Quantitative liquid sampling instrument

Abstract: Quantitative liquid sampling instrument comprising: (1) a main body of a pipet device constituted of a pipet tip having an opening at the lower end for sucking and discharging a liquid, being in a shape of a tube shell having a cavity for holding a liquid sucked therein through the opening and being made of a transparent semi-transparent material; a pipet tip fitting portion to which a pipet tip is fitted detachably; and means for applying a gas pressure for sucking or discharging the liquid to or from the inside of the cavity of the pipet tip fitted to the pipet fitting portion, (2) liquid level detecting means installed under the main body of the pipet device, (3) vertical movement control means for moving the main body of the pipet device or a container of the liquid to make them approach relatively until a predetermined length of the lower end of the pipet tip is dipped into the liquid in the container, and (4) computing means for computing the end point of relative approaching movement of the main body of the pipet device and the container and for outputting a signal regarding the end point to the vertical movement control means.

Description: Background of the Invention: Field of the Invention: The present invention relates to a quantitative liquid sampling instrument for taking out an extremely small quantity of a liquid sample, a liquid reagent or the like quantitatively. Description of the Related Art: In microanalysis of biological samples, or the like, it is necessary to take out an extremely small amount of a sample or a reagent quantitatively from a container with high accuracy, and to dispense the taken sample into a reaction vessel or the like. Specially constituted systems and apparatuses are used therefor. Usually in analysis and measurement employing such systems, only a little amount of a sample is available, or an extremely minor component like an antibody or protein in blood is determined, and furthermore variations in quantity of the sample taken out will introduce critical errors in to the measurement result. Accordingly micropipettes are generally used which enable taking out a micro-quantity of samples with high accuracy. Further, since contamination in the sample will interrupt the achievement of correct analysis data, automated apparatuses are desired for treating a number of samples successively. For the purpose of taking out and dispensing successively a number of samples, many systems are developed in which tips of disposable type are used for the pipettes of the micropipette instrument, or a tip washing step is incorporated in the system. The micropipette instruments are operated in such a manner that the tip (or the lower end) of the pipette is inserted into a sample container, and a predetermined quantity of a liquid sample is sucked into the pipette by a negative air pressure, and subsequently the taken sample is dispensed into a reaction vessel or the like by a positive air pressure. In this operation, the pipette tip is washed, or is preferably replaced by a new one for every sample. In the aforementioned micropipette type of quantitative liquid sampling instrument, special consideration should be given to control of the position of the pipette tip relative to the level of the liquid sample in order to take out a liquid sample with high accuracy. In immune reaction measurement, biochemical analysis, or the like, sample containers have generally a small opening such as of a diameter of as small as 15 mm or less, so that quantitative sampling of relatively viscous liquid samples like blood are greatly affected by the meniscus of the liquid. Unless the level of the liquid to be brought into contact with the pipette tip is precisely detected, the inconvenience arises that an excessively large amount of the sample may be introduced into the pipette, or a required amount of the liquid cannot be taken out precisely. When an excessive amount of the liquid sample is introduced to the pipette, the liquid sample attached to the wall of the pipette may run down during dispensation of the sample, causing disadvantageous variation in the amount dispensed. Frequently in immune reaction measurement, samples are taken out from one sample container repeatedly to utilize the whole sample effectively. In such a case, the pipette may inconveniently strike against the bottom of the sample container at the final quantitative sampling from the same container. Accordingly, the depth of dipping of the pipette needs to be detected precisely. Provided that an "invariable quantity" of samples such as biological samples are always contained in an "invariable shape" of sample containers, it is generally understood that mere control of descending of the pipette to a fixed position enables accurate quantitative sampling with high accuracy. Practically, however, the quantities of a liquid sample in sample containers are rarely constant and the level of the liquid usually varies in every container, even if the container has a uniform diameter (namely having a constant horizontal sectional area). Therefore, in order to insert the pipette to a predetermined depth precisely into the liquid for quantitative sampling, it is greatly important in such types of quantitative liquid sampling instrument to detect the liquid level in the container for every sample and to decide the pipette inserting depth depending on the detected information. In the fields of biochemistry and analytical chemistry, as discussed above, especially in immune reaction measurement, liquid level height (position) in the container needs to be measured for sampling quantitatively a microquantity of liquid samples, and in particular, mechanical or automated