GB2112664A - Liquid sample dilution means and process and a tip therefor - Google Patents

Abstract

An improved process and device for facilitating microanalysis of samples under ten microliters with good accuracy-precision. A sample together with a dilution liquid are simultaneously delivered into a receptacle in a suitably small volume. The delivering means comprises a liquid receiver (12) with a metering means (13) for dispensing dilution liquid into the bore of a tip (11) at the lower end of the reservoir. A probe (23) extends from the lower end of the tip so that liquid from the tip will run down the probe. The free end of the probe has a longitudinally extending slot which may be filled by dipping with a liquid sample to be diluted. <IMAGE>

Description

SPECIFICATION Liquid handling means and process and a tip therefor This invention relates to dilution devices and processes, and more particularly to a device and process for making accurate dilutions of small volumes of liquid, as used in microanalysis chemical procedure and the like.

There has been a trend in recent years toward miniaturization of laboratory tests involving chemical reactions in order to conserve expensive reagents and limited quantities of samples to be analyzed, and to facilitate mechanical automation, for example as set forth by applicant in "Semi-automation of Immunoassays by Use of Magnetic Transfer Devices", Method in Enzymology, Volume 70, pages 388-416, 1 980. Each of the reaction wells in a typical 96-well tray in aerologic work can contain a maximum of approximately 300 microliters; the volume of reactants actually dispensed in each such well normally ranges from 100-200 microliters.

For this reason, precision made microliter pipettes or other small-volume liquid transfer devices are used for handling and diluting the reactants.

In cases where samples need to be initiaily diluted minimally, e.g. 1:2-1:20, several volumetric liquid transfer devices have been used. United States Letters Patent No.

3,252,331 teaches apparatus which accomplishes serial dilution of a sample by fluid trapped in said apparatus and, after mixing in a receptacle containing a diluent fluid, a fixed volume of the diluted material can be transferred to another receptacle, mixed, and such steps repeated.

In cases where samples must be substantially diluted (as for example 1:50 or more) before being tested in one of these miniature analytical systems, the dilutions are usually made in intermediate, larger vessels, then small volumes of the diluted materials are transferred to the miniature reaction wells in which the test is to be conducted.

Heretofore there has been no satisfactory way to make accurate-precise dilutions of small volumes, principally of ten microliters or less, directly into miniature reaction wells.

This problem can be readily understood, for example, when one considers the need to make, in one step, a 1:50 initial dilution of a serum sample so that the total volume will be only 100-200 microliters; the volume of serum sample required is either 2 or 4 microliters, volumes not conveniently or easily measured accurately-precisely with existing equipment. A presently used alternative, as heretofore mentioned, is to use existing equipment to make the initial dilution in an intermediate vessel in suitably large volumes, then transfer a small volume of the diluted sample into the miniature reaction well for testing.This obviously requires an extra step in the assay process, necessitates the use of one or more extra intermediate vessels, extra pipettes and/or pipette tips, and additional time is required, all of which invites human errors inevitably associated with multiple manipulations.

The present invention provides a process and device for micro-analysis wherein under ten microliters of a sample can be taken up accurately-precisely and delivered, with a convenient volume of diluent, directly into a miniature reaction vessel.

Thus according to one aspect of the present invention, there is provided a tip for a liquid handling means to receive liquid dispensed therefrom, said tip comprising: a continuous wall defining an axial bore which extends through the entire length of said tip, said bore having an upper portion and a lower portion.

an elongate probe extending axially beyond the lower extremity of said wall, the probe being positioned so as to receive on its surface liquid issuing from the lower end of said bore, a longitudinally extending slot in said probe, and located axially beyond the lower extremity of said wall.

In a preferred embodiment of the invention, the elongate probe is attached to the wall of the tip. Thus it may be a separately manufactured part which is subsequently attached to the outer or inner surface of the wall of the tip, or it may be an integral extension of the tip wall.

According to a second aspect of the present invention, there is provided a probe for use in association with liquid handling means for effecting dilutions of volumetrically small liquid samples with diluent from said liquid handling means, said probe comprising: an elongated body portion having an upper end and a lower end; means for attaching the upper end of said body portion to the outlet end of a liquid handling means so that liquid issuing downwardly from said liquid handling means outlet passes down the surface of said probe; a lowermost longitudinal slot in the lower end of said probe, adapted to retain therein a small liquid sample for dilution with diluent from said liquid handling means.

