Liquid Transfer Pipetting Device

Abstract

A hand-held pipetter for extracting a predetermined volume of liquid from a body of liquid, including a movable piston within a piston chamber that is in fluid communication with a liquid receiving tip. A plunger connected at one end to the piston is provided with two positive calibrating stops which limit the travel of the piston within the piston chamber to withdraw and dispense predetermined amounts of liquid. The piston and plunger assembly are normally resiliently urged toward a rest position, this position being determined by an adjustable stop which thereby adjusts the liquid volume transfer capabilities of the pipetter.

Description

BACKGROUND OF THE INVENTION

This invention is related generally to hand-held liquid transfer devices and, more specifically, to such devices of a precision calibrated pipetting type.

The present hand-held liquid pipetting instruments are exemplified by U.S. Pat. No. 3,494,201 - Roach (1970), reissued as Pat. No. Re 27,637 on May 8, 1973, and a present device manufactured by Oxford Laboratories of San Mateo, Calif. under the registered trademark SAMPLER. Such a device includes a tube-like barrel outer structure having a plunger extending outward of one end thereof and a piston attached to the other end of the plunger and positioned within a piston chamber. This piston chamber communicates with an aperture at an end of the barrel handle which is shaped for frictionally engaging a disposable tip. The piston is held in a normal rest position by a pair of springs within the barrel handle.

As the piston is first depressed by pushing on the plunger from its rest position, a first spring that is relatively soft is compressed until a sudden increase in resistance is reached wherein the piston is in a calibrated position. When in this position, a tip attached to the pipetter is immersed in a liquid and the plunger is allowed to slowly return the piston to its normal rest position and thereby to draw liquid into the tip. The volume of liquid drawn into the tip is directly related to the volume of fluid displaced within the piston chamber. The pipetter is then positioned over another container and liquid is discharged from the tip by again depressing the plunger into the barrel handle which displaces fluid within the piston chamber and forces liquid out of the tip. The plunger, in addition to compressing the relatively soft spring, is further depressed beyond the point of a sudden increase in resistance to compress an additional relatively hard spring. This causes the piston to travel an "overshoot" distance beyond its calibrated position and thereby to displace a greater amount of fluid within the piston chamber upon liquid discharge from the pipetting device than it displaced prior to drawing liquid into the tip. The result is that any liquid adhering around the opening of the disposable tip is pushed outward into the second container.

Also, single spring hand-held pipetter instruments are availble on the market which do not provide an overshoot feature for full discharge of liquid. The result is that these instruments are subject to errors in liquid volumes transferred, especially with liquids of high viscosity such as serums.

It is a primary object of the present invention to provide a hand-held pipetter with a simplified construction using a single spring but still providing the desirable piston overshoot which may be used to discharge liquid from a tip.

It is another object of the present invention to provide a pipetter construction that permits simple calibration of the liquid volumes transferred thereby so that each pipetter may be individually calibrated.

It is yet another object of the present invention to provide a new method of using liquid transfer pipetters.

SUMMARY OF THE INVENTION

These and additional objects are accomplished by the various aspects of the present invention wherein a pipetter is provided with a single return spring of a piston/plunger assembly within a barrel handle assembly. Two positive stops are provided on either the barrel assembly or the plunger to limit travel of the piston from its resiliently held rest position to either a normal calibrated position or an overshoot position, depending upon which of the two positive stops is utilized by the operator. In a preferred form, the positive stops are provided on the plunger for engaging an end surface of the barrel handle. In normal use, the plunger is depressed until its first stop engages the end of the barrel handle wherein the piston is in its calibrated position. Liquid is drawn out of a container by the piston slowly traveling back to its rest position under the urging of the single spring. For discharge of liquid therefrom according to the usual method of using such devices, the plunger is depressed to the first stop, moved laterally with respect to the handle to release the first stop and the plunger is then depressed an additional amount to cause the piston to overshoot its calibrated position until the second stop on the plunger strikes the barrel end surface. This structure is a simple and economical mechanical means for providing an overshoot fluid displacement feature without the necessity of having two springs. There is an additional advantage for large volume pipetters since the larger the pipetter, the greater the force required to be exerted by the second overshoot spring. This force has been found to be so great in large devices of the 1-5 milliliter liquid transfer range that some individuals cannot easily depress the piston to its overshoot position.