quantitative liquid sampling instruments require indispensably a liquid level detection device. In the fields to which the present invention is directed, since a liquid level detection technique is necessary, many specific instruments have been proposed, and developed for practical use with the view of prevention of contamination. The liquid level detecting devices of conventional quantitative liquid sampling instruments, however, are not suitable for repeated measurements of liquid levels since the steps of liquid level detection and liquid suction are separately conducted, resulting disadvantageously in lower operating efficiency. One example of a conventional quantitative liquid sampling instrument for biological samples of the type mentioned above employs a non-contact type of optical liquid level detecting device which utilizes an optical reflection type of sensor, in which the distance to the liquid level is detected by bringing the sensor close to the liquid surface with downward light projection and the distance to the liquid surface is detected from the change in the measured intensity of the reflected light. This system, however, involves difficulties in achieving consistently stable measurement of various kinds of samples because the reflection from the liquid surface depends on the shape and inclination of the liquid surface, and further involves difficulty in conducting accurate measurement owing to ambiguity in detection of an abrupt change of intensity of the reflected light which is used as the liquid level detection information. In the case where the sample in the container to be taken out is an extremely small quantity, the diameter of the opening of the container for sucking with a pipette is small, since a large diameter of the container would give insufficient depth of the small quantity of the liquid, and would cause the pipette tip to strike against the bottom of the container, preventing suction of the liquid sample. With a small diameter of the container, it is not easy to bring the pipette and the sensor, etc. of a liquid detection device simultaneously close to the liquid face in the container, which gives disadvantage in operation in that the liquid sucking step is necessarily separated from the liquid level detecting step. On the other hand, liquid level detecting devices of contact type are also known which employ an electrode as the sensor, and detect the liquid level by an abrupt change of the electric current in the electrode upon contact with the liquid. Although this type of device gives advantageously a higher detection sensitivity in comparison with the non-contact types, the devices necessarily cause contamination by contact of the electrode with the liquid, even if a disposable type of pipette tip is employed as the pipet for the device. Therefore, such contact type devices are not suitable for microanalysis such as immune reaction measurement. Naturally, the disadvantage of contamination can be avoided by exchanging the electrode for every sample. Such a device, however, is not practical. Further, in the contact type of liquid level detecting device, the contact position of the pipette with the liquid is usually separated from the position of liquid level detection in the horizontal direction, whereby the shape or the inclination of the meniscus of the liquid sample gives a much more remarkable influence in comparison with the above-mentioned non-contact optical liquid level detecting device. For uses other than those to which the present invention is directed, another contact type of liquid level contacting devices is known which utilizes the phenomenon that a ray of light introduced into a transparent body at an angle larger than the critical angle and reflected totally will be emitted outside when the transparent body is brought into contact with a different external substance. (See, for example, Japanese patent Application Laid-Open Nos. Sho 53-55191, Sho 53-91779, and Sho 60-22730). This type of liquid level detecting device is capable of detection at higher sensitivity in comparison with the non-contact reflection type of device using light similarly. however, in using this type of device for sampling a micro-quantity of a liquid sample, the transparent body for liquid level detection is inserted together with the pipette simultaneously into the sample container of a small diameter. This simultaneous insertion is difficult, so that the liquid level detection and the liquid suction have to be conducted disadvantageously in separate and successive steps. Further, this type of device involves the problem of horizontal deviation of the liquid level detection point from the liquid sucking pipette position, and the problem of contamination by the previous samples is encountered similarly as that in the above electrode type liquid level detection device because of the contact type detection. EP-A-0038912 discloses a method of controlling immersion in a liquid of the truncated cone shaped end of a pipette which is to take a sample of the liquid. Light guides surround the pipette and have ends inclined at 45° to the axis of the pipette, these ends being located some distance along the pipette from the end of the truncated cone shaped end of the pipette. Light is transmitted down inside the material of the light guides and reflected at the inclined ends of the light guides and returned back up inside the light guides. DE-A- 3934344 describes a sample aspirating pipe arrangement (Fig.4) in which light is projected down inside the walls of the pipe, is reflected off the fluid surface, and returns back up inside the pipe walls to a detector. Summary of the Invention: The present invention intends to provide a novel quantitative liquid sampling