail, which is mounted on the cover 11 adjacent the side wall 7 of the housing, comprises a rod 18 which supports the underside of the carriage 16 along one side (see Fig. 2). The opposite side of the carriage 16 is secured to an upper portion 25 of a carriage arm 24. The upper portion 25 of the arm is enlarged and engages the groove 19 in the cover to support the carriage on the right hand side and guide the carriage for linear movement along the track. A spring 27 mounted on the arm 24 bears against the underside of the cover 11 to hold the carriage 16 on the cover and maintain the enlarged portion

25 of the arm in the groove 19. Between the groove 19 and the rail 18 the cover 11 is shaped as a shallow funnel to form a drip tray 26 which extends for the length of travel of the carriage 16. The tray

26 collects fluids spilt from the carriage and funnels then into a central drain hole 20 which- is connected via a pump (not shown) to a suitable reservoir. A gauze filter 21 is located within the drain hole 20.

The carriage 16 is moved along its linear track by the stepper motor 36 via a lead screw mechanism 28. The narrower lower portion of the carriage arm 24 extends downwardly from its enlarged upper portion 25 through a slot 29 in the cover 11 alongside the drip tray 26 and is secured to a lead screw sleeve 23 of the lead screw mechanism 28. A lead screw 30 is rotatably mounted at one of its ends in a ball race bearing 32 mounted on the rear wall 6 of the housing 9 and is attached at its other end to the shaft of the stepper motor 36 mounted adjacent the front wall 5 of the housing 9. The lead screw sleeve 23 engages the lead screw thread by means of a pair of spaced threaded plastics bushes 38 secured to the inside of the sleeve 23 to provide smoother running characteristics than would a metal equivalent. To maintain a clean thread on the lead screw 30 it is enclosed within a first bellows-like gaiter 40 attached to the sleeve 23 at one end and to the bearing 32 at the other end, and a second bellows-like gaiter 44 attached to the sleeve 23 at one end and to a sleeve 35 on the stepper motor 36 at the other end. As the sleeve 23 is caused to travel along the lead screw 30 due to rotation of the stepper motor shaft each gaiter 40, 44 will extend or retract in order to accommodate the changing position of the sleeve 23.

The carriage 16 can be moved by the stepper motor 36 from a "home position" at the front of the cover 11 to a rear operating position beneath a dispensing head 46. The rinsing fluid supply nozzles 48 and suction nozzles 50 project downwardly from the dispensing head in pairs. The spacing of the pairs of nozzles corresponds to the spacing of the wells in a row in the microtitration plate. In this embodiment the dispensing head 46 and carriage 16 are constructed so as to be used on the standard microtitration plate mentioned previously with an array of 8 x 12 rows. Thus 12 pairs of washing liquid supply and suction nozzles 48, 50 are provided. Clearly more or less nozzle pairs could be mounted in a row. The dispensing head is interchangeable and can be swapped for a head having, say, 8 pairs of nozzles. It is also possible to have more than one row of nozzle pairs in order to increase the speed of the washing process. Each suction nozzle 50 projects vertically downwardly from the dispensing when the dispensing head 46 is in an evacuating position and is of sufficient length to enable it to draw fluid from the bottom of a well in the microtitration plate 14. Each supply nozzle 48 is mounted at a slight angle to and is terminated short of its corresponding suction nozzle 50. The supply nozzles 48 may be mounted at an angle to the suction nozzles 50 so as to allow the washing liquid to set up a swirling motion as it enters the well from the nozzle 48 to enhance its cleaning action. Figures 8 to 11 illustrate two forms of supply nozzle 48 which may be used in the dispensing head 46, with the dispensing head lowered to a filling position with respect to the well to be washed. Alternatively, the supply nozzle 48 could be arranged as shown in Figures 12 and 13 in which the supply and suction nozzles lie parallel to one another.

The dispensing head 46 is detachably connected to an interconnect 52 illustrated in Figure 5, such that suction and supply bores 54, 56 in the dispensing head 46 are communicated through the interconnect with a suction pump (not shown) and a supply pump 80. The suction and supply bores are connected within the head to the suction and supply nozzles 50 and 48. respectively. The cross-section of the supply bore 56, connecting the nozzles 48 with the interconnect 52, is of sufficiently large diameter in relation to the cross-section of the nozzles that there is no significant pressure difference between the fluid at the different nozzles.