The return of the piston to its normal rest position by the single spring of the device according to the present invention is limited by an adjustable stop between the plunger and the barrel handle. This adjustable stop may either be positioned along the plunger or along the length of the barrel handle. In the latter case, the liquid volume range of an instrument is displayed by markings along one side of the plunger so that the volume capability of each pipetter may be accurately set by the adjustable stop. When volume markings are not used, the liquid volume capacity may be simply set on each pipetter individually by temperarily positioning a calibrating block between the first plunger stop and the end of the barrel handle against which the plunger is depressed while the adjustable stop is fastened in position. In either case, volume size of pipetters of the present design are less dependent on tolerances of their internal parts since each pipetter can be individually adjusted as to fluid volume displaced by one stroke of a piston.

According to another aspect of the present invention, a pipetter of the present invention and others may be utilized in a reverse transfer method wherein the piston is initially set to its overshoot position before liquid is drawn thereby from a container. More liquid is thereby removed from the container than is desired to be transferred to another container. When the discharge of the liquid is effected, the piston is moved to its normal calibrated position to discharge a precise amount of liquid. An excess amount of liquid remains in the liquid tip as a result of initially drawing in more liquid volume than is desired. By dispensing the exact desired amount of liquid from a slightly greater liquid volume than that desired, thick liquids (high viscosity) such as serum that tend to adhere to the surface of the disposable tips may be more accurately dispensed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows generally the use of a hand-held pipetting instrument according to the present invention;

FIG. 2 is a cross-sectional view of an improved pipetter construction according to the present invention;

FIG. 3 shows the pipetter of FIG. 2 with its plunger and piston in a different position;

FIG. 4 is a cross-sectional view of the pipetter of FIGS. 2 and 3 taken across section 4--4 of FIG. 2;

FIG. 5 is an outside view of the upper portion of the pipetter shown in FIGS. 2 and 3; and

FIG. 6 shows a cross-sectional view of another pipetter embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, one method of using a pipetting instrument 11 of the present invention is illustrated. A plastic disposable tip 13 has initially been positioned onto one end of the pipetter 11 by frictional engagement with a section of stainless steel small bore tubing 15. A knob 17 on the free end of a plunger 19 is then depressed by the thumb of an operator until a first stop 21 engages an end surface 23 of the pipetter device wherein a piston (not shown in FIG. 1) attached to the opposite end of the plunger 19 is in a predetermined calibrated position. The tip 13 is then partially immersed below the surface of liquid within a vessel 25 and the plunger 19 is allowed to return slowly to its normal rest position upon the urging of a resilient means within the pipetter 11. A volume of liquid is thereby drawn into the tip 13 that is related to a displacement of fluid by piston movement with the pipetter 11. The rest position of the piston is determined by a volume adjustment mechanism 27 which thereby determines the volume of liquid drawn into the tip 13 during one operation of the pipetter.

The pipetter and tip assembly with liquid therein is then transferred to a second container 29 for discharge of liquid from the tip 13. The knob 17 and the plunger 19 are again depressed by the thumb of an operator until the calibrated stop 21 hits the end 23 of the pipetter. To remove any liquid that may adhere to the opening of the disposable tip 13, the piston within the pipetter 11 is displaced an additional amount (overshoot) upon liquid discharge by moving the plunger laterally with respect to the pipetter 11 in a direction away from a finger hold 31 to release the positive stop 21. This permits travel of the plunger 19 until a second calibrated stop 33 strikes the surface 23 at the end of the pipetter 11.