instrument which can conduct precise liquid sampling by detecting the liquid level of the sample without the problems involved in the prior art. The present invention further intends to provide a novel quantitative liquid sampling instrument which can conduct simultaneously and rapidly liquid level detection and liquid sampling instrument. The present invention still further intends to provide a novel quantitative liquid sampling instrument which is free from the contamination between samples by simply employing a disposable type of pipette tip. The present invention also intends to provide an improved sampling method for liquid coupling. Thus the present invention provides a method of taking a sample of liquid from a container with a pipette which comprises detecting the level of the surface of the liquid in the container and immersing the tip of the pipette into the liquid a predetermined distance determined by reference to the detected surface level of the liquid, drawing liquid into the pipette, withdrawing the pipette from the liquid and discharging the liquid in the pipette into a desired location, the pipette tip being of truncated cone shape, characterised in that the detection of the surface level of the liquid is achieved by projecting radiation down inside the cavity of the pipette tip and repeatedly measuring the intensity of the radiation reflected back by the inside face of the lower end of the pipette tip as it approaches the liquid surface and detecting the abrupt reduction in the intensity of the measured radiation which occurs when the end of the pipette tip contacts the surface of the liquid, and in that the end of the pipette tip, from which the radiation is reflected, functions as a total reflection surface and is preferably located in a plane at right angles to the longitudinal axis of the pipette tip, and in that the radiation whose intensity is measured is that which is reflected from the said end of the pipette tip and is guided back up within the walls of the pipette tip and which emerges from the end of the pipette tip opposite to the end at which the reflection occurs. The present invention also provides a quantitative liquid sampling instrument comprising: (a) a main body of a pipette device having a pipette tip having an opening at the lower end for sucking and discharging a liquid, and being made of a transparent or semi-transparent material; a pipette tip fitting portion to which a pipette tip is fitted detachably; and means for applying a gas pressure for sucking or discharging the liquid to or from the inside of the cavity of the pipette tip fitted to the pipette fitting portion, (b) liquid level detecting means associated with the main body of the pipette device, optionally being mounted for movement with but independently of the main body, (c) vertical movement control means for moving the main body of the pipette device or a container of the liquid to make them approach relatively until a predetermined length of the lower end of the pipette tip is dipped into the liquid in the container, and (d) computing means for computing the end point of relative approaching movement of the main body of the pipette device and the container and for outputting a signal regarding the end point to the vertical movement control means; the liquid level detecting means comprising radiation projecting means for projecting radiation to the lower end face of the pipette tip and radiation receiving means for receiving radiation reflected back from the said lower end face, the instrument employing the intensity of the received radiation as the detection information, wherein the pipette tip is of the shape of an inverted truncated cone, and the lower end of the pipette tip is a horizontal face, characterised in that the light projecting means is installed at the inside of the pipette tip, and projects radiation down inside the cavity of the pipette tip and the light receiving means is opposed to the upper end face of the pipette tip, and in that the light receiving means comprises a light transmitting body constituted of an annular portion opposed to the annular upper end face of the pipette tip and a columnar portion provided continuously on the annular portion, the light receiving sensor being opposed to the upper end face of the columnar portion. Desirably the computing means for computing the end point of the relative approaching movement of the main body of the pipette device and the container detects an abrupt reduction in the intensity of the reflected light received by the light receiving means on contact of the horizontal lower end of the pipette with the liquid. Brief Description of the Drawings: Fig. 1 schematically illustrates the constitution of the quantitative liquid sampling instrument of Example 1. Fig. 2 illustrates the changes in intensity of the detected reflected light. Fig. 3(a) and Fig. 3(b) illustrate schematically the changes in reflected light intensities in the cases where the liquid sampling is conducted quantitatively and accurately, and is conducted less quantitatively and less accurately. Detailed Description of the Preferred Embodiment: The instrument which realizes the intended objects of the present invention has been provided on the basis of the experience below. In consideration of the operation of quantitative sampling and liquid level detection combinedly, the pipette is indispensably brought into contact with the liquid in the liquid sampling operation. The contact of a body with a liquid is conducted also in the liquid detection with an electrode and in the detection with a contact type optical liquid level detecting device in different application fields. Having noticed this fact, the inventors of the present invention