METHODS AND DEVICES FOR CONDUCTING ASSAYS

Abstract: Methods and devices for separating bound label from unbound label within an assay mixture and for predispensing assay reactants in self-contained assay vessels, as well as a method for detecting the presence and/or amount of an analyte within a fluid sample, and a reusable detection vessel for use therein and with specific binding assays in general are disclosed. Significant to the separation of bound label from unbound label is the use of a cushion comprising generally one primary layer and in some cases one or more secondary layers.

Description: METHODS AND DEVICES FOR CONDUCTING ASSAYS Cross-Reference to Related Application This application is a continuation-in-part of applicant's copending application Serial No. 768,108, filed August 21, 1985. Technical Field This invention relates generally to specific chemical, biochemical and immunochemica 1 assays in self-contained assay vessels, and more particularly, to methods for storing and mixing reactants, and for separating labeled components bound to a solid phase from unbound labeled components in binding assays, followed by measurement of the bound labeled components. This invention also relates to methods for detecting the presence and/or amount of an analyte within a fluid sample using either a homogeneous or heterogeneous assay performed in a self-contained assay vessel, where the assay vessel contains a reaction mixture and a cushion which are predispensed in one or more layers. Background of the Invention Specific chemical, biochemical and immunochemica I assays have found widespread application in the fields of biomedical research and clinical diagnostics where they are used to determine the presence or amount of a variety of substances (analytes) common] encountered in biological fluids. Such substances may include proteins, drugs, hormones, metabolites, nutrients, vitamins, microorganisms, etc. in addition, such specific assays may find ut lity in other fields, such as food processing and environmental quali. ty control, for example, the detection of microorganisms and their toxins, or for detecting organic wastes. Specificity is very important because of the plethora of substances which may be present in samples. Such specific assays are commonly divided into "chemistries" (chemical and biochemical assays) and binding assays (e.g. i unoassays) . Such assays are further classified as "homogeneous" or "heterogeneous." In a homogeneous assay, a single reaction solution containing the sample is incubated to develop a detectable signal. For homogeneous binding assays, in which the amount of a bound label must be detected, the signal emitted by the bound labeled component is different from the signal emitted by the unbound labeled component. Hence, the two can be distinguished without the need for a physical separation step. The classical homogeneous specific binding assay is the enzyme-multiplied i munoassay technique (EMIT), described in U.S. Patent 3,817,837, issued to Rubenstein. Homogeneous assays are typically rapid and easy to perform, usually requiring ten minutes or less to complete. They can be carried out either manually or with automated instruments. Various methods for homogeneous assays are described in Methods in Clinical Chemistry, A.M. Pesce and L.A. Kaplan editors, C.V. Mosby Co., 1987. Homogeneous immunoassays can be more complex to perform than other assays, in that they typically require sequential additions and mixing of reagents with careful timing. Automation is preferable and has been achieved with various large clinical analyzers (e.g. DuPont aca™, Roche Cobas Bio" 1 } . In spite of their simplicity, homogeneous assays have several disad antages: they are prone to interferences, and are generally limited in sensitivity to detection of approximately one nanomolar analyte. Binding assays are the most specific homogeneous assays, but they are still subject to interferences, they are typically only compatible with low molecular weight analytes, and they require extended incubations (20-40 minutes) to detect nanomolar levels of analyte. Heterogeneous assays are usually binding assays. In many such assays, both large and small molecules can be detected. At least one labeled ligand is present and a second phase (typically solid) is employed to separate bound from unbound label. Since the signal emitted by the bound and unbound labeled ligands is identical, the two must be physically separated in order to distinguish between them. The classical heterogeneous specific binding assay is the competitive radioimmunoassay (RIA), described by Yalow (Science 200: 1245, 1978). Other heterogeneous binding assays are the radioreceptor assay, described by Cuatrecasas (Ann. Rev. Biochem., 43: 109-214, 1974), and the sandwich radioimmunoassay, described by Wide (pp. 199-206 of Radioimmυnoassay Methods, edited by Kirkham and Hunter, E. & S. Livingstone, Edinburgh, J 970) . Heterogeneous binding assays can be significantly more sensitive and reliable than homogeneous assays: interferences are usually eliminated, signal-to-noise ratios are improved because unbound label is eliminated, and excess binding reagents can sometimes be used to speed binding reactions involving very dilute analyte. In a typical heterogeneous ("double antibody") competitive RIA, a known amount of radiolabeled ligand and ligand_ present in the sample compete for a limited amount of antibody. Sufficient time is allowed for specific binding to occur, after which the antibody and bound ligand are precipitated by addition of anti-immunoglobulin, washed to remove unbound label by repeated centrifugation, and the amount of labeled ligand present in the bound phase is determined. Heterogeneous competitive binding assays work equally well for low and high molecular weight substances. A sandwich assay can be used to achieve greater sensitivity for analytes such as antigen in an immunoassay. In a sandwich assay, excess ligands are used to force binding at concentrations below the dissociation constant of the binding pair. Such assays usually employ a solid phase consisting of a plastic head to which an antibody is permanently attached. In the typical sandwich immunoassay, two antibody types are required, each of which can bind simultaneously to the antigen. The antibody which is not bound to the solid phase is labelled. As with competitive RIAs, one or more discrete washing steps to separate bound and unbound label are required, and sequential addition of reagents is typical. Sandwich assays are typically used for high molecular weight substances. Because in heterogeneous assays the solid phase must be isolated and washed, and because sequential reagent additions are frequently required, they tend to be time consuming and labor-intensive. However, they offer desirable results because they can be used for low and high molecular weight compounds, are less prone to interferences than homogeneous assays, and can be sensitive to subpicomolar antigen concentrations. Automation of heterogeneous immunoassays has been accomplished with limited commercial success (ARIA II by Becton Dickinson, CentRIA by Union Carbide) . Hunter describes such an automated device in U.S. Patent No. 4,125,375 (issued November 14, 1978). However, these devices have required either sophisticated and expensive instrumentation to carefully control liquid flow and to monitor bound and unbound fractions, or it has resulted in the detection only of the unbound label flowing through a rapidly hydrated antibody solid phase. Several attempts have been made to eliminate the inconvenience of washing steps in heterogeneous binding assays. For example, Glover et al., GB 1,411,382, describe a method for measuring the amount of unbound radiolabel, after partial separation from bound label, by shielding the bound (lower) phase. However, it is well known in the art that the sensitivity and precision of specific binding assays is severely limited if changes in the unbound rather than the bound labeled component are measured. Furthermore, methods which lack a washing step have the disadvantage of detecting both tight-binding (specific) and weak-binding (nonspecific) label, resulting in very high nonspecific signal. Charlton et al. , U.S. Patent 4,106,907, issued August 15, 1978, disclose another container for radioactive counting which consists of a tapered reaction tube having a radiation shield extending up from the bottom of the tube to a uniform height, such that only radiation from the supernatant (the unbound labeled fraction) can be detected. This method is subject to the same limitations as Glover et al., supra. Chantot et al., Analyt. Biochem. 