Laboratory Dispensing Apparatus

Abstract

Laboratory dispensing apparatus including a base, a liquid reservoir and well assembly mounted on the base, and a microtitration plate carrier device mounted on the base for reciprocating movement between a forward retracted position and a rearward operative position overlying a portion of the well. A vertically movable pipette dispensing assembly is positioned above the base in operative alignment with the portion of the liquid well so that with the carrier in the forward retracted position the plural pipettes may withdraw microquantities of liquid from the well and then dispense the same into the microtitration plate wells when the carrier is moved to the rearward position. The plural pipettes are supported and contained in a head unit which is readily removable from the dispensing assembly to permit quick changing of heat units for cleaning purposes. A liquid agitator assembly is associated with the liquid reservoir and well assembly to circulate and stir the liquid and thereby insure uniformity and homogeneity of the liquid being dispensed by the plural number of pipettes. An electrical and pneumatic control system operates the stirring assembly and dispensing assembly, with the various control elements and components being mounted within the base structure of the dispensing apparatus to provide a compact self-contained portable system.

Description

BACKGROUND of THE INVENTION

This invention is related generally to laboratory analyzing and titrating apparatus and more particularly to an automatic sampling and dispensing apparatus capable of precisely and simultaneously withdrawing into a plurality of needles or pipettes predetermined reproducible microquantities of liquid from a liquid source and delivering the same to a plurality of corresponding wells of a microtitration plate.

In the laboratory, quite often it is necessary to perform analytical and/or experimental tests or procedures on a large number of laboratory specimens, the procedures frequently involving the measuring and dispensing of microquantities of liquid in the nature of 25 microliters. It is, of course, desirable that such procedures be performed within a minimum amount of time and with extreme accuracy and precision to eliminate error.

In the past, laboratory tests of this nature have usually been performed manually and individually and have required a great deal of time and produced questionable results. The operator simply could not, time after time, reproduce with accuracy the microquantitative liquid measurements necessary for these tests, nor could he perform an adequate number of tests within a reasonable period of time.

Automatic laboratory devices have been developed in attempting to overcome these problems and one such device which was quite successful is described in my copending U.S. Pat. application Ser. No. 740,289 filed on June 26, 1968, now U.S. Pat. No. 3,568,735. While the apparatus disclosed in application Ser. No. 740,289, now U.S. Pat. No. 3,568,735 represented a significant improvement over prior conventional devices, it was not entirely satisfactory from several standpoints. For example, when the apparatus was used with liquids consisting of two different constituents, such as a saline solution, no provision was made in the apparatus to maintain uniform characteristics of the liquid so that all the microquantities of the liquid dispensed are the same. Also, each of the dispensing pipettes was individually mounted in the pipette dispensing assembly, thus requiring a great deal of time and labor in removing each pipette for cleaning or replacement purposes. In addition, the components of the electrical and pneumatic control systems were mounted separate from the dispensing apparatus itself, which as a result was not as compact as desired.

Accordingly, while this invention includes several of the unique features of the apparatus disclosed in Application Ser. No. 740,289, now U.S. Pat. No. 3,568,735 it is intended to be an improvement thereover and includes various novel features which contribute significantly to a more reliable, more practical and more compact dispensing apparatus.

SUMMARY OF THE INVENTION

A primary object of the invention resides in the provision of a novel, more compact, self-contained laboratory dispensing apparatus capable of reliable use with any type liquid and functioning to simultaneously withdraw from a liquid reservoir into each of a plurality of needles or pipettes a predetermined reproducible microquantity of liquid, for example, a drop of 25 microliters, and simultaneously dispense the same into corresponding specimen containers or wells of a microtitration plate.

Another object of the invention resides in the provision of a more compact, self-contained laboratory dispensing apparatus in which the liquid supply source is readily mounted as part of the apparatus and liquid agitating means stirs and circulates the liquid to ensure that it is uniform and homogeneous throughout, thereby ensuring that the plural number of pipettes withdraw precise microquantities of liquid identical in characteristics and dispense the same into the associated microtitration plate wells.