The arrows at the top of FIG. 1 show the different stages of travel of the knob 17 as liquid is discharged from the pipetter tip 13. The knob is initially depressed an amount represented by the arrow 35 until the stop 21 positively engages the end surface 23. In this position, the piston within the pipetter 11 has been returned to the same position that it occupied prior to withdrawing liquid from the vessel 25. To drive the piston to an overshoot position, the knob 17 is moved laterally as represented by the arrow 37 which releases the first calibrated latch 21 and permits it to be moved inward of the pipetter 11. The knob 17 is thereafter depressed an additional distance represented by the arrow 39 which is the amount of overshoot given to the piston within the pipetter 11.

Referring to FIGS. 2-5, the structure of the pipetter 11 which permits such a liquid transfer operation is shown in detail. A generally cylindrical elongated barrel handle member provides an outer shell of the pipetter and includes an upper portion 41 and a lower portion 43 threadedly attached to one another in a manner that access may be had when desired to the interior of the pipetter for cleaning or replacement of parts. A glass cylindrical sleeve 45 is positioned within the member 43 to form a piston chamber 47. The glass sleeve 45 assures a precision in fluid displacement within the piston chamber 47 that is required for accurate liquid volume transfer. A stainless steel piston 49 having a sealing O-ring 51 thereabout is rigidly attached to the lower end of the plunger 19 for movement therewith.

A barrel piece dividing member 53 through which the plunger 19 is permitted to pass holds one end of a stainless steel spring 55 in compression. An opposite end of the spring 55 is held by a collar 57 which is rigidly attached in some convenient manner to the plunger 19. The spring 55 tends to drive the plunger and thus the piston 49 upward to their normal rest positions. This normal rest position is determined by the setting of the volume adjustment assembly 27.

The volume adjustment assembly 27 includes a stop 59 on the interior of the barrel handle member 41. The collar 57 that is attached to the plunger 19 is limited in its travel by the stop 59 as shown in FIG. 2. The stop 59 is held in position by an adjustment screw 61 that is threadedly engaged therewith through a cover plate 63. It will be noted best from FIG. 5 that the entire volume adjustment assembly 27 slides up and down within a slot 65 provided along a portion of the length of the barrel handle member 41. The cover plate 63 is wider than the slot 65 so that a tightening of the adjustment screw 61 by turning it relative to the stop 59 compresses a portion of the barrel wall structure 41 and holds the stop 59 firmly in place. The cover plate 63 is made in an elongated shape in order to cover a portion of the slot 65 for better appearance and also in order to reduce the possibility that foreign matter will enter the barrel handle member 41 and interfere with operation of the spring 55.

It will be noted primarily from FIG. 5 that one side of the upper portion of the plunger 19 is inscribed with volume markings 67 which are used in conjunction with the volume adjustment assembly for setting the pipetting instrument to transfer a particular predetermined volume of liquid. A particular volume marking of the marking 67 that is immediately adjacent the barrel end portion 23 represents the volume of liquid 71 of FIG. 2 that will be drawn into a tip 13 as the piston 49 is moved from its position shown as dotted in FIG. 2 to its position shown in solid lines, a distance represented by the arrow 71 of FIG. 2. When the piston is in its dotted position, the calibration ledge 21 engages the end surface 23 of the barrel member 41.

Thus, when the rest position of the piston 49 is to be adjusted, the volume adjustment screw 61 is loosened and the plunger 19 is positioned so that a desired volume marking 69 is made to be coincident with the surface 23. The volume markings 67 represent directly the amount of liquid that will be withdrawn when the pipetter is operated. This volume may be, for instance, in the range of 1-5 milliliters wherein the lower portion of the range is marked at the top of the calibration 67 and the higher end of the range is marked on the lower end of the calibration 67. Once the desired liquid transfer volume has been set by positioning the plunger 19, the adjustment mechanism 27 is slid within its slot 65 until it engages the flange 57 that is attached to the plunger 19. While still holding the plunger 19 in the desired position, the adjustment screw 61 is tightened, thereby to position the stop 59 so that the plunger will always return under the urging of the spring 55 to a rest position. The desired liquid transfer volume will be indicated on the scale 67 at the end surface 23 of the barrel member 41 whenever the piston 49 is in its rest position.