84:256, 1978, describe a radioreceptor assay method for measuring the binding of radiolabeled ligands to membrane receptors. The technique involves counting the total amount of radiolabel present, centrifuging the sample, and recounting with an externally mounted copper screen which serves to absorb radiation from a defined volume of the supernatant. The screen itself consists of a copper sleeve mounted on the outside of a custom-made test tube having a small knob precisely positioned above the base to support the screen. This method suffers from the disadvantage of requiring double detection, and suffers as well from high nonspecific binding as described above for the Glover and Charlton methods. Furthermore, the tube as disclosed is vulnerable to jamming and breaking in standard gamma counters. As with the above-described "screening" methods, the large diameter of the screen allows significant scattered radiation from within the screened volume to impinge on the detector, resulting in inaccurate measurements of the unscreened label. Also, because bound label is directly adjacent to and in contact with unbound label, normal and unavoidable variability in the position of the screen or in the volumes of the unbound and bound phases can cause significant variability in signal. Bennett et al. , (J. Biol. Chem. 252: 2753, 1977) describe a radioreceptor assay in which, after mixing and incubating reagents, the asssay mixture is transferred to a centrifuge tube to wash the solid phase containing bound label. They employed prolonged (30 minutes) high speed centrifugation to force the solid phase into a solution of 20% sucrose, followed immediately by freezing the assay tube in liquid nitrogen and excising the tip of the tube containing the solid phase and bound label. This method provides more effective separation of bound and unbound label than those described above, but has several significant disadvantages. The assay mixture cannot be incubated in situ on top of the sucrose solution, thus requiring separate incubation and separation vessels, because reactants would diffuse into the solution. Care must be used in loading the assay mixtures onto these sucrose solutions because mixing will cause dilution of the assay mixture, thus changing the equilibrium for assay reactants. The separation is relatively lengthy, and assay tubes must be frozen immediately after centrifugation because the bound label can dissociate from the solid phase and diffuse away from the tip of the separation tube. Finally, excising the tip of separation tubes is inconvenient, time-consuming, difficult to perform reproducibly, exposes the user to the risk of liquid nitrogen burns and radioactive contamination from fragments of frozen tubes and their, contents, and would be very difficult to automate. In U.S patent 4,125,375 (issued Nov 14, 1978), Hunter describes a method and automated instrumentation for performing heterogeneous immunoassays by carefully injecting a sucrose solution underneath a previously equilibrated immunoassay mixture containing particles of higher density than the sucrose solution. The particles are allowed to settle into the injected subphase, thereby separating the particles from the unbound label. This method potentially eliminates some of the disadvantages inherent in the Bennett et al. method, but suffers from several significant shortcomings. These shortcomings include that: (1) it requires separate preequilibration of the assay mixture prior to separation of bound and free label, plus removal of liquid waste, and thus cannot be self-contained, (2) the method is not readily adaptable to the most rapid (centrifugal) separations, (3) it suffers from potential dilution and diffusion artifacts as in the Bennett et al. method, (4) it is not suitable for convenient and reproducible manual assays, and (5) any automated instrument would require pumps, tubing, plus reservoirs for reagents, wash solution, and liquid waste. Linsley et al., Proc. Natl. Acad. Sci . (USA) 82: 356,1985, describe a radioimmunoassay for type I transforming growth factor in tissue cultures. The solid phase is S_^ aυreυs, and the bound label is separated from the unbound label by rapid sedimentation into a solution of 10% sucrose, followed by freezing in liquid nitrogen and excision of the tip of the centrifuge tube to determine the sedi ented bound label. This method is essentially an immunoassay embodiment of the rad io-receptor assay described by Bennett et al. , with the inherent disadvantages of the former method. Although each of the binding assay methods described above have brought minor improvements to the state of the art, there remains a need in the art for a method of specific binding assay which combines the ease and rapidity of homogeneous techniques with the enhanced sensitivity typical of heterogeneous techniques, for both isotopic and nonisotopic applications, without the undesirable variability, delay, labor, and dissociation which occur during the wash steps. Further, the method should allow rapid separations, should be convenient for manual use with standard detection instruments, and should be readily adaptable to semi-automated or fully automated instrumentation. Ideally the method should be self-contained, have minimal plumbing and moving parts, and be compatible with fully predispensed reagents. Furthermore, there is a growing need for compact and inexpensive instrumentation for use in decentralized test sites (e.g. physicians' offices). Preferable features for testing systems adapted to decentralized, low volume sites include: (1) the use of unprocessed samples (e.g. whole blood); (2) elimination of instrument calibration and maintenance; (3) unit-dispensing with no multi-use reservoirs for reagents or waste; (4) containment of waste within a sealed reaction vessel for protection from infectious samples; (5) precision and accuracy attainable by unskilled users which is equivalent to that attainable on larger, automated clinical lab analyzers by skilled operators. Advances and available products designed to meet the needs of decentralized test sites are reviewed by J.A. Jackson and M.E. Conrad in American Clinical Products Review (August, 1987). However, no product is disclosed which solves the problems discussed above or provides these needed features. The present invention fulfills the needs described above for both homogeneous and heterogeneous assays and further provides other related advantages. Disclosure of Invention Briefly stated, the present invention discloses methods and associated devices for separating bound label from unbound label within a binding assay mixture, and for predispensing a cushion in one or more liquid or solid layers, as well as in some cases a reaction mixture, which, with the addition of sample, forms a self-contained system for both heterogeneous and homogeneous assays. In addition, methods for detecting the presence and/or amount of analyte within a fluid sample, as well as assay vessels and a reusable detection vessel for use therein, and within specific binding assays in general are disclosed. For purposes of the present invention, the term "cushion" is defined to include a primary layer plus any secondary layers within any one embodiment. The "assay mixture" generally comprises a reagent mixture plus a sample containing analyte, in which mixture the assay process has been initiated. The "reaction mixture" comprises all reaction components (except sample) required for initiation of an assay. Other reaction components may be added subsequently. The reaction mixture can be in the form of one or more layers in an assay vessel, a layer being in the form of a droplet, or varying from a thin film to several centimeters thick depending on the volume of the reaction mixture and the dimensions of the assay vessel. For most heterogeneous assays, the assay mixture includes at least one or more labels and one or more binding components, and sample. In some heterogeneous assays, the assay mixture contains only sample and binding components added subsequent to assay initiation. Binding components normally comprise two parts: a solid phase and a specific binding agent attached thereto, which confers specific binding activity. Additional specific binding agents may be present which are not initially attached to the solid phase, as long as substantially all of the additional binding agent becomes attached to the solid phase prior to separation of bound label from unbound label. In addition, other binding agents may be added subsequent to the initial separation of bound label from unbound label. Once the reaction mixture and sample are combined to form the assay mixture, an incubation period is usually required. The incubation period can range from one second to several days, depending in part upon factors such as the sensitivity required, and the binding affinity and concentration of binding components. Following incubation of a heterageneous binding assay mixture, at least some label and/or analyte is bound to at least some of the binding components. Said incubated binding assay mixture typically includes some unbound label and/or unbound analyte, and in addition also includes other components such as water, buffer, preservative, and proteins - these components typically comprising a largely aqueous solution. As an alternative to forming the complete assay mixture within an assay vessel, an assay mixture and/or 10 cushion may be prepared outside of the assay vessel. This alternative is especially appropriate for non-isotopic binding assays, thereby avoiding the potential hazards associated with handling of radioisotopes outside of the assay vessel. This alternative is especially advantageous, for instance, when automated liquid-handling apparatus is available to the user for dispensing reagents. Briefly, for heterogeneous binding assays these methods comprise: (a) contacting a primary layer with an assay mixture, both the binding components and the unbound label being immiscible with the primary layer and the binding components being of a different density than the primary layer; and (b) subjecting the assay mixture in contact with the primary layer to conditions sufficient to cause the binding components and the unbound label to separate. Typically the binding components have a density greater than that of the primary layer and the aqueous solution component of the assay mixture has a density less than or equal to that of the primary layer. In some embodiments, a barrier layer is positioned between the assay mixture and the primary layer. The barrier layer, when liquified, is miscible with the assay mixture, but while in the solid or gel form, it serves to separate the reaction mixture from the primary layer. A barrier layer is especially useful when predispensing reaction components is desired for an embodiment utilizing a liquid primary layer. In particular embodiments of heterogeneous assays, either or both the binding components and the unbound label may be of the same density as the primary layer. In one such embodiment, the binding components are formed by immobilizing specific binding agents to the surface of a vessel containing the remainder of the assay mixture, and thus the density- of the binding components is not relevant to the assay. Upon completion of an incubation step, addition of a cushion displaces the aqueous phase of the reaction mixture containing unbound 11 label. In another such embodiment, the binding components comprise magnetic particles and are separated from unbound label by magnetic forces. In such cases, the binding components need