The longitudinally extending slot in the probe has a very small volumetric size, e.g.

about 0.1about 10 microlitres, and dimensions of the order of 0.1-3.0 millimeters in width and 1.0-20 millimeters in length. Preferably the slot is 0.5 millimeters in width and 5.0 millimeters in length. When it is dipped into a liquid sample in a container and then withdrawn, it picks up and holds a predetermined volume of the liquid. Then this volume of the liquid sample can be washed out of the slot in the probe with an accurately metered amount of diluent liquid dispensed from the liquid handling means. The volume of liquid sample held by the slot can be determined by an initial calibration, as described below, so that by use of the device of the invention accurate dilution of very small volumes of sample liquids can be achieved, in a simple and rapid manner.

Another aspect of the invention comprises the use of a device described above for effecting serial dilutions of a very small liquid sample, utilising a single volumetric quantity of liquid contained in the slot, but repeated washings thereof with predetermined equal volume aliquots of diluent. Thus the invention provides a process for making serial dilutions of a liquid sample, utilizing a liquid handling means having attached thereto a downwardly extending probe with a lowermost longitudinal slot therein, which comprises:: (a) filling the liquid handling means with a selected diluent; (b) dipping the end of the probe into the liquid sample so as to fill the slot therein with liquid sample; (c) removing the probe and slot from the sample; (d) holding the probe and slot containing the sample vertically above a receptacle; (e) releasing a predetermined volume of said diluent from the liquid handling means to pass over, around and down said probe and slot into said receptacle; and (f) repeating steps (d) and (e) and desired number of times, into separate receptacles, without refilling said slot with sample.

It is to be understood that throughout the specification and claims the term "liquid handling means ' is used in a generic sense and includes, but is not specifically limited to burettes, pipettes, micropipettes, syringes which deliver a desired volume and then automatically refill from a reservoir, and other dispensing apparatus comprising a liquid reservoir, metering means for accurately and reproducibly controlling the dispensing of said liquid, and a section or a separate fitting to receive the tip of the present invention.

In the preferred embodiments of the invention, the greatest diameter and over-all length of the tip is such as to permit insertion into clinical specimen containers such as blood sample tubes or the like. Additionally, the probe ranges in length from 3-100 millimeters, preferably 20 millimeters, as measured from the lower edge of the respective tip; said probes are desirably of corrosion resistant metallic composition such as gold, cadmium or nickle plated steel, and stainless steel.

Furthermore, the probe suitably ranges from 0.3-5.0 millimeters in diameter, preferably approximately 1.0 millimeter. The radial distance between the circumference of a respective probe and inner side wall of lower bore is desirably 0.5 millimeter but may range from 0.1-5.0 millimeters. It is understood that the dimensions of the several members are variable within the specified ranges to accomplish the ends of the invention; in general, however, where the probe is provided within the lower opening of the tip, the annular spacing around said probe within the lower bore of the tip must be sufficiently great to permit unrestricted flow of the diluent liquid over, around and down said probe, yet not so great as to cause uncontrolled leakage or release of the diluent through the respective lower bore due to breaking the surface tension thereof.

The inner wall of the tip is preferably conical, converging downwardly, or cylindrical.

Specific preferred embodiments of the invention are illustrated in the accompanying drawings, by way of example and illustration, in which: Figure 1 is a side elevational view, partly broken away, of conventional liquid handling means with a preferred embodiment of the tip of the subject invention secured thereto; Figure 2 is a fragmentary, side elevational view, partly broken away of conventional liquid handling means with a preferred embodiment of the tip of the subject invention remotely connected thereto; Figure 3 is a fragmentary, vertical sectional view taken along the medial, longitudinal axis of the preferred embodiment of the invention; Figure 4 is a top plan view of the embodiment of Fig. 3; Figure 5 is a fragmentary, greatly enlarged side elevational view of the lower portion of the probe of the embodiment of Fig. 3;; Figure 6 is a fragmentary, greatly enlarged side elevational view of the lower portion of another embodiment of probe; Figure 7 is a fragmentary, vertical sectional view taken along the medial, longitudinal axis of another embodiment of the invention; and Figure 8 is a curve showing the counts per minute for a serially diluted sample of human serum plotted against the relative concentration of each sample.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in Fig. 1 a conventional liquid handling device 10 to which tip 11 of the subject invention is adapted to be secured.

This device is, in Fig. 1, a conventional burette. A different form of conventional liquid dispensing device in Fig. 2, which additionally includes an elongated tube 1 5 or other transfer means (see Fig. 2) secured to section 14 with adapter 16 on the remote end thereof to receive tip 11.

