Fractional-fill Pipette Assembly

Abstract

A fractional-fill pipette assembly adapted to fill to a predetermined mark to obtain a desired known volume of liquid such as blood, plasma, test reagents or the like is disclosed. A means formed of a hydrophobic material is positioned within the tube bore, a predetermined distance from one end thereof so as to divide the capillary tube into two segments, one of the segments having a precise known volumetric capacity for collecting a desired known volume of liquid so that when the liquid fills the segment containing the precise known volume the liquid contacts the hydrophobic means defining the interface between the segments, the hydrophobic means breaks or resists the forces acting on the liquid filling the bore and prevents liquid from filling the pipette beyond the hydrophobic means by maintaining the surface tension of the liquid at the interface between the fluid and the hydrophobic means.

Description

BACKGROUND OF THE INVENTION

Pipette assemblies for delivering precise known volumes of fluid such as blood are in use in laboratories and hospitals. Where blood samples are taken for analytical purposes such as blood cell counting, a Trenner pipette is employed. Also, there are micropipettes which are made in the form of a syringe which will fill to a mark by retracting the piston from the barrel portion of the syringe. The barrel is in the form of a capillary so that as the piston is retracted, a volume of liquid will fill the displaced portion of the barrel.

Other micropipette assemblies are employed in which a capillary of known length and volume is filled throughout its entire length by capillary action such as is described in U.S. Pat. No. 3,045,494. However, it is difficult to handle and measure accurately precise volumes of fluid employing presently available micropipettes since they require dexterity by the technician using them and, in many cases, an error in operation of a micropipette leads to erroneous test results.

SUMMARY OF THE INVENTION

With the foregoing in mind it is an object of the present invention to provide a fractional fill pipette assembly having a means formed of a hydrophobic material which is disposed within the tube bore a predetermined distance from one end thereof in which the distance between the end of the capillary and the hydrophobic means defines a zone of known volumetric capacity. When the liquid filling the tube bore reaches the hydrophobic means the forces acting on the liquid are equilibrated thereby forming an interface between a wettable zone and a non-wettable zone.

It is a further object of the invention to provide a partial fill capillary assembly which will automatically fill to a predetermined point or mark so that a precise known volume of fluid will be measured and the precise volume of fluid will be available for dilution in a premeasured volume of diluent to obtain a precise known ratio of solution so that a test can accurately be performed.

My invention generally contemplates providing a partial fill pipette assembly which includes a tube with a bore extending therethrough and a hydrophobic means positioned in the tube bore a predetermined distance between the ends thereof to define a zone from one end of the tube to the hydrophobic means having a precise known volumetric capacity so that when a column of fluid fills the tube bore to the hydrophobic means a precise volume of liquid can be quantitated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side plan view of the fractional-fill pipette assembly of the invention herein.

FIG. 2a is a side plan view of the hydrophobic means greatly enlarged.

FIG. 2b is a side plan view greatly enlarged of another form of the hydrophobic means of FIG. 2a.

FIG. 3 is a sectional view of FIG. 1 with portions of the pipette assembly broken away.

FIG. 4 is an exploded view of the pipette assembly in which apparatus means is illustrated for positioning the hydrophobic means within the tube bore.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fractional-fill pipette assembly is illustrated in FIG. 1 and is best illustrated in FIG. 3 in which the fractional-fill pipette assembly 10 is shown fully assembled in sectional view. The pipette or capillary assembly 10 includes a hydrophobic means 12 positioned in the capillary bore 13 between ends 14 and 16 of capillary tube 11. Holder 20 is mounted on the capillary tube in frictional engagement therewith and between ends 14 and 16 and is employed to facilitate holding the pipette during use such as for filling and subsequently dispensing the predetermined volume of fluid contained in zone or segment 17 defined between end 14 and hydrophobic means 12 of pipette 11.

Hydrophobic means 12 is shown in FIG. 2a and is illustrated mounted in position in FIG. 3 in detail. Hydrophobic means 12 is preferably made of a hydrophobic material such as polyethylene or other type polyolefins or any other plastic material which has hydrophobic characteristics or may even be a wettable material which is coated with a hydrophobic coating such as paraffin wax, beeswax or various plastic made into dilute lacquers which will render the surfaces of the hydrophobic means non-wettable. Hydrophobic means 12 may be formed having cylindrical mid-section or zone 30 of a diameter such that it will frictionally engage the surfaces defining bore 13 and is mounted in fixed position in capillary 11 by an interference fit. Extending from cylindrical zone 30 is a pair of truncated conical members 32 with ends 34 and 35 formed having a diameter less than zone 30 of hydrophobic means 12. A bore 37 is formed through hydrophobic means 12 to provide an air passage through capillary 11.

As noted above, the outer diameter of hydrophobic means 12 is slightly larger than the internal bore diameter of capillary tube 11 such that when hydrophobic means 12 is inserted into capillary bore 13, hydrophobic means 12 will frictionally engage the surfaces of bore 13 and when positioned will be a fixed predetermined distance from end 14 and will not move unless a positive force is applied thereto. The mechanism for properly positioning hydrophobic means 12 within capillary bore 13 is illustrated in FIG. 4 in exploded orientation which illustrates the sequence of steps for assembling the fractional-fill micropipette of FIG. 1.