decided to utilize a pipette simultaneously for a constituting member of a liquid level detecting device, although the pipette is a tool for sucking a liquid and is normally not for liquid level detection. As a result, the inventors have succeeded in using the liquid-sampling pipette simultaneously as a constituting member of a liquid level detecting device. Consequently, the following facts were confirmed: the numbers of the constituting members of the devices are reduced; a liquid level can be detected with high accuracy and further, quantitative sampling of a liquid with high accuracy are made feasible; the operations of liquid level detection and quantitative liquid sampling are unified so as to result in quickening of the operations; contamination between samples is readily prevented by use of a pipette of a disposable type; and the unified constitution of the device is highly useful for quantitative sampling of a micro-quantity of liquid such as a biological sample. The instrument of the present invention may be simplified by unifying the pipette tip with the main pipette instrument in one body and combining a washing means for washing the liquid-contacting portion of the pipette tip. Such a simplified instrument of the present invention requires less work for washing operation with less liability of contamination because the liquid-contacting member is limited only to the sampling pipette which serves also as a contact type of liquid level detecting device. In the instrument of the present invention, the shape of the pipette tip constituting the pipette device is not limited provided that its lower end is capable of being inserted into a small container (such as a cupshaped container and a test tube) and is capable of sucking a liquid into the pipette. Generally those of a shape approximately of an inverted truncated cone are preferably used. The shape of the lower end face of the pipette tip is such that the face functions as a total reflection surface for a downward light beam from the top in a non-liquid-contacting state, and is formed as a horizontal plain end face. The lower end face of the pipette tip in the present invention naturally includes end faces visible to the naked eye, but is not limited thereto. For example, a pipette tip, which is made of a thermoplastic material and is stretched with heating into a sharp needle shape, has no distinct end face to the naked eye. Such a pipette tip is shown experimentally to be useful for the liquid level detection by use of reflected light from the pipette lower end face of the present invention. Accordingly, the above-mentioned "lower end face" denotes collectively the interface between the lower end portion of the pipette tip and the surrounding space. The material for the pipette tip includes materials which are transparent as well as semi-transparent excluding opaque bodies which intercept light. Particularly preferred materials specifically include polypropylene, polystyrene, acrylics, polymethacrylates, polyethylene, and poly-4-methyl-1-pentene. An example of the tip made of such a material is a CRYSTAL LONG TIP (1 to 200µl) made by Wakenyaku K.K. Other examples of a disposable type of tips are those made of a commercial polypropylene such as STANDARDTIPS (1 to 100µl microlitres) made by Eppendorf Co. With a pipette instrument having a pipette tip of such a shape and material, light projected toward the lower end face of the pipette tip is reflected strongly (e.g. reflected totally) at the lower end face to be detected by a light-receiving means as a large reflection intensity. When this pipette tip is brought into contact with a liquid, the interfacial condition changes to decrease the difference of the refractive indexes, thereby abruptly decreasing the reflective light intensity. As a typical example in which the refractive index of the material of the pipette tip is nearly equal to that of the liquid, almost all of the light totally reflected in the non-liquid contacting state comes to be introduced into the liquid on contact with the liquid, whereby the intensity of the reflected light abruptly drops from a certain level. The instrument of the present invention is constituted by applying this principle to the pipette of the instrument for quantitative sampling of micro-quantity of liquid. In practical operations of the quantitative liquid sampling instrument of the present invention, a diluting liquid may be preliminary (or later) sucked into the pipette without any inconvenience in operation of the instrument since the diluting liquid is normally light-transmissive. Further, as described later, an instrument which is not affected by existence of a liquid in the pipette tip can be constituted by establishing a selective light path for light projection and light receiving. In the case where a transparent dilution liquid is sucked firstly into the pipette and liquid sample is sucked subsequently therein, the intensity of the received light is larger advantageously owing to the light reflected by the lower interface of the diluting liquid. Otherwise with a light source provided inside a pipette tip, the light efficiency of the light source is advantageously higher because the light from the light source is readily introduced by the upper interface of the diluting liquid into the tip material. The instrument of the present invention is particularly effective in taking out quantitatively a sample in an extremely small amount from 1 to 1000µl microlitres, particularly from 1 to 150 µl microlitres, but is not limited by the handling volume of sample in principle. In many cases, apparatuses for treating an extremely small amount of liquid like an apparatus for immune measurement handle a number of sample containers