The volume adjustment mechanisms described herein minimize the amount of residual airspace within the piston chamber 47 during operation of the pipetter. A minimum of airspace between the piston 49 and liquid being transferred in the tip 13 reduces the changes of error in the volume of liquid transferred since the amount of air subject to compression is kept low.

As can be seen from FIG. 2, a predetermined volume of liquid 71 is drawn into the detachable and disposable tip 13 as the piston 49 is permitted to return from the dotted position shown in FIG. 2 to its rest position shown in solid lines while the lower tip of the disposable tip 13 is immersed within a liquid. Before immersing the opening of the tip 13 in liquid to be sampled, the operator has depressed the knob 17 so that the first calibrating edge 21 has struck the barrel end surface 23. This is the dotted position of the elements shown in FIG. 2. The liquid 71 is drawn into the tip 13 since the interior of the tip 13 is in fluid communication with the piston chamber 47 through the central bore of the tubing 15 that interconnects the two elements.

To discharge the liquid 71 from the tip 13, the piston 49 is again depressed so that its calibrating stop 21 strikes the surface 23 as shown in FIG. 3. Although the plastic material of the tip 13 is chosen so that most liquids do not readily adhere thereto, there may be some residue liquid 73 (FIG. 3) remaining in the tip 13. Since it is desired to transfer to the second container a precise volume of liquid, it is necessary to remove as much of the residue volume 73 from the tip 13 as is possible. This is accomplished by further depressing the piston 49 from the position shown in solid lines of FIG. 3 an overshoot distance indicated by an arrow 75 to the dotted position shown in FIG. 3.

This overshoot is accomplished by the operator moving the plunger 19 laterally with respect to the barrel member 41 within an aperture 77 in the end surface 23 through which the plunger passes. This releases the impediment to further depressing of the plunger 19 by positioning the stop surface 21 within the aperture 77 and permits movement of the plunger until the overshoot calibrating ledge 33 strikes the barrel end surface 23. It is this latter condition that is represented by the dotted outline of FIG. 3. It will be noted that since the finger hold 31 and the plunger 19 are oriented so that the stop surface 21 and the finger hold 31 are on the same side of the plunger 19, this transverse motion of the plunger 19 within the aperture 77 is easy for an operator to accomplish when the pipetter 11 is held in the manner shown in FIG. 1.

The structural shape of the plunger 19, which permits easy motion to overshoot the piston 49 when discharging liquid from the tip 13, may be observed with respect to FIG. 2 which shows one side of the square cross-sectional plunger 19. In this one direction, the majority of the plunger has a dimension substantially the same as the opening of the aperture 77 in the top end 23 of the barrel member 41. This dimension or width is represented by the arrow 79 in FIG. 2. Between the segment including the arrow 79 and the calibration ledge 21 is a section having a width indicated by the arrow 81 that is substanitally less than the width indicated by the arrow 79. Thus, the width 81 is also substantially less than the width of the aperture 77 into the barrel member. The plunger 19 maintains the width indicated by the arrow 81 for at least a distance along its length equal to the thickness of the barrel member 41 surrounding the opening 77 at the end surface 23. This permits translational movement of the plunger 19 with respect to the barrel member 41 after the calibration stop 21 strikes the end surface 23. A gradual change in thickness between the segments indicated by the arrows 79 and 81 is accomplished on one side of the plunger 19 by sloping the side at 83. The thickness of the plunger 19 in this one direction and extending along its length between the calibrating ledge 21 and the second (overshoot) calibrating ledge 33, is indicated by an arow 85 and is again substantially the same width as the opening 77. Therefore, it is seen that the right hand side of the plunger 19 jogs in the one direction an amount equal to the width of the calibrating ledge 21.

Except for the glass sleeve 45 and the stainless steel elements 15, 49, 55 and 123, the remaining parts of the pipetter 11 are preferably constructed from a glass filled Nylon having high resistance to chemicals.