The interconnect 52 is pivotably supported on a post 49 secured to the rear wall 6 of the housing 9, by means of a leaf spring 55 and is raised and lowered about that pivot by means of a second stepper motor 57 (see Fig. 5). On a shaft 58 of the second stepper motor 57 is mounted a scroll cam plate 60 shown in Figure 4. In the surface of the plate 60 facing away from the stepper motor 57 is a spiral scroll groove 62 which progresses outwardly from about the centre of the plate 60. Engaged in the groove 62 on the cam plate 60 is a pin 64 which projects from a downstop arm 66 secured to the base of the interconnect 52. By rotating the second stepper motor 57 the fixed pin 64 is moved upwardly or downwardly as the path of the groove 62 moves past it. This causes the down-stop arm 66 to rise and thus tilt the interconnect 52 and dispensing head 46 about the pivotal leaf spring 55. The advantage of the leaf spring 55 is that it does not suffer from backlash when the direction of movement of the head 46 is changed.

In order to maintain the pin 64 bearing against one wall of the groove 62 during travel of the head 46, the interconnect/ dispensing head assembly 68 is biased downwardly by means of a spring 70 secured between the interconnect/dispensing head assembly 68 and a bracket 71 fixed to the rear wall 6 of the housing 9. The inner end of the spiral groove 62 in the cam plate 60 is flared in its width such that when a lower face on the downstop arm 66 abuts a stop surface 73 on an adjustable lever 72 as the down stop arm 66 and hence the dispensing head 46 are lowered into the evacuating position, the pin 64 is allowed to disengage from the said wall of the groove 62 against which it bears as a result of gravity and the additional restraining force exerted by the spring 70. The lever 72 is pivoted about a point 76 such that rotating a graduated dial 98 on a threaded shaft 74 mounted in the side wall 7 draws the lever 72 upwardly or downwardly about the pivot point 76. By this, the lowered evacuating position of the dispensing head 46 can be adjusted without having to reprogramme the movement of ^'the second stepper motor 57 and with the pin 64 out of contact with the side wall of the groove 62. To lift the head 46, the plate 60 is rotated by the stepper motor 57 so that the pin 64 eventually engages with that wall of the spiral groove 62 raising the downstop arm 66 from a position of abutment with the arm of the lever 72. By this, the time taken by the head 46 to reach a height above a given microtitration plate 17 is independent of the setting of the evacuating position of the suction nozzles 50. This greatly simplifies the programming of the controls for the stepper motor 57.

Referring also to Figure 6, the suction bore 54 is connected with a suction pump through a solenoid operated suction valve 78 which controls the use of the suction pump. In the present embodiment the suction pump is a separate pump (not shown) located outside the apparatus but in another embodiment the suction pump is also located within the housing In the space 79,

The supply pump 80, illustrated schematically in Figure 7, comprises a piston 81 which moves in a cylinder 83. Movement of the piston 81 from right to left causes a first one-way valve 85 in the piston crown to close and fluid to he drawn in to the space vacated by and to the right of the piston 81 via a second one-way valve 87. Once the piston 81 has travelled from right to left to the end of the cylinder 83 and the space to the right of the piston 80 is charged with fluid, the piston returns to its original position allowing the fluid to pass through the first one-way valve 85 into the space to the left of the piston 81. Repeating the motion of the piston from right to left forces the fluid in the space to the left of the piston 81 out through a third one-way valve 87 in the cylinder 83 and the charging of the space to the right of the piston 81 to be repeated. A solenoid surrounding the cylinder 83 is energised to move the piston 81 to and fro.

The delivery response of the pump 80 on start up is immediate. This enables the amount of fluid metered to the supply nozzles 48 to be calculated on the basis of the amount of time the pump is in operation, assuming a virtually constant rate of supply over the time. This constant supply rate is compromised, however, by the pulsating nature of the output characteristic of the pump 48. To overcome this the output of the supply pump 80 is connected with one end of a length of silicone rubber tubing 82 which has its other end connected with an adjustable restrictor valve 91. The silicone rubber tubing 82 absorbs energy from the pulses of wash fluid such that, by tuning the restrictor valve 91 to the length of tubing 82, an optimum smoothness is achieved. The smoothed wash supply is connected by hardwall tubing 92 to a solenoid operated fill control valve 84. Hardwall tubing is used to prevent pressure being trapped between the restrictor valve 91 and the solenoid valve 84 which would cause a flow surge immediately the solenolid valve 84 was opened.

The washing operation can include as many wash and rinse cycles for each row as required. These are programmed in software in a microprocessor (not shown) which issues the appropriate commands to the first and^' second stepper motors 36 and 57 and the solenoid valves 78 and 84.