holding about 200µl of a sample and being about 1 cm in diameter. In such cases, it is practically not feasible to bring a pipette together with a separate liquid-level detecting member simultaneously close to the liquid face in the container. However, the above constitution of the present invention enables detection of the liquid level and suction of the liquid practically in one step, since only a pipette tip which serves both to suck the liquid and to detect the liquid face has to be brought close to or into contact with the liquid face in a container having a small inside diameter. The instrument of the present invention having the above-mentioned constitution includes various embodiments in practical use. For example, the container may be moved vertically to bring the pipette tip into contact with the sample in the container without any inconvenience, although a pipette vertical-driving mechanism is generally employed. The path of the light projected to or reflected by the pipette tip end face for liquid level detection is in the inside of the tube shell of the pipette tip. Particularly preferred is the type utilizing the pipette tip tube shell as the light transmitting medium. In the case where a pipette tube shell which is made of a highly light-transmissive material is utilized as the light transmitting material to transmit the reflected light, the influence such as diffusion of the light is reduced, enabling decrease of light intensity of the light source. The mechanical constitution may be made spatially roomy by arranging the received light sensor in opposition to the upper end face of the pipette tip as the light transmitting medium. Further, in addition to the use of the pipette tip as a light-transmitting medium, a particularly preferred constitution includes a light-transmitting medium having a columnar portion and an annular portion opposed to the annular upper end face of the pipette tip, and a received-light sensor provided at the upper end face of the columnar light-transmitting medium. With such a constitution, the reflected light from the lower end face of the pipette tip is effectively utilized as the light for received-light sensor, thereby the abrupt change of the reflected light intensity can be detected more precisely, or the intensity of the light source can be reduced. For the light source of light projecting means and the received light sensor mentioned above, a light emitting element such as various LEDs, semiconductor lasers, and photodiodes and the like are appropriately employed without limitation. The projection of light may be continuous or pulsed. The use of optical fiber for projection and reception of light is often preferred for securing freeness of mechanical design of the instrument. The light for liquid level detection is not limited, but is preferably light in the region of ultraviolet light of from 200 to 1100 nm, visible light, and infrared light, particularly preferred being light in the infrared region. In a bright place like a bright room where operation of the instrument is usually conducted, the received light sensor may react to room illumination or other light. Therefore the wavelength of light is selected as above to stabilize the operation of the apparatus and to improve the accuracy without external light disturbance. If a sample to be sucked into the pipette tip is turbid, a longer wavelength of light is generally advantageous since it causes less light scattering. The selection of the wavelength can be made by selection of the light source and the received light sensor, or by use of an appropriate light filter. The computing means for detecting the pipette/liquid contact from the detection information of abrupt change (or abrupt fall) of the reflected light intensity is generally constituted by a microprocessor, but is not limited thereto. The time of the abrupt change is judged from various phenomena such as a fall of the reflected light intensity below a predetermined threshold value, and a rise of the decreasing rate of the reflected light intensity above a predetermined rate. The end point of the downward movement of the pipette can be designed so as to be sufficient for taking out the sample. Specifically the quantitative liquid sampling instrument of the present invention is particularly useful, for example, for immune diagnosis apparatus which conduct diagnosis by determining quantitatively a minute quantity of protein or the like in blood by immune reaction, and biochemical reaction apparatuses and analysis apparatuses which require quantitative sampling of an extremely small amount of liquid. The instrument of the present invention, in which a pipette only is brought into contact with a liquid, enables simultaneous operations of a contact type liquid level detection and of liquid sucking with high accuracy. The present invention is described in reference to examples shown in the drawings. Example 1 Fig. 1 illustrates in outline the constitution of a quantitative liquid sampling instrument of the present invention employing a disposable type of pipette tip used, for example, for immune reaction measurement. Fig. 1 shows the states of the instrument before the pipette tip is inserted into a sample solution (namely a solution to be detected). In the drawings, the sample container 1 contains a liquid sample 2 such as a small amount of blood which may be as little as about several µl microlitres. The liquid sample 2 is introduced into the quantitative liquid sample instrument which is incorporated in an immune reaction measurement apparatus not shown in the drawing. A pipette tip fitting tube portion 4 and a frame 6 integrated therewith constitute the main body of the pipette instrument. A disposable type