not differ significantly in density from the primary layer, though typically the aqueous solution component of the assay mixture will have a density less than that of the primary layer. For homogeneous assays, additional embodiments are employed. In one such embodiment, the density of the entire assay mixture may be greater than the density of the primary layer. Such an embodiment typically utilizes a barrier layer or primary layer which is in a solid or gel form during assay initiation, but which is displaced by the assay mixture during a subsequent centrifugation step. The barrier or primary layers may be selectively liquifiable, especially if gravitational force alone is used to displace the primary layer with the assay mixture. When the assay mixture has ' displaced the primary layer, it typically mixes with a secondary layer containing additional reagents such as color developer. One advantage of such a configuration is that homogeneous assays can be conveniently performed in the same type of self-contained assay vessel as heterogeneous assays (e.g. Figure 4B) . Another homogeneous assay embodiment exhibits the above-mentioned advantage plus additional advantages. This embodiment utilizes an assay mixture having a lower density than the primary layer. The cushion volume may be optimized to maintain the assay mixture at the appropriate height in the assay vessel for effective mixing and for measurement of signal (e.g. color intensity) . The assay vessel may also serve as a centrifuge tube for clarification of the assay mixture. In some embodiments the cushion may be omitted entirely and a homogeneous assay may be performed in the empty tube, especially if no mixing reaction components are required (e.g. direct measurement of sample color intensity) . 12 Within another aspect of the present invention, a method for detecting the presence or amount of an analyte within a sample containing insoluble components is disclosed. The method generally comprises: (a) combining a sample with a partial reaction mixture to form an incomplete assay mixture, the incomplete assay mixture being in contact with a primary layer; (b) subjecting the incomplete assay mixture to conditions sufficient to cause any insoluble components present in the incomplete assay mixture to separate therefrom; (c) combining additional reaction components with the incomplete assay mixture to form a complete assay mixture, the complete assay mixture containing one or more binding components and label, at least some of the label and some of the analyte binding, directly or indirectly, to the binding components, the binding components and unbound label being immiscible with and of different density than the primary layer; (d) subjecting the complete assay mixture to conditions sufficient to cause the binding components and the unbound label to separate; and (e) detecting the label bound to the binding components and therefrom determining the presence or amount of the analyte. The insoluble component may be, for example, blood cells within whole blood samples, or a precipitate (such as a phosphotungstate lipoprotein precipitate) . Within a related aspect of the present invention, a method for detecting the presence or amount of an analyte within a sample is disclosed. In one embodiment, performed in a heterogeneous format, the method generally comprises: (a) combining a sample with a partial reaction mixture to form an incomplete assay mixture, the incomplete assay mixture being in contact with a primary layer; (b) combining additional reaction components with the incomplete assay mixture to form a complete assay mixture containing a precipitate, the complete assay mixture containing one or more binding components and label, at least some of the label and some of the analyte binding, 13 directly or indirectly, to the binding components, the binding components and unbound label being immiscible with and of different density than the primary layer; (c) subjecting the complete assay mixture to conditions sufficient to cause the binding components to separate from the unbound label and the precipitate; and (d) detecting the label bound to the binding components and therefrom determining the presence or amount of the analyte. In another embodiment, performed in a homogeneous format, additional reaction components are combined with the incomplete assay mixture to form a complete assay mixture, containing a precipitate, and the complete assay mixture incubated under conditions and for a time sufficient to generate signal in proportion to the amount of analyte in the sample. Prior to or subsequent to incubation, the complete assay mixture is subjected to conditions sufficient to cause the precipitate to separate from the complete assay mixture. Subsequently, the signal within the complete assay mixture is detected, and the presence or amount of the analyte determined. In certain related embodiments, additional reagents may be positioned as one or more secondary layers within the cushion. In such embodiments, solids separated into the cushion from the reaction mixture will typically remain separated by water-immiscible material from the assay mixture. This can be achieved by selection of cushion components with appropriate densities and irascibility properties. In an embodiment of the present invention which includes the predispensing of the cushion and the reaction mixture, the reaction mixture is contacted with the primary or barrier layer, as described above, substantially prior to the addition of sample and the subsequent incubation of the assay mixture. For both heterogeneous and homogeneous assays, this provides advantages to the user of greater convenience compared to assays where each reactant must be dispensed as needed. Furthermore, where precise and 14 automated equipment is used to predispense the assay reactants during manufacture of the assay system, greater precision is to be expected compared to manual dispensing of reactants by the user as they are needed. In some embodiments, one or more reactants are contained separately from the main reaction mixture in the assay vessel. For example, in a competitive immunoassay, reaction components may be predispensed to form a reaction mixture. Following addition of sample to the reaction mixture, the reaction is initiated by addition of binding components (analyte-specific antibody and particles) . This analyte-specific antibody can be added manually or automatically from an external source. However, most conveniently antibody can be contained in a reagent reservoir within the assay vessel prior to initiation of the reaction. For example, as will be discussed more fully below, a secondary layer in the cushion can serve as a reagent reservoir containing additional reaction components. Alternatively, a hollow cap for the assay vessel , having a removable seal, may be provided to serve as a reagent reservoir. One kind of seal could be fashioned by providing a small orifice in the hollow cap, where the orifice is plugged with a selectively liquifiable or penetrable material (preferably water-immiscible). The seal is weakened or breached by increasing the temperature or otherwise applying a concentrated source of energy (e.g. a beam from a laser diode) . Such a beam may be conveniently applied to all assay vessels within an assay using a centrifuge rotor as sample transporter, and may advantageously be operated in a synchronized fashion to selectively target the reservoir seals. A moderate force such as an air pressure or low-speed centrifugation cou Ld be employed to force the liquid from the cap reservoir into the reaction mixture. In a related aspect of the present invention, a method is disclosed for detecting the presence and/or amount of analyte within a fluid sample using either a 15 homogeneous or heterogeneous assay performed in a self-contained assay vessel. The assay vessel typically contains reaction components which are predispensed in one or more layers. In some embodiments for detecting the presence or amount of analyte within a fluid sample, the label may comprise the analyte itself, where the analyte is capable of emitting a detectable signal. Such analytes include hemoglobin as well as enzymes (prostatic acid phosphatase, creatine kinase) when the latter are combined with appropriate signal-producing substrates. For example, one heterogeneous assay for detecting the percentage of glycosylated hemoglobin present in blood typically involves separating most or all of this analyte from a blood sample using a binding component in the form of an ion exchange or affinity column, then measuring the absorbance of the bound analyte (glycosylated hemoglobin) as well as the absorbance of the unbound label (which includes nonglycosylated hemoglobin) using a suitable colorimeter. In this embodiment, the same particles used in commercially available columns (Pierce, Rockford, Illinois) can be used as binding components in the present invention. After a suitable incubation, the assay mixture is subjected to conditions sufficient to cause the binding components and the unbound label to separate and the bound label (analyte) is detected. Preferably both bound and unbound label are measured to allow the calculation of the percentage of analyte which is bound. The bound label may be eluted from the particles (such as with a sugar solution for glycosylated hemoglobin), prior to the absorbance measurement. In an additional related aspect of the present invention, several devices for separating bound label from unbound label within an assay mixture as described above are disclosed. In one embodiment, the device comprises an assay vessel having an open proximal end, preferably resealable, and a closed distal end, the vessel defining an elongated chamber therewithin. In another embodiment, the 16 device comprises a multiwell plate. In a further embodiment, the device comprises elongated assay vessels or strips of connected elongated assay vessels. The assay vessels are typically positioned such that they have substantially the same spacing as the wells in a multiwell plate. These devices have a primary layer which most often