Still another object resides in the provision of a novel, more compact, and self-contained laboratory dispensing apparatus in which the electrical and pneumatic control components for automatically controlling a dispensing operation are mounted within the base section of the apparatus as an integral part of the system thereby producing a readily portable apparatus which is easier to handle and conserves laboratory space.

A further object resides in the provision of a novel pickup and dispensing assembly including a pipette head unit having a plurality of needles or pipettes mounted therein, with the pipette head unit being readily removable from the dispensing assembly to permit quick simultaneous replacement of all the pipettes, or simultaneous cleaning thereof in a device such as an autoclave.

A still further object of the invention resides in the provision of a novel, vacuum-operated pneumatic control system for actuating the dispensing assembly to simultaneously control the pickup and dispensing of liquid from the plurality of pipettes and for accurately and precisely determining the exact microquantities of fluid which are to be picked up and dispensed.

Still other objects and advantages of the invention will become more apparent from reading the following detailed description of the invention as it proceeds with reference to the accompanying drawings in which like numerals indicate like elements. However, it is understood that the described embodiment is for illustrative purposes only, and that the scope of the invention is to be determined and limited solely by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general perspective view of the novel laboratory dispensing apparatus of the invention;

FIG. 2 is a fragmentary, partially sectioned front elevation view of the laboratory dispensing apparatus shown in FIG. 1 and particularly illustrating the base construction of the apparatus;

FIG. 3 is a fragmentary, partially sectioned view taken generally along line 3--3 of FIG. 2 and illustrating particularly the manner in which several of the electrical and pneumatic control components and indicators and the liquid stirring device are mounted within the base structure;

FIG. 4 is a fragmentary, partially sectioned plan view taken generally along line 4--4 of FIG. 2 with the upper bed plate removed and illustrating the general positions in which the various electrical and pneumatic control components are mounted on the lower base plate of the laboratory dispensing apparatus;

FIG. 5 is a top plan view of the liquid reservoir base and bottle receiver forming a part of the invention;

FIG. 6 is a sectional elevation view taken generally along line 6--6 of FIG. 5;

FIG. 7 is an enlarged fragmentary, partially sectioned view of the novel pickup and dispensing assembly which forms part of the invention;

FIG. 8 is a fragmentary, partially sectioned side elevation view of the dispenser unit shown in FIG. 7;

FIG. 9 is an enlarged fragmentary, partially sectioned view of an individual pipette illustrating the manner in which it is mounted within the dispensing assembly; and

FIG. 10 is a schematic diagram of the electrical and pneumatic control systems by which the laboratory dispensing apparatus is automatically operated.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, the novel automatic pickup and dispensing apparatus 20 comprises a base assembly 22 having an upper, Bakelite mounting bed-plate 24 covered by a thin decorative brass plate 25, and a pickup and dispensing pipette assembly 26 which includes a plurality of pipettes 27. The assembly 26 is mounted above bed-plate 24 via vertical post 28, control gear housing 30, gear rack 32 mounted within housing 30 and driven by a suitable pinion within housing 30 by handle 34, and L-bracket 36 fixed to the lower end of rack 32 and the rear end of dispensing assembly 26 via screws 38. The pickup and dispensing assembly 26 is moved vertically relative to the mounting bed-plate 24 by rotating handle 34 to drive the rack 32 up and down.

A sample carrier plate 40 is slidably mounted within longitudinal recesses provided in guide rails 42 and 44 which are spaced above the bed-plate 24 by spacers 46 secured to the bed-plate. Carrier plate 40 has a readily accessible knob 48 by which an operator may slide the plate from its forward retracted position shown in FIG. 1 to a rearward operational position in which the plate underlies the dispensing assembly 26.