On first switching the apparatus on, the microprocessor is programmed to cause the stepper motors 36 and 57 both to execute verification routines, each in conjunction with a corresponding microswitch.

The microswitch 90 mounted on the sleeve 23 trips when the carriage 16 has reached the end of its travel toward the rear wall 6 by means of a lever 92 abutting a stop (not shown) mounted on the wall 6. The travel of the dispensing head 46 is inhibited by a similar microswitch (not shown) which is closed when the head is fully raised.

The programme checks for the presence of a closed microswitch corresponding to the dispensing head being fully raised. If such a closure is not detected the head is raised up by a maximum of 61 steps or increments of the second stepper motor. Once again, the microswitch is interrogated for a closed state and if one is not forthcoming an error condition is indicated on a display panel and the entire wash process is inhibited. If, however, a closure is detected, the head is moved twenty steps of the second stepper motor downwardly and the microswitch is interrogated for an open circuit. Once this is detected the head is raised once more to within 10 steps of the fully raised position. The microprocessor then conducts a similar verification routine on the movement of the carriage; if the microswitch corresponding to the carriage movement is not made the carriage is moved back a maximum of 1549 steps of the first stepper motor, while the processor surveys the microswitch to detect its closure. Once the closure is detected the carriage is moved 20 steps of the first stepper motor forward and the apparatus is ready to be programmed according to the number of rows and the nature of the wash cycle to be performed.

The number of wash routines are programmed into the microprocessing controller by means of a number of switches on a display panel 86 on a cover 88 on the housing 9. By selecting the appropriate button, the type of washing cycle can be selected, the number of times the cycle is repeated can be selected, the volume of washing fluid used for each well can be selected, soaking periods in between washing and evacuating the wells may also be programmed in.

The microprocessor offers the following mode of washing programs:- a) A normal cycle, suitable for most applications. In this cycle the microprocessor causes each row of wells in turn to be filled with rinsing fluid and allowed to soak. After all rows have been filled the suction nozzles are lowered to the bottom of wells for each row in turn to evacuate the wsells. The cycle is repeated a number of times. b) A moving carriage cycle. This cycle is the same as the normal cycle except that during the evacuation of the wells the program follows a sub-routine whereby the carriage moves such that first one side 100 of the microplate well is moved up to the suction nozzle 50 and then the other side of the microplate well 101 is brought to the suction nozzle (see Fig. 15). Finally the carriage is moved to bring the nozzle to the centre of the well before moving on to the next row to be washed. These movements are effected by the microprocessor instructing the stepper motor 36 to move back, forth and back again the requisite number of steps to move the plate by distances corresponding to the size of the standard microtitration well. c) A special washing cycle to allow efficient washing of any residue at the top of the well. The apparatus is arranged so that the suction nozzles are about 2mm above the level of the top of the microtitration plate when in the raised position (see Fig. 16). The microwell plates are over-filled while the head is in this position but suction is applied to the suction nozzle to remove any possibility of overflowing. Thus the well is washed to the top. d) A single row washing cycle. This cycle is the same as the normal cycle except that each row is washed on all cycles before moving on to the next.

Describing the washing cycle in greater detail, with the dispensing head 46 raised, the verification routines checked and the washing mode programmed, the carriage 16 moves the microtitration plate 17 beneath the dispensing head 46 until the first row of wells programmed to be washed is aligned beneath the row of twelve nozzle pairs. The dispensing head 46 is lowered into a dispensing position such that the nozzle pairs are above the final level of the wash fluid which is supplied to the supply nozzles 48 on a command from the microprocessor opening the solenoid operated fill control valve 84 for a given amount of time. The angle of each supply nozzle 48 with respect to the plate allows the fluid to swirl around the well as previously described, and the smoothed pump characteristic ensures that the amount of fluid metered is accurate and does not spill into adjacent wells. The force of the jet of fluid emitting from the supply nozzle 48 is arranged to be insufficient to wash off the reacted antigen/antibody adsorbed to the well wall.

After a soak period the dispensing head is lowered toward the wells of the plate to the evacuating position. As this is done the solenoid operated suction control valve 78 is opened to start the suction procedure before each suction nozzle 50 meets the surface of the fluid in its corresponding well. By this sequence the liquid is drawn into the suction nozzles 50 without touching, (and thereby possibly contaminating) the sides of the nozzles, as the rate of descent of the dispensing head 46 is such that the level of the fluid goes down at a rate greater than the nozzles are lowered.