extends generally transversely within the chamber or across the well to form a selective barrier therein, the primary layer being immiscible with both the binding components and the unbound label, and typically of different density than the binding components. For narrow, elongated vessels, the orientation can be vertical, horizontal, or intermediate between the two extremes without mixing any liquid layers. Stability of liquid cushion layers to resist mixing during agitation or inversion may be enhanced by selection of cushion materials with appropriate properties such as surface tension, hydrophobicity, and viscosity. Additional stability may be achieved by using at least one selectively liquifiable solid or gel in the cushion, or as a barrier layer. A barrier layer is typically positioned in the assay vessel between the proximal end and the primary layer. In another aspect of the present invention, a method is disclosed using heterogeneous binding assay for detecting the presence or amount of an analyte within a fluid sample. Briefly, the method comprises (a) incubating the fluid sample with a reaction mixture to form an assay mixture, the assay mixture being formed within an assay vessel, with the assay mixture containing one or more binding components, label, analyte, and other components, and with at least some of the label and some of the analyte binding, directly or indirectly, to the binding components; (b) contacting a primary layer with the assay mixture, the binding components having label and/or analyte bound thereto, and the unbound label being immiscible with the primary layer; (c) subjecting the assay mixture in contact with the primary layer to conditions sufficient to cause 17 the binding components and the unbound label to separate; and (d) detecting the label bound to the binding components and therefrom determining the presence or amount of the analyte. A particularly preferred embodiment of the method disclosed above is self contained, and comprises contacting the primary layer with the sample and reaction mixture prior to incubation of the resultant assay mixture. Within this embodiment, the formation and incubation of the assay mixture occurs in the assay vessel in which the separation is carried out. An additional preferred embodiment of the method disclosed above comprises including, in one or more second¬ ary layers, supplementary assay components which are prefer- ably added to the binding components after bound label is separated from unbound label. Supplementary assay components may include label such as an enzyme-conjugated antibody, specific binding agent such as unconjugated antibody, enzyme substrate color developer, and enzymes such as proteases. Other substances contained in secondary layers such as those listed in Table 2 may be considered, in some cases, to be supplementary assay components if they perform an additional function beyond adjusting the density of the secondary layer solution. In another aspect of the present invention, an alternative method is disclosed using a self-contained homogeneous assay for detecting the presence or amount of an analyte within a fluid sample. Briefly, the method comprises: (a) mixing the fluid sample with reaction components to form a reaction mixture in contact with a primary layer and within an assay vessel; (b) delivering additional reaction components from a reservoir within the assay vessel to form an assay mixture; (c) incubating said assay mixture and detecting the signal produced, and therefrom determining the presence or amount of the analyte. 18 An additional preferred embodiment of the heterogeneous and homogeneous methods disclosed above is especially appropriate for unprocessed samples as described above, and comprises: (a) mixing the fluid sample with reaction components to form a reaction mixture in contact with a primary layer and within an assay vessel; (b) applying a -force sufficient to cause solid components of the resultant reaction mixture to penetrate the primary layer; (c) delivering additional reaction components from a reservoir within the assay vessel to form an assay mixture; (d) incubating said assay mixture and detecting the signal produced, and therefrom determining the presence or amount of the analyte. An additional aspect of the present invention discloses a reusable detection vessel for use in specific binding assays which use radioactive labels. The detection vessel generally comprises an elongated container having an open end and a closed end, and a radiation shield adapted to fit within the elongated container and positioned therein to provide a shielded portion, and an unshielded portion toward the closed end. In most instances it is preferable to use a shield which has a substantially cylindrical bore, which better provides effective and uniform shielding. Although not essential, it may be convenient to provide the shield with a cylindrical exterior. In one embodiment, this design allows a portion of an assay vessel, which has been inserted into the detection vessel, to protrude downward from the shield a distance sufficient to allow detection of the label within the exposed portion of the assay vessel. In another embodi¬ ment, the detection vessel is provided with a substantially cylindrical member positioned between the shield and the distal end of the container, the cylindrical member being adapted to support and maintain the position of the shield within the container. In still another embodiment, the cylindrical member is closed at the distal end to support an additional thin radiation shield in the form of a disk. 19 The disk allows more effective shielding when using certain detection instruments such as certain well-type gamma counters. Other aspects of the present invention will become evident upon reference to the following detailed description and attached drawings. Brief Description of the Drawings Figure 1A is a side elevation view of an assay vessel and related closures of the present invention. Figure IB is a side elevational view of an alternative assay vessel of the present invention. Figure 2 is a fragmentary side elevational view of a multiwell plate assay vessel of the present invention. Figure 3 is a side elevational view of a reusable detection vessel of the present invention, with an assay vessel placed therein. Figures 4A and 4B are side elevational views of self-contained assay vessels of the present invention, for heterogeneous (A) and homogeneous (B) assays. Best Mode for Carrying Out the Invention The following terms are defined herein for clarity: REACTION COMPONENT: a substance or reagent which participates in and is a component of the assay; typically one of several solvents, solutes, or solids which are mixed to form a reaction mixture or an assay mixture. A reaction component also may be stored within a reagent reservoir, and may be added to the assay mixture subsequent to initiation of the reaction. REAGENT RESERVOIR: an enclosure, matrix, or device in the assay vessel in which reaction components are predispensed or stored, separate from the main reaction mixture or the assay mixture. A reagent reservoir may be 20 located distal to the primary or a secondary layer, relative to the proximal end of the assay vessel, or within a secondary layer. Alternatively, it may be located in or adjacent to the proximal end of the assay vessel. REACTION MIXTURE: the primary solution or suspension of reaction components which, upon addition of sample, becomes the assay mixture (or incomplete assay mixture) . The reaction mixture is typically in contact with or in proximity to the primary layer. The phrase "partial reaction mixture" refers to a mixture lacking one or more of the reaction components necessary for the main reaction to be initiated. ASSAY MIXTURE: the combination of a reaction mixture with a sample results in the formation of an assay mixture. The sample or other reaction components may serve as reaction activators. If other reaction components serve as reaction activators, they are typically added subsequent to the addition of sample. The phrase "incomplete assay mixture" refers to the combination of sample and a partial reaction mixture. BINDING COMPONENTS: these include specific binding agents (such as antibody) and a solid phase (nascent or preformed, particulate or a continuous surface) . CUSHION: includes the primary and any secondary layer(s), as well as any assay components which are not in the reaction mixture or assay mixture, except those that are contained in one or more discrete reagent reservoirs. PRIMARY LAYER: a substance which is not miscible with water, and extends generally transversely within an assay vessel (typically in contact with the assay mixture) . In heterogeneous binding assays, the primary layer serves 21 to separate bound from unbound label by allowing the penetration of binding components without allowing the penetration of unbound label. Generally, the primary layer is a liquid in the temperature range of 15°-50°C, or may be selectively penetrable while in a gel or semi-solid state. SECONDARY LAYER: any layer or material distal from the primary layer relative to the reaction mixture, assay mixture, or proximal end of the assay vessel. A secondary layer may be water miscible or immiscible. BARRIER LAYER: located between a reaction or assay mixture and the primary layer, the barrier layer serves to prevent contact and/or mixing between the mixture(s) and the primary layer, is selectively liquifiable or selectively penetrable, and typically does not separate bound from unbound label in heterogeneous binding assays. SELECTIVELY LIQUIFIABLE: A substance is selectively liquifiable when it can be converted to a liquid from a solid or gel under specific conditions. Generally this is accomplished by melting, in the temperature range of 15°-50°C. SELECTIVELY PENETRABLE: A substance is selectively penetrable if one or more insoluble components or materials (but not aqueous liquids) can penetrate the substance, usually upon the application of centrifugal force. Such substances include a gel or semi-solid material. Insoluble components or materials in this