Carrier plate 40 may support a plurality of separate vials or tubes or may preferably support a transparent microtitration plate 50 having a plurality of wells 52 corresponding to the number of pipettes 27 in the dispensing assembly 26. The microtitration plate 50 may be located on carrier 40 via a titration plate locator and guide assembly (not shown) similar to that disclosed in application Ser. No. 740,289. The use of such a locator assembly enables an operator to readily replace one titration plate for another and ensures that the wells 52 of each plate 50 will be accurately aligned with their respective corresponding pipettes 27 during a dispensing operation.

Mounted across the rear portion of mounting bed 24 is a clear acrylic, liquid reservoir plate 54 (FIGS. 5 and 6) which is positioned on bed-plate 24 by an end recess 56 that engages a guide pin 58 extending upwardly from bed-plate 24 and by a locking pin 60 which extends downwardly through rail 44 into a blind hole 62 provided in the top surface of reservoir plate 54. Reservoir plate 54 has a cylindrical recess or well 64 communicating with a liquid supply trough 66 located underneath the dispenser assembly 26 via a pair of channels 68 and 70 arranged substantially tangentially to the outer periphery of cylindrical recess 64 to provide for recirculation of the liquid between recess 64 and trough 66 by an agitating device described below. Another passage 72 extends diagonally between the supply passage 68 and trough 66 to ensure that the liquid at the center of trough 66 remains in an agitated state. A liquid reservoir tank or bottle 74 is mounted in an inverted state above cylindrical recess 64 via bottle receiver 76 having a central opening 77, the receiver 76 being fixed to the upper surface of reservoir plate 54 by screws 78.

A permanent magnet bar stirrer 80 is supported on the bottom of recess 64 via a non-friction bearing stud 82 and is rotated by the magnetic drive assembly 90 (FIGS. 2-4) to circulate the liquid from recess 64 through channel 68 into trough 66 and then back through channel 70 to recess 64. A portion of the liquid from channel 68 passes through channel 72 toward the center of trough 66 to ensure that all the liquid in trough 66 remains agitated and prevents any occurrence of dead spots within the trough.

Referring now particularly to FIGS. 2-4, the base assembly 22 also includes a lower base plate 84 supported from a table or other suitable support by mounting pads 86, with the mounting bed-plate 24 being spaced above base plate 84 and supported thereon by vertical posts 88. The various electrical and pneumatic control components, which will be described later with respect to a typical dispensing operation, and the agitator magnetic drive assembly 90 are supported on base plate 84 within the confines of an annular shroud or cover member 92 connected at its upper end to the peripheral edge of mounting bed 24.

Mounting bed 24 has an elongated slotted opening 93 estending along one side thereof. The agitator drive assembly 90 includes a motor 94 mounted on base plate 84 for rotating a suitable holder for a permanent bar magnet 96 which is positioned within slot 92 under the brass plate 25 directly beneath the cylindrical recess 64 of reservoir plate 54. As the drive magnet 96 is rotated, stirrer magnet 80 in recess 64 will follow magnet 96 and will thereby circulate the fluid between recess 64 and through 66 to ensure that the liquid, for example a saline solution, will be of uniform and homogeneous composition at all times.

