GB2343673A

GB2343673A - Device for the splitting of liquid flow

Info

Publication number: GB2343673A
Application number: GB9823710A
Authority: GB
Inventors: Clive Lawrence Ayling; Ian Pestell; Neil James Butt
Original assignee: Cambridge Molecular Technologies Ltd
Current assignee: Cambridge Molecular Technologies Ltd
Priority date: 1998-10-29
Filing date: 1998-10-29
Publication date: 2000-05-17
Also published as: GB9823710D0

Abstract

A device for splitting an input volume of liquid into a plurality of output volumes comprises a liquid inlet passage (5, fig.6), a divider, and a plurality of liquid outlet passages 10, wherein the aggregate area of the outlet passages 6 is no more than twice the cross-sectional area of the inlet passage (5, fig.6). These dimensions ensure that the surface tension of the liquid flowing through the device prevents air becoming trapped in the flow resulting in the dispensing of less accurate volumes. Preferably the output volumes of liquid are all equal and the inlet (5, fig.6) and outlet 6 passages are of uniform cross-section along their length. The device may divide the flow in two stages, having intermediate passages 8 extending from the inlet passage, and outlet passages 6 extending from the intermediate passages 8. In this case, the aggregate cross-sectional area of the intermediate passages 8 is preferably not more than 1.5 times the cross-sectional area of the inlet (5, fig.6).

Description

SPLITTING DEVICE The present invention relates to a device for the splitting of input volumes of liquid into a plurality of output volumes of liquid. More specifically, the present invention relates to a device that can accurately deliver controlled, small volumes of liquid simultaneously to multiple receptacles.

In many biochemistry laboratories apparatus is used to conduct biochemical reactions in order to, for example, purify protein or nucleic acid from various starting materials.

Manual methods for purifying protein or nucleic acid are time consuming and labour intensive. Consequently, apparatus has been developed which can enable the simultaneous purification of multiple protein or nucleic acid samples. One such apparatus is described in WO 95/02049.

According to Wo 95/02049, reagents in reservoirs R1 to R8 aredelivered in a controlled manner to each of the twelve vessels in which the purification procedure is conducted. In order to deliver reagent from one reservoir to each of the twelve reaction vessels, the valve controlling the opening of the reservoir is opened for a period of time in order to draw a controlled volume of reagent into the liquid inlet line. This liquid inlet line divides at one point into twelve liquid outlet lines. The device used for this purpose is illustrated in Figures 1 and 2. The output volumes of liquid which flow through the device are delivered to each of the twelve reaction vessels. The liquid lines are subsequently flushed with air or another inert gas to remove any residual reagent from the lines. The splitting device is thereafter ready for the delivery of a second reagent from a second reservoir to each of the reaction vessels.

It has been found by the applicant that this configuration of splitting device fails to flush fully the liquid delivery line, produces a bubbly flow path, and consequently does not deliver an accurate balanced dose between the twelve output lines.

Furthermore, in some cases, it has been found that the output lines are temporarily left blocked by small"slugs"of liquid of the previously dispensed reagent separated by air gaps.

Consequently, different doses are delivered by this device resulting in poor repeatability of the very sensitive biochemical reactions.

Accordingly, it is an object of the present invention to provide a device for the splitting of an input volume of liquid into a plurality of output volumes of liquid in which it is possible to produce highly-repeatable balanced output doses of liquid.

According to the present invention there is provided a device for the splitting of an input volume of liquid into a plurality of output volumes of liquid comprising: (a) a liquid inlet passage; (b) a divider for dividing flow of liquid; and (c) a plurality of liquid outlet passages communicating via the divider with the liquid inlet passage, wherein the aggregate cross-sectional area at any distance along the pathway of the liquid through the splitting device is no greater than twice the minimum cross-sectional area of the inlet passage.

It has been found by the-applicant that by designing the pathway of the liquid through the splitting device to be such that the sum or aggregate cross-sectional area at any distance along the pathway of the liquid through the splitting device is no greater than twice the minimum cross-sectional area of the inlet passage, the liquid passes through the device and is split into the required number of output volumes of the liquid in a controlled manner. It is thereby possible to obtain consistent, accurate and controlled output volumes of the liquid.

As used herein, the term"pathway"of the liquid is used to refer to the complete pathway of the liquid through the splitting device, comprising all passages after each point at which flow of liquid is split, and not to a single passageway thereof.