regard include, for example, cells, precipitates, bacteria, latex particles and other binding components. As noted above, heterogeneous specific binding assays are typically more sensitive than homogeneous assays. However, in practice this advantage is often outweighed by 22 the labor-intensive and time consuming manipulations of the assay mixture which are typically required. Even with homogeneous assays, several separate, sequential additions of assay reagents are often required. The present invention is concerned with materials and methods for the performance of more convenient and less labor-intensive assays, including both homogeneous and heterogeneous assays. Such assays can be performed manually or with simplified automated instruments, even with unprocessed samples such as whole blood. In heterogeneous assays, a binding component is typically employed which comprises a solid phase and attached specific binding agent, and typically binds at least some of the label to produce both bound and unbound label. In homogeneous binding assays, a binding agent is employed which is typically dissolved in solution. An important advantage for heterogeneous assays provided by the present invention is that the reaction mixture or the assay mixture can be stored or incubated in contact with a barrier layer or a primary layer. Such storage of the reaction mixture is advantageous because it allows the reaction components and cushion to be prepackaged. This reduces the number of manipulations by the user in preparing for and performing the assay, and can improve both convenience, speed, and precision. Because separate storage of wash buffer as well as collection and disposal of waste liquids are eliminated, the present invention reduces the space requirements, increases the safety of laboratory testing, and simplifies field testing. Another important advantage provided by the present invention which is relevant to both homogeneous and heterogeneous assays is that supplementary assay components can be predispensed in one or more layers separate from the assay mixture layer, to create a completely self-contained assay vessel for determining the presence and/or level of an analyte. In the prior art, such supplementary assay components (for example, enzyme substrate color developer for homogeneous immunoassays, and labeled antibody in 23 sandwich immunoassays) typically are added after an incubation step and in some cases after the separation of bound from unbound label and/or analyte. There exists significant commercial advantage in the present invention for predispensing all assay reactants so that such features as user convenience are increased compared to the prior art. In another aspect, the invention is concerned with methods and devices for selectively measuring bound label after the separation has been performed. In some embodiments, measurement of bound label is facilitated by shielding the unbound label from the detector. A. BARRIER LAYERS The barrier layer serves to prevent contact and/or mixing between the reaction and/or assay mixture(s) (complete or incomplete) and the primary layer. Typically the layer is selectively liquifiable or selectively penetrable. In liquid form the barrier layer is usually miscible with the aqueous solution of the reaction mixture. A preferred barrier layer is an agarose or gelatin gel which is selectively penetrable or which can be melted in the temperature range of 15°-50°C. A low-melting agarose gel (Seaprep'" agarose, from FMC Corporation, Rockland, Maine) is one example of a substance which can be selectively penetrable in the gel state as well as selectively liquifiable in the temperature range of approximately 20°-50°C. Barrier materials for other applications have been described in U.S. Patent No. 4,522,786 (issued June 11, 1985, to R.C. Ebersole), which is herein incorporated by reference. Certain barrier layer materials are selectively liquifiable and are located between a reaction or assay mixture and the primary layer, but are water immiscible. These layers are not termed primary layers because they are less dense than the liquid portion of the assay mixture and thus, in certain embodiments, do not 24 serve to separate bound from unbound label. Instead, when they are liquified these layers are displaced by the more dense reaction or assay mixture. One added advantage of such water immiscible barrier layers is that they can seal the air/water interface of the assay mixture at the conclusion of the assay. B. PRIMARY AND SECONDARY LAYERS OF THE CUSHION The methods of this invention generally employ a largely aqueous assay mixture containing sample, one or more reaction components, and a primary layer. A primary layer is immiscible with water and extends generally transversely within an assay vessel (typically in contact with the assay mixture) . Generally, the primary layer is a liquid in the temperature range of 15°-50°C. Preferred are primary layers which are selectively liquifiable, or selectively penetrable. An example of a selectively penetrable primary layer is a silicon paste as described in U.S. Patent No. 4,049,692; issued to Anthony R. Zine, Jr. on September 10, 1977. Such a material allows the penetration of blood cells as well as binding components even in a gel or semi-solid state. One or more secondary layers may also be employed. The primary layer serves at least one of two functions depending on whether the assay is homogeneous or heterogeneous. In a heterogeneous assay, the primary layer serves to separate bound from unbound label by allowing the penetration of binding components without allowing the penetration of unbound label. In a homogeneous assay, the primary layer typically positions the reaction solution at the appropriate height for photometric measurement. The primary layer may further serve to separate the reaction and assay mixtures from other reaction components (such as color developer) which must be added to the assay mixture after an initial incubation step. In certain embodiments, the assay mixture passes around or through the primary 25 layer to contact or mix with other reaction components contained in a secondary layer. However, in a typical homogeneous assay embodiment, the cushion has a greater density than the assay mixture. For assays employing unprocessed samples, such as whole blood, the cushion serves an important function as a receptacle for solids such as blood cells removed from the reaction mixture by centrifugation. The cushion may serve a similar function as to other insoluble components, such as precipitates. In certain cases a composite primary layer can be produced which has certain surprising advantages. A composite primary layer is one which contains two, mutually immiscible, water immiscible substances. Such composites can improve the stability of cushion layers for shipping, handling and storage. Composite primary layers can, for example, also allow the cushion to sequester blood solids or other insoluble components separately from binding components after the separation of bound and unbound label. In one exemplary embodiment, a composite primary layer is comprised of a primary layer which contacts the reaction mixture (or a barrier layer) and has properties which allow penetration by blood solids and by binding components. A second water immiscible layer is in contact with the first, but distal from the reaction mixture. This second layer has properties which prevent penetration by blood solids, but allow penetration by binding components. A preferred embodiment in this regard utilizes as the primary layer a silicone oil or paste (d-1.03-1.08) , while the second water- immiscible layer is comprised of a fluorosilicon fluid (d-1.09-1.30) . With the appropriate assay vessel design (such as shown in Figures 4A-B) , the surface tension, viscosity, and other properties of the cushion fluids stabilizes the layers during shipping, handling, and storage. An additional preferred embodiment in this regard utilizes as the second water-immiscible layer a substance 26 which is selectively liquifiable and is normally in a solid form during shipping and storage. Such a substance need not necessarily be immiscible with the first primary layer material. In some instances it can be advantageous to have a "soft" (diffuse) interface between the two water-immiscible layers. Dimethyl itaconate is an example of such a material when used with a silicone oil primary layer. Prior to separation of bound from unbound label, the lower layer can be liquefied to allow penetration by binding components. One related embodiment which functionally serves a similar purpose, does not use adjacent water-immiscible layers. In this embodiment, the second layer is aqueous, but has a density such that blood solids do not penetrate (e.g. , 50% glycerol). The barrier layer, if present, and the cushion are in a liquid form, or are selectively penetrable, at least during the separation of bound from unbound label by the primary layer and during any subsequent steps in which binding components penetrate additional layers. The primary layer is also largely or totally immiscible with the assay mixture. These two features allow effective contact of the binding components with the primary layer, with the concomitant exclusion of the aqueous components of the assay mixture. In many embodiments, the primary and secondary layers may also be of a density different than the assay mixture (typically the densities are greater than that of the assay mixture), so that the relative positions of the assay mixture and the cushion layers can be maintained under the forces of gravity or centrifugation. Separated from the assay mixture layer by the primary layer, one or more additional layers may be employed which may be miscible or immiscible with aqueous solutions. These additional layers are hereinafter referred to as "secondary layers". Each secondary layer typically is of different density than the other layers employed, and in addition is largely or totally immiscible 27 with any adjacent layers. In general, all layers should be resistant to mixing or inversion, or should return to their