Referring now particularly to FIGS. 1 and 7-9, the dispensing assembly 26 includes an upper manifold plate 100, a piston guide plate 102 secured to manifold 100 via screws 104, and a removable dropper or pipette head unit 106 which includes a dropper plate 108 and a dropper spring retainer plate 110 secured to plate 108 via screws 112. The dropper head unit 106 which supports all the pipettes 27 as a removable, self-contained unit is removably clamped to manifold 100 and guide plate 102 by a pair of side clamps 114 and 116, each of which has a horizontal lip 118 engaging the bottom side edges of retainer plate 110. As shown best in FIGS. 1 and 7, a pair of adjusting jack screws 120 is connected to each clamp 114 and 116. Each jack screw has an upper, large-diameter cylindrical portion 122 which is slidable within an opening 124 in manifold 100 and a reduced diameter portion 126 which extends through a complimentary hold in guide plate 102, with the lower end of section 126 being threaded through a mating threaded opening in clamp 114 or 116. A compressed spring 128 surrounds screw section 126 of each of the adjusting screws in position between the opposed faces of guide plate 102 and clamps 114 and 116. A locking cam 130 is pivotally supported between the upper ends of each cooperating pair of jack screws 120 on a pin 132 which extends between the screws and is retained thereon by snap rings 134. A washer 136 surrounds each of the screws 120 in resting engagement with the top surface of manifold 100, and each of the locking cams 130 includes a locking cam surface 138 which, when engaged with washer 136 as shown in FIG. 7, fastens the dropper unit 106 in operative relation to manifold 100 and guide plate 102. When the locking cams 130 are pivoted to an upright position, jack screws 120 will slide downwardly relative to manifold 100 and plate 102 and thereby cause the clamps 114 and 116 and dropper unit 106 to be lowered to a release position (shown in phantom lines in FIG. 7) in which the entire unit 106 may be removed simply by sliding the unit forwardly over the retaining lips 118. The dropper unit 106 is properly positioned in place during operation of the unit by abutment of the rear ends of plates 108 and 110 against the bottom edge of the support bracket 36 as shown best in FIG. 8.

The dispenser assembly is operated by a vacuum source and, to accomplish this, the manifold 100 includes a horizontal passageway 140 extending through one side face of the manifold and a vertical passageway 142 intersecting and depending downwardly from passage 140 through the bottom face of manifold 100. The outer end of passage 140 is connected to a vacuum source by way of conduit 144 and a suitable threaded coupling 146.

Piston plate 102 has an uppermost rectangular annular recess 148 which receives a sealing gasket 150 providing a seal between the opposed faces of manifold 100 and plate 102. Plate 102 also has an enlarged central rectangular recess 152 communicating with passage 142, with recess 152 being intersected by a plurality of bores 154 which open through the bottom face of plate 102. In the embodiment illustrated, there are 96 bores 154 corresponding in number to the 96 pipettes 27.

A piston 156 is slidably received in each of bores 154 and is biased downwardly by a spring 158. A thin flexible diaphragm 160 is positioned between the opposing faces of the piston guide plate 102 and the dropper plate 108 when the dropper assembly 106 is positively fastened to plate 102. In such a position, diaphragm 160 closes the bottom ends of bores 154 and thereby separates bores 154 from aligned openings 162 provided through the upper face of dropper plate 108 which also has an enlarged rectangular bottom recess 164. The dropper spring retainer plate 110 closes the bottom of recess 164 and itself includes a plurality of openings 166 which are aligned with openings 162 and bores 154.

Each of the actuating pistons and the corresponding pipettes is identical to that illustrated in FIG. 9 and therefore only one piston and its actuating effect on its associated pipette 27 will be described. The pistons 156 include an upper annular flange section 168 adapted to seat on the bottom of recess 152 around bore 154. A reduced diameter piston section 170 is loosely received within bore 154 to provide a clearance therebetween and spring 158 is received within an axial blind bore 172 to normally bias the piston 156 downwardly so that its bottom end firmly engages the upper side of the diaphragm 160.

Each pipette 27 includes a large tubular section 174 having at its upper end an enlarged diameter section 176 which is slidable but closely fitted within opening 162 of dropper plate 108. The upper end of section 176 is formed with an annular flange 178 which defines an upper recess 180 communicating with the central passageway 182 of tubular section 174. The lower end of section 176 is formed with a shoulder 184, and a compressed spring 186 acts between shoulder 184 and the upper face of the spring retainer plate 112 to bias the dropper 27 upwardly so that the annular flange 178 sealingly engages the underside of diaphragm 160. The lower end of tubular section 174 extends outwardly through opening 166 of plate 110 and has a reduced diameter tubular stem 188 press-fitted within its bottom end in flow communication with the passage 182.

In operation, when a vacuum is applied through passages 140 and 142 to recess 152, piston 168 will be moved upwardly within bore 154 and the vacuum will be applied to the top of diaphragm 160, which then will be retracted within bore 154. As a result, a suction effect will be created within recess 180 and passage 182 of pipette 27 and, if the bottom end of the pipette stem 188 is immersed in the liquid in trough 66, a predetermined quantity of the liquid will be sucked into the pipette.