A somewhat different embodiment is shown in FIG. 6. The pipetter of this Figure has certain variations in the design described with respect to FIGS. 105 which are desirable in a smaller instrument such as those designed to transfer less than 1 milliliter of liquid. A narrow bore piston chamber 87 formed in a portion of the housing 89 contains a piston 91 that is sealed thereto by an O-ring 93 held by the housing portion 89. The piston is attached to a plunger 95 having a shape and principle of operation similar to the plunger 19 that is described above with respect to FIGS. 1-5. A calibrating ledge 96 and an overshoot ledge 98 are provided on the plunger 95 for engaging a barrel end surface 111. A single spring 97 (FIG. 6) is normally held in compression between an adjustment block 99 and a structural cross member 101 that is rigidly held to a barrel housing member 103. The adjustment block 99 is selectively attached to the plunger 95 by a screw 105 that is accessible through an opening 107 in the barrel housing member 103.

To set the liquid volume that may be transferred by the device of FIG. 6, the screw 105 is loosened so that the plunger 95 can move with respect to the adjustment block 99. The spring 97 will maintain the adjustment block 99 in the position shown. The plunger 95 is then moved until a desired distance beteween the calibration ledge 96 and the end surface 111 is reached. This desired distance is conveniently established by positioning a calibration block (not shown) between the ledge 96 and the end surface 111 and depressing the plunger 95 to hold the calibration block under compression. The screw adjustment 105 is then tightened so that the adjustment block 99 is ridigly attached to the plunger 95. The pipetter will then operate to have a primary movement indicated by the arrows 113 and an overshoot movement indicated by the arrow 115 of FIG. 6. The distance between the ledge 96 and the surface 111 determines the liquid transfer volume of the pipetter. The liquid transfer capabilities may be in the range of 10 microliters to 1 milliliter.

It will be noted that the smaller pipetter of FIG. 6 is tapered at its lower end for frictional engagement within conically shaped disposable tips such as the tip 117 shown in FIG. 6. These smaller tips preferably contain abrupt stops (not shown) on their outside for loading in storage trays for rapid insertion onto a pipetter and separation therefrom, in a manner described and claimed in aforementioned U.S. Pat. No. 3,494,201 - Roach, without a user having to handle the tips.

In the case of both instrument embodiments described, the one shown in FIGS. 105 and the variation shown in FIG. 6, the disposable tips 13 and 117 are preferably used only once to prevent contamination between volumes of liquid that are transferred from one container to another. A usual application of such pipetters and tips is in medical laboratories of hospitals and elsewhere. In the case of the large sampler of FIGS. 105, the tips 13 may be too large for convenient rack mounting and thus are supplied with a large number of them sealed in a single bag in random positions. It is desirable, of course, that none of the liquid being transferred actually gets into any part of the pipetter to prevent contamination. Thus, for larage samples, the effective internal volume of the disposable tip 13 when attached to a pipetter must be in excess of 5 milliliters in order to handle the full range of liquid volumes (1-5 milliliters) for which the instrument is designed.

Referring primarily to FIG. 2, the disposable tip 13 includes an outer cylindrically shaped plastic shell 119 for maximum volume capability in a short length. The shell 119 is reduced in outside diameter at its lower end in order to provide a very small opening 121 at its bottom. The opening 121 is made as small as possible for precision volume liquid transfers. A small opening 121 prevents loss of liquid therethrough from shaking, wiping the outside of the tip 13, etc. The opposite end of the tip 13 is circular in cross-section for frictionally engaging the circular outside surface of the tip member 123 that is attached to the tubing 15 as part of the pipetter 11. The tip member 123 thus serves as a transition element for matching the large diameter of the tip 13 to a small diameter of the tubing 15.

The walls of the disposable tips 13 and 117 are made preferably of polypropylene in order to reduce adherence of liquid to tip sides.