For example, where the divider comprises a single manifold with 6 outlet passages extending therefrom, the pathway comprises all 6 outlet passages. The aggregate cross-sectional area at a distance after the point of splitting along this pathway is the sum of the cross-sectional areas of the 6 passages through which liquid flows at said distance from the point of splitting.

One principle behind the invention is that the liquid pathway must be such as to"keep together"the leading edge of the liquid passing through the splitting device and to ensure that liquid surface tension would not be broken thereby allowing air to enter the"slug"of liquid as bubbles. According to the present invention, in order to avoid this problem, it is required that the aggregate cross-sectional area at any distance along the total pathway of the liquid through the splitting device be no greater than twice the minimum cross-sectional area of the inlet passage. Preferably, the aggregate cross-sectional area at any distance along the pathway of the liquid through the splitting device does not increase at any stage. Nevertheless, it has been found that provided the aggregate cross-sectional area at any distance along the pathway of the liquid through the splitting device is not greater than twice the cross-sectional area of the inlet passage, breakup of the liquid"slug"is~avoided.

Preferably, the inlet passage has a cross-sectional area of not greater than 1. 7mm2 and the outlet passages each have a crosssectional area of not less than 0.05mm2. For an inlet passage of circular cross-section, a cross-sectional area of 1. 7mm2 corresponds to a diameter of approximately 1. 5mm. A crosssectional area of 0.05mm2 corresponds to a passage of circular cross-section of approximately 0.25mm2 in diameter. It is, of course, possible to have inlet passages and outlet passages of cross-section that is other than circular. However, for simple manufacturing of the device, it is preferred that the crosssection of these passages be circular.

It has been found by the applicant that for liquids having a surface tension similar to or higher than that of water, the advantageous properties of the splitting device of the present invention are difficult to obtain when the passage of the device is too wide. An inlet passage width of a maximum of 1. 5mm in diameter is preferred to avoid breaking of the"leading edge"of the liquid passing through the inlet passage.

It has also been found by the applicant that, for liquids with a viscosity near to that of water, tubes of smaller than 0.25mm in diameter suffer from tolerance variations leading to variance in flow resistance, hence speed and accordingly variance in the output volume of liquid.

It is possible to have outlet passages of different crosssectional areas in order to deliver different but controlled doses to receptacles such as reaction vessels. However, it is preferred that the output volumes of liquid be equal. Usually this is controlled by ensuring that the cross-sectional area of the outlet passages are equal. Preferably, the outlet passages are also equal in length to ensure that liquid flows through the passages at the same velocity.

It is also possible for the inlet and outlet passages to be of varying cross-sections along their length. For example, the inlet tube may be tapered. However, it is preferred that the inlet passages and each of the outlet passages be substantially uniform in cross-section along their length.

Preferably, the divider comprises a first manifold associated with the inlet passage comprising a plurality of apertures. It is more preferred that the divider further comprises a series of outlet manifolds from which the outlet passages extend.

Accordingly, it can be envisaged that the divider includes a series of"stages"of splitting. A divider comprising a single splitting stage corresponds to a divider having one manifold only with a number of apertures corresponding to the total number of outlet passages. A divider having two stages of splitting according to one embodiment of the invention corresponds to a divider having a first manifold associated with the inlet passage comprising a plurality of apertures, and a second stage of splitting manifolds ("outlet manifolds") corresponding in number to the number of apertures in the first manifold. It is envisaged that the splitting device of the present invention may include between one and five stages of splitting in order to divide the flow of a liquid a suitable number of times in order to obtain the required number of output volumes of the liquid.

According to a particularly preferred embodiment of the invention, the divider means comprises: -an inlet manifold associated with the inlet passage comprising a plurality of apertures, -intermediate passages extending one from each aperture in the inlet manifold, and -outlet manifolds associated with each of the intermediate passages having a plurality of outlet apertures; wherein an outlet passage extends from each of the outlet apertures of the outlet manifolds.

It is preferred that the inlet passage and the outlet passages be substantially circular in cross-section.

It is preferred that the apertures of each manifold be spaced circumferentially about an axis to one end of the passage with which it is associated. It is more preferred that the apertures be spaced circumferentially about an axis at the end of the passage with which it is associated.