The actuating pressure acting on diaphragm 160, in addition to being set by the pneumatic control components described hereinbelow, is determined by properly adjusting jack screws 120 to set the clamping force applied by claims 114 and 116 to hold the pipette head unit 106 against diaphragm 160 and plate 102. For example, jack screws 120 may be adjusted to provide approximately 7 to 10 pounds actuating pressure with the head unit in place.

It should be especially noted that each of the pipettes 27 is permanently mounted within the dropper unit 106 so long as the plates 108 and 110 are maintained in assembled relationship. When unit 106 is to be removed from manifold 100 and guide plate 102 by releasing the locking cams 130 and clamps 114 and 116, the annular shoulder 184 of each of the pipettes will engage the wall surfaces of dropper plate 108 surrounding the openings 162 and thereby maintain the pipettes in assembled relationship between plates 108 and 110. Consequently, the entire dropper unit 106 may be removed and placed in a device such as an autoclave for simultaneously cleaning all the pipettes together without having to disturb the individual mounting or assemblage of each individual pipette.

A typical pickup and dispensing operation will now be described with special reference to FIGS. 1 to 4 and 10, with FIG. 4 generally illustrating the position of the various electrical and pneumatic control components as they are mounted on the base plate 84 enclosed within shroud 92, and FIG. 10 schematically illustrating the electrical and pneumatic connections to the various components. Flexible conduit 144 extends from manifold 100 downwardly through suitable openings in the decorative coverplate 25 and mounting bed 24 within shroud 92 and is connected to a solenoid operated valve 188. Valve 188 is connected to a suitable vacuum source via vacuum gauge 190 and conduit 192 which has a coupling end 194 extending outwardly through a slot 196 provided in the rear wall of annular shroud 92. The head of vacuum gauge 190 is positioned so as to lie within the elongated slot 93 of mounting bed 24 and decorative plate 25 has a suitable opening and protective transparent cover through which the gauge 190 may be read by an operator as illustrated in FIG. 1. It should be noted that the valve 188, gauge 190, and connecting conduits are all supported on base plate 84 within the annular shroud 92.

The electrical operating circuit for the dispensing apparatus includes an on-off switch 196 and indicator light 198 mounted on the top of decorative plate 25 and a foot operated switch 200 conveniently located for use by an operator. The main electrical system of the apparatus includes a terminal board 202 having contacts C1, C2, C3, C4, C5 and C6, with terminal board 202 being mounted on the top of base plate 84. A three-wire electrical conductor 204 has its ground lead connected to contacts C1 and its other leads 208 and 210 connected to contacts C2 and C3, respectively. The electrical system also includes a transformer 212 for reducing the supply voltage of either 110 volts or 220 volts to a predetermined control voltage used to operate the agitating motor 94 and the other components of the control system. The transformer 212 has an output terminal board 214 having contacts A, B and C, with contact B being a ground terminal connected to ground contact C1 of terminal board 202 by a conductor 216 and to a machine ground 218 by a conductor 220.

To use the apparatus, an operator will initially close switch 196 to pass current from contact C2 through lines 222 and 224, fuse 226, line 228, the appropriate contacts of a slide switch 230 to the primary side T1 of transformer 212. Slide switch 230 is of a conventional type by which either 110 volts or 220 volts can be supplied to the primary side T1 of transformer 212. Current will then be supplied from the secondary side T2 of transformer 212 to the output terminals A, B and C of terminal board 214. Current then flows from terminal C of board 214 to the agitating drive motor 94 via conductor 232, contacts C4 of board 202, conductor 236 through motor 94 to conductor 238, resistor 240, potentiometer 242, and conductor 244 back to contact C5 of board 202. The speed of motor 94 may be adjusted by proper adjustment of potentiometer 242 to vary the speed of rotation of drive magnet 96 and follower magnet 80 within recess 64. In this way, the rate of circulation of the liquid in reservoir 54 may be properly adjusted.