Claims

I claim:

1. In a hand-held pipetter comprising an elongated barrel shaped handle, a piston chamber within said barrel, means at one end of said barrel for holding a disposable tip and for providing fluid communication between said piston chamber and an interior portion of said tip, a piston in said piston chamber, a plunger operably communicating at one end thereof with said piston and extending out of said barrel to a free end thereof through an aperture at another end of the barrel thereby to permit operation of said piston for displacing fluid within said piston chamber by moving the free end of the plunger, and spring means normally urging said piston and plunger toward said another end of the barrel, the improvement comprising:

means providing two positive stops of the plunger relative to the barrel shaped handle for selectively limiting travel of the piston with respect to the piston chamber, whereby fluid displacement within the piston chamber is accurately determined by selection of one of said two positive stops for limiting travel of said plunger and piston,

a collar on said plunger portion within said barrel handle,

a slot oriented with its length extending along a length of said handle, and

an adjustable stop carried by and manually movable along the slot to limit the spring return of said plunger collar to a predetermined position engaging the stop, said stop including means operable from outside the handle to fix the stop to the handle at a selected position along the length of said slot.

2. The improved pipetter according to claim 1 wherein said spring means consists of only one spring.

3. The improved pipetter according to claim 1 wherein the two positive stops of said selective piston travel limiting means includes first and second calibrated ledges on the plunger portion normally held outside said barrel handle by said spring means, said two positive stops being positioned to engage an outside surface of said another end of said barrel as the plunger is depressed in a manner to operate against said spring means.

4. The improved pipetter according to claim 3 wherein the aperture at said another end of the barrel and said plunger are of cooperating shape so that when the plunger is pushed against said spring, said first stop hits said another end of the barrel and the plunger can then be released for further movement into the barrel by first moving said plunger laterally with respect to the barrel and then pushing said plunger further into said barrel until the second stop hits said another end of the barrel.

5. The improved pipetter according to claim 1 wherein the improvement additionally comprises:

an adjustment block held by and slidable along the plunger within said barrel handle, said block being selectively tightenable onto said plunger for engaging some portion of said barrel handle interior structure to limit the distance of plunger spring return, said spring means urging said adjusting block toward said another end of said handle, whereby said plunger is urged by the spring only through its connection with said adjusting block, and

an opening in the side of said barrel handle adjacent said another end thereof to permit access to said calibrating block for securely fastening it to the plunger.

6. In a hand-held pipetter comprising an elongated barrel shaped handle, a piston chamber within said barrel, means at one end of said barrel for holding a disposable tip and for providing fluid communication between said piston chamber and an interior portion of said tip, a piston in said piston chamber, a plunger connected at one end thereof with said piston and extending out of said barrel to a free end thereof through an aperture at another end of the barrel thereby to permit operation of said piston for displacing fluid within said piston chamber by moving the free end of the plunger, and spring means normally urging said piston and plunger toward said another end of the barrel, the improvement comprising means providing two positive stops of the plunger relative to the barrel shaped handle for selectively limiting travel of the piston with respect to the piston chamber, whereby fluid displacement within the piston chamber is accurately determined by selection of one of said two positive stops for limiting travel of said plunger said piston, said plunger including a cross-sectional shape in a first portion along its length that fills substantially all of the aperture in said another end of the barrel in at least one direction, a section portion adjacent to said first portion and outward thereof toward the free end of the plunger that is smaller in said one direction than the aperture size in the barrel end member in said at least one direction, a third portion including a first ledge forming one of said positive stops adjacent the second portion of the plunger and extending outward therefrom toward the free end of the plunger with a dimension greater than that of the second section in said at least one direction but still small enough to fit through said aperture of said another end of the barrel handle, a fourth portion immediately adjacent said third portion toward the free end of the plunger and including a second ledge forming the other of said positive stops and having a dimension in excess of the aperture at said another end of the barrel handle in said at least one direction thereby to limit travel of the plunger into the barrel, whereby the small dimension of said second portion permits lateral movement of the plunger with respect to the barrel after said first ledge contacts said another end of the barrel handle.

7. The improved pipetter according to claim 6 wherein said barrel handle includes a finger hold piece extending outward of said another end thereof and curving to the side and further extending generally in said at least one direction, thereby to permit gripping said barrel handle between the fingers and operating of said plunger with the thumb.