Preferably, the sum of the cross-sectional areas of the intermediate passages is not greater than 50% more than the cross-sectional area of the inlet passage, and preferably not greater than 20%.

It is also preferred that the sum of the cross-sectional areas of the outlet passages be not greater than 50% more than the sum of the cross-sectional areas of the intermediate passages, more preferably not greater than 20% more.

Preferably, the outlet passages are not less than 0.3mm in diameter.

Preferably, the inlet passage is not less than 0.7mm in diameter.

Preferably, the inlet passage is not greater than l. Omm in diameter.

Preferably, the intermediate passages are from 0.3 to l. Omm in diameter, more preferably from 0.4 to 0.9mm in diameter.

Preferably, the intermediate passages are each of the same length.

Preferably, the input volume of liquid is split into output volumes of between 100nl and 1 litre. It is more preferred that the output volumes of liquid be between 30y1 and lml.

The applicant has found-that at the point~of splitting there is a danger that the cross-sectional area expands too quickly, especially if the passages which extend from the manifold merge with each other to effectively form a large chamber at the end of the inlet passage. Accordingly, it was found by the applicants that, for splitting devices requiring at least ten outlet passages or tubes, splitting the pathway of the liquid in two stages gives rise to particularly good dose control.

It is also preferred that the sum of the diameters of the outlet passages be less than the circumference of the inlet passage.

As will be understood by persons skilled in the art of the invention, a large number of combinations of stages of splitting may be used in the divider according to the present invention.

If it is desired that the splitting device have twelve liquid outlet passages, this may be obtained by a two stage splitter having one 1: 2 and two 1: 6 manifolds, possibly built into the same block. Alternatively, one 1: 3 and three 1: 4 manifolds could be used to obtain twelve liquid outlet passages from a single inlet passage. Where different numbers of outlet passages are required, various permutations and combinations of manifolds and passages may be used to split the input volume of liquid into the desired plurality of output volumes of the liquid.

According to the present invention there is also provided apparatus for the simultaneous purification of nucleic acid from a plurality of nucleic acid-containing samples comprising: (a) a series of assemblies in which the nucleic acid containing samples can be purified; (b) one or more liquid reagent reservoirs; (c) a liquid delivery means communicating with each of the assemblies for the delivery of output volumes of liquid reagent from a liquid reservoir to each of the assemblies; and (d) outlets for the elution of the purified nucleic acid from each assembly, wherein the liquid delivery means comprises the splitting device as described above.

Preferably, the liquid delivery means of the apparatus comprises a valved gas inlet for flushing of the passages of the liquid delivery means. Preferably, the gas comprises air.

The presence of a valved air-inlet in the apparatus of the present invention allows for emptying of the liquid passages, and air flushing in between dispenses (which may be of differing reagents).

According to the present invention there is also provided a method for the simultaneous purification of nucleic acid from a plurality of nucleic acid-containing samples comprising: (a) delivering the nucleic acid-containing samples to a series of assemblies; (b) delivering volumes of liquid reagent from a liquid reagent reservoir simultaneously to each of the assemblies through a splitting device as described above; and (c) eluting isolated nucleic acid from each of the assemblies, wherein, in step (b), an input volume of liquid reagent is channelled into the splitting device and the output volumes of the liquid reagent are subsequently dispensed into the assemblies.

Preferably, the method involves the delivery of more than one liquid reagent through the splitting device to each of the assemblies. Where more than one liquid reagent is to be delivered to the assembly through the splitting device, it is preferred that, following the delivery of the first liquid reagent to the assemblies, the splitting device is flushed with air to remove residual liquid therefrom. The air flushing step may include flushing with a series of bursts of air. It has been found by the applicant that this method of forcing bursts of air through the splitting device is particularly effective in removing residual liquid from the passages of the splitting device.

The present invention will now be described in further detail with reference to the following Figures, in which: Figure 1 is a side view of one splitting device of the prior art; Figure 2 is cross-section on the centre line a-a of the splitting device of the prior art illustrated in Figure 1; Figure 3 is a side view of the splitting device of a preferred embodiment of the invention; Figure 4 is a cross-section along line B-B of the splitting device illustrated in Figure 3; Figure 5 is a cross-section along line A-A of the splitting device illustrated in Figure 4; Figure 6 is a cross-section along line C-C of the splitting device illustrated in Figure 3; Figure 7 is a schematic illustration of the flow path of liquid through the splitting device illustrated in Figure 3; Figure 8 illustrates an array of twelve assemblies into which liquid divided through the splitting device of the present invention can be delivered; and Figure 9 shows a circuit diagram for the Qperation of an apparatus fitted with the splitting device of the present invention.