As best seen in Figs. 3-5 of the drawings, tip 11, which is made of plastic and is generally conical shape, includes upper edge 17, downwardly converging outer wall 18, inner wall 1 9 generally parallel thereto, lower edge 20, upper bore 21 of slightly greater inside diameter than the corresponding outside diameter of tapered section 14 or fitting heretofore mentioned, and lower bore 22. Axially extending probe 23 is secured upwardly to the approximate medial portion of wall 1 9 on bracket 24 or the like; the medial portion of probe 23 is axial with respect to bore 22 and bracket 24 does not impede liquid flow around the proble.The probe extends substantially downwardly of lower edge 20 of the tip 11 and includes a longitudinally extending and downwardly opening slot 25 of predetermined volumetric capacity in the lower portion thereof.

In use, tip 11 thus formed is detachably or fixedly secured in tapered section 14 or other fitting on liquid handling device 10.

As viewed in Fig. 6, the lower portion of probe 23' includes a generally ovally shaped and laterally opening slot 26, the parameters of which will hereinafter be more fully set forth.

There is shown in Fig. 7 another embodiment of tip 27 constructed in accordance with the principles of the invention wherein longitudinally extending probe 28 is fixedly secured as by an adhesive or the like to downwardly converging outer wall 29 with inner wall 30 generally parallel thereto; the probe extends angularly downwardly and terminates in a longitudinally extending and downwardly opening slot 33 of predetermined volumetric capacity which is spaced approximately vertically below lower bore 32 in the tip. At least the inner wall 30 is conical, converging downwardly, or is cylindrical.

The method of calibrating probes 23, 23' and 28 will be described in detail; for purposes of convenience only, reference will be made to the calibration of probe 23 and slot 25 therein; it is understood that similar steps are utilized in calibrating probe 23' and slot 26 as well as probe 28 and slot 31.

A radioactive material such as iodine-125 is dissolved in human serum, and the serum is sampled by use of a conventional microliter pipette capable of measuring a volume of 50 microliters with a proven precision and accuracy of better than 5% coefficient of variation, hereinafer designated as CV. This sample, with an appropriate volume of diluent liquid, is placed in a vial for counting. The amount of gamma radiation in the known volume of serum is measured in counts/minute, hereinafter designated as CPM, by means of a gamma counter. The average CPM of a certain 50 microliter sample, for example, was 140,848.

Probe 23 is first lowered into the said serum to take up a sample of the radioactive serum within slot 25 and is then withdrawn therefrom. An appropriate volume, e.g. 200 microliters of diluent liquid, is then allowed to flow downwardly, around and over slot 25 so that the mixture of ratio-active serum and diluent falls or streams from the probe into a counting vial; said vial is then placed in a gamma counter and the CPM of the sample of unknown volume is determined. This process is repeated several times and the average CPM calculated for the replicate samples of unknown value.

TABLE I Unknown % Deviation Sample No. CPM from the Avg.

1 10,853 4.1 2 10,340 0.8 3 10,564 1.3 4 10,749 3.1 5 10,342 0.8 6 10,786 3.5 7 9,741 6.5 8 10,520 0.9 9 10,144 2.8 10 10.230 1.9 Avg. 10,424 2.5 As shown in Table I, the average volume of serum sample deliverable by probe 23 is calculated as follows, wherein: x = the volume of serum sample delivered by the device of the invention; 10,424 CPM = the average activity of the samples delivered by the process and device of the invention; 140,848 CPM = the average activity of the known 50 microliter sample; x 10,424 50 = 140,838 therefore, x = 3.71 microliters, A dose-response experiment was conducted to ascertain if liquids widely different in protein concentration, hence with slightly different viscosities, behave differently when sampled by the device of the subject invention, in this experiment, radioactive serum was serially diluted in 0.85 isotonic saline buffered to pH 7.3. As shown in Fig. 8 of the drawings, a wide range of serum protein concentrations in the specimens had no detectable effect upon the accuracy or reproducibility of volume measurements. The best fit curve 33 of CPM in the serially diluted samples plotted against the relative concentration of each sample is a straight line which goes through the origin, a classic indication of a valid, precise quantative procedure.

The process and device of the subject invention may be conveniently used for making serial dilutions. As a given volume of diluent liquid is released by measuring means 1 3 heretofore mentioned, it bathes slot 25 and end of probe 23, then falls into a receptacle; the liquid remaining within said slot still contains some of the original serum in a diluted form. Therefore, the amount of original serum remaining in the slot 25 is steadily decreased as more and more diluent passes over, around and through it.

Specifically, serial dilution of a specimen comprises the following steps: 1. Filling reservoir 1 2 of liquid handling means 10 with a diluent including but not limited to water, phosphate buffered saline (PBS), PBS containing 1% bovine serum albumen, and PBS containing 10% bovine serum or Nessler's reagent.