relative positions on brief standing. This can be achieved by selecting at least one layer material which is a solid at the temperature of storage, or by using layers which differ in density and are immiscible (e.g., butyl phthalate and fluorocarbon oil) . Miscible mixtures of materials such as those materials listed in Table 1 also have useful properties. For example, by blending two or more substances one could obtain a mixture with a desirable density and a desirable melting temperature in the 15°-50°C range. Surprisingly, the inclusion of detergent in one or more layers or in the assay mixture in some cases facilitates spontaneous separation of mixed liquid layers. Suitable detergents include nonionic (such as Nonidet P-40, Triton X-100 or silicon glycol detergents) and ionic detergents (such as taurodeoxycholate or dodecyl sulfate) and various mixtures of detergents. The primary layer can be composed of any of a variety of compounds provided that it is substantially immiscible with the components of the assay mixture, and typically will have a density different than the solid and liquid components of the assay mixture. In instances where the primary layer has a density equal to or greater than the aqueous solution of the assay mixture, the density of the primary layer is usually approximately 1.01 or greater. For such instances involving centrifugal separations, the density of the primary layer typically does not exceed 1.20, and is most preferably greater than 1.03 and less than 1.15. For whole blood, the density should be approximately 1.03-1.09. Furthermore, for heterogeneous binding assays the density of the primary layer typically will be less than the apparent density of the binding components. In addition, the primary layer typically will be in a liquid form at least for the separation step or supplementary 28 reagent mixing step following incubation. Secondary layers typically also will be in a liquid form, at least for any supplementary reagent mixing steps, and/or during such periods that the binding components are desired to penetrate or pass through the secondary layers. Liquification of solid primary layers typically involves melting, generally in the range of 15°-50°C. In applications utilizing gravity or centrifugation to achieve separations in heterogeneous binding assays, or where a supplementary reagent mixing step is desirable in homogeneous binding assays, the density of the primary and any secondary layers should be different than the density of the assay mixture. A representative listing of water-immiscible dense oils suitable for use as primary layers is shown in Table 1. These materials may also be used as components of secondary layers. In some embodiments, primary or secondary layers may be selectively penetrable in a non-liquid (e.g. gel) form. Table 1 : REPRESENTATIVE LIST OF WATER- IMMISCIBLE SUBSTANCES 30 For embodiments where the primary layer is more dense than the liquid components of the assay mixture, the primary layer materials will have the properties of oils with densities greater than water (d>1.00). However, for some homogeneous assays requiring a supplementary reagent mixing step, in which the entire reaction mixture penetrates the primary layer to mix with one or more supplementary assay components in a secondary layer, an oil with a density less than or equal to water can be employed if it can be maintained in a solid form during incubation, then subsequently liquified. In such embodiments, the reaction mixture may contain one or more materials which form dense aqueous solutions. A representative list of such water-miscible materials forming dense aqueous solutions is shown in Table 2. Dense oil-like materials are typically synthetic esters (usually methyl, ethyl, propyl or butyl.) of bioorganic acids, and usually contain substantial oxygen, nitrogen, or sulfur, or they are fluorocarbon oils or silicon based oils, or combinations (e.g., fluorosilicon fluids) . Many dense oil-like materials are miscible with each other and can be used alone or in various mixtures in primary or secondary layers. However, in some embodiments it is possible and desirable to create adjacent water-immiscible layers which are not miscible with each other and which differ in density (e.g. a hydrocarbon-based material or mixture plus a fluorocarbon- based material or mixture) . In such embodiments a water-immiscible layer which is not in contact with the assay mixture would be called a secondary layer. To those experienced in organic chemistry and others skilled in the art, related water-immiscible materials which have desirable properties, other than those materials listed in Table 1, will be readily apparent. Such properties include partial or complete immiscibility in water and aqueous solutions and lack of objectionable odor or toxicity. A further desirable property of a 31 primary layer material is the ability to rapidly and spontaneously reform a homogeneous phase when mixed with a reagent mixture or assay mixture and a lack of damaging effect on reaction components. Further, the primary layer must be in a liquid form during the separation step (in heterogeneous assays) and the supplementary reagent mixing step (in homogeneous assays), unless it is selectively penetrable. While most of the applications described for the present invention can utilize dense oils as primary layer materials, it will be apparent to those skilled in the art that water-immiscible substances with densities less than water could be blended with materials such as dense oils to form mixtures with useful densities. Certain organic solvents could be used, as well as fats and waxes. Amphiphilic substances which would disperse in water on melting could be useful additives to barrier layers or primary - layers (Hargreaves et al. , Bi ochemi s t ry 17:3759-3768, 1978, herein incorported by reference) For water-immiscible dense oils which are useful as solids in the methods of the present invention, 1iquification typically takes place within the range from 15 to 50 C. In some cases the temperature of liquification for meltable water-immiscible dense oils can be controlled by blending two or more substances which individual ly melt at different temperatures than the mixture of the substances. It will also be apparent to those skilled in the art that liquification can be achieved in some cases by means other than melting, such as by depolymerization of a solid polymer. Particularly preferred for centrifugal applications are primary layer materials or mixtures thereof with densities in the approximate range of 1.03 to 1.15, such as various silicon fluids available from Dow Corning (e.g. 550 fluid) , and dipropyl or dibutyl phthalate, methyl cinna ate, ethyl cinnamate, butyl cinnamate, butyl citrate, diethyl fumarate, dimethyl 32 itaconate, diethyl maleate, diethyl oxalate, diethyl succinate, and dibutyl tartrate. Where a detergent is used in the assay mixture with a liquid primary layer, the preferred primary layers include butyl phthalate, ethyl cinnamate, ethyl salicylate, silicon oil (e.g., Table 1, #39 and dimethyldiphenylpoly-siloxane) , because materials such as these do not form unwanted emulsions with reaction mixtures containing detergents. Where no detergent is used, in addition to silicon oils or fluids, preferred primary layer materials include diethyl succinate, methyl adipate, dimethyl succinate, ethyl salicylate, dimethyl alonate, and diethyl malonate, because they readily separate into two or more clear phases when mixed with aqueous reaction mixtures that lack detergent. Particularly preferred for embodiments in which the binding component is attached to the surface of the assay vessel are primary layers of fluorocarbon, fluorosilicon or silicone oils, because of the low viscosity and high density of these oils, which properties aid in the complete displacement of water from the binding components in such embodiments. Fluorocarbon and silicon oils are also attractive for such applications because polystyrene assay vessels can be used with such oils. For other embodiments in which the assay vessel is desired to be clear plastic such as polystyrene, preferred primary layer materials include methyl cinnamate or methyl itaconate (stored below 36°C) , silicon oils (such as Table 1 #39, dimethyldiphenylpoly-siloxane, or "high temperature" melting point bath oil, from Sigma Chemical Co., St. Louis, MO, or from Aldrich Chemical Co . , Milwaukee, WI) , and dj methyId Iphenylpolysiloxane . Preferred for embodiments in which the primary layer is desired to be in a solid form within some part of the temperature range from 15°-50°C, are primary layers containing methyl cinnamate, dimethyl itaconate, dimethyl oxalate, dimethyl succinate, dimethyl, diethyl, and dibutyl tartrate, or diphenyImethane , or mixtures of these 33 substances. For those embodiments which utilize both centrifugal separations and solid primary layers in the range of 15-50 C, the preferred primary layer materials are methyl cinnamate and dimethyl itaconate and mixtures of these substances. Depending on the nature of the signal emitted or produced by the label, the washing effectiveness or supplemental reaction required, it may be desirable to include a secondary layer or layers. While a secondary layer may be formed using an appropriate water-immiscible material from Table 1, a secondary layer may also be water soluble. To form water soluble secondary layers, or to increase the density of an assay mixture for applications such as homogeneous assays, typically a material is dissolved in water to increase its density. A representative listing of materials appropriate for this purpose is shown in Table 2. These materials are especially well suited for use as components of secondary layers or assay mixtures as described above. However, in certain instances, a material which is soluble both water and in water-immiscible substances (e.g. formamide or dimethylsulfoxide) may be used within a primary layer. In another embodiment, formamide may be included in a DNA hybridization assay mixture and/or a primary layer for such an assay to facilitate the hybridization of polynucleotide strands. For use with certain homogeneous assays, an aqueous secondary layer containing enzyme substrate may have the same or similar density as the assay mixture. In such an embodiment, the primary layer will typical ly be a solid or a selectively penetrable gel during incubation prior to color development. For example, if the primary layer is less dense than both the assay mixture and the secondary layer, the primary layer will float to the top of the assay vessel upon melting. This will allow the assay mixture and the substrate-containing secondary layer to merge in the bottom of the assay vessel. In this 34 embodiment, the primary layer material can be less dense than water if it can be solidified after dispensing onto an immiscible secondary layer of greater density. An electromagnet can be used to obtain effective mixing of the assay mixture and secondary layer after liquification of the primary layer, if several paramagnetic particles are included in the assay vessel. 