Known devices of the prior art for splitting an input volume of liquid into twelve equal output volumes of liquid, such as that illustrated in Figures 1 and 2, include an inlet passage 1 of 2.2 mm in diameter (corresponding to a cross-sectional area of 3.8 mm2) and twelve outlet tubes 2 of 1 mm in diameter. At the point where the twelve outlet tubes 2 merge with each other and meet with the inlet tube 1, there is formed a plenum or manifold chamber 3 of 6 mm in diameter, corresponding to a cross-sectional area of 28.3 mm2. When the fluid passing through the inlet tube 1 reaches the manifold chamber 3, the liquid will not immediately fill the manifold chamber 3 with liquid, but-will dribble through into one or more of the outlet tubes 2 in a poorly controlled way. As a result of this, air bubbles will be formed in the liquid pathway, resulting in poor dose control from each of the twelve outlet tubes 2. In addition, it has been found that in some instances a droplet of water in the outlet tube may block the outlet tube preventing liquid from flowing therethrough.

According to one preferred embodiment illustrated in Figures 3-6, there is provided a device for the splitting of an input volume of liquid into a plurality of output volumes of liquid comprising a liquid inlet passage 5 and twelve liquid outlet passages 6 separated by a dividing means or divider for dividing the flow of liquid. The divider comprises an inlet manifold formed at the end of the inlet passage 7 comprising two apertures (see Figure 5); two intermediate passages 8 of equal length extending one from each of the two apertures in the inlet manifold 7; and two outlet manifolds 9 one at each end of the intermediate passages 8 and each having six outlet apertures, from which the outlet passages 6 extend. At the end of each outlet passage 6 there is an outlet port 10 which is adapted to receive tubes (not shown) leading to vessels to which the controlled doses of liquid are to be delivered.

The liquid inlet passages is of 1 mm in diameter (cross-sectional area 0.8 mm2) ; the intermediate passages 8 are each of 0.7 mm in diameter (cross-sectional area of 0.4 mm2 each; total crosssectional are of 0.8 mm2) and the outlet passages 6 are each of 0.3 mm in diameter (cross-sectional area of 0.07 each-, corresponding to an aggregate cross-sectional area of 0.8 mm2).

The outlet passages are also equal in length.

At inlet manifold 7 the pathway of the liquid splits into two, defined by each of the intermediate passages. At a distance of, for example, 1 mm from the inlet manifold in the direction of flow of liquid into each intermediate passage, the aggregate cross-sectional area of the pathway of liquid is the sum of the cross-sectional areas of the two intermediate passages: 0.4 mm2 + 0.4 mm2 = 0. 8 mm2. At each of the two outlet manifolds the pathway of the liquid splits into six (a total of 12), defined by each of the outlet passages. At a distance of, for example, 1 mm from the outlet manifold in the direction of flow of liquid into each outlet passage, the aggregate cross-sectional area of the pathway of the liquid is the sum of the cross-sectional areas of the 12 outlet passages: 12 x 0.07 mm2 = 0. 8 mm2.

With reference to one of the outlet manifolds 9 located at the end of one of the intermediate passages 8, the six 0.3 mm outlets can extend from 0.3 mm circular apertures in the intermediate passage without forming a large chamber, since the circumference of the 0.7 mm diameter intermediate tubes 8 is large enough to accommodate six equally circumferentially spaced circular apertures of 0.3 mm in diameter. Accordingly, in order to avoid a large chamber from being formed at the manifold, it is preferred that the sum of the diameters d of the apertures in the outlet manifold 9 be less than the circumference of the intermediate passage of diameter D: nd < xD.

The splitting device according to the present invention is formed from perspex, and the passages thereof are formed by drilling into the perspex. Accordingly, it is preferred that the passages of the splitting device be straight and not too long. The splitting device also comprises an inlet port 11 which is adapted to receive a tube or the like (not illustrated) which is connected via a valve to a plurality of reagent reservoirs, as explained in further detail below.

Figure 7 illustrates schematically the pathway of the liquid through the splitting device.

According to one preferred embodiment of the present invention illustrated in Figure 8, there is. provided a series of twelve assemblies, in the form of columns; into which the output volumes of liquid are dispensed. A reagent, fluid delivery and control system is present to control the delivery of liquid reagent to each of the columns and the operational steps for the purification of nucleic acid or protein in the columns.

The liquid delivery system shown in Figure 9 comprises a number of reagents in reservoirs (R1 to R8) which are pressurised typically at 0.5 bar fine pressure regulated air. The exact number of reagents used will depend on the exact purification protocol to be adopted. The reagents are divided into two blocks: reservoirs R1 to R4 contain reagents to be delivered to inlet 43 of the column and are controlled by corresponding valves 61 to 64; and reservoirs R5, R6 and R8 contain reagents to be delivered to the second chamber of the column 42 through inlet 45 under control of valves 65,66 and 75. Reservoir R7 contains particulate matrix material to be delivered by an independent line to the top of chamber 42 through inlet 55 under control of valves 74 and 76.

Taking delivery of reagent from reservoir R4 as an example, this reagent is delivered by opening valve 64 for a selected period of time (e. g. 250 ms). Because the reservoir is under pressure and there is a drop in the pressure on open sided valve 64, reagent enters the system until the valve closes. The reagent passes into the line and through a splitting device D1 which is of the configuration illustrated in Figures 3-6. There is now a volume of liquid in the line from valve 64 to valve 68 which runs through the splitting device. The length of this line is chosen to accommodate the maximum likely volume. The diameter of the line leading from valve 64 to the splitting device D1 is 1 mm in diameter, and the diameter of the line leading from the splitting device D1 to valve 68 is 0.3 mm in diameter. The volume of reagent is then moved to the top of the first chamber by switching valve 68 on and switching valve 67 on for a sufficient time to allow the reagent to reach the top of the chamber. The pressure of the air force in the liquid down the line can be either 0.5 bar or, if valve 77 is switched on, 1.7 bar. The air in front of the liquid to be moved is vented to atmosphere via valve 69 and 70. All valve timings are controlled by a microprocessor (not shown).

Figure 9 illustrates just one of the twelve columns shown in Figure 8. The eleven other outlet lines extending from each of the splitting device D1 and D2 leave via corresponding valves to each of the other eleven columns in the apparatus.

After reagent from reservoir R4 has been passed into the column by this means, the column is flushed with air which is forced through the lines in a series of sharp bursts. The liquid lines are now ready for the delivery of a second reagent through the lines into the columns.

The apparatus illustrated in the Figure is particularly useful in the field of biochemistry since: (i) the doses do not contaminate each other; (ii) the outlet doses are controlled to be identical; and (iii) the liquid line is flushed by air and not an inert liquid such as water. In some biochemical reactions, liquid flushing agents are not tolerated, and therefore cannot be used to flush the system.

The system including an inert gas purge (such as air in most cases) demands small tubes so that boundary layer effects do not prevent the gas from scouring the reagents of the tube walls.

It has been found by the applicants that for liquids with a surface tension near to that of water or higher, the liquid lines between the valves, including the passages of the splitting device must be smaller than 1.5 mm diameter, preferably not greater than 1 mm in diameter.

Claims

CLAIMS 1. A device for the splitting of an input volume of liquid into a plurality of output volumes of liquid comprising: (a) a liquid inlet passage; (b) a divider for dividing flow of liquid; and (c) a plurality of liquid outlet passages communicating via the divider with the liquid inlet passage, wherein the aggregate cross-sectional area at any distance along the pathway of the liquid through the splitting device is no greater than twice the minimum cross-sectional area of the inlet passage.
2. The device as claimed in claim 1, wherein the inlet passage has a cross-sectional area of not greater than 1.7 mm2 and the outlet passages each have cross-sectional areas of not greater than 0.05 mm2.
3. The device as claimed in claim 1 or claim 2, wherein the output volumes of liquid are equal.
4. The device as claimed in any one of the preceding claims, wherein each of the inlet passage and the outlet passages are substantially uniform in cross-section along their length.
5. The device as claimed in any one of the preceding claims wherein the divider comprises a first manifold associated with the inlet passage comprising a plurality of apertures.
6. The device as claimed in any one of the preceding claims, wherein the divider further comprises a series of end manifolds from which the outlet passages extend.
7. The device as claimed in any one of the preceding claims, wherein the divider divides the flow of liquid in two stages.
8. The device as claimed in claim 7, wherein the divider comprises: an inlet manifold associated with the inlet passage comprising a plurality of apertures; intermediate passages extending one from each aperture in the inlet manifold; and outlet manifolds associated with each of the intermediate passages having a plurality of outlet apertures, wherein an outlet passage extends from each of the outlet apertures of the outlet manifolds.
9. The device as claimed in claim 8, wherein the inlet passage and the outlet passages are substantially circular in crosssection.
10. The device as claimed in claim 8 or claim 9, wherein the apertures of each manifold are spaced equally circumferentially about an axis to one end of the passage with which it is associated.
11. The device as claimed in any one of claims 8 to 10, wherein the sum of the cross-sectional areas of the intermediate passages (Aint) is not greater than 50% more than the cross-sectional area of the inlet passage (Ain) : E Aint S 1. 5 Ain
12. The device as claimed in claim 11, wherein the sum of the cross-sectional areas of the intermediate passages (Aint) is not greater than 20% more than the cross-sectional area of the inlet passage (Ain) E Aint 5 1. 2 Ain
13. The device as claimed in claim 11 or claim 12, wherein the sum of the cross-sectional areas of the outlet passages (Aout) is not greater than 50% more than the sum of the cross-sectional areas of the intermediate passages (Aint) : E Aout s 1-5 Aint
14. The device as claimed in claim 13, wherein the sum of the cross-sectional areas of the outlet passages (Aout) is no greater than 20t more than the sum of the cross-sectional areas of the intermediate passages (Aint) : E Aout s 1.2 E Aint
15. The device as claimed in any one of the preceding claims, wherein the outlet passages are not less than 0.3 mm in diameter.
16. The device as claimed in any one of the preceding claims, wherein the inlet passage is not less than 0.7 mm in diameter.
17. The device as claimed in any one of the preceding claims, wherein the inlet passage is not greater than 1.0 mm in diameter.
18. The device as claimed in any one of claims 8 to 17, wherein the intermediate passages are from 0.3 to 1.0 mm in diameter.
19. The device as claimed in claim 16, wherein the intermediate passages are from 0.4 to 0.9 mm in diameter.
20. The device as claimed in any one of the preceding claims wherein the input volume of liquid is split into output volumes of from lOnl to 1 litre.
21. The device as claimed in claim 20, wherein the output values of liquid are between 30p1 and lml.
22. A device for the splitting of an input volume of liquid into a plurality of output volumes of liquid, comprising: (a) a liquid inlet passage; (b) a divider for dividing flow of liquid; and (c) a plurality of liquid outlet passages communicating with the liquid inlet passage via the divider, wherein the aggregate cross-sectional area at any point of splitting along the pathway of the liquid does not increase by more than 50%.
23. Apparatus for the simultaneous purification of nucleic acid from a plurality of nucleic acid-containing samples comprising: (a) a series of assemblies in which the nucleic acid containing samples can be purified; (b) one or more liquid reagent reservoirs; (c) a liquid delivery means communicating with each of the assemblies for the delivery of output volumes of liquid reagent from a liquid reservoir to each of the assemblies; (d) outlets for the elution of the purified nucleic acid from each assembly, wherein the liquid delivery means comprises the device as defined in any one of claims 1 to 22.
24. The apparatus as claimed in claim 23, wherein the liquid delivery means comprises a valved gas inlet for flushing of the passages of the liquid delivery means.
25. A method for the simultaneous-purification of nucleic acid from a plurality of nucleic acid-containing samples comprising: (a) delivering the nucleic acid-containing samples to a series of assemblies; (b) delivering volumes of liquid reagent from a liquid reagent reservoir simultaneously to each of the assemblies through a splitting device as defined in any one of claims 1 to 22; (c) eluting isolated nucleic acid from each of the assemblies, wherein, in step (b), an input volume of liquid reagent is drawn into the splitting device and the output volumes of the liquid reagent are subsequently dispensed into the assemblies.
26. A method as claimed in claim 25, wherein more than one liquid reagent is delivered via the splitting device to each of assemblies.
27. A method as claimed in claim 26, wherein following the delivery of a first liquid reagent to the assemblies, the splitting device is flushed with air to remove residual liquid therefrom.
28. A method as claimed in claim 27, wherein the air flushing step includes flushing with a series of bursts of air.