Holder 20 is preferably formed of a plastic material such as polyethylene and comprises a body portion 21 having a bore or passage 22 formed therethrough and is of a diameter such that it will slidably receive capillary 11 and when mounted in position, as seen in FIG. 3, will be held in fixed position by frictional engagement with exterior surfaces 15 of capillary 11 and the interior surfaces of bore 22 of holder 20. Holder 20, as shown in FIGS. 1 and 3, is positioned on capillary 11 a greater distance from end 14 than hydrophobic means 12 for facilitate the operator's vision of the entire zone which contains the accurately filled fluid in capillary 11 between end 14 and hydrophobic means 12. Also holder 20 may be positioned between end 14 of capillary 11 and hydrophobic means 12 which will also permit visual inspection of the fluid column contained in capillary 11 between end 14 and hydrophobic means 12. The positioning of holder 20 on capillary 11 will generally depend upon the size of a resilient reservoir which holds a desired volume of diluent disclosed and illustrated in copending application of Waldemar Ayres filed even date herewith entitled "Fractional-Fill Capillary Pipette and Method."

Holder 20 as indicated is preferably formed of a suitable plastic material and is molded having tubular body portion 21. Integrally formed therewith and extending from body portion or sleeve 21 is a finger engaging member or tab 24. Tab 24 presents a suitable surface for a label for receiving indicia such as is used to identify the sample to be tested or to specify the fractional fill capacity. Sleeve 21 at its forward end terminates in a reduced flange 26 which is adapted to couple to a reservoir (not shown) having a predetermined measured volume of diluent such as is disclosed in the aforesaid copending patent application.

In FIG. 2b another form of hydrophobic means 12 is illustrated and is generally referred to as 12'. Hydrophobic means 12' is preferably made of a suitable plastic material which is made non-wettable. Hydrophobic means 12' is molded in the shape of a frustro conical cone having a passage 37' extending between the forward end 35' and rear end 30'. Forward end 35' is of reduced diameter and rear end 30' has a diameter slightly greater than the bore diameter of capillary 11 so that hydrophobic means 12' when positioned in bore 13 of capillary 11, an interference fit is formed to prevent accidental displacement thereof. Hydrophobic means 12 and 13' is mounted in capillary 11 by the assembly device 40 illustrated in FIG. 4.

The assembly device 40 comprises a rod 44 mounted on handle 42. Rod 44 has a diameter less than the diameter of bore 13 of capillary 11 to facilitate insertion of rod 44 therein. Assembly device 40 is made of a rigid material which may be plastic with high impact strength or may be formed of metal with rod 44 being mounted to handle 42 in any convenient manner such as by threading, or swaging. The forward end 46 of rod 44 terminates in a reduced flange 48 having a diameter substantially equal to but less than the passage formed in hydrophobic means 12 or 12'. Slidably mounted on rod 44 is an adjustable depth insertion guide 50 for inserting hydrophobic means 12 or 12' to the exact distance from end 14 in capillary bore 13. Insertion guide 50 comprises an annular ring 52 having a passage therethrough with a diameter slightly greater than the diameter of rod 44. Ring 52 is held at the desired position by any convenient means such as a lock adjusting set screw 54 which is threadedly engaged in a threaded opening in annular ring 52. When set screw 54 is threaded through annular ring 52 the forward end biases against rod 44 to hold insertion guide 50 in a fixed predetermined position. Thus, the insertion depth as depicted in FIG. 4 is the distance between end 46 and rod 44 and insertion guide 50. The flange portion 48 is inserted into passage 30 of hydrophobic means 12 and then the hydrophobic means is inserted to the proper depth. Holder 20, as noted, is slidably mounted on capillary 11 by simply passing either end 14 or 16 of capillary 11 into passage 22 of holder 20, at end 56 or 58.

Having described the illustrative embodiments of the invention, variations of the invention may be had by changes in design in materials without parting from the spirit and scope of the invention.

Claims

I claim:

1. A fractional-fill pipette assembly for the quantitation of a liquid comprising:

a. a capillary tube capable of being filled with fluid by capillarity when the one end of the tube is immersed in the fluid to be quantitated;

b. a hydrophobic means associated with the tube bore and positioned a predetermined distance from said one end of said tube to define a zone having a predetermined known volume;

c. said hydrophobic means having a passage formed therethrough so that said other end of said capillary is open to the atmosphere whereby when said fluid contacts said hydrophobic means the fluid ceases filling to provide a precise known volume of fluid in said zone.

2. The fractional-fill pipette assembly of claim 1 wherein said hydrophobic means is formed of plastic.

3. The fractional-fill pipette assembly of claim 1 wherein said hydrophobic means is coated with a hydrophobic material.

4. The fractional-fill pipette assembly of claim 1 wherein said hydrophobic means is a tubular member made of molded plastic, said member having a zone of increased diameter slightly greater than the bore diameter of the capillary so as to provide an interference fit in the capillary after being positioned a predetermined distance from said one end.

5. The fractional-fill pipette assembly of claim 1 wherein said capillary assembly includes a holder slidably mounted on said capillary and positioned between the ends thereof.

6. The fractional-fill pipette assembly of claim 5 wherein said holder comprises an integrally molded plastic body including a sleeve having a passage therethrough, said passage having a zone where the diameter is substantially equal to the outer diameter of the capillary tube so that when slidably mounted in position an interference fit is formed.

7. The fractional-fill pipette assembly of claim 1 wherein said hydrophobic means is a tubular member made of a molded plastic material having a central zone, the outside diameter of which is slightly greater than the bore diameter of the capillary tube and extending from said zone is at least one frustro conical cone integrally formed therewith.