Following closure of switch 196, the indicator lamp 198 will be lighted as current flows from contact C4 through the lamp and back to contact C5 of board 202 and contact A of board 214 via conductor 246. Thus, the operator is informed of proper operation of the system by the lighting of lamp 198.

With the agitating drive system 90 continually operating and the liquid being circulated between recess 64 and trough 66 to ensure uniformity of the characteristics of the liquid, the operator may then effect a dispensing operation as follows. With the carrier 40 and microtitration plate 50 in its inoperative retracted position shown in FIG. 1, the dispensing assembly 26 may be lowered by rotation of handle 34 until the open ends of pipettes 27 are immersed in the liquid in trough 66. The operator then depresses the foot operated switch 200 which causes current to flow from contact C4 through the closed switch 200 to contact C6, through conductor 248 to the operating solenoid 250 of valve 188, and back to contact C5 via conductor 252. Energization of solenoid 250 will open valve 188 to thereby connect conduit 144 to the vacuum in conduit 192.

This vacuum will then be applied through the passages 140 and 142 of manifold 100 to the enlarged guide plate recess 152 which acts as a vacuum chamber. Pistons 156 will be raised within their respective bores 154 and the vacuum will then be applied to the top of diaphragm 160 which will be retracted within the bores 154. Consequently, a suction will be created within each of the pipettes 27 and a predetermined amount of liquid will be withdrawn into the tubular passageways of each pipette. While still holding the foot operated switch 200 depressed to continue application of the vacuum to manifold 100, the operator will then raise the dispensing assembly 26 out of trough 66 and thereafter slide carrier plate 40 along guide tracks 42 and 44 to its rearward operative position in which it overlies trough 66 and in which the wells 52 of microtitration plate 50 are accurately aligned with respective ones of pipettes 27. Dispensing assembly 26 is again lowered so that each of the pipettes 27 is positioned within an aligned well 52 of plate 50. The operator then releases foot-operated switch 200 to deenergize solenoid 250 and close valve 188 to disconnect conduit 144 from conduit 192. Valve 188 includes a conventional means (not shown) which, when the valve is in the closed position, will automatically vent conduit 144 to atmosphere. Thus, when valve 188 is closed, atmospheric pressure will again be applied to manifold 100 through conduit 144 to thereby cause pistons 156 and diaphragm 160 to be biased downwardly by springs 158. Downward movement of diaphragm 160 will cause the liquid in each of the pipettes 27 to be simultaneously expelled from the pipettes into wells 52 of microtitration plate 50.

The dispensing assembly 26 is then raised to its uppermost position and carrier 40 returned to its forward retracted position in which microtitration plate 50 may be readily removed and another plate placed on the carrier 40 for the next dispensing operation.

From the above description, it is apparent that the laboratory dispensing apparatus of the invention accomplishes the objects and provides the advantages initially set forth by providing a device which is highly reliable and accurate in simultaneously dispensing reproduceable microquantities of liquid; is compact and self-contained and therefore readily portable and manipulatable; is adapted for usage with various type liquids including those having several constituents; and is easily cleaned when necessary due to the fact that the dispenser head unit supporting the plural number of pipettes is quickly and easily removed from the apparatus for placement in a device such as an autoclave.

It is understood of course, that various other modifications and embodiments are encompassed within the scope of the invention which, of course, is not limited by the detailed description above. For example, when used with some liquids, it may be necessary to provide a suitable coating on the external surfaces of the pipettes to prevent any adherence of the liquid thereto which might adversely affect the accuracy of the microquantities of liquid being dispensed.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

What is claimed and desired to be secured by Letters Patent is:

1. Dispensing apparatus comprising a base section, liquid reservoir means on said base section for containing a liquid to be dispensed, agitator means associated with said reservoir means pick up the liquid therein, carrier means mounted on said base section for horizontal movement between a first non-dispensing position at one side of said liquid reservoir means and a second dispensing position in which said carrier means overlies a portion of said liquid reservoir means, liquid receptacle means supported on said carrier means, pick up and dispensing means mounted on said base section in operative alignment above said portion of said reservoir means, means for moving said pick up and dispensing means vertically relative to said base section, whereby, when said carrier means is in said first position, said pick up and dispensing means may be lowered to a first position to withdraw liquid from said portion of said reservoir means, then raised to permit said carrier means to be placed in said second position, and then lowered to a second position to dispense the withdrawn liquid into said receptacle means.