35 Table 2 Representative Dense, Water-Miscible Liquids CHEMICAL NAME COMMENTS CESIUM CHLORIDE CESIUM SULFATE DIETHYLENE GLYCOL DIMETHYLSULFOXIDE MP-18 DEGREES ETHYLENE GLYCOL FICOLL FORMAMIDE MP=2.6 DEGREES GLYCEROL LITHIUM BROMIDE SOL. IN .6 PARTS H20 LETHIUM CHLORIDE SOL. IN 1.3 PARTS H20 LITHIUM SULFATE SOL. IN 2.6 PARTS H20 METRIZAMIDE DENSITY AT 15 DEGREES PERCOLL SELF-FORMING GRADIENTS POTASSIUM ACETATE POTASSIUM BROMIDE POTASSIUM CITRATE POTASSIUM IODIDE POTASSIUM TARTRATE PROPYLENE GLYCOL MISC WITH H20, CHCL3 SODIUM BROMIDE SORBITOL SOLUBLE TO 83% SODIUM IODIDE SUCROSE DIETHANOLAMINE Further, it ay be desirable to include supplementary assay components in either primary or secondary layers which aid in signal production or detection. An example of an additive for a primary or secondary layer is a scintillation fluor, such as 2,5 diphenyloxazole (PPO) or 1 , 4-bis [ 5-pheny-l-2-oxozoly1 ] 36 benzene (POPOP) , which may be included in a primary or secondary layer if the label can be detected in a scintillation counter using such fluors. Additives to a secondary layer can also include enzymes, proenzymes (zymogens) , or enzyme substrate, where the label is an enzyme substrate, a zymogen activator, or an enzyme, respectively. In some embodiments (e.g. certain sandwich binding assays) where a label is added to the assay mixture after an initial incubation and separation of bound from free analyte, a secondary layer may contain label (e.g. labelled antibody) . Secondary layers can also be formulated to contain harsh or chaotropic agents, such as salts, urea, diethanolamine, guanidinium chloride, nonionic or ionic detergents, etc. , for example to reduce nonspecific binding. In any case, the concentrations of these additives typically should not be sufficient to cause 'significant dissociation of specifically bound label from its binding component during the movement of the binding component through such layers. However, in some embodiments, dissociation of label from its binding component is desirable and can be achieved by inclusion of a suitable dissociating agent in a secondary layer. For example, sorbitol will dissociate glycosylated hemoglobin from the boronic acid particles used in a commercial column chromatography kit from Pierce (Rockford, Illinois). For embodiments employing non-isotopic labels, it may be useful to utilize secondary layers which serve to mix, redistribute, or spread the binding components, in order to maximize assay sensitivity or precision. Typically, mechanical mixing may be achieved by including, for example, dense particles combined with agitation of assay vessels. Magnetic mixing can be achieved with appropriate use of, for example, paramagnetic or metallic particles with an external magnet. A preferred embodiment, especially for use with centrifugal methods lacking mechanical mixing of binding components within the cushion, 37 utilizes a spreading layer with a density greater than that of the binding components. Typically, such a layer is a liquid, and is water immiscible. A surprising and advantageous property of such layers is that binding components forced against the "spreading" layer can be spread into a uniform layer, while bathed in an adjacent aqueous solution (e.g. enzyme substrate). Both signal intensity and precision are enhanced. Useful spreading layers vary with assay properties, such as different solid phases and different detergents, and include halogenated solvents (e.g. choloform, iodoform) , fluorinated silicone fluids, and fluorocarbon fluids (e.g. FC-40 from 3M Corp.). A preferred spreading ayer for latex particles, such as bromostyrene particles, is comprised of fluorocarbon fluid. As noted above, for the purpose of the present invention, the term "cushion" is defined to include all primary or secondary layers, alone or used in combination. The volume of the cushion in different embodiments is variable and will depend on a number of factors, including the particular label employed, the detection mode, the required sensitivity of the assay, and the assay mixture volume. Both the volume and formulation of the cushion can be determined empirically. For most isotopic applications, however, a ratio of 2.5 volumes of cushion to one volume of sample will be adequate where it is required to shield radiation emanating from unbound label. For multilayer cushion embodiments, and in cases where nonspecific binding is adequately low, the volume of the primary layer need only be enough to completely isolate the assay mixture from the secondary laye (s) under the conditions used. Where no secondary layers are used, the primary layer need only isolate binding components from the assay mixture after the separation step. A ratio of assay mixture:cushion volumes greater the 1:1 can be used in some cases. Typically for competitive assays, approximately 3-4% nonspecific binding is acceptable, while 1-2% is very good. For sandwich assays where excess label may be used, nonspecific binding may be required to be 0.2% or below. Nonspecific binding is determined largely by the physical properties of the label and the binding components and will vary. For example, in the use of a 96-well plate, a ratio of one volume of primary layer to one volume of sample will usually be adequate. A smaller amount of primary layer may be usable if it is in a solid form during sample loading, if a barrier layer is employed, or if the assay mixture is immiscible with all primary and secondary layers. The geometry and orientation of the assay vessel, the assay mixture, and the cushion will be governed by particular applications. In a typical use involving centrifugal or gravity separation, one of many types of test tubes or multiwelled plates can be used. In most uses, the sample, binding components, and secondary components are conveniently added, mixed, and incubated in contact with the predispensed primary layer. In some cases, binding components and/or reaction components can be predispensed along with the cushion in sealed assay/separation vessels. In such cases, fewer components (as few as one, the sample) need be added by the user prior to mixing and incubation. Homogeneous assays in some cases can require or benefit from a sample extraction step, while the separation step in certain heterogeneous assays can include a liquid/liquid extraction step. Such extraction steps can be performed within the methods of the present invention, typically by employing cushion materials which are immiscible with the extraction solvents applied to the reaction mixture or assay mixture. For example, fluorocarbon liquids (e.g. FC-40, 3M Corporation) can serve as primary layer materials, while chloroform and ethyl acetate are commonly used extraction solvents which are essentially immiscible with water. Typical fluorocarbon liquids are essentially immiscible with both chloroform and 39 ethyl acetate. Depending on the requirements of a particular embodiment, it may be desirable to use an extraction solvent which is denser than water (such as chloroform) or one which is less dense than water (such as ethyl acetate) . For heterogeneous assay embodiments in which reaction mixture solids undergo a separation step prior to assay initiation, the properties of the cushion and the binding components are typically selected so different types of solids ultimately occupy separate positions. For example, in an embodiment in which blood cells are forced into the cushion by high speed centrifugation, solid phase binding components after the separation of bound and unbound label are typically caused to occupy a different location than the cells within the cushion. This can be achieved by employing two adjacent layers within the cushion which differ in density sufficiently to allow the passage of both types of solids through the first layer, but only of one type of solid into the second layer. For example, heterogeneous assays using whole blood samples and bromostyrene binding component particles can employ the combination of a silicone oil (d = 1.04-1.09) with an immiscible fluorosilicon oil (d = 1.09-1.30) . Other related embodiments employ a selectively liquifiable material for the layer of greater density, such as the combination of a silicon oil (d = 1.04-1.09) with dimethyl itaconate (mp = 44-45°C, liquid density = 1.11) . Additional embodiments utilize the combination of a silicon oil (d = 1.04-1.09) with a higher density water-miscible or aqueous solution (e.g. 50% glycerol, d = 1.12) . In heterogeneous assay embodiments where water-immiscible layers of different density are employed, a dissolution material may be placed between the two layers. For example, to dissolve blood cells centrifuged against a high-density (d>1.09) water-immiscible layer, examples of dissolution material include aqueous detergent solutions, neat diethylamine (pH >11), or combinations of detergent 40 and chaotropic materials, typically at high pH (>pH 10) . An advantage of this embodiment is that the dissolved solids can provide additional washing of the binding components, removing unbound or nonspecifically bound label from the solid phase prior to the detection of bound