2. Inserting slot 25 of probe 23 into a specimen, including but not limited to serum, spinal fluid and urine.

3. Removing said probe and slot from said specimen.

4. Holding said probe containing said specimen vertically above a receptacle, such as a test tube or well of a 96-well plastic tray.

5. Actuating metering means 1 3 to release a predetermined volume of diluent of step 1 to pass over, around and down said probe and fall into the receptacle of step 4; and 6. Repeating steps 4 and 5 any desired number of times, using separate receptacles to collect the diluted material after each release of diluent.

Tests were conducted to determine the reproducibility of serial dilution of a sample, utilizing the steps heretofore specified, wherein: 1. Slot 25 was inserted into a serum sample containing iodine-125, withdrawn, moved directly above a test tube and a diluent liquid which does not contain a surfactant allowed to flow from reservoir 1 2 over, around and down said slot so that only one drop was collected in a given tube.

2. The slot of said probe was placed directly above another test tube and a single drop collected in said tube; this step was repeated four more times so as to serially collect one drop in each of the different test tubes.

3. Each of the six tubes was placed in a gamma counter to determine the relative amount of iodine-1 25 contained in each drop.

Additionally, the amount of iodine-125 in 50 microliters of the original, undiluted serum was determined. The results of such experiment are shown in Table II.

TABLE II Drop No. CPM (x 104) 1 3.4 2 1.1 3 0.44 4 0.16 5 0.084 6 0.050 Fifty microliters of the original, undiluted serum had a count of 7.18 x 105 CPM, therefore, the selected slot was calculated to contain 2.33 microliters.

When this data was plotted on semilogarithmic, 3-cycle graph paper, the curve which best fit the data was slightly parabolic and very closely approximated each point of the data. Since 45 drops from the slot equals 1 ,000 microliters, the volume of each drop was 22 microliters. The slot of said probe had previously been calibrated, as above, and contained 2.33 microliters of the original, undiluted serum sample. The calculated serum dilutions are set forth in Table Ill.

TABLE 111 Drop No. CPM 1 1:9.44 2 1:29.2 3 1:73 4 1:201 5 1:383 6 1:643 The results of these and other identical experiments show that the dilution effect was constant, predictable and reproducible.

In still another experiment, wherein two successive drops were collected sequentially and pooled in one vessel, the dilution effect again was found to be reproducible. The dilution factor differences between each pair of drops was, as expected, greater than between single drops.

It is concluded from such experiments that reproducible serial dilutions of a serum can be obtained by allowing diluent to flow over the sample containing slot 25 and collecting selected volumes of the mixture for testing.

If the volumes of single or double drops are too small for an assay, an alternative method is to use a separate repetitive pipetting device to add a fixed additional volume of diluent to each vessel in which the drops were collected; this gives the same fold-dilution increment differences between samples, but at a higher dilution level.

The dilution effect can be achieved without collecting individual drops, by adding a detergent such as 0.25% Tween 80 to the diluent, then releasing appropriate volumes of the diluent and collecting these volumes sequentially in separate receptacles. Detergents tend to inhibit drop formation so that very small volumes of liquid fall or stream over, around and from the probe; the liquid does not fall from the probe as uniform drops; the volume of each increment of diluent can be somewhat more easily controlled and varied in this manner. The reproducibility of this serial dilution process when employing a detergent is slightly increased by keeping reasonably uniform the rate of diluent flow over and across the probe.

A major advantage of the subject invention is its "self-cleaning" feature. The dilution effect produced by allowing a 200 microliter stream of diluent to pass over the probe and fall into a receptacle reduces the amount of residual specimen in slot 25 to such a low level as to be insignificant for many analytical purposes. For example, the last drop in a typical dilution experiment contained a 1:1,603 dilution of the original specimen after a total volume of only 1 32 microliters of diluent had passed over the probe. Numerous experiments indicate that a diluent volume of 200 microliters passing over the probe reduces the amount of residual sample to an almost undetectable level.If, in some unusual circumstances, further reduction in residual sample is desired, use of a "flushing" volume of another 200 microliters of diluent, which is discarded, reduces the contamination level to well below measurable levels. Therefore, the mechanical design and process of the invention provides a "self-cleaning" capability which, being a result of the normal operation of the device and process, facilitates its use in situations requiring dilutions of specimens one after the other.