8. In a hand-held pipetter comprising an elongated barrel shaped handle, a piston chamber within said barrel, means at one end of said barrel for holding a disposable tip and for providing fluid communication between said piston chamber and an interior portion of said tip, a piston in said piston chamber, a plunger connected at one end thereof with said piston and extending out of said barrel to a free end thereof through an aperture at another end of the barrel thereby to permit operation of said piston for displacing fluid within said piston chamber by moving the free end of the plunger, and spring means normally urging said piston and plunger toward said another end of the barrel, the improvement comprising:

means providing two positive stops of the plunger relative to the barrel shaped handle for selectively limiting travel of the piston with respect to the piston chamber, whereby fluid displacement within the piston chamber is accurately determined by selection of one of said two positive stops for limiting travel of said plunger and piston,

a collar on said plunger portion within said barrel handle,

a stop adjustable along the side of said barrel to limit the return of said plunger by said spring means to a predetermined position, said adjustable stop being located in the interior of said barrel handle and providing an abutment for said collar,

a tightening screw threaded to said stop and extending outside of said barrel handle so that the stop and screw are slidable along a slot extending a portion of the length of said barrel for selectively adjusting the stop inside of the barrel, and

markings along the one side of the plunger which normally extends outward of said aperture to denote the volume displacement of said piston within said piston chamber, thereby to permit adjustment of said stop for a predetermined volume displacement of said piston.

9. In a hand-held pipetter comprising an elongated barrel shaped handle, a piston chamber within said barrel, means at one end of said barrel for holding a disposable tip and for providing fluid communication between said piston chamber and an interior portion of said tip, piston in said piston chamber, a plunger operably communicating at one end thereof with said piston and extending out of said barrel handle to a free end thereof through an aperture at another end of the barrel in a manner to permit operation of said piston for displacing fluid within said piston chamber by moving the free end of the plunger, and spring means normally urging said piston and plunger toward said another end of the barrel handle, the improvement comprising:

said spring means normally urging the piston and plunger toward said another end of the barrel including a single strength spring element,

a first calibrating ledge extending outward of said plunger and in the vacinity of its free end for striking said another end of said barrel handle adjacent said aperture, and

a second calibrating ledge extending outward of said plunger and positioned along its length between its free end and said first calibrating ledge, said second calibrating ledge having dimensions which prohibit further movement of said plunger into the barrel handles when said second calibrating ledge strikes said another end of the barrel handle,

the plunger having a dimension immediately adjacent the first calibrating ledge but on its side opposite to that of the second calibrating ledge that is significantly less than the aperture dimension in at least one direction in a manner permitting sufficient lateral movement of the plunger with respect to the barrel handle for movement of the first calibrating ledge through said aperture to permit engagement of the second calibrating ledge with said another end of the barrel handle.

10. The improved hand-held pipetter of claim 9 which additionally comprises adjustable means within said barrel defining an extreme return position of said plunger and piston toward said barrel another end, whereby, a stroke distance of said plunger from said extreme return position to its depressed position wherein the first calibrating ledge strikes said barrel another end is variable while a stroke distance of said plunger as it is depressed between its said first and second calibrated ledges is fixed.

11. In a hand-held pipetter comprising an elongated hollow handle, a piston chamber within said handle means at one end of said handle for holding a disposable tip and for providing fluid communication between said piston chamber and an interior portion of said tip, a piston in said piston chamber, a plunger operably connected at one end thereof with said piston and extending out of said handle at another end thereof, thereby to permit operation of said piston for displacing fluid within said piston chamber by moving the plunger, and spring means normally urging said piston and plunger toward said another end of the handle, the improvement comprising:

a collar rigidly attached to said plunger along a portion thereof within said barrel handle,

a slot oriented with its length extending along a length of said handle, and

an adjustable stop carried by and manually movable along the slot to limit the spring return of said plunger to a predetermined position engaging the stop, said stop including means operable from outside the handle to fix the stop to the handle at a selected position along the length of said slot.