label. An additional option is available with magnetic separations, wherein the cushion layer(s) can be oriented lateral to the assay mixture, or above the assay mixture if the density of assay mixture is greater than that of the layer(s) . In the case where the binding components are attached to the surface of the assay vessel, the assay mixture can be preequilibrated in contact with the binding components at the bottom of the assay vessel. To achieve separation of bound label from unbound label, a primary layer material can be poured or pipetted into the assay vessel to displace the less dense secondary ' components (including unbound label) *to the top of the primary layer. In some cases, secondary layers can be added simultaneously with or subsequent to the primary layer addition. In embodiments intended especially for whole blood samples or for unskilled operators, it is advantageous to automatically monitor the amount of sample delivered to the reaction mixture. This can be accomplished, for example, by including in the reaction mixture a measurable reference material which is modified by the added sample in proportion to its volume. A specific example is the use of a laser dye solution (e.g. oxazine -1- perchlorate. The dye is diluted by the liquid sample volume and its optical density is reduced proportionately. An advantage of dyes, such as typical laser dyes, is that maximal absorbance occurs in a wavelength range (e.g. infrared) where biological samples rarely absorb, thus eliminating interference from light-absorbing substances present in the sample. A preferred embodiment for sample volume monitoring exhibits a relatively large change in signal with a relatively small 41 (e.g. 1-10%) dilution due to sample. For example, fluorescence resonance energy transfer (F.R.E.T.) can be used to measure the average distance between appropriate pairs of fluorescent molecules. Small changes in this average distance (i.e. concentration) cause large changes in F.R.E.T., because efficiency of F.R.E.T. is a function of the fifth power of the distance. C. SOLID PHASES USED IN BINDING COMPONENTS FOR HETEROGENEOUS ASSAYS Binding components normally comprise two parts: a solid phase and a specific binding agent attached thereto, which confers specific binding activity. Several types of solid phases are useful in performing specific binding assays. In general they are of three types: preformed particles, the surface of a vessel, and soluble polymers which can be attached to specific binding components and which can be made insoluble during the binding assay. For each of these solid phase types, the specific binding activity may be an inherent property or it may be generated by covalent or noncovalent attachment of materials, hereinafter called "specific binding agents", which confer specific binding properties on a solid phase. Preformed particle solid phases include stabilized microbial suspensions such as a Staphylococcus aureus strain which naturally produces the immunoglobulin- binding molecule, "Protein A". Alternatively, the solid phase can be non icrobial particle suspensions of minerals (hydroxyapatite, glass, or metal), beaded insoluble poly- mers (such as dextran fSephadex G-10 or G-25] , agarose, polystyrene) . An especially preferred particle type comprises broynostyrene latex particle from the JSR America Corp. , N.Y., N.Y. Some of these non-microbial particles naturally exhibit useful binding activity (e.g. hydroxyapatite) . However, most others must be coated with a suitable agent, using coating procedures well known in the art. These solid phases noted above can also be prepared with or may exhibit inherent magnetic or paramagnetic properties which may be exploited for separating bound from unbound label or for mixing. Small particles confer rapid reaction kinetics on solid phase assays, but excessively small particles are not ideal for centrifugal applications. For most centrifugal applications, particles should have average diameters of 0.5-3 microns and densities of 1.1 g/L or greater. Preferred particles have relatively uniform diameters of approximately 1 micron and densities of 1.3-3.5 g/L. The preferred use of micron-sized microparticulate sol id phases results in surprisingly fast reaction kinetics, comparable to liquid phase assays. For grav ty separation embodiments, preferred solid phase materials include very high-density particles, such as glass or plastic-coated metal beads (typically 0.3-6 microns diameter). Large coated metal beads can easily be produced by immersing the metal beads in a solution such as polystyrene dissolved in acetone or chloroform, then draining the beads, allowing the solvent evaporate, then incubating the beads with one or more specific binding agent such as antibody, as is well known in the art. Small micron-sized particles can be chemically derivatized (e.g. , silanization) to allow coating with specific binding agent. Some particles specifically bind analyte with a non—biologica1 mechanism. In one such embodiment, glycosylated hemoglobin binds to ion exchange particles from BioRad (Richmond CA) , and especially to particles wi h boronic acid on their surfaces such as those from Pierce Chemical Co. (Rockford IL) . Such particles are used for determining the percentage of this analyte in blood using column chroma ography, and these or related particles are suitable for serving as binding components in the methods of the present invention. Binding components can also be produced by precoating the assay vessel. The most stable precoated assay vessels will be produced by chemically cross-linking 43 the molecules which contribute binding activity to each other and/or to the assay vessel, surface. Such coated assay vessels (anti-lgG for mouse, rabbit, goat) are commercially available, for example, from Micromedi.c Systems, Inc. (Horsham, PA). Alternatively, the solid phase can be produced during or subsequent to incubation of the assay mixture, by polymerization or aggregation of soluble subunits coupled to a useful binding agent. Since reactions equilibrate more rapidly when all reactants are in solution, such an approach offers shorter incubation times than traditional methods using large, preformed, insoluble binding components. In immunoassays, binding components will typically contain specific binding agents such as antibody, antigen, protein A, avidin, or biotin, either adsorbed or chemically coupled to the solid phase. A preferred solid phase coating for immunoassays is species-specific anti-immunoglobulin (for example, goat anti-rabbit IgG) . Anti-immunoglobulin coated particles can be produced using bacterial particles (Frohman et al., J. Lab. Clin. Med. , 93:614-621, 1979, and Bennett and O'Keefe, J. Biol. Chem. , 25_3 :561-568, 1980 herein incorporated by reference). For maximum stability, such preadsorbed binding components can be chemically stabilized (e.g. with glutaraldehyde or carbodiimide) to cross-link binding agent molecules to each other and/or to the binding component particle surface. These modified "biological" solid phases have the advantage that they do not experience interference from immunoglobulin molecules such as occur at high levels in serum samples, and are commercially available (Tachisorb, from Behring Diagnostics, La Jolla, CA) . Preferred particulate solid phases for centrifugal applications are those which have appropriate density and particle size to spin down rapidly through primary layer materials, preferrably in standard laboratory and clinical centrifuges. These include carboxylated ά d bromostyrene latex particles (JSR America Corp, New York, N.Y.) and similar sized carboxylated magnetic copolymer particles (Seragen, Indianapolis, IN), and silica particles (3 micron average diameter, Baker Chemical Company). For example, the particles can be rapidly pelleted at 2000-3000 x g (at 45°C) using primary layers comprised of dibutyl phthalate, dimethyl cinnamate, or dimethyl itaconate and silicon fluids (d 1.03-1.1.1). Surprisingly, the silica particles will even sediment through such primary layers without cent ifugation. Nonspecific binding using immunoglobulin-activated bromostyrene particles can be less than 0.1%. Preferred also are the characteristics of low non-specific binding of the label to be used (usually 1-2% or less) and a high, reproducibly manufacturable binding capacity (typically 10-50 microgram IgG per mL of 10% wt/v suspension). Commercial preparations of S. aureus (Behring Diagnostics, San Diego, CA and I re Corp., Seattle, WA) exhibit these desirable properties. Chemically stabilized, anti-immunoglobulin coated S. aureus suspensions with the seproperties are also available from Behring Diagnostics (Tachisorb) . Other desirable solid phases for embodiments employing centrifugal separations include Sephadex G10, G15, and G25 (Pharmacia), which can be oxidized with periodate to form aldehydes suitable for chemically coupling with amino groups on proteins and other molecules. Because large molecules are excluded from the matrix of these particles, nonspecific binding of most labels is very low and can be further minimized by including in the assay solution appropriate chemical agents (such as sodium chloride >0.1M) . D. BINDING ASSAY METHODS For simplicity, the specific binding assays of this invention will be described in terms of antigens and antibodies. However, it will be appreciated by those 45 skilled in the art that any substantially specific binding pair can be employed in the methods of this invention, including, but not limited to, the following: the binding of complementary nucleic acid sequences; the binding of lectins with carbohydrates; the binding of hormones with receptors; the binding of vitamins with transport proteins; and the binding of immunoglobulins with nonimmunogiobulin, antibody-binding proteins. The binding assays of this invention can employ any of a variety of labeling substances which are well-known in the art. These can include, but are not limited to, the following: radioisotopes (eg. 32-P, 3-H, 125-1, 35-S, 14-C) ; enzymes (eg. horseradish peroxidase, urease, beta galactosidase, alkaline phosphatase, