Pipetting head for a liquid dispensing apparatus

Abstract

A pipetting head 100 comprising a pipette body mounting assembly 120 with an array of pipette body mounts, and a moveable pipette guide assembly 180 having a guide plate 182 comprising a plurality of

Description

FIELD OF THE INVENTION The present invention relates to a pipetting head for a liquid dispensing apparatus for use with an array of removable pipettes, each having a pipette body or "pipette tip". The present invention relates in particular to a pipetting head having a pipette body mounting assembly having an array of pipette body mounts, each arranged to hold one of the pipette bodies, and to a method of connecting an array of removable pipettes to the pipetting head of a liquid dispensing apparatus. BACKGROUND OF THE INVENTION It is known to use a pipette to aspirate or dispense a liquid sample. It is also known to use a pipette with automated machines, such as liquid dispensing apparatuses, to improve accuracy and repeatability of pipetting actions. Such machines often use an array of removable pipettes mounted on a pipette body mounting assembly of the machine. A typical pipette, known as a "positive displacement" pipette, uses a plunger or piston to aspirate or dispense liquid either through direct contact with the sample liquid or via a small air gap between the plunger and the liquid. Typically, an array of removable pipette bodies is held in a rack or "tip box" beneath the pipetting head and is connected to the pipetting head by moving the pipetting head in position over the tip box and lowering the pipetting head to engage the array of removable pipette bodies with the pipette body mounting assembly. Once the pipette bodies are connected to the pipette body mounting assembly, the pipetting head can perform liquid dispensing and / or aspirating operations as desired. During use, the pipetting head of a liquid dispensing apparatus drives a plunger relative to the pipette bodies to aspirate or dispense the liquid from an aperture at the distal end of the pipette body. The inventors have identified several problems with the manner in which pipettes are connected with known pipetting heads for liquid dispensing apparatuses. There is a need for an improved pipetting head. SUMMARY OF THE INVENTION A first aspect of the invention provides a pipetting head for a liquid dispensing apparatus for use with an array of removable pipettes, each having a pipette body, the pipetting head comprising a pipette body mounting assembly comprising an array of pipette body mounts, each extending in an axial direction and arranged to hold one of the pipette bodies, a pipette guide assembly positioned below the pipette body mounting assembly, wherein the pipette guide assembly comprises a guide plate comprising a plurality of apertures configured to receive the pipette body mounts, and an array of guide protrusions which extend in the axial direction 5 from an underside of the guide plate wherein the array of guide protrusions each have an upper end adjacent to the plurality of apertures and a lower end which is laterally positioned between adjacent pipette body mounts; and a guide plate drive mechanism arranged to translate the guide plate axially towards and away from the pipette body mounting assembly between a lowered position in which the lower 10 ends of the guide protrusions are positioned below the lower ends of the pipette body mounts, and a raised position in which the pipette body mounts extend through the plurality of apertures and the lower ends of the pipette body mounts are positioned below the lower ends of the guide protrusions. With existing pipetting heads, it has been found that damage can sometimes 15 occur to some of the pipettes in the array due to misalignment of the pipette bodies during the mounting the pipettes to the pipetting head. Pipettes are often held only loosely in the tip box prior to mounting to the pipetting head and this can lead to lateral misalignment of one or more of the pipettes relative to the pipette body mounting assembly. Where the pipette bodies have flexible arms or wings at their 20 proximal (i.e. upper) ends to facilitate snap-fit engagement with the pipette body mounting assembly, these flexible arms can be caused to plastically deform or break if misaligned with the pipette body mounting assembly. If picked by the pipetting head, damaged pipettes can ruin the assay. Consequently, damaged pipettes may need to be replaced prior to operating the pipetting head, potentially 25 causing delays to the procedure. With the present invention, the guide plate can be moved to the lowered position prior to mounting of the pipette bodies to the pipette body mounting assembly, such that the downward extending protrusions of the pipette guide assembly nudge any misaligned pipette bodies towards the correct position, in 30 which the pipette bodies are substantially co-axial with the pipette body mounting assembly, to reduce or prevent those pipette bodies from becoming trapped and / or damaged when the pipette bodies are received by the pipette head. Once the pipette bodies are no longer misaligned, the guide plate can be raised relative to the pipette body mounts so that the pipette body mounts extend below the guide 35 protrusions and can connect with the pipette bodies in the tip box. In this manner, the pipette guide assembly is configured to align the pipette bodies with the pipette body mounting assembly during mounting to the pipetting head. The raising of the guide plate relative to the pipette body mounts can be achieved by also raising the guide plate relative to the pipette bodies in the tip box. Alternatively, since the guide plate and the pipette body mounts can be moved separately, the guide plate may be raised at the same time as the pipette body mounts are lowered, and at 5 the same rate. This holds the guide plate static relative to the pipette bodies in the tip box so that the distal (i.e. lower) ends of the guide protrusions can remain in contact with the pipette tips, thereby ensuring that the pipette bodies are aligned with the mounts during the process of connecting the pipette bodies to the pipette body mounts. 10 This arrangement has been found to be especially helpful for arrays in which the removable pipettes are positioned closely together, and / or if there are a large number of pipette bodies, such as a 384 array in which 384 pipettes are positioned to perform dispensing / aspirating operations in all of the wells of a standard 384 well microplate. With pipetting heads designed for a 384 positive displacement 15 design, packaging of both the plunger engagement mechanism and the pipette body engagement mechanism can be very difficult due to the limited space available. Consequently, the pipette tips have little to no clearance between them. This can exacerbate the potential for tip damage if alignment is not controlled. The pipette body mounting assembly is configured to hold, i.e. connect to and 20 retain, the pipette bodies in relation to the pipetting head. The pipette body mounting assembly may be configured to hold a pipette body connector portion of the pipette body of each of the array of removable pipettes. The pipette body mounting assembly may comprise a pipette body mounting plate having a plurality of pipette body mounts each configured to couple with one of the pipette bodies 25 during use. The pipette body mounting plate may be defined by a lower surface of the pipetting head chassis. The pipetting head is configured for use with an array of pipettes each having a pipette body. The array of pipettes may each further include a removable plunger disposed within each pipette body. The use of pipettes with both removable 30 plungers and removable pipette bodies can help to reduce the risk of cross contamination of samples. The pipetting head may comprise a pipetting head chassis which forms the main body and / or support structure of the pipetting head by which the various components of the pipetting head are supported, either directly or indirectly. The pipetting head will typically be connected to the liquid 35 dispensing apparatus via the head chassis, such that when the head chassis is moved relative to the liquid dispensing apparatus so too is the rest of the pipetting head and its remaining components. The pipetting head chassis may comprise a single, unitary head chassis component. The pipetting head chassis may comprise a plurality of head chassis parts which are fixed together to form the head chassis. For example, the head chassis may comprise an upper head chassis part, a mid 5 head chassis part and a lower head chassis part which are fixed together to form the pipetting head chassis. The pipetting body mounting assembly may defined, at least in part, by the pipetting head chassis. For example, the array of pipette body mounts may be fixed to a lower surface of the pipetting head chassis. The pipette body mounting assembly may comprise a pipette body mounting plate by which the 10 pipette body mounts are supported. The pipetting body mounting plate may be fixed to, integral with, and / or defined by the lower surface of the pipetting head chassis. Optionally, the array of guide protrusions are arranged such that a plurality of guide openings are defined between adjacent guide protrusions, and the plurality 15 of guide openings are co-axial with the plurality of apertures in the guide plate and are wider at the lower end of the guide protrusions than the plurality of apertures. £\j Optionally, the array of guide protrusions are tapered, such that the plurality of guide openings narrow from the lower ends of the guide protrusions towards the plurality of apertures. 20 Optionally, the array of guide protrusions each have a polygonal cross- 00 sectional shape defining a plurality of side walls, each of which faces towards one of the plurality of apertures. Optionally, the side walls are concave such that the plurality of guide openings each have a circular or part-circular cross-sectional shape. 25 With this arrangement, as the guide plate is moved towards an extended position, the guide openings easily receive the pipette bodies. The concave shape of the side walls can ensure that the guide protrusions abut against the pipette bodies as the guide plate is extended so that the pipette bodies become co-axial with guide apertures in the guide plate. This provides an efficient aligning system. 30 Optionally, the side walls are continuous with the plurality of apertures. That is, the side walls and their respective apertures together form a continuous surface. Optionally, each guide protrusion has a pyramid shape. Optionally, each guide protrusion has a truncated pyramid shape. Optionally, the plurality of apertures is an array of apertures, which 35 corresponds in number and position to the array of pipette body mounts, such that each of the plurality of apertures is configured to receive a single pipette body mount of the array of pipette body mounts. Optionally, the array of apertures and the array of pipette body mounts are both arranged in a grid, and wherein the array of guide protrusions extend from 5 the guide plate such that each guide protrusion is laterally positioned between each group of four adjacent pipette body mounts. With this arrangement, a large number of pipette bodies may be aligned by the pipette guide assembly and be received by the pipette mounting assembly. As the pipette bodies are arranged in a square pattern, a gap is formed between any 10 group of pipettes. This gap is relatively large in comparison to the closest distance between any two adjacent pipettes. The guide plate design 'targets' this hole, nudges the pipettes into alignment avoiding damage to the parts. Optionally, the array of guide protrusions have substantially the same size and shape. In other embodiments, one or more of the guide protrusions may have 15 a different size and / or shape to the other guide protrusions, as desired. Optionally, each of the array of pipette body mounts comprises a radially extending feature on its outer surface for forming one half of a snap-fit connection, wherein the plurality of apertures in the guide plate define a plurality of sleeves each having a clamping region and being co-axial with one of the array of pipette 20 body mounts. In such embodiments, the guide plate drive mechanism may be operable to move the guide plate between a clamp position and a release position, wherein: when the guide plate is in the clamp position, each of the plurality of sleeves circumscribes an associated one of the array of pipette body mounts and is axially positioned such that the clamping region is adjacent to the radially extending 25 feature on its associated pipette body mount, and, when the guide plate is in the release position, the plurality of sleeves is axially positioned such that the clamping region of each sleeve is axially offset from the radially extending feature on the outer surface of its associated tip mount. In this manner, the guide plate has a dual function of guide plate for guiding the pipette bodies onto the pipette body mounts, 30 and clamp plate for clamping the pipette bodies on their respective pipette body mounts. In other embodiments, the function of clamp plate may be provided by a separate clamp plate which may be moveable independently to the guide plate. As used herein, the terms "clamping" and "clamp" refer to arrangements by which the pipette body or plunger (or both) are mounted and locked in place to 35 prevent inadvertent removal without first releasing the clamp. This is in contrast to arrangements in which the pipette body or plunger are held in place by a single mechanism, without a separate clamp or locking means, for example using only an interference fit or a snap-fit connection in which the pipette body or plunger can be removed by pulling hard enough in an axial direction, or in which the pipette bodies are held In a magazine and mounted to the pipetting head by pressing the magazine 5 against the underside of the pipetting head. The use of a clamping mechanism can also improve the accuracy of aspirating and dispensing operations by preventing small relative movements that might otherwise occur between the pipetting head and the plunger or the pipette body. As used herein, the terms "above", "upper", "low", "below", and "lowest" 10 refer to the normal orientation of the pipetting head during use. A second aspect of the invention provides a liquid dispensing apparatus comprising a main body with a deck for receiving one or more microplates and a pipetting head as described according to the first aspect, the pipetting head being positioned above the deck. 15 Although the deck is configured to receive one or more microplates, other laboratory equipment may be received on the deck instead of, or in addition to a microplate. For example, one or more arrays of pipette bodies, well plates or microplate transfer devices may be received on the deck, as well as an array of vials or sample tubes, a liquid reservoir, or any other laboratory equipment with 20 which the liquid dispensing apparatus may be used. A third aspect of the invention provides a method of connecting an array of removable pipettes to the pipetting head of a liquid dispensing apparatus, the method comprising providing a liquid dispensing apparatus according to the second aspect of the invention, placing an array of removable pipettes in a container on 25 the deck of the liquid dispensing apparatus, each removable pipette having a pipette body, positioning the pipetting head over the container such that the array of pipette body mounts is laterally aligned with the array of removable pipettes, translating the guide plate to the lowered position in which the lower ends of the array of guide protrusions are positioned below the lower ends of the pipette body 30 mounts, engaging the pipette bodies with the array of guide protrusions, translating the guide plate to the raised position in which the pipette body mounts extend through the plurality of apertures and the lower ends of the pipette body mounts are positioned below the lower ends of the array of guide protrusions, connecting the pipette bodies to the array of pipette body mounts by moving the pipetting head 35 axially towards the container to insert the array of pipette body mounts into the upper ends of the pipette bodies. Optionally, each of the array of pipette body mounts comprises a radially extending feature on its outer surface which forms a snap-fit connection with a cooperatively shaped radially extending feature on an inner surface of each of the pipette bodies when the pipette bodies are connected to the array of pipette body 5 mounts. In such embodiments, the plurality of apertures in the guide plate may define a plurality of sleeves each having a clamping region and being co-axial with one of the array of pipette body mounts, the method comprising clamping the pipette bodies onto the array of pipette mounts by translating the guide plate to a clamp position in which the clamping region of each of the plurality of sleeves 10 circumscribes an associated pipette body mount and pipette body connected to the pipette body mount and is axially positioned adjacent to the snap-fit connection to resists release of the snap-fit connection by restricting radially outward movement of the pipette body. In this manner, the guide plate has a dual function of guide plate for guiding the pipette bodies onto the pipette body mounts, and clamp plate 15 for clamping the pipette bodies on their respective pipette body mounts. In other embodiments, the function of clamp plate may be provided by a separate clamp plate which may be moveable independently to the guide plate. Optionally, the method may further comprise raising the pipetting head to remove the array of removable pipettes from the container after the step of 20 connecting the pipette bodies to the array of pipette body mounts and before the step of clamping the pipette bodies onto the array of pipette mounts. This enables the pipetting head to connect to (or "pick") and clamp single, part row, part columns, rows, columns, sections and full arrays of pipette bodies by offsetting the pipetting head relative to the array of pipette bodies in the tip box without the 25 clamping mechanism fouling against unpicked pipette bodies. Removing the pipette bodies from the tip box can also be advantageous in situations in which the pipette bodies are not at a uniform height within the container, since the guide plate can effectively "nudge" unclamped pipette bodies which are positioned slightly higher downwards into the correct axial position during the step of clamping. 30 Alternatively, the step of raising the pipetting head to remove the array of removable pipettes from the container may be carried out after, or simultaneously with, the step of clamping. Optionally, the steps of translating the guide plate to the raised position and connecting the pipette bodies to the array of pipette body mounts are carried out 35 simultaneously. Optionally, the steps of translating the guide plate to the raised position and connecting the pipette bodies to the array of pipette body mounts are carried out simultaneously and at the same speed, such that the axial position of the guide plate relative to the container is generally constant as the pipetting head is moved axially towards the container to insert the array of pipette body mounts into the upper ends of the pipette bodies. In this manner, the guide plate can more 5 readily support the pipette bodies and maintain their correct position during the process of connecting to the array of pipette body mounts. Alternatively, the step of translating the guide plate to the raised position is carried out before the step of connecting the pipette bodies to the array of pipette body mounts. BRIEF DESCRIPTION OF THE DRAWINGS 10 Further features and advantages of the present invention will be further described below, by way of example only, with reference to the accompanying drawings in which: FIGURE 1 is a front view of a liquid dispensing apparatus with a pipetting head in accordance with the present invention; , 15 FIGURE 2 is a perspective view of a pipetting head in accordance with the present invention; FIGURE 3 is a perspective view of the pipetting head of FIGURE 2, in which a top side of the guide plate is shown; FIGURE 4 is a perspective view of the pipetting head of FIGURE 2, in which CO 20 an underside of the guide plate is shown and in which an array of pipette body mounts is shown extending through the guide plate; FIGURE 5 is a perspective view of the pipetting head of FIGURE 2, in which the top cover is omitted to show the components beneath; FIGURE 6 is an exploded perspective view of the pipetting head of FIGURE 5; 25 FIGURE 7 is a cross-sectional view through the pipetting head of FIGURE 2, with the head chassis removed for clarity; FIGURE 8 is a plan view of the guide plate of the pipetting head of FIGURE 2; FIGURE 9 is an underside view of the guide plate of FIGURE 8; FIGURE 10 is an enlarged underside view of the guide plate of FIGURE 8; 30 FIGURE 11 is an enlarged side perspective view of part of the pipetting head, showing the pipette body mounts extending through the guide plate; FIGURE 12 is an underside side perspective view of part of the pipetting head, showing the guide plate in the lowered position; FIGURE 13 is an underside side perspective view of part of the pipetting head, showing the guide plate in the raised position; FIGURE 14 shows an enlarged cross-sectional view of a lower portion of the pipetting head, showing a side region of the guide plate and the pipette body mounting assembly; FIGURE 15 shows an enlarged cross-sectional view of an upper region of the pipetting head, showing the guide plate drive mechanism in greater detail; FIGURE 16 is a cross-sectional view of an exemplary plunger mounting assembly and plunger clamping mechanism for the pipetting head of FIGURE 2; FIGURE 17 is an enlarged cross-sectional view of the drive mechanism of the plunger clamping mechanism of Figure 16; FIGURE 18 is an enlarged cross-sectional view of part of the plunger mounting assembly and the plunger clamping mechanism of Figure 16, showing the plunger mounting plate and the plunger clamp plate in more detail; FIGURE 19 is an enlarged view of a central portion of the cross-sectional perspective view of FIGURE 7, showing the dispense drive actuator assembly; FIGURE 20 is a cross-sectional view of a pipette for use with the liquid dispensing system of FIGURE 1; FIGURE 21 is a perspective view of the pipette body and the plunger of the pipette of FIGURE 20 in which the plunger has been removed from the pipette body; FIGURE 22 is a cross-sectional view of the pipette mount in greater detail; FIGURE 23 is a perspective view of the pipette and plunger for use with the liquid dispensing system of FIGURE 1; FIGURE 24 is a schematic flowchart showing a method of connecting an array of removable pipettes to the pipetting head of a liquid dispensing apparatus in accordance with the invention; and FIGURE 25 is an enlarged cross-sectional view of part of the plunger mounting assembly and the pipette body mounting assembly to which a pipette is connected and clamped. DETAILED DESCRIPTION Figure 1 shows a liquid dispensing apparatus 10 for use with an array of pipettes 1010. The apparatus 10 comprises a main body 12 with a microplate receiving area, or deck, 14 and a pipetting head 100 positioned above the microplate receiving area 14. The microplate receiving area 14 is has a substantially horizontal upper surface 16 arranged to receive a laboratory microplate. The receiving area 14 can be located on a height-adjustable support structure 18 which enables the height of the microplate receiving area 14 to be varied as required. The receiving area 14 may be configured to retain a laboratory microplate is a fixed position. For example, the upper surface 16 of the receiving area 14 may comprise one or more recesses (not shown) arranged to receive a microplate and to prevent lateral translation of the microplate with respect to the receiving area 14. The apparatus 10 will generally be used in the orientation shown in Figure 1 to retain products in wells of the microplate by gravity. The axis marked Z in Figure 1 therefore represents an upward direction, with gravity acting in the opposite direction to retain the products in the wells of the microplate. References to upward and downward directions or to an axial direction therefore refer to movement along the axis marked Z in Figure 1, while references to lateral or transverse directions refer to movement in the directions marked X (width) and Y (depth) in Figure 1. References to vertical direction or height also therefore refer to dimensions or movement along the axis marked Z in Figure 1. The pipetting head 100 is configured to hold an array of pipettes, as discussed below in relation to Figures 2 to 24, and may be moveable in relation to the deck 14 to bring pipettes mounted on the pipetting head 100 into close proximity to a microplate supported on the deck 14 to allow liquid to be aspirated from or dispensed into the wells of the microplate. The pipettes may be positive displacement pipettes (which include a pipette body and a plunger) or standard pipettes (which only include a pipette body). As illustrated, an array of removable pipettes 1010 is held in a container 1012 placed on the deck 14 ready for connection to the pipetting head 100. Figure 2 is a perspective view of the pipetting head 100. The pipetting head comprises a head chassis 101. In the illustrated embodiment, the head chassis 101 includes an upper head chassis part 102, a lower head chassis part 103, and mid head chassis part 104 between the upper and lower head chassis parts, all of which are fixed together to form the main body of the pipetting head 100. In other embodiments, the head chassis 101 may comprise more or fewer head chassis parts. For example, the head chassis may be formed of a single part. The head chassis essentially forms the main body or support structure of the pipetting head by which the various components of the pipetting head are supported, either directly or indirectly. The pipetting head will typically be connected to the liquid dispensing apparatus via the head chassis, such that when the head chassis is moved relative to the liquid dispensing apparatus so too is the rest of the pipetting head and its remaining components. A top cover 105 may be removably mounted over the top surface of the upper head chassis part 102, as shown. The head chassis 101 is connected to the body of the apparatus by chassis support plates (not shown), which preferably enable the entire pipetting head 100 to be moved in the X, Y and Z directions relative to the receiving area of the apparatus in a conventional manner using one or more head actuators (not shown). The pipetting head 100 further comprises a pipette guide assembly 180. The pipette guide assembly 180 includes a guide plate 182 which is moveably positioned at the underside of the pipetting head. The upper head chassis part 102, lower head chassis part 103, and mid head chassis part 104 may be fixed together using any suitable method, for example using bolts and tie-rods. Figure 3 shows the pipetting head 100 of Figure 2, showing the guide plate 182 and the pipette body mounts with all other features of the pipetting head 100 omitted for clarity. This allows the top surface of the guide plate 182 to be seen. Figure 4 shows an underside of the pipetting head shown in Figure 3, again showing the guide plate 182 and the pipette body mounts with all other features of the pipetting head 100 omitted for clarity. The pipette guide assembly 180 is discussed in greater detail below in relation to Figures 8 to 13. Figure 5 is a perspective view of the pipetting head 100 with the top cover removed, and Figure 6 is an exploded view of the head chassis components of the pipetting head 100. Referring to Figures 5 and 6, the upper head chassis part 102 forms, on its top surface, a support for the dispense drive motor 161 of the dispense drive actuator assembly 160 of the pipetting head 100 and for the pipette body clamp motor 132, which are discussed in more detail below. Control circuitry 106 for the operation of the pipetting head is also fixed to the top surface of the upper head chassis part 102. All of the components mounted on the top surface of the upper head chassis part 102 may be enclosed beneath the top cover 105 when this is placed over the upper head chassis part 102. The mid head chassis part 104 includes an outer wall 107 which is rectangular in transverse cross-sectional shape. The outer wall 107 includes an aperture 110 in each of its four corners. Each aperture 110 may comprise a linear bush 115. The lower head chassis part 103 has an outer wall 111 which defines an internal chamber 112 and, like the upper head chassis part 102 and the mid head chassis part 104, is rectangular in transverse cross-sectional shape. The outer wall 111 includes an aperture 113 in each of its four corners. Each aperture 113 may comprise a linear bush 116. The pipetting head 100 further comprises a pipette body mounting assembly 120 for holding the pipette bodies of an array of pipette bodies during use. The pipette body mounting assembly 120 includes a pipette body mounting plate which is identified as feature 121 in Fig. 14. As mentioned above in relation to Figures 3 and 4, the pipetting head 100 further comprises a pipette guide assembly 180 for aligning the pipette bodies of an array of pipette bodies before use. The pipette guide assembly 180 is positioned below the pipette body mounting assembly 120. Figure 7 shows a cross-sectional view of an exemplary pipetting head 100 with the upper and mid head chassis parts removed for clarity. The pipetting head 100 may include additional assemblies depending on the type of pipettes to be used. For example, if positive displacement pipettes are to be received by the pipetting head, the pipette head 100 may further include a plunger mounting assembly 140 and a plunger clamping mechanism 150. The plunger mounting assembly 140 is used holding the plungers of an array of positive displacement pipette bodies during use. The plunger clamping mechanism 150 is used for securely clamping the plungers on the plunger mounting assembly 140. The pipetting head 100 may also include a dispense drive actuator assembly 160 operable to move the plunger mounting assembly 140 relative to the pipette body mounting assembly 120 in an axial direction. The guide plate 182 of the pipette guide assembly 180 is positioned directly below the pipette body mounting assembly 120. The pipette guide assembly 180 also comprises a guide plate drive mechanism 133 including a linear actuator and a guide plate drive linkage 134, 135. The linear actuator includes a guide plate drive motor 132 fixed in relation to the head chassis (see feature 101 in Figure 5) and coupled to the guide plate 182 by guide plate drive linkage 134, 135 to move the guide plate 182 in the axial direction in relation to the head chassis 101 and the pipette body mounting assembly when the guide plate drive motor 132 is actuated. In the illustrated embodiment, the guide plate drive motor 132 is secured to the top surface of the upper head chassis part (see feature 102 in Figures 5 and 6). As will be discussed below, the pipette guide assembly 180 may further function as a pipette body clamping mechanism for securely clamping the pipette bodies on the pipette body mounting assembly 120. In this manner, the guide plate has the dual function of aligning pipette bodies during their connection to the pipette body mounting assembly and locking the pipette bodies in place on the pipette body mounting assembly once connected. In other embodiments, a discrete pipette body clamping mechanism may be provided. This may comprise a clamp plate (not shown) which is discrete to both the pipette body mounting assembly and the guide plate, for example located between the mounting assembly and the guide plate, and a clamp plate drive mechanism for translating the clamp plate independently of the guide plate. Figure 8 shows a top view of the guide plate 182. As shown, the guide plate has a plurality of apertures 184 configured to receive the array of pipette body mounts and arranged in an array 185. The array 185 is arranged in a grid with a plurality of rows and columns. In the illustrated embodiment, the plurality of apertures is an array of apertures which corresponds in number and position to the array of pipette body mounts, such that each aperture 184 in the guide plate 182 is arranged to receive a single pipette body mount (see feature 123 in Figures 11 and 13). However, it will be understood that each aperture may be configured to receive multiple pipette body mounts. For example, the apertures may be elongate and arranged to receive a row of two or more pipette body mounts. In the illustrated embodiment, the plurality of apertures comprises 384 apertures arranged in a gridtype array 185 with 16 rows and 24 columns of apertures. The pipetting head in this embodiment is configured for use with a standard 384 well microplate. However, it will be understood that the pipetting head may be configured for use with other types of microplate / well plate with the plurality of apertures and pipette mounts arranged as appropriate. Figure 9 shows a bottom perspective view of part of the guide plate 182. Figure 10 shows an enlarged bottom view of part of the guide plate 182. Figure 11 shows an enlarged side perspective view of the guide plate, showing the pipette body mounts extending through the apertures 184 in the guide plate 182. With reference to Figures 9 to 11, the guide plate 182 further comprises an array of guide protrusions 186 which extend in the axial direction from the underside of the guide plate 182. Each of the guide protrusions 186 has a distal end 187 which is laterally positioned between adjacent apertures 184 in the guide plate 182 and, therefore, between adjacent pipette body mounts 123. The array of guide protrusions 186 are arranged so that a plurality of guide openings 188 are defined between adjacent protrusions 186. As shown most clearly in Figure 10, the guide openings 188 are co-axial with the plurality of apertures 184 in the guide plate 182. The guide openings 188 are wider at the distal ends 187 of the protrusions 186 than the plurality of apertures 184. The plurality of guide openings 188 therefore narrow from the distal ends 187 of the protrusions 186 towards the plurality of apertures 184. This type of guide opening geometry is provided by forming the guide plate 182 such that each guide protrusion 186 is tapered, as best seen in Figure 11. The cross-sectional area of the distal end 187 of each guide protrusion 186 is smaller than the cross-sectional area of the proximal end of each guide protrusion 186 adjacent to the apertures 184. Each guide protrusion 186 has a polygonal cross-sectional shape which defines a plurality of side walls 189 which each face, i.e. extend towards the plurality of apertures 184. The side walls 189 are continuous with the plurality of apertures 184. The side walls 189 form a sleeve that extends from the distal ends 187 of the protrusions 186 towards the apertures 184. As shown most clearly in Figure 10, each protrusion 186 has four side walls 189 that extend towards a respective aperture 184 in the guide plate 184. Thus, in the illustrated embodiment, each guide protrusion 186 generally has a truncated pyramid shape. However, it will be understood that the guide protrusions may have any suitable shape. For example, a frustoconical (i.e. a tapered circular cross-sectional) shape, or a polygonal shape with more or fewer than four side walls. The side walls 189 are concave so that the guide openings 188 each have a circular or part-circular cross-sectional shape. The array of protrusions 186 have substantially the same size and shape. This can help to ensure that the protrusions 186 contact and align the pipette bodies generally at the same time. In this example, each protrusion generally has a truncated pyramid shape. The array of apertures 184 are co-axial with the array of pipette body mounts 123, and the arrays of apertures, guide openings and guide protrusions 186 are all arranged in a grid. The array 185 of guide protrusions 186 extends from the guide plate 182 such that each guide protrusion 186 is laterally positioned between each group of four adjacent pipette body mounts 123. This enables the side walls 189 of each guide protrusion 186 to form a continuous surface with four surrounding guide openings 188 and thereby help steer the pipette bodies into each of the four apertures 184 associated with the guide openings 188. The guide plate drive mechanism 133 is operable to translate the guide plate 182 axially towards and away from the pipette body mounting assembly between a lowered position and a raised position. Figure 12 is an underside side perspective of part of the pipetting head, showing the guide plate 182 in the lowered position. Figure 13 is an underside side perspective of part of the pipetting head, showing the guide plate 182 in the raised position. As illustrated, when the guide plate 182 is in the lowered position (as in Figure 12), the distal ends of the array of guide protrusions 186 are positioned below the distal ends of the pipette body mounts 123. When the guide plate 182 is in the raised position (as in Figure 13), the pipette body mounts 123 extend through the plurality of apertures 184 such that the distal ends of the pipette body mounts 123 are positioned below the distal ends 187 of the array of guide protrusions 186. In this manner, the guide protrusions 186 can be moved to an advanced position (the lowered position) to help to align the pipette bodies in the container with the pipette body mounts 123 - or at least guide the pipette bodies closer to the correct position with respect to the pipette body mounts 123 - before being moved out of the way to a retracted position (the raised position) relative to the pipette body mounts so that the pipette bodies can fully connect with the pipette body mounts 123. As the guide plate 182 is lowered towards an array of pipette bodies in the container - by lowering the guide plate with respect to the pipette mounting assembly or by lowering the enter pipetting head, or both - the side walls 189 of the protrusions 186 can be brought into contact with laterally misaligned pipette bodies in the container and guide those pipette bodies into a more central position with respect to their respective pipette body mount. The guide protrusions 186 are therefore arranged to abut against the pipette bodies to help position the pipette bodies in a co-axial arrangement with the apertures 184 in the guide plate 182 and the pipette body mounts 123. Aligning the pipette bodies of the array of pipettes before the pipette body mounts 123 fully receive the pipette bodies can help to prevent the pipette bodies from being misaligned and reduce the risk of damage to the pipettes and / or incorrect or incomplete connection of those pipette bodies to the pipetting head during the mounting process. The process by which pipette bodies are aligned with the guide plate and connected to the pipette body mounts is discussed in detail below in relation to the flowchart of Figure 24. Figure 14 shows an enlarged cross-sectional view of a lower portion of the pipetting head, showing a side region of the guide plate 182 and the pipette body mounting assembly (feature 120 in Figures 6 and 7). In this view, the guide plate 182 is in the raised position and abuts against the pipette body mounting plate 121 directly above. The array of pipette body mounts 123 of the pipette body mounting assembly 120 extend in the axial direction through the thickness of the guide plate 182 such that a connector portion of each mount 123 is located below the distal ends of the guide protrusions 186. Each pipette body mount 123 is in the form of a tubular sleeve extending axially from the underside of the pipette body mounting plate 121. In the illustrated embodiment, the pipette body mounting plate 121 comprises an array of mounting apertures 122 in which the pipette body mounts are received. The mounting apertures 122 each have a shoulder towards their upper end which is defined by a restriction with a diameter which is less than the outer diameter of the pipette body mount sleeves 123. The restrictions prevent the pipette body mount sleeves 123 from being forced through the pipette body mounting plate in an upward direction when the pipette body mount sleeves are initially engaged with the pipette bodies. The lower end of each pipette body mount sleeve 123 preferably has an outwardly tapered end surface to assist with the insertion of the lower end of the pipette body mount sleeve into a pipette body. The outer surface of each pipette body mount sleeve 123 comprises a radially extending feature on its outer surface which is configured to form one half of a snap-fit connection with a correspondingly shaped feature on the inner surface of a pipette body. In this example, the radially extending feature is in the form of an annular groove 124 which circumscribes the pipette body mount sleeve 123 and is configured to form a snap-fit connection with an annular rib protruding from an inner surface of the pipette body. In other examples, the radially extending feature may be a protrusion, such as an annular rib (not shown), which circumscribes the pipette body mount sleeve 123 and is configured to form a snap-fit connection with an annular groove on an inner surface of the pipette body. Optionally, in addition to defining an array of apertures through which the pipette body mounts 123 extend, the guide plate 180 may further comprise a pipette body clamping mechanism by which the pipette bodies may be locked into place on the pipette body mounts. In this manner, the guide plate 182 may also define a pipette body clamp plate having an array of pipette body clamping members 127 by which the array of apertures 184 is defined. The array of pipette body clamping members is provided in the form of a plurality of clamp sleeves 127 defined by the regions of the guide plate 182 which immediately surround the apertures 184, each of which is associated with one of the plurality of pipette body mounts 123. Each clamp sleeve 127 has a clamping region 128 with an inner diameter which is greater than the outer diameter of its respective pipette body mount sleeve. In this manner, a small clearance is provided between the outer surface of the pipette body mount sleeve 123 and the inner surface of the clamping region 128 when the pipette body clamping mechanism is engaged. When the guide plate 182 is positioned against or close to the underside of the pipette body mounting plate 121 (as shown in Figure 14), the clamping regions 128 of the clamp sleeves 127 are axially offset from the annular grooves 124 on the pipette body mounts 123. This is the disengaged state of the clamping mechanism 120, in which pipette bodies can be placed over or removed from the pipette body mounts 123 without interference from the guide plate 182. To clamp the pipette body mounts 123, the guide plate 182 is moved axially away from the pipette body mounting plate 121 until the clamping regions 128 of the clamp sleeves 127 are adjacent to the annular grooves 124 on the pipette body mounts 123. This is the engaged state of the clamping mechanism 120, in which the clamp sleeves resist or prevent removal of the pipette bodies from the pipette body mounts 123 by preventing radial movement of the pipette bodies and thereby preventing the snap-fit connection from disconnecting. In the illustrated embodiment, the functions of pipette body alignment and pipette body clamping are provided by the same plate; the guide plate 182. However, it will be understood that the pipetting head 100 may comprise a discrete clamping plate which is distinct from the guide plate and is positioned between the pipette body mounting assembly 120 and the guide plate 182. The clamping plate may be used to secure the pipette bodies to the pipetting head 100, while the guide plate 182 would be used to align the pipette bodies with the pipette body mounts. Figure 15 shows an enlarged cross-sectional view of an upper region of the pipetting head 100, showing the guide plate drive mechanism in more detail. As discussed above in relation to Figure 7, the guide plate drive mechanism 133 includes a linear actuator and a guide plate drive linkage 134, 135. The linear actuator includes a guide plate drive motor 132 fixed in relation to the head chassis 101 and coupled to the guide plate 182 by the guide plate drive linkage 134, 135 to move the guide plate 182 in the axial direction when the guide plate drive motor 132 is actuated. The guide plate drive motor 132 is secured to the top surface of the head chassis 101, in this example to the top surface of the upper head chassis part 102. The guide plate drive linkage comprises a bearing plate 134 coupled to the linear actuator and a plurality of standoffs 135 extending between the bearing plate 134 and the guide plate 182. The linear actuator further includes a screw mechanism 136 connected to the bearing plate 134 and configured to convert rotation of the clamp motor 132 into axial displacement of the standoffs 135 and the guide plate 182. In this embodiment, the standoffs are provided in the form of guide rods 135 which are optionally supported radially at their upper ends by the linear bushes 115 of the mid head chassis part (feature 104 in Fig.7) and at their lower ends by the linear bushes 116 of the lower head chassis part (feature 103 in Fig.7). The guide rods 135 are also radially supported between their upper and lower ends by linear bushes (feature 173 in Fig.7) provided in each of the four corners of the plunger mounting assembly 140. In this embodiment, there are four guide rods, one at each corner of the guide plate 182. In other embodiments, fewer or more guide rods or standoffs may be provided. For example, the plurality of standoffs may consist of two standoffs, for example positioned on opposite sides of the guide plate 182, for example at the mid-point between front and back of the guide plate 182. The screw mechanism 136 comprises a ring gear 137 which is coupled to the bearing plate 134 by a threaded connection. The ring gear 137 is supported by a bearing 131 which is provided between the upper head chassis part 102 and the ring gear 137. In this manner, the ring gear 137 is fixed in the axial direction relative to the head chassis 101 and is rotatable about the drive axis 190. The clamp motor 132 drives a driving gear 138, the teeth of which mesh with the teeth of the ring gear 137. The ring gear 137 comprises a hollow shaft 137A with a central bore extending in the axial direction, i.e. along the drive axis 190, and a radial flange 137B with a plurality of gear teeth 137C. The radial flange 137B may extend radially outwardly from the hollow shaft 137A. In the illustrated embodiment, the hollow shaft 137A of the ring gear 137 extends within a threaded bore 134A of the bearing plate 134, and the threaded connection between the ring gear 137 and the bearing plate 134 is provided by a screw thread on the outer surface of the hollow shaft 137A of the ring gear 137 and a complimentary screw thread on the inner surface of the threaded bore 134A of the bearing plate 134. The bores of the bearing plate 134 and the hollow shaft 137A together define a central aperture or axial bore. In other embodiments, the bearing plate 134 may include a hollow shaft 134A extending within the bore of the hollow shaft 137A of the ring gear 137, with the threaded connection being provided by a screw thread on the outer surface of the hollow shaft 134A of the bearing plate 134 and a complimentary screw thread on the inner surface of the hollow shaft 137A of the ring gear 137. When the guide plate drive motor 132 is operated, the driving gear 138 rotates the ring gear 137 and this causes the bearing plate 134 to move axially in relation to the ring gear 137 by virtue of the threaded connection between the ring gear 137 and the bearing plate 134. Rotation of the guide plate drive motor 132 in a first direction causes the bearing plate 134 to move upwards in the axial direction. Rotation of the guide plate drive motor 132 in an opposite, second direction causes the bearing plate 134 to move downwards in the axial direction. Since the bearing plate 134 is fixed to the guide plate 182 by the guide rods 135, the guide plate 182 moves axially with the bearing plate 134 to move towards or away from the pipette body mount plate 121. With this configuration of guide rods, the plunger mounting assembly 140 may slide axially along the guide rods when the plunger mounting assembly 140 is moved by the dispense drive actuator assembly and the guide rods may move axially along the linear bushes when the guide plate is moved. In this manner, the standoffs perform the dual function of transferring axial drive from the bearing plate 134 to the guide plate 182 and providing a guide along which the plunger mounting assembly 140 is slidably supported. The guide rods 135 help to maintain a parallel relationship between the pipette body mounting plate 121, the guide plate 182, and the plunger mounting assembly 140. Due to the dual function of the guide rods 135, it is not necessary to provide separate components to guide the movement of the plunger mounting assembly 140 and to transfer the axial displacement of the bearing plate 134 to the guide plate 182. This frees up space within the head chassis 101, allowing larger diameter guide rods 135 to be used while still maintaining a compact structure within the pipetting head. This can further improve performance of the pipetting head by reducing the amount of flex in the guide rods and thereby maintaining a parallel relationship between the moving plates. For example, the guide rods 135 may have a diameter of between 8mm and 16mm, e.g. 12mm. The pipette head 100 may be used with pipettes that include plungers (so called positive displacement plungers). In this example, the pipette head 100 may further include a pipette plunger mounting assembly 140. This is shown and discussed in greater detail in relation to Figures 16 to 18. Figure 16 is a cross-sectional view of an exemplary plunger mounting assembly 140 and the plunger clamping mechanism 150 in an exemplary pipette head 100. Figure 17 is an enlarged cross-sectional view of the drive mechanism of the plunger mounting assembly of Figure 16. The plunger mounting assembly 140 and the plunger clamping mechanism 150 are movable together in the axial direction by the dispense drive actuator assembly 160, as discussed below in relation to Figure 19. The plunger clamping mechanism 150 is moveable in the axial direction relative to the plunger mounting assembly 140. The plunger mounting assembly 140 comprises a plunger mounting plate 141 with a plurality of apertures 142. The plunger mounting assembly 140 comprises a box-shaped housing 170 within which the components of the plunger mounting assembly 140 and the plunger clamping mechanism 150 are at least partly housed. For ease of assembly, the housing 170 preferably has a two-part construction comprising an upper housing and a lower housing which are fixed together around the plunger clamping mechanism 150. The plunger mounting plate 141 forms the lower surface of the housing 170. The housing 170 is rectangular in cross-sectional shape view and comprises an axially extending linear bush 173 in each corner. The linear bushes 173 are supported within apertures 174 in the housing 170 so that the entire assembly shown in Figure 16 can move up and down along the guide rods of the guide plate drive mechanism. The plunger clamping mechanism 150 comprises a plunger clamping plate 145 positioned above the plunger mounting plate 141, a plurality of plunger clamping members 147 (see Fig.18) on the plunger clamping plate 145, and a plunger clamp drive mechanism 153, operable to selectively engage the plunger clamping mechanism. The plunger clamp drive mechanism comprises a linear actuator 153 configured to drive relative axial movement between the plunger clamping plate and the plunger mounting assembly to selectively engage the plunger clamping mechanism. The linear actuator 153 comprises a plunger clamp motor 152 fixed in relation to the plunger clamp plate 145, and coupled to the plunger mounting assembly 140 to move the plunger clamp plate 145 relative to the plunger mount plate 141 in the axial direction to selectively engage the plunger clamping mechanism 150 when the clamp motor 152 is actuated. The plunger clamping mechanism 150 further includes a plunger clamp support plate 154 and a motor support plate 154A which is fixed to the plunger clamp support plate 154. The plunger clamp plate 145 is fixed to the underside of the plunger clamp support plate 154. The plunger clamping mechanism may further include plunger clamp guide rods 155 which guide the plunger clamp plate 145 during movement of the plunger clamping mechanism 150. In this embodiment, the plunger clamp guide rods 155 are fixed in relation to the plunger clamp support plate 154 and slide within apertures 175 in the housing 170 which are located inboard of the apertures 174. In other embodiments, the plunger clamp guide rods may be fixed to the housing 170 and the support plate comprise apertures which slide along the outside of the guide rods. The plunger clamp drive mechanism 153 comprises a screw mechanism 156 configured to convert rotation of the clamp motor 152 into axial displacement of the clamp plate 145. The screw mechanism 156 comprises a ring gear 157 and a hollow shaft 176 which are coupled together by a threaded connection. The hollow shaft 176 extends along the drive axis 190 and is fixed to the housing 170 of the plunger mounting assembly 140. The ring gear 157 is supported by a bearing 151 which is provided between the plunger clamp support plate 154 and the ring gear 157. In this manner, the ring gear 157 is fixed in the axial direction relative to the plunger clamp support plate 154 and is rotatable about the drive axis 190. The ring gear 157 comprises a central bore 157A extending in the axial direction, i.e. along the drive axis 190, and a radial flange 157B with a plurality of gear teeth 157C. The radial flange 157B may extend radially outwardly from the hollow shaft 157A. The bores of the hollow shaft 176 and of the ring gear 157 together define a central aperture or axial bore. In the illustrated embodiment, the bore 157A of the ring gear 157 extends around the hollow shaft 176 fixed to the housing 170, and the threaded connection between the ring gear 157 and the hollow shaft 176 is provided by a screw thread on the outer surface of the hollow shaft 176 and a complimentary screw thread on the inner surface of the bore 157A of the ring gear 157. In other embodiments, the ring gear 157 may include a threaded shaft which extends within the bore of the hollow shaft 176 of the housing 170, with the threaded connection being provided by a screw thread on the outer surface of the hollow shaft of the ring gear 157 and a complimentary screw thread on the inner surface of the hollow shaft 176. The clamp motor 152 drives a driving gear 158, the teeth of which mesh with the teeth 157C of the ring gear 157. When the clamp motor 152 is operated, the driving gear 158 rotates the ring gear 157 around the hollow shaft 176. Due to the threaded connection between the ring gear 157 and the hollow shaft 176, rotation of the ring gear 157 causes the ring gear 157 to move axially up or down the hollow shaft 176. Due to the axial fixation of the ring gear 157 in relation to the plunger clamp support plate 154, axial movement of the ring gear 157 causes axial movement of the rest of the plunger clamping mechanism 150, with the exception of the hollow shaft 176 which is fixed in relation to the housing 170. Rotation of the clamp motor 152 in a first direction causes the ring gear 157 to move upwards in the axial direction along the hollow shaft 157. Rotation of the clamp motor 152 in an opposite, second direction causes the plunger clamping mechanism 150 to move downwards in the axial direction. Since the clamp plate 145 is fixed to the plunger clamp support plate 154, the clamp plate 145 moves axially with the plunger clamp support plate 154 to move towards or away from the plunger mount plate 141. Figure 18 is an enlarged cross-sectional view of part of the plunger mounting assembly 140 and the plunger clamping mechanism 150, showing the plunger mounting plate 141 and the plunger clamp plate 145 in more detail. The plunger mounting plate 141 has an array of apertures 142 extending in the axial direction through the thickness of the plunger mounting plate 141. Secured within each aperture 142 is a plunger mount 143 extending in a downward axial direction from the plate 141 and configured to couple with one of the plungers during use. Each plunger mount 143 is in the form of a tubular plunger mount sleeve retained in one of the array of apertures 142. In this example, the plunger mounts 143 each have a shoulder having an outer diameter which is greater than the diameter of the apertures 142 and which abuts against the underside of the plunger mount plate 141. The shoulder helps to set the height of the plunger mount sleeve 143 when initially inserted into an aperture 142 in the plunger mount plate 141. The lower end of the pipette body mount sleeve 143 preferably has an inwardly tapered end surface to assist with the insertion of a plunger into the bore of the plunger mount sleeve 143. Each plunger mount sleeve 143 comprises a radially extending feature on its inner surface which is configured to form one half of a snap-fit connection with a correspondingly shaped feature on the outer surface of a plunger. In this example, the radially extending feature is in the form of an annular groove 144 in the inner surface of the plunger mount sleeve 143 towards its lower end. The plunger clamping plate 145 has an array of recesses or apertures 146 extending in the axial direction. The array of apertures 146 of the plunger clamping plate 145 correspond in number and position to the array of apertures 142 of the plunger mounting plate 141. The array of apertures 146 of the plunger clamping plate 145 may also correspond in number to the arrays of apertures 122 and 126 of the pipette body mounting and clamping mechanisms. The plunger clamping plate 145 further comprises an array of plunger clamping members 147 each associated with one of the plurality of plunger mounts 143. The array of plunger clamping members is provided in the form of a plurality of clamping rods 147 which extend axially from the plunger clamping plate 145 and extend into the bores defined within the plunger mount sleeves 143. Each clamping rod 147 has an enlarged head 148 at its lower end which extends from a narrower neck region 149A. The enlarged head 148 has an outer diameter which is less than the inner diameter of the plunger mount sleeve 143. In this manner, a small clearance is provided between the outer surface of the enlarged head 148 and the inner surface of the plunger mount sleeve 143 when the plunger clamping mechanism is engaged. The neck 149A has an outer diameter which is less than that of the enlarged head 148. Preferably, each clamping rod 147 also has a main shaft 149B with an outer diameter which is substantially the same as the inner diameter of the region of the plunger mount sleeve 143 in which it is located. The main shaft 149B moves axially along the bore of the plunger mount 143 as the plunger clamping plate 145 is moved up and down in the axial direction relative to the plunger mount plate 141. The relative movement between the plunger clamping plate 145 and the plunger mounting plate 141 allows the plunger clamping mechanism 150 to be moved into any position between an engaged state, in which a plunger is clamped in position, and a disengaged, or "released" position or state, in which a plunger may be mated with or removed from the plunger mounts in an axial direction. The plunger clamping mechanism 150 may be configured to move to an ejection state, in which the plunger clamping plate 145 is moved towards the plunger mounting plate 141 beyond the clamping position. In this position, the enlarged heads 148 of the clamping rods 147 are axially below the radially extending features on the inner surfaces of the plunger mount sleeves 143, which are instead in alignment with the narrower neck portions 149A of the clamping rods. This allows the top of the plunger, i.e. the plunger connector portion, to flex inwards. Further downward movement of the clamping plate 145 and clamping rods 147 pushes the plunger downwards to release it from the snap-fit connection with the plunger mounting sleeves. When the guide plate 182 is also in a released position, i.e. a position in which the clamping region is not axially aligned with the snap-fit connection between the pipette bodies and the pipette body mounts 123 to prevent radially outward flexing of the pipette body connector portion, further downward movement of the clamping rod using the plunger clamping drive mechanism pushes the plunger against one or more inside surfaces of the pipette body, for example the region around the aperture at the distal end of the pipette body, to push the pipette body from the pipette body mounting assembly. This ejects the entire pipette assembly from the pipetting head. Alternatively, the clamping rod and the plunger may be moved down together using the direct drive actuator 160 to push against the pipette and thereby eject the pipette body from the apparatus. In this ejection mode, the plunger may remain on the apparatus even after the pipette body has been ejected. As best seen in Figure 19, the dispense drive actuator assembly 160 comprises a dispense drive motor 161 and a dispense drive mechanism 162 by which the dispense drive motor 161 is directly coupled to the plunger mounting assembly 140 to drive axial displacement thereof. The dispense drive motor 161 is mounted on the top surface of the upper head chassis part 102. The dispense drive mechanism 162 includes the output shaft 165 of the dispense drive motor 161, a threaded rod 163, and a ball screw actuator nut 164 with which the threaded rod 163 forms a ball screw actuator. The output shaft of the drive motor 161 may be integral with, i.e. unitary with, the threaded rod 163, or a discrete component which is fixed with respect to the threaded rod 163 for rotation therewith. The ball screw nut 164 is axially fixed in relation to the plunger mounting assembly 140 so that the two move simultaneously along the threaded rod 163 when the threaded rod 163 rotates. In the illustrated embodiment, the ball screw nut 164 is fixed within the hollow shaft 176 of the housing 170 of the plunger mounting assembly 140. The threaded rod 163, ball screw actuator nut 164, screw mechanism 136 and screw mechanism 156 are all co-axial. In other words, the central axes of all of these components are aligned along a common axis. One or both of the output shaft 165 of the drive motor 161 and the threaded rod 163 extend through the axial bore defined by the screw mechanism 136 of the guide plate drive mechanism. One or both of the threaded rod 163 and the ball screw nut 164 may extend through the axial bore defined by the screw mechanism 156 of the guide plate drive mechanism. In this manner, the dispense drive mechanism 162 is concentric with the screw mechanism 136 and with the screw mechanism 156 along at least a part of the length of the dispense drive mechanism 162. With this arrangement, the forces applied by the various drive mechanisms all act along the same single axis, which is preferably central. This can help to keep the movable plates parallel with each other to promote smooth operation and reduce the risk of crabbing. Further, each drive mechanism applies a force in a single location. This avoids the risk of crabbing due to the uneven application of force at multiple locations. For example, where the force is applied at each of the four corners of a plate by a pulley and belt arrangement, failure to keep the force at each pulley even can lead to crabbing of the plate. Figures 20 to 23 show a pipette 1010 for use with the pipetting head 100. The pipette 1010 comprises a pipette body 1100. In some examples, the pipette body 1100 may further include a plunger 1200. The pipette body 1100 may be a receptacle for receiving and / or containing a sample fluid, or a sample liquid. The pipette body 1100 may be configured for insertion into a container of sample liquid, or into sample liquid. The pipette body 1100 has a proximal end 1101 and a distal end 1102 defining a longitudinal axis 1001 therebetween. The pipette body 1100 has an aperture 1108 at its distal end 1102. The aperture 1108 may be defined by an inner wall surface 1111 of the pipette body 1100 at the distal end 1102, specifically at an outermost distal point of the pipette body 1100. The aperture 1108 may be any suitable shape, e.g. elliptical, oval, or circular. The pipette body 1100 has a fluid cavity 1109 extending from the aperture 1108 at least partially towards the proximal end 1101. The fluid cavity may be configured to receive and / or retain a fluid, such as an airgap or sample liquid. The fluid cavity 1109 may be substantially elongate. The fluid cavity may be defined by the inner wall surface of the pipette body 1100. The longitudinal axis 1001 may be a central axis about which the pipette body 1100 and / or plunger 1200 is disposed. The longitudinal axis 1001 may define a central axis about which the pipette body 1100 and / or plunger 1200 are disposed uniformly or symmetrically. The pipette body 1100 and / or plunger 1200 may be configured so that the direction of aspirating and / or dispensing is along the longitudinal axis 1001. The pipette body 1100 may be substantially elongate, extending between its proximal and distal ends, 1101, 1102. The pipette body 1100 may be substantially or entirely hollow. The pipette body 1100 may comprise or consist of a polymeric material. The pipette body 100 may comprise or consist of a homogeneous material. The pipette body 100 may comprise or consist of a translucent or transparent material. The pipette body 1100 may comprise a series of portions, each having a different function, distinguishing features and / or a different shape or dimension. The pipette body 1100 may comprise a pipette body connector portion 1103. The fluid cavity 1109 may extend through one, more or all of the portions of the pipette body 1100. The pipette body connector portion 1103 is configured for connection to a liquid handling system, for example with a snap-fit connection. The pipette body connector portion 1103 may comprise a split tubular wall 1120 which may be defined by a plurality of flexible segments 1121. The flexible segments 1121 may be configured to resiliently deflect in a radially outward direction to increase the outer diameter of the proximal end 1101 of the pipette body 1100 from a first outer diameter, in which the flexible segments are undeflected and the pipette body connector portion is in a rest state, to a second outer diameter, in which the flexible segments are deflected radially outwardly and the pipette body connector portion is in an expanded state. In the embodiment depicted, the pipette body connector portion 1103 comprises four axially extending discontinuities or slots 1122 in the tubular wall 1120 which separate four flexible segments 1121. The pipette body connector portion 1103 may comprise any suitable number of axially extending discontinuities 1122 to define any number of flexible segments 1121, such as two, three, four, five, or six. The arrangement of flexible segments 1121 and slots 1122 enables the pipette body connector portion to expand without requiring significant forces to be exerted on the pipette body connector portion. This can reduce the insertion force required to engage the pipette body connector portion 1103 with the pipetting head. The pipette body connector portion 1103 may further comprise one or more radially extending features 1123 on its inner surface by which the pipette body may be coupled to the pipetting head. The radially extending feature on the inner surface of the pipette body connector portion 1103 may comprise a protrusion, which extends radially inward, and / or a recess or groove, which extends radially outward. The radially extending feature may extend in a circumferential direction. In the depicted embodiment, the radially extending feature on the inner surface of the pipette body connector portion 1103 comprises a part-annular rib 1123 which protrudes from the inner surface of the pipette body connector portion 1103. Preferably, the second outer diameter to which the pipette body connector portion is increased is larger than the first outer diameter by at least the radial extent of the radially extending feature 1123. The rib 1123 preferably comprises angled upper and lower surfaces along which the inner diameter of pipette body connector portion gradually increases from the rib 1123 to the regions of the pipette body connector portion above and below the rib 1123. If the pipette body includes a plunger 1200, then the plunger 1200 may have a proximal end 1201 and a distal end 1202 extending along the longitudinal axis 1001. The plunger 1200 is configured to extend at least partly between the proximal and distal ends 1101, 1102 of the pipette body 1100 into the end portion 1110. The plunger 1200 may be configured to extend substantially or wholly between the proximal and distal ends 1101, 1102 of the pipette body 1100 into the end portion 1110. The plunger 1200 is movable towards and away from the aperture 1108 to aspirate or dispense fluid from the pipette body 1100. The plunger 1200 may have an end portion outer wall surface 1212 configured for alignment with the inner wall surface 1111 of the pipette body end portion 1110. The plunger 1200 may comprise or consist of a polymeric material. The plunger 1200 may comprise or consist of a homogeneous material. The plunger connector portion 1203 is configured for connection to a liquid handling system, for example with a snap-fit connection. The plunger connector portion 1203 may be configured to be received entirely within the pipette body connector portion 1103 of the pipette body 100. The centering portion 1104 may be configured to centre the plunger 1200 within the pipette body 1100. The centering portion 1104 may have an inner surface which may have a tapered or conical shape. In other embodiments, the plunger 1200 may be centred with a different mechanism, such as an enlarged cylindrical section on the plunger shaft which centres the plunger 1200 within the bore extending below the centering portion 1104 shown in Fig. 20. The support portion 1105 may comprise one or more structural ribs or rings, configured to reduce flexibility and / or improve structural integrity of the pipette body 1100. The one or more ribs may extend on an outer surface of the pipette body, along at least part of the length between the proximal end 1101 and distal end 1102, for example axially, as best seen in Figure 23. The main body portion 1106 may be substantially cylindrical and / or elongate. The main body portion 1106 may have a substantially uniform inner diameter and / or outer diameter. The main body portion 1106 may extend along at least half of the length of the pipette body 1100. The main body portion 1106 may have a larger inner and / or outer diameter than any diameter of the end portion 1110. The bridging portion 1107 may be configured to bridge the main body portion 1106 to the end portion 1110. The bridging portion 1107 may define a gradual transition or a step between the main body portion 1106 and the end portion 1110. The bridging portion 1107 may be substantially tapered, conical and / or dome shaped. The plunger 1200 has a proximal end 1201 and a distal end 1202 extending along the longitudinal axis 1001. The plunger 1200 is configured to extend at least partly between the proximal and distal ends 1101, 1102 of the pipette body 1100 into the end portion 1110. The plunger 1200 may be configured to extend substantially or wholly between the proximal and distal ends 1101, 1102 of the pipette body 1100 into the end portion 1110. The plunger 1200 is movable towards and away from the aperture 1108 to aspirate or dispense fluid from the pipette body 1100. The plunger 1200 may have an end portion outer wall surface 1212 configured for alignment with the inner wall surface 1111 of the pipette body end portion 1110. The plunger 1200 may comprise or consist of a polymeric material. The plunger 1200 may comprise or consist of a homogeneous material. The plunger 1200 may comprise a series of portions, each having a different function, distinguishing features and / or a different shape or dimensions. The plunger 1200 may comprise one or more of: a plunger connector portion 1203, a centering portion 1204, a main body portion 1206, and a sealing portion 1207. From the proximal end 1201 to the distal end 1202, each portion may be arranged in the following order: connector portion 1203, centering portion 1204, main body portion 1206, sealing portion 1207 and end portion 1210. The plunger connector portion 1203 is configured for connection to a liquid dispensing device, for example with a snap-fit connection. The plunger connector portion 1203 may be configured to be received entirely within the pipette body connector portion 1103 of the pipette body 1100. The plunger connector portion 1203 may comprise a split tubular wall 1220 which may be defined by a plurality of flexible segments 1221. The flexible segments 1221 may be configured to resiliently deflect in a radially inward direction to decrease the inner diameter of the proximal end 1201 of the plunger 1200 from a first inner diameter, in which the flexible segments are un-deflected and the plunger connector portion is in a rest state, to a second inner diameter, in which the flexible segments are deflected radially inwardly and the plunger connector portion is in a compressed state. In the embodiment depicted, the plunger connector portion 1203 comprises three axially extending discontinuities or slots 1222 in the tubular wall 1220 which separate three flexible segments 1221. The plunger connector portion 1203 may comprise any suitable number of axially extending discontinuities 1222 to define any number of flexible segments 1221. The arrangement of flexible segments 1221 and slots 1222 enables the plunger connector portion to be compressed without significant compressive stresses. The plunger connector portion 1203 may further comprise one or more radially extending features 1223 on its outer surface by which the plunger may be coupled to the pipetting head. The radially extending feature on the outer surface of the plunger connector portion 1203 may comprise a protrusion, which extends radially outward, and / or a recess or groove, which extends radially inward. The radially extending feature may extend in a circumferential direction. In the depicted embodiment, the radially extending feature on the outer surface of the plunger connector portion 1203 comprises a bulbous head 1223 at the upper end of the plunger which is defined above an annular groove in the outer surface of the plunger connector portion 1203. Preferably, the second inner diameter to which the plunger connector portion is decreased is smaller than the first inner diameter by at least the radial extent of the radially extending feature 1223, i.e. by at least half the difference between the outer diameter of the bulbous head 1223 and the outer diameter at the narrowest part of the annular groove. The plunger connector portion 1203 may further comprise an internal wall 1224 extending transversely from the inner surface of the plunger connector portion 1203. The internal wall 1224 may be positioned distally of the flexible segments 1221. The internal wall 1224 may be positioned proximally of the main body portion 1206. The internal wall 1224 forms a restriction across the bore of the plunger connector portion. This provides a surface against which the head of the clamping rods can abut to eject the pipettes. The internal wall 1224 may extend across the full width of the interior bore of the plunger 1200, as illustrated in Figure 22. However, this is not necessarily the case. The inner wall 1224 need only restrict the size of the bore to less than the outer diameter of the head of the clamping rod. The centering portion 1204 may comprise an annular shoulder. The centering portion 1204 may be substantially conical or dome shaped.In the depicted embodiment, the centering portion 1204 is a radially extending frustoconical shoulder. The centering portion 1104 of the pipette body 1100 is preferably configured to receive and engage with the centering portion 1204 of the plunger 1200 to centre the alignment of the plunger. In the embodiment depicted in Figure 14, the two centering portions have corresponding geometries. The main body portion 1206 of the plunger 1200 may be configured to be received in the main body portion 1106 of the pipette body 1100. The main body portion 1206 may be substantially cylindrical and / or elongate. The main body portion 1206 may have a substantially uniform diameter. The main body portion 1206 may extend along at least half of the length of the plunger, optionally at least two thirds of the length of the plunger 1200. The main body portion 1206 may have a larger diameter than the diameter of the end portion 1210. The sealing portion 1207 of the plunger 1200 may be configured to be received in the bridging portion 1107 of the pipette body 1100. The sealing portion 1207 may be configured to form a seal against an inner wall of the pipette body 1100. The sealing portion 1207 may form a fluid-tight seal within the pipette body, such that fluid cannot pass from a proximal side of the sealing portion 1207 to a distal side of the sealing portion 1207 when the plunger 1200 is installed in the pipette body 1100. The sealing portion 1207 may be at least partly flexible. The sealing portion 1207 may be configured to bridge the main body portion 1206 to the end portion 1210. The sealing portion 1207 may define a step between the main body portion 1207 and the end portion 1210. The sealing portion 1207 may be substantially tapered, conical and / or dome shaped. The end portion 1210 of the plunger 1200 may be configured to be received in the end portion 1110 of the pipette body 1100. The end portion 1210 may have a smaller diameter than the sealing portion 1207 and / or the main body portion 1206. With reference to Figure 24, a method 2300 of connecting an array of removable pipettes to the pipetting head 100 of the present invention will be described. The final, connected, and clamped configuration for a single pipette 101 is illustrated in Figure 25. Although the Figures to which the below discussion relates show only a single pipette, it will be understood that the alignment and connection steps may be applicable to multiple pipettes simultaneously. For example, where the pipetting head comprises 384 mounts, the below discussion may apply to the simultaneous alignment of 384 pipettes, or any subset thereof. At step 2301, a liquid dispensing apparatus 10, such as the apparatus described above in relation to Figures 1 to 19, is provided. At step 2302, an array of removable pipettes 1010 held in a container 1012 is placed on the deck 14 of the liquid dispensing apparatus 10. Each removable pipette 1010 has a pipette body 1100 and may further comprise a plunger 1200 which is removable from the pipetting head and moveable within the pipette body. At step 2303, the pipetting head 100 is positioned over the container (by use of the associated drive mechanisms within the liquid dispensing apparatus 10). During this process, the array of pipette body mounts 123 is laterally aligned with the array of removable pipettes 1010. As will be understood, some of the pipettes might be misaligned with respect to their respective pipette body mount. At step 2304, the guide plate 182 is translated to the lowered position (using associated drive mechanisms within the liquid dispensing apparatus 10). In the lowered position, the distal ends 187 of the guide protrusions 186 extend below the distal ends of the pipette body mounts 123. At step 2305, the pipette bodies 1100 are engaged with, i.e. contacted by, the array of guide protrusions 186 on the guide plate 182. The pipette bodies 1100 therefore physically contact the guide protrusions 186, which guide the pipette bodies 1100 to an aligned position in which they are co-axial and aligned with the apertures 184 in the guide plate 182 and, therefore, the pipette body mounts 123. At step 2306, the guide plate 182 is translated to a raised position in relation to the pipette body mounts 123. In the raised position, the pipette body mounts 123 extend through the plurality of apertures 184 in the guide plate 182 such that the distal ends of the pipette body mounts 123 are positioned below the distal ends of the guide protrusions 186. At step 2307, the pipetting head 100 is moved axially towards the container 1012 (using the associated drive mechanisms within the liquid dispensing apparatus 10) to insert the array of pipette body mounts 123 into the proximal ends of the pipette bodies 1100 and thereby connect the pipette bodies 1100 to the array of pipette body mounts 123. It will be understood that steps 2306 and 2307 may be carried out subsequently, or simultaneously so that the axial position of the guide plate 182 relative to the container 1012 is generally constant as the pipetting head 100 is moved axially towards the container 1012 to insert the array of pipette body mounts 123 into the proximal ends of the pipette bodies 1100. In a preferred method, the guide plate 182 is raised in relation to the pipette body mounts 123 at the same time and same rate as the pipette body mounts 123 are lowered relative to the pipette bodies in the container. In this manner, the guide plate remains broadly static in relation to the pipette bodies and the distal ends of the guide protrusions can remain in contact with the pipette bodies as the pipette body mounts are inserted into the end of the pipette bodies. This enables the guide plate to maintain alignment of the pipette bodies as the pipette bodies are engaged by the pipette body mounts. At step 2308, the pipetting head 100 is raised to lift the array of removable pipettes 1010 from the container 1012. Optionally, at step 2309, the pipette bodies 1100 are clamped onto the pipette mounts 123 by translating the guide plate 182 to a clamp position in which the clamping region 128 of each of the plurality of sleeves 127 circumscribes an associated pipette body mount 123 in the region of the snap-fit connection. In this manner, the sleeves 127 resist release of the snap-fit connection by restricting radially outward movement of the pipette body 1100. Step 2309 may be carried out after step 2308. Optionally, step 2309 is carried out before step 2308. In other words, the step of raising the pipetting head to remove the array of removable pipettes from the container may be carried out after the step of connecting the pipette bodies to the array of pipette body mounts and before the step of clamping the pipette bodies onto the array of pipette mounts. This enables the pipetting head to connect to (or "pick") and clamp single, part row, part columns, rows, columns, sections and full arrays of pipette bodies by offsetting the pipetting head relative to the array of pipette bodies in the tip box without the clamping mechanism fouling against unpicked pipette bodies during the step of clamping. Removing the pipette bodies from the tip box before subsequently clamping the pipette bodies onto the mounts can also be advantageous in situations in which the pipette bodies are not at a uniform height within the container, since the guide plate can effectively "nudge" the pipette bodies downwards into the correct axial "snap-fit" position during the step of clamping if those pipette bodies are not correctly seated on their respective mount. Alternatively, the step of raising the pipetting head to remove the array of removable pipettes from the container may be carried out after, or simultaneously with, the step of clamping. With the above method, the guide plate 182 is moved axially in the Z-direction towards an array of pipettes so that the protrusions 186 on the guide plate 182 contact with the pipette body connector portions of laterally misaligned pipette bodies 1100 and guide those pipette bodies towards a co-axial lateral position. This helps ensure that the pipette bodies 1110 are aligned with the pipette body mount sleeves 123 so that the pipette bodies 1100 can be correctly connected to the pipetting head. After aligning the pipette bodies 1100, the guide plate 182 is retracted back to the raised position relative to the pipette body mounts and the entire pipetting head 100 is moved using the pipette head Z motor (not shown) to bring the pipette body mounting assembly towards an array of pipettes 1010 such that the pipette body mount sleeves are received within the pipette body connector portions of the pipette bodies 1100. During insertion of the pipette body mount sleeves 123 into the proximal ends of the pipette bodies 1100, the annular ribs 1123 on each pipette body 1100 ride over the outer surface of the pipette body mount sleeve 123, causing the pipette body connector portion of each pipette body to be outwardly deflected to the expanded position. Once the annular rib 1123 is received in the annular groove 124, the flexible segments return to an un-deflected, or only partially deflected, state. In this position, the pipette body connector portion is in a rest state and each pipette body 1100 is coupled to its respective pipette body mount sleeve 123 in a snap fit connection. The guide plate 182 is then lowered to a clamp position in which the pipette body clamping sleeves 127 extend around the pipette body connector portion 1103 of each pipette body 1100 and the clamping region of each pipette body clamping sleeve is adjacent to the snap fit connection between each pipette body 1100 and its respective pipette body mount sleeve 123. The inner diameter of the clamping region 128 is less than the expanded diameter of the pipette body connector portion 1103, for example it may be substantially the same as the unexpanded, or "first", outer diameter of the pipette body connector portion. With this arrangement, the pipette body connector portion 1103 is prevented from expanding fully, thus preventing the radially extending feature 1123 on the inner surface of the pipette body 1100 from being released from the groove 124 on the outer surface of the pipette body mount sleeve 123. In this manner, the pipette body 1100 is "locked" or "clamped" in position without the need to exert any significant forces on the pipette body connector portion of the pipette body 1100. This can reduce the force required to engage the pipette body clamping mechanism. If the pipettes 1100 include removable plungers which are held by a plunger mounting assembly 140 of the pipetting head, connection between the plungers and the pipetting head typically occurs during the process of connecting the pipette bodies with their respective pipette body mounts. In this process, the plunger mount sleeves 143 are brought into contact with the plunger connector portion 1203 at the upper, or proximal, ends of the plunger 1200. Further downward movement of the plunger mounting assembly 140 (either independently to the pipetting head or together with the pipetting head) causes the inner surface of the plunger mount sleeve 143 to ride over the outer surface of the plunger connector portion, inwardly deflecting the plunger connector portion to the contracted or compressed position and allowing the bulbous head 1223 at the top of the plunger 1200 to be received in the groove 144 on the inside surface of the plunger mount sleeve 143. Once the bulbous head 1223 is received in the groove 144 on the inside surface of the plunger mount sleeve 143, flexible segments 1221 of the plunger connector portion 1203 return to an undeflected, or partially deflected state. In this position, the plunger connector portion is in a rest state and each plunger 1200 is coupled to its respective plunger mount sleeve 143 in a snap fit connection. Then, the plunger clamping plate 145 is moved in the axial direction towards the plunger mounting plate 141 by the plunger clamp drive mechanism until the heads 148 of the plunger clamp rods are adjacent to the snap fit connection between the plunger and the plunger mount sleeve. The outer diameter of the head 148 of each clamping rod is greater than the inner diameter of the plunger when in the contracted state, or "second inner diameter". With this arrangement, the plunger connector portion is prevented from contracting fully to the contracted position, thus preventing the bulbous head 1223 of the plunger 1200 from being released from the groove 144 on the inner surface of the plunger mount sleeve. Figure 25 is an enlarged cross-sectional view of a lower region of the pipetting head 100 showing the removable pipette mounted and clamped on the pipetting head using the method discussed above in relation to Figure 24. In this position, the pipette body 1100 is mounted to the pipette body mounting assembly 120 and clamped in position by the guide plate 182, and the plunger 1200 is mounted to the plunger mounting assembly 140 and is clamped in position by the plunger clamping mechanism. Once the aspirating and dispensing operations are complete, the pipette or pipettes can be ejected as follows. Firstly, the plungers are each moved to the bottom of their travel inside their respective pipette body and the pipette body clamping function provided by the guide plate 182 is disengaged by moving the guide plate upwards towards the pipette body mounting plate 121 to bring the clamp sleeves 127 out of alignment with the snap-fit connection between each pipette body and its respective pipette body mount 123. Next the plunger clamping plate 145 is moved towards the plunger mounting plate 141, beyond the engaged position, such that the head 148 of each clamping rod 147 is moved below and out of alignment with the snap-fit connection between each plunger 1200 and its respective plunger mount 143 and instead bringing the neck portion 149A of each clamping rod 147 into alignment with the snap-fit connection and causing the distal end surface of the head of the clamping rod to abut against the internal wall 1224 of the plunger connector portion 1203. Since the neck portions 149A of the clamping rods 147 have an outer diameter which is less than the second, compressed, inner diameter of the plunger connector portions 1203, when the clamping rods are in this position, the plunger connector portions are not restricted from deflecting inwardly to the compressed state. Thus, continued downward movement of the plunger clamping plate 145 releases the bulbous heads 1223 of the plunger connector portions 1203 from the corresponding radially extending features 144 of the plunger mounts 143 to remove the plungers 1200 from the pipetting head. Further downward movement of the plunger clamping plate, and therefore of the clamping rods and the plungers themselves, pushes the distal end portions of the plungers against the respective portions of the pipette bodies within which they are located. In this manner, the plungers and the plunger clamping mechanism can be used to release the pipette bodies 1100 from the pipette body mounts 123, thereby ejecting the pipettes 1010 from the pipetting head. In the above example and method described in relation to Figures 18-25, the pipette head 100 includes a plunger mounting assembly 140 and a plunger clamping mechanism 150. It will be understood that neither is essential. Instead, the pipette head 100 may be absent these components if the pipette bodies 1100 do not include plungers 1200 and / or if the pipetting head does not include a plunger clamping mechanism. In other examples, the pipette head 100 may be provided without a pipette body clamping mechanism. Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A pipetting head for a liquid dispensing apparatus for use with an array of removable pipettes, each having a pipette body, the pipetting head comprising:5 a pipette body mounting assembly comprising an array of pipette body mounts, each extending in an axial direction and arranged to hold one of the pipette bodies;a pipette guide assembly positioned below the pipette body mounting assembly, wherein the pipette guide assembly comprises:10 a guide plate comprising a plurality of apertures configured to receivethe pipette body mounts, and an array of guide protrusions which extend in the axial direction from an underside of the guide plate, wherein the array of guide protrusions each have an upper end adjacent to the plurality of apertures and a lower end which is laterally positioned between adjacent15 pipette body mounts; anda guide plate drive mechanism arranged to translate the guide plate axially towards and away from the pipette body mounting assembly between a lowered position in which the lower ends of the array of guide protrusions are positioned below the lower ends of the pipette body mounts, and a raised20 position in which the pipette body mounts extend through the plurality of apertures and the lower ends of the pipette body mounts are positioned below the lower ends of the array of guide protrusions.

2. The pipetting head according to claim 1, wherein the array of guide 25 protrusions are arranged such that a plurality of guide openings are defined between adjacent guide protrusions, wherein the plurality of guide openings are coaxial with the plurality of apertures in the guide plate and are wider at the lower end of the guide protrusions than the plurality of apertures.30 3. The pipetting head according to any preceding claim, wherein the array ofguide protrusions are tapered, such that the plurality of guide openings narrow from the lower ends of the guide protrusions towards the plurality of apertures.

4. The pipetting head according to claim 3, wherein the array of guide35 protrusions each have a polygonal cross-sectional shape defining a plurality of side walls, each of which faces towards one of the plurality of apertures.

5. The pipetting head according to claim 4, wherein the side walls are concave such that the plurality of guide openings each have a circular or part-circular cross-sectional shape.5 6. The pipetting head according to claim 4 or claim 5, wherein the side wallsare continuous with the plurality of apertures.

7. The pipetting head according to any of claims 4 to 6, wherein each guide protrusion has a truncated pyramid shape.

108. The pipetting head according to any preceding claim, wherein the plurality of apertures is an array of apertures which corresponds in number and position to the array of pipette body mounts, such that each of the plurality of apertures is configured to receive a single pipette body mount of the array of pipette body 15 mounts.xl 9. The pipetting head according to claim 8, wherein the array of apertures andthe array of pipette body mounts are both arranged in a grid, and wherein the array of guide protrusions extend from the guide plate such that each guide protrusion is "1” 20 laterally positioned between each group of four adjacent pipette body mounts.1—CO 10. The pipetting head according to any preceding claim, wherein the array ofguide protrusions have substantially the same size and shape.25 11. The pipetting head according to any preceding claim, wherein each of thearray of pipette body mounts comprises a radially extending feature on its outer surface for forming one half of a snap-fit connection, wherein the plurality of apertures in the guide plate define a plurality of sleeves each having a clamping region and being co-axial with one of the array of pipette body mounts, wherein30 the guide plate drive mechanism is operable to move the guide plate between a clamp position and a release position, wherein:when the guide plate is in the clamp position, each of the plurality of sleeves circumscribes an associated one of the array of pipette body mounts and is axially positioned such that the clamping region is adjacent to the radially extending 35 feature on its associated pipette body mount, andwhen the guide plate is in the release position, the plurality of sleeves is axially positioned such that the clamping region of each sleeve is axially offset from the radially extending feature on the outer surface of its associated tip mount.5 12. A liquid dispensing apparatus comprising:a main body with a deck for receiving one or more microplates; anda pipetting head according to any of claims 1-11, the pipetting head being positioned above the deck.10 13. A method of connecting an array of removable pipettes to the pipetting headof a liquid dispensing apparatus, the method comprising:providing a liquid dispensing apparatus according to claim 12;placing an array of removable pipettes in a container on the deck of the liquid dispensing apparatus, each removable pipette having a pipette body;15 positioning the pipetting head over the container such that the array ofpipette body mounts is laterally aligned with the array of removable pipettes;xl translating the guide plate to the lowered position in which the lower endsof the array of guide protrusions are positioned below the lower ends of the pipette body mounts;"1” 20 engaging the pipette bodies with the array of guide protrusions;-j— translating the guide plate to the raised position in which the pipette bodyCO mounts extend through the plurality of apertures and the lower ends of the pipettebody mounts are positioned below the lower ends of the array of guide protrusions; connecting the pipette bodies to the array of pipette body mounts by moving 25 the pipetting head axially towards the container to insert the array of pipette body mounts into the upper ends of the pipette bodies.

14. The method of claim 13, wherein each of the array of pipette body mounts comprises a radially extending feature on its outer surface which forms a snap-fit 30 connection with a co-operatively shaped radially extending feature on an inner surface of each of the pipette bodies when the pipette bodies are connected to the array of pipette body mounts, and wherein plurality of apertures in the guide plate define a plurality of sleeves each having a clamping region and being co-axial with one of the array of pipette body mounts, the method comprising:35 clamping the pipette bodies onto the array of pipette mounts by translatingthe guide plate to a clamp position in which the clamping region of each of the plurality of sleeves circumscribes an associated pipette body mount and pipettebody connected to the pipette body mount and is axially positioned adjacent to the snap-fit connection to resists release of the snap-fit connection by restricting radially outward movement of the pipette body.5 15. The method of claim 14, further comprising:raising the pipetting head to remove the array of removable pipettes from the container after the step of connecting the pipette bodies to the array of pipette body mounts and before the step of clamping the pipette bodies onto the array of pipette mounts.1016. The method of any of claims 13 to 15, wherein the steps of translating the guide plate to the raised position and connecting the pipette bodies to the array of pipette body mounts are carried out simultaneously such that the axial position of the guide plate relative to the container is generally constant as the pipetting head 15 is moved axially towards the container to insert the array of pipette body mounts into the upper ends of the pipette bodies.31 10 24

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