2. Dispensing apparatus as in claim 1, said liquid reservoir means comprising a trough positioned underneath said pick up and dispensing means, a liquid supply well remote from said trough and communicating therewith through connecting channel means, and said agitator means causing agitation of the liquid in said trough.

3. Dispensing apparatus as in claim 2, wherein said channel means comprises a pair of channels connecting said supply well with said trough, and said agitator means causes recirculation of the liquid between said supply well and said trough via said channels.

4. Dispensing apparatus as in claim 3, said agitator means comprising a permanent magnet rotatably mounted in said supply well for stirring said liquid, a drive assembly for said magnet including motor means mounted on said base section, a drive magnet rotated by said motor means, said drive magnet being operatively associated with said stirring magnet to rotate the same upon actuation of said motor means.

5. Dispensing apparatus as in claim 4, wherein said pair of channels extends substantially tangentially from the outer periphery of said supply well to the outer portions of said trough, and another channel intersects and extends from the inlet one of said pair of channels toward the central section of said trough to ensure that all of the liquid in said trough is agitated.

6. Dispensing apparatus as in claim 1, wherein said agitator means comprises a permanent magnet rotatably mounted in said liquid reservoir means for stirring said liquid, a drive assembly for said stirring magnet including motor means mounted on said base section, a drive magnet rotated by said motor means, said drive magnet being operatively associated with said stirring magnet to rotate the same upon actuation of said motor means.

7. Dispensing apparatus as in claim 1, wherein said base section includes a lower base plate and an upper mounting bed-plate supported above said base plate, said liquid reservoir means, said carrier means, and said pick up and dispensing means being mounted on said upper mounting bed-plate, said mounting bed-plate having an elongated slot at least part of which extends underneath said reservoir means, said agitator means comprising a permanent magnet rotatably mounted in said reservoir means for stirring the liquid therein, motor means mounted on said lower base plate, a drive magnet rotated by said motor means and positioned within said slot of said reservoir means, said drive magnet being operatively associated with said stirrer magnet to rotate the same upon actuation of said motor means.

8. Dispensing apparatus as in claim 7, comprising an electrical control system for said agitator motor means, a pneumatic control system for operating said pick up and dispensing means, and at least some of the components of said electrical and pneumatic control systems being mounted on said base plate, thereby providing a compact, self-contained dispensing apparatus.

9. Dispensing apparatus as in claim 1, said pick up and dispensing means comprising manifold means, a pipette head unit including a plurality of dispensing pipettes normally retained in assembled relationship in said head unit, means for removably connecting said pipette head unit to said manifold means, flexible diaphragm means operatively positioned between said manifold means and said pipette unit, said manifold means having air passage means leading to one side of said diaphragm means, and a pneumatic supply system connected to said air passage means for creating a differential pressure on opposite sides of said diaphragm means to simultaneously pick up predetermined quantities of liquid from said reservoir means into said pipettes and thereafter simultaneously dispense said quantities from said pipettes.

10. Dispensing apparatus as in claim 9, said manifold means comprising a manifold having an air passageway, an actuating guide plate connected to said manifold and having a plurality of apertures communicating at one end with said manifold air passageway and terminating at their other ends at said one side of said diaphragm means, piston means mounted in each of said apertures and normally biased into engagement with said one side of said diaphragm means, said pipette head unit being removably connected to said manifold and piston guide plate, with each pipette being operatively aligned on the other side of said diaphragm means with a corresponding piston.