12. In a hand-held pipetter comprising an elongated hollow handle, a piston chamber within said handle, means at one end of said handle for holding a disposable tip and for providing fluid communication between said piston chamber and an interior portion of said tip, a piston in said piston chamber, a plunger operably connected at one end thereof with said piston and extending out of said handle at another end thereof, thereby to permit operation of said piston for displacing fluid within said piston chamber by moving the plunger, and spring means normally urging said piston and plunger toward said another end of the handle, the improvement comprising:

a collar rigidly attached to said plunger along a portion thereof within said barrel handle,

an adjustable stop located in the interior of said handle to limit the return of said plunger by said spring means to a predetermined position upon engagement of said collar with said stop,

a tightening screw threaded to said stop and extending outside of said handle, the stop and screw being slidable along a slot extending a portion of the length of said handle for selectively adjusting the stop inside of the barrel, and

markings along the one side of the plunger which normally extend outward of said handle to denote the volume displacement of said piston within said piston chamber, thereby to permit adjustment of said stop for a predetermined volume displacement of said piston.

13. In a hand-held pipetter comprising an elongated hollow handle, a piston chamber within said handle, means at one end of said handle for holding a disposable tip and for providing fluid communication between said piston chamber and an interior portion of said tip, a piston in said piston chamber, a plunger operably connected at one end thereof with said piston and extending out of said handle at another end thereof, thereby to permit operation of said piston for displacing fluid within said piston chamber by moving the plunger, and spring means normally urging said piston and plunger toward said another end of the handle, the improvement comprising:

a calibrating block held by and slidable along the plunger within said handle, said block being selectively tightenable onto said plunger for engaging a portion of the handle's interior structure to limit the distance of the plunger spring return, said spring means urging said adjusting block toward said another end of said handle, whereby said plunger is urged by the spring only through its connection with said adjusting block, and

an opening in the side of said barrel handle adjacent said another end thereof to permit access to said calibrating block for securely fastening it to the plunger.

14. In a hand-held pipetter comprising an elongated barrel shaped handle, a piston chamber within said barrel, means at one end of said barrel for holding a disposable tip and for providing fluid communication between said piston chamber and an interior portion of said tip, a piston in said piston chamber, a plunger operably communicating at one end thereof with said piston and extending out of said barrel handle to a free end thereof through an aperture at another end of the barrel in a manner to permit operation of said piston for displacing fluid within said piston chamber by moving the free end of the plunger, and spring means normally urging said piston and plunger toward said another end of the barrel handle, the improvement comprising:

said spring means normally urging the piston and plunger toward said another end of the barrel including a single strength spring element,

a single finger hold attached to the barrel adjacent said another end thereof and extending outward of a side of the barrel in one direction,

a first calibrating ledge extending outward of said plunger in said one direction and in the vacinity of its free end for striking another end of said barrel handle adjacent said aperture,

a second calibrating ledge extending outward of said plunger in said one direction and positioned along its length between its free end and said first calibrating ledge, said second calibrating ledge having dimensions with prohibit further movement of said plunger into the barrel handle when said second calibrating ledge strikes said another end of the barrel handle,

the plunger having a dimension immediately adjacent the first calibrating ledge but on its side opposite to that of the second calibrating ledge that is significantly less than the aperture dimension in said one direction in a manner permitting sufficient lateral movement of the plunger with respect to the barrel handle in a direction away from said finger hold to permit movement of the first calibrating ledge through said aperture to permit engagement of the second calibrating ledge with said another end of the barrel handle,

a collar on said plunger portion within said barrel handle,

a slot oriented with its length extending along a length of said handle, and

an adjustable stop carried by and manually movable along the handle slot to limit the spring return of said plunger collar to the predetermined position engaging the stop, said stop including means operably from outside the handle to fix the stop to the handle at a selected position along the length of said slot.

15. The improved pipetter according to claim 14 wherein the improvement additionally comprises markings on one side of the plunger along a length thereof adjacent said free end to denote the volume displacement of said piston within said piston chamber, whereby the volume displacement of the pipetter may be set with reference to said markings.