pipette tip 3 is fitted detachably to the pipette tip fitting tube portion 4 of the main body of the instrument, and is attached to and detached from the pipette tip fitting tube portion 4 at a predetermined waiting position by an attaching-detaching device not shown in the drawing. The pipette tip 3 is made of a light transmissive material as described above. A vertical movement mechanism 11 (not shown) moves the frame 6 constituting the main body of the pipette instrument from an upper waiting position (see Fig. 1) downward to a lower movement end point (not shown) by a pulse-motor type mechanism (or a turning screw type, or cylinder type), and then returns it to the waiting position, under control of a driving control device 12 (not shown). The pipette fitting tube portion 4 is fixed to the frame 6 through a light transmitting body 22. The light transmitting body 22 has a columnar portion 221 as the upper portion which is fixed to the frame 6, and an annular portion 222 which extends downward from the lower end periphery of the columnar portion 221. The pipette tip fitting tube portion 4 and a light-emitting element (e.g. an LED 25) constituting a light-projection means are fixed in the inside of the annular portion 222. The lower end face of the annular portion 222 is opposed to the upper end face of the pipette tip 3 fitted to the pipette tip fitting tube portion 4. A cover 223 is provided to cover the light transmitting body 22. The pipette tip fitting tube 4 has a through-hole 41. A photodiode 10 which is a light-receiving element of the light receiving means is fixed on the top of the frame 6 so as to be juxtaposed to and confront the upper end face of the columnar portion 221 of the light-transmitting body 22. It is preferable that the photodiode 10 contacts face-to-face with the surface 221. A detection circuit 8 (not shown) detects the signal derived from the electromotive force, and outputs a signal to the driving control device 12. In this example, the light-emitting element which is the light source 25 is made to emit light in pulses which are synchronized with the detection timing of the above detection circuit 8 to cancel the influence of external disturbance such as room illumination and the like. A volume controller 14 (not shown), such as an air cylinder, is connected to the cavity in the pipette tip 3 through a tube 13 to generate a negative pressure for sucking liquid and a positive pressure for discharging the liquid. The stroke length of the piston in the air cylinder is adjustable. The operation of the above-mentioned quantitative liquid sampling instrument is described below. The instrument firstly is waiting at the state shown in Fig. 1. In this state, the pipette tip 3 is attached to the pipette tip fitting tube portion by a pipette tip attaching-detaching device not shown in the drawing. The instrument is waiting in this state at the upper waiting position (see Fig. 1). Thereto a predetermined sample container 1 is introduced and placed at a lower position for taking the liquid by means of a suitable conveyor or the like. Subsequently, the frame 6 starts its downward movement with the aid of the vertical movement mechanism 11, and the light-emitting source 25 starts emitting pulses of light. A portion of the light emitted downwardly from the light-emitting source 25 is transmitted through the inside of the pipette tip 3 and is reflected by the lower end face. In the pipette instrument having the structure described above, the light reflected by the lower end face of the pipette tip is transmitted through the inside of the pipette tip to the upper end thereof, introduced to the annular portion 222 of the light transmitting body 22 via the end face 31 of the tip and the end face 224 of the body 22, and then transmitted through the columnar portion 221 of the light-transmitting body to the photodiode 10. With such a constitution, the light reflected by the whole of the bottom end face of the pipette tip is to be received by the photodiode 10 through the above-mentioned path, whereby the utilization rate of the reflected light is greatly increased in comparison with the case where only a portion of the light reflected at the annular portion is detected (for example, the case where a photodiode is directly opposed to a limited portion of the annular upper end portion of the pipette tip). Accordingly, the more clear detection of the abrupt change of the intensity of the reflected light is made feasible, or the intensity of the light of the light source may be decreased for obtaining the same level of detection accuracy, advantageously. As the pipette moves downward, light reflected by the liquid surface also comes to be introduced to the photodiode 10 thus increasing gradually the received light intensity which is detected in the detection circuit 8. As soon as the lower end of the pipette tip 3 is brought into contact with the liquid sample as the result of further downward movement, the reflection condition abruptly changes because of the decrease of the difference in the refractive index at the interface caused by the change from the refractive index of air with which the lower end of the pipette was surrounded to the refractive index of the liquid sample brought into contact with the tip. Therefore, the reflection at the lower end face of the pipette tip is practically cancelled, resulting in a reflected light intensity of nearly zero at the photodiode 10. Therefore the contact of the pipette tip 3 with the liquid can be readily detected by the abrupt decrease of the reflected light intensity. The pipette device brought into contact with the liquid is further moved downward and stopped at a predetermined depth of the tip. At this position, the liquid is sucked by driving the air cylinder. The liquid suction is stopped by stopping the cylinder drive. With the constitution of this example, the liquid sample can be sucked into the pipette accurately by the air cylinder stroke at the position where the downward movement of the pipette tip has been stopped. The predetermined distance of the downward movement of the pipette after the contact with the liquid is decided in relation to the amount of the sample, the diameter of the container, the amount of the liquid to be taken out, and so forth. In immune reaction measurement apparatuses, for example, the distance is about several mm in many cases for the purpose of obtaining a high accuracy of detection. Fig. 2 is a plot of the measurement results of the intensity of the reflected light detected by the detection circuit 8 as a function of the time with a pipette instrument having a pipette tip moving downward as mentioned above. This drawing shows clearly that the intensity of the reflected light abruptly falls at the time of contact with the liquid surface. The instrument of this example gives an advantageous effect that the position of the lower end of the pipette tip in the liquid sample is precisely decided at the start of the liquid suction, and enables accurate quantitative sampling of a liquid. The instrument of this example gives an additional effect that the excess or insufficiency of the amount of the taken-out liquid can be confirmed by confirming the starting time and the finishing time of the liquid suction. This is explained by reference to Fig. 3. In Fig. 3, the abscissa shows the lapse of time, and the ordinate shows the detected intensity of reflected light. Both Fig. 3(a) and Fig. 3(b) illustrate real measurement results of certain samples. In Fig. 3(a), the reflected light intensity gradually increases as the pipette tip is moved down toward the liquid sample. At the time when the tip comes to touch the liquid sample (at the time t 1 in the drawings), the light to be reflected comes to penetrate into the liquid sample, resulting in rapid fall of the light detection intensity. Then liquid suction is conducted at a predetermined negative pressure for a predetermined time T. After the lapse of time T (the time t 2 in the drawings), the suction is stopped, and the pipette is moved up to become apart from the liquid sample, resulting in the abrupt rise of the intensity of the detected reflected light up to the intensity level before the time t 1 . In Fig. 3(b), a case is shown where the pipette tip comes to be separated from the liquid sample by some reason during the suction (at a time t s ). This separation of the pipette tip from the liquid sample is readily confirmed by restoration of the intensity of the detected reflected light at the time t s . In this case, the amount of the sucked liquid becomes insufficient in correspondence with the shortage of time in comparison with the predetermined time T (as shown by the shadowed portion in the drawing). In such a manner, the time length of the contact with the liquid is controlled and confirmed by measuring the change pattern of detected reflected light in applying negative pressure to the insider of the pipette tip contacting with the liquid sample, whereby sufficiency or insufficiency of the sucked amount of the liquid sample to be measure is confirmed in comparison with the predetermined sucked amount. Thus the instrument of the present invention gives excellent effect that an influence of a sampling error can be avoided practically which affects, for example, the result of diagnosis by an immune reaction measurement using liquid sample taken with this instrument, since the operation is conducted with confirmation of quantitative sampling of each liquid sample. Thus a reflected light pattern as in Figure 3(a) indicates that accurate sampling has occurred and 3(b) that it has not. The quantitative liquid sampling instrument of the present invention gives advantages below by solving various problems from which conventional instruments could not be free. The instrument enables the accurate detection of the liquid surface level of a sample or the like in a container, realizing high accuracy in quantitative liquid sampling, which make it possible to provide a quantitative liquid sampling instrument of high accuracy suitable, for example, for an immune diagnosis apparatus. The instrument enables liquid level detection and liquid sucking simultaneously in one step with a micro quantity of sample as a measurement object, realizing rapid operation, which makes it feasible to constitute a quantitative liquid sampling instrument of automated or mechanised immune diagnosis apparatuses. The instrument can prevent contamination between samples by use of a disposable type of pipette tip. In the preferable constitution of the present invention, utilizing the tube shell of the pipette tip, the arrangement of the constituting members can be designed with high freedom, since a mechanical member for liquid level detection need not be placed around the pipette tip of the pipette instrument and interference of the pipette tip with other circumferential devices and members is avoided. In another preferable constitution utilizing a light-transmitting body of the type of collecting light transmitted to the upper end face through the columnar portion, almost all of the reflected light is utilized for detection, giving high detection sensitivity and enabling decrease of intensity of the light source. In embodiment utilizing, for detection, light of nonvisible wavelength region such as light of infrared light region, the detection is not affected by external disturbance like room illumination, realizing high accuracy of the detection.