It is a characteristic of the flat bottomed wells in some forms of microtitration plate that a proportion of the washing liquid remains in the corners due to surface tension. In order to minimise the amount of fluid remaining in the well and thus markedly improve the dilution of unreacted material therein, the microprocessor can be set in mode (c) to move the carriage to and fro with the head 46 in the evacuating position, allowing the suction nozzles 50 to suck up a greater amount of the fluid. Once the suction operation is performed the head 46 is raised and the wash cycle repeated in the same wells or those of the next row in the plate 14.

Each supply nozzle 48 is located beside its ^'corresponding suction nozzle 50 in order to allow by the movement of the carriage 16, the removal of the fluid adhering to the. corners of the wells without it being limited by the presence of the supply nozzles 48 and hence impair the completeness of the evacuating operation.

The spacing of each well will be the same for a given type of microtitration plate, but it is necessary to be able to finely adjust the attitude of the dispensing head 46 in the factory. This is done by loosening screws 93 securing the interconnect to the downstop 66 and setting the- position in a horizontal plane. To allow the depth of movement of the suction nozzles 50 into the wells to be altered the adjustment lever is used as previously described. The depth of such wells may vary according to the nature of the plate used; if the bottom was not flat the shuffling backwardly and forwardly of the suction nozzles 50 would be reduced or programmed out of the cycle and the working depth of the suction nozzles 50 changed accordingly.

Claims

CLAIMS :

1. A method for evacuating fluid from wells in a microtitration plate including the steps of lowering suction nozzles into wells in the plate, applying suction to the nozzles, and moving the microtitration plate horizontally relative to the suction nozzles so that the suction nozzles traverse the bottoms of the wells.

2. Apparatus for evacuating fluid from wells in a microtitration plate comprising suction nozzles mounted in a head, a support for the microtitration plate, means for moving the support horizontally to position selected wells in a microtitration plate beneath the suction nozzles and means for lowering the head to lower the nozzles to the bottoms of the selected wells, a vacuum source for applying suction to the nozzles, and control means arranged to cause the support-moving means to operate whilst the head is lowered and suction is applied to the nozzles so that the suction nozzles traverse the bottoms of the selected wells.

3. Apparatus according to claim 2 in which the head also carries wash-fluid supply nozzles, the wash-fluid supply nozzles being arranged with the suction nozzles in pairs so that one supply nozzle and one suction nozzle is lowered into each well in the microtitration plate.

4. Apparatus according to claim 3 in which the supply nozzles are connected to a source of wash fluid through a supply valve and the suction nozzles are connected to vacuum source through a suction valve, the control means being arranged to cause the supply valve and suction valve to operate in a cycle.

5. Apparatus for removing fluid from reservoirs comprising a support for the reservoirs, suction n'ozzles for removing fluid from the reservoirs on the support, a suction means connected to the suction nozzles, means for lowering the suction nozzle towards the reservoirs, and control means for applying suction to the suction nozzles before the nozzles are lowered to the fluid in the reservoirs and for lowering the nozzles sufficiently slowly- that the nozzles do not penetrate the surface of fluid in the reservoirs as the fluid is drawn up into the suction nozzles.

6. Apparatus according to claim 5 In which the reservoirs are wells in a microtitration plate.

7. Apparatus for washing wells in a microtitration plate including filling nozzles and means for supplying rinsing fluid to the filling nozzles, suction nozzles and means for moving the suction nozzles relative to a support for carrying the microtitration plate, control means being adapted to position the suction nozzles just above the level of the upper surface of the microtitration plate and to actuate the supply means to supply rinsing fluid through the filling nozzles to the wells of the microtitration plate whilst suction is being applied to the suction nozzles whereby the wells are filled to their rims with rinsing fluid without overflowing.

8. Apparatus for supplying liquid comprising a pump, a length of tubing of flexible, resilient material connected to the output of the pump, and a restrictor valve connected to the downstream end of the tubing to receive the output of the pump through the tubing, the walls of the tubing being so elastic as to dampen pulses in the output of the pump.

9. Apparatus according to claim 8 in which the restrictor valve is adjustable.

10. Apparatus according to claim 8 or 9 in which the output of the restrictor valve is connected to a metering valve.

11. Apparatus for washing wells in a microtitration plate, the apparatus including a plurality of supply nozzles, a supply of washing fluid and apparatus for supplying liquid according to claims 1, 2 or 3, the pump inlet being connected to the supply of washing fluid and the restrictor valve being connectable to the nozzle for delivering washing fluid through the nozzles to the wells in the microtitration plate.