Measuring and testing – Sampler – sample handling – etc. – Capture device
Reexamination Certificate
1999-12-08
2001-10-02
Raevis, Robert (Department: 2856)
Measuring and testing
Sampler, sample handling, etc.
Capture device
Reexamination Certificate
active
06295880
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to pipettes, and more particularly to an adjustable pipette having an improved stop arrangement for sample aspirating and dispensing strokes of different lengths.
DESCRIPTION OF THE PRIOR ART
Pipettes are used for transferring liquid samples having a precisely measured volume. A piston assembly is moved in an aspirating stroke, first down into a pipette body and then up to draw or aspirate a sample into a sample receiving region, such as a disposable tip. Then the piston assembly is again moved down in a dispensing stroke to dispense the sample from the sample receiving region. It is desirable that the piston travel farther in the downward direction during the dispensing stroke than during the intake stroke so that the resulting overtravel positively expels the entire sample.
U.S. Pat. No. 3,827,305 discloses a pipette typical in many respects of the prior art. A main spring biases the piston in the upward direction, and the user overcomes the force of this spring when moving the piston down. An overtravel spring biases a lower stop to a normal position where it limits downward travel of the piston during the intake, aspiration stroke. During the discharge or dispensing stroke, the user forces the piston to overtravel by overcoming the force of the overtravel spring as well as the main spring so that the lower stop is moved down from its normal position.
This known lower stop arrangement requires skill and strength. During the aspirating stroke the user must move the piston down all the way to the lower stop to assure that a full sample is drawn up. But the user must not move the piston beyond its initial contact with the lower stop, or else the aspirated sample is too large. A user therefore must detect by feel the proper downward limit of the aspiration stroke.
Typically the overtravel spring is stronger than the main spring. A stiff overtravel spring aids the user in detecting contact of the piston against the lower stop without moving the lower stop during the aspiration stroke. In pipettes where the overtravel spring serves an additional function of applying force to a lower seal, a stiff spring also increases the sealing effect. The use of a strong overtravel spring requires the user to apply a relatively large force during the dispensing stroke to overcome the overtravel spring force and move the lower stop.
U.S. Pat. Nos. 3,506,164; 3,766,784 and 4,435,989 disclose pipettes having indexing mechanisms with rotatable members responsive to piston movement for alternately limiting downward motion with spaced lower stops to achieve relatively shorter and longer aspiration and dispensing strokes respectively. The alternate action is automatic, with no user input other than repetitive, purely axial piston operation. Although these devices eliminate the need for the user to overcome a large spring force for overtravel movement, they are undesirably complicated, expensive and delicate. In addition, the automatic alternate selection by the internal mechanism of either an aspiration or a dispensing stroke can lead to inadvertent interchange of the strokes and resulting error.
U.S. Pat. No. 4,117,728 discloses a pipette with cooperable plunger and casing shoulder means for regulating relative movement of the pipette plunger and casing to permit the sequential and separate pickup of different liquids, the mixing of the liquids, and the dispensing of the mixed liquids. This pipette does not a provide a mechanism for achieving a dispensing stroke that is essentially longer than the aspirating stroke.
SUMMARY OF THE INVENTION
A principal object of the present invention is to provide an adjustable pipette having an improved lower stop arrangement for providing sample aspirating and dispensing strokes of different lengths. Other objects of the invention are to provide a pipette in which an increased dispensing stroke length is achieved without requiring great operator skill or strength; to provide a pipette in which the thumb button is shaped to facilitate rotation of the plunger assembly to alternate positions corresponding to different aspirating and discharging plunger strokes; to provide a pipette permitting the user to see the mechanism that provides either an aspirating or discharging stroke; to provide a pipette with positive, solid stops at the bottom of both the aspirating and dispensing strokes; to provide a pipette eliminating the necessity for the user to compress a heavy spring or to compress more than one spring while performing aspirating and dispensing operations; to provide a pipette wherein the need for a single repeated user thumb movement is avoided; and to provide an improved pipette overcoming disadvantages of known adjustable pipettes.
In brief, in accordance with the invention there is provided a pipette for aspirating and dispensing liquid samples. The pipette includes an elongated body having a longitudinal axis, the body having an upper handle portion and a lower portion. The body defines a cylinder communicating with the lower portion. A plunger assembly moves axially in the body and includes a piston movable axially in the cylinder away from the lower portion to aspirate a liquid sample and toward the lower portion to dispense a liquid sample. An upper stop engages the plunger assembly and defines an upper position. First and second lower stops engage the plunger assembly and define first and second discrete lower positions of the piston assembly The first and second lower stops are spaced apart axially and circumferentially relative to the axis. A return spring connected between the body and the plunger assembly biases the plunger assembly upward toward the upper position. The plunger assembly includes an axially extending operating shaft for moving the plunger assembly downward to one of the lower positions. The plunger assembly includes a radially extending and rotatably mounted lower stop engaging portion, the stop engaging portion being rotatable in a first direction for engagement with the first lower stop to define a relatively shorter aspiration stroke of the plunger assembly, and the stop engaging portion being rotatable in a second direction for engagement with the second lower stop to define a relatively longer dispensing stroke of the plunger assembly.
REFERENCES:
patent: 3492876 (1970-02-01), Bull et al.
patent: 3506164 (1970-04-01), Weichselbaum et al.
patent: 3766784 (1973-10-01), Walker
patent: 3827305 (1974-08-01), Gilson et al.
patent: 4117728 (1978-10-01), Johnson
patent: 4263257 (1981-04-01), Metsala
patent: 4435989 (1984-03-01), Meyer et al.
patent: 5104624 (1992-04-01), Labriola
Kolehmainen Philip M.
Mason, Kolehmainen Rathburn & Wyss
Raevis Robert
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