Claims: A method of taking a sample of liquid from a container with a pipette which comprises detecting the level of the surface of the liquid in the container and immersing the tip of the pipette into the liquid a predetermined distance determined by reference to the detected surface level of the liquid, drawing liquid into the pipette, withdrawing the pipette from the liquid and discharging the liquid in the pipette into a desired location, the pipette tip being of truncated cone shape, characterised in that the detection of the surface level of the liquid is achieved by projecting radiation down inside the cavity of the pipette tip and repeatedly measuring the intensity of the radiation reflected back by the inside face of the lower end of the pipette tip as it approaches the liquid surface and detecting the abrupt reduction in the intensity of the measured radiation which occurs when the end of the pipette tip contacts the surface of the liquid, and in that the end of the pipette tip, from which the radiation is reflected, functions as a total reflection surface, and in that the radiation whose intensity is measured is that which is reflected from the said end of the pipette tip and is guided back up within the walls of the pipette tip and which emerges from the end of the pipette tip opposite to the end at which the reflection occurs. A method as claimed in claim 1 characterised in that the end of the pipette tip from which the radiation is reflected is located in a plane at right angles to the longitudinal axis of the pipette tip. A quantitative liquid sampling instrument comprising: (a) a main body (4) of a pipette device having a pipette tip (3) having an opening at the lower end for sucking and discharging a liquid, and being made of a transparent or semi-transparent material; a pipette tip fitting portion (4) to which a pipette tip (3) is fitted detachably; and means for applying a gas pressure (13,14) for sucking or discharging the liquid to or from the inside of the cavity of the pipette tip (3) fitted to the pipette fitting portion (4), (b) liquid level detecting means (25,10) associated with the main body of the pipette device, optionally being mounted for movement with but independently of the main body, (c) vertical movement control means (6) for moving the main body of the pipette device or a container of the liquid to make them approach relatively until a predetermined length of the lower end of the pipette tip is dipped into the liquid in the container, and (d) computing means for computing the end point of relative approaching movement of the main body of the pipette device and the container and for outputting a signal regarding the end point to the vertical movement control means; the liquid level detecting means comprising radiation projecting means (25) for projecting radiation to the lower end face of the pipette tip and radiation receiving means (10) for receiving radiation reflected back from the said lower end face, the instrument employing the intensity of the received radiation as the detection information, wherein the pipette tip (3) is of the shape of an inverted truncated cone, and the lower end of the pipette tip is a horizontal face, characterised in that the light projecting means (25) is installed at the inside of the pipette tip (3), and projects radiation down inside the cavity of the pipette tip (3) and the light receiving means (10) is opposed to the upper end face of the pipette tip (3), and in that the light receiving means comprises a light transmitting body constituted of an annular portion (222,224) opposed to the annular upper end face (31) of the pipette tip and a columnar portion (221) provided continuously on the annular portion, the light receiving sensor (10) being opposed to the upper end face of the columnar portion (221). A quantitative liquid sampling instrument as claimed in claim 3, characterised in that the computing means for computing the end point of the relative approaching movement of the main body of the pipette device and the container to detects an abrupt reduction in the intensity of the reflected light received by the light receiving means on contact of the horizontal lower end of the pipette with the liquid.

Inventors: MITSUHISA YUKIO, SAKKA TOSHIAKI, MITSUHISA, YUKIO, SAKKA, TOSHIAKI
Applicants: TOSOH CORP, TOSOH CORPORATION
Application year: 1991
Main classification: G01F23
IPC codes: G01F23, B01L3, G01N35, G01N1