As heretofore specified, the subject invention makes it possible to make direct, accurate-precise initial dilution of a sample-specimen in a relatively small volume. It is evident that a probe and slot can be fabricated so as to take up a predetermined volume of a sample-specimen which, used with an appropriate volume of diluent, achieves a resulting single, one-step dilution which is ideal for a particular "screening" essay, i.e., a quantative assay requiring only one concentration of a sample-specimen.For example, an assay may require a serum dilution of 1:50 in a final volume of 150-200 microliters, a mechanically convenient volume for the test; the following steps accomplish this purpose: 1. a slotted probe is fabricated and cali brated in the manner heretofore described to take up and deliver a volume of 3.2 microliters; and 2. the liquid handling means 10 is regulated to deliver a diluent volume of 1 60 microliters which will flush the sample-specimen from slot 25 into a receptacle to give a 3.20:160 or 1:50 final serum dilution.

In normal laboratory practice, the diluent for a clinical specimen is relatively inert. The subject invention is advantageously used in mixing specimens with appropriate chemically reactive reagents for purposes other than simple dilution however. For example, a serum sample can be mixed with a color-producing reagent and simultaneously dispensed, by use of the subject invention, into containers or receptacles suitable for spectrophotometric analysis of the colored reaction product. Instrumentation for such spectrophotometric analysis is commercially available to accommodate miniature 96-well plastic trays, thereby making small-volume combinations of clinical specimens and color-producing reagents practicable, when considered in the light of the subject invention.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the process and device herein chosen for the purpose of the disclosure which do not constitute departures from the spirit and scope of the invention.

Claims (11)

1. A tip for a liquid handling means to receive liquid dispensed therefrom, said tip comprising: a continuous wall defining an axial bore which extends through the entire length of said tip, said bore having an upper portion and a lower portion; an elongate probe extending axially beyond the lower extremity of said wall, the probe being positioned so as to receive on its surface liquid issuing from the lower end of said bore; a longitudinally extending slot in said probe, and located axially beyond the lower extremity of said wall.

2. A tip as claimed in claim 1, wherein the elongate probe is attached to the wall of the tip.

3. A tip as claimed in claim 2 wherein the longitudinally extending slot in said probe has volumetric capacity from about 0.1 to about 10 microlitres.

4. A tip as claimed in claim 3 wherein the continuous wall thereof is generally cylindrical.

5. A tip as claimed in claim 4 wherein the axial bore is circular in cross section and generally downwardly convergent, the upper portion thereof being larger than the lower portion.

6. A tip as claimed in claim 3, wherein the probe is attached to the inner surface of the wall of the tip, and passes downwardly through the lower extremity of the wall.

7. A tip as claimed in claim 3, wherein the probe is attached to the outer surface of the wall of the tip, and extends axially and angularly downwardly to dispose the longitudinally extending slot therein approximately vertically below the lower extremity of the wall.

8. A tip as claimed in claim 3, claim 6 or claim 7, wherein the slot opens downwardly and ranges in width from 0.1 to 3.0 millimeters, and in length from 1.0 to 20 millimeters.

9. In combination, a liquid handling means comprising a reservoir for liquid, metering means to control accurately and reproducibly the dispensing of liquid from the reservoir, and connector means; and a tip connected to said connector means and comprising: a continuous wall defining an axial bore which extends through the entire length of said tip, the bore having an upper portion and a lower portion; an elongate probe extending axially beyond the lower extremity of said wall, the probe being positioned so as to receive on its surface liquid issuing from the lower end of said bore; and a longitudinally extending slot in said probe, and located axially beyond the lower extremity of said wall.

1 0. A process for making serial dilutions of a liquid sample, utilizing a liquid handling means having attached thereto a downwardly extending probe with a lowermost longitudinal slot therein, which comprises: (a) filling the liquid handling means with a selected diluent; (b) dipping the end of the probe into the liquid sample so as to fill the slot therein with liquid sample; (c) removing the probe and slot from the sample; (d) holding the probe and slot containing the sample vertically above a receptacle; (e) releasing a predetermined volume of said diluent from the liquid handling means to pass over, around and down said probe and slot into said receptacle; and (f) repeating steps (d) and (e) and desired number of times, into separate receptacles, without refilling said slot with sample.

11. A probe for use in association with liquid handling means for effecting dilutions of volumetrically small liquid samples with diluent from said liquid handling means, said probe comprising: an elongated body portion having an upper end and a lower end; means for attaching the upper end of said body portion to the outlet end of a liquid handling means so that liquid issuing downwardly from said liquid handling means outlet passes down the surface of said probe; a lowermost longitudinal slot in the lower end of said probe, adapted to retain therein a small liquid sample for dilution with diluent from said liquid handling means.