11. Dispensing apparatus as in claim 10, said pipette head unit comprising a dropper plate having a plurality of openings which operatively align with said piston guide plate apertures, a retainer plate connected to said dropper plate and having openings aligning with said dropper plate openings, each of said pipettes having an enlarged diameter section at one end slidably received within one of said dropper plate openings and a smaller diameter section extending outwardly through the aligned retainer plate opening, and spring means biasing said enlarged diameter section into engagement with the other side of said diaphragm means in operative alignment with one of said pistons.

12. Dispensing apparatus as in claim 10, said means for removably connecting said pipette head unit to said manifold means comprising clamp means for supporting said head unit, shaft means slidably extending through said manifold and piston guide plate and fixed at one end to said clamp means, locking means connected to the other end of said shaft means and movable between locked and unlocked positions, whereby when said locking means is in said locked position said head unit is operatively clamped to said manifold and said piston guide plate, and when said locking means is in said unlocked position said head unit is unclamped and may be readily removed from said pick up and dispensing means.

13. Dispensing apparatus as in claim 12, wherein said shaft means is adjustable screw means adjustably connected to said clamp means so that the effective actuating pressure acting on said diaphragm means may be varied by adjustment of said screw means.

14. Dispensing apparatus as in claim 9, wherein said pneumatic supply system includes a vacuum source.

15. Dispensing apparatus comprising a base section including a lower base plate and an upper mounting bed-plate supported above said base plate; liquid reservoir means on said mounting bed-plate for containing a liquid to be dispensed, said reservoir means including a trough and a liquid supply well remote from said trough and connected therewith through connecting channel means; liquid agitator means comprising a permanent stirring magnet rotatably mounted in said reservoir means, motor means mounted on said lower base plate, a drive magnet rotated by said motor means and operatively associated with said stirring magnet to rotate the same upon actuation of said motor means; carrier means mounted on said mounting bed-plate for movement between a first non-dispensing position at one side of said trough and a second dispensing position in which said carrier means overlies said trough; liquid receptacle means supported on said carrier means; pick up and dispensing means mounted for vertical movement on said bed-plate in operative alignment above said trough, said pick up and dispensing means comprising manifold means, a pipette head unit including a plurality of dispensing pipettes normally retained in assembled relationship in said head unit, means for removably connecting said pipette head unit to said manifold means, flexible diaphragm means operatively positioned between said manifold means and pipette head unit, said manifold means having air passage means leading to one side of said diaphragm means; a pneumatic control system connected to said air passage means for creating a differential pressure on opposite sides of said diaphragm means to permit simultaneous pick up of predetermined quantities of liquid from said trough when said carrier means is in said first non-dispensing position and then dispensing of said predetermined quantities of liquid from said pipettes into said liquid receptacle means when said carrier means is in said second dispensing position; an electrical control system for said agitator motor means; and at least some of the components of said pneumatic control system and said electrical control system being mounted on said lower base plate to provide a compact, self-contained dispensing apparatus.

16. Dispensing apparatus as in claim 15, said pneumatic control system comprising an air supply source, a solenoid-operated air valve mounted on said lower base plate and connected between said supply source and said air passage means in said manifold means, manually operated electrical switch means connected to said air solenoid valve means for operating said valve means, said switch means being accessible to an operator, and a pressure indicator means mounted on said base plate and extending through said mounting bed-plate so as to be readily visible by an operator.

17. Dispensing apparatus as in claim 16, said electrical control system comprising an electrical supply source, a transformer mounted on said base plate, second switch means connecting the primary side of said transformer to said electrical supply source, said second switch means being mounted above said bed-plate for ready access by an operator, first circuit means connecting the secondary side of said transformer to said agitator motor means and including a variable resistance mounted on said lower base plate for varying the speed of said motor means, and second circuit means including an indicator lamp mounted above said bed plate and readily visible to an operator to indicate operation of the apparatus when said second switch means has been placed in an on position.

18. Dispensing apparatus as in claim 16, wherein said air supply source is a vacuum source and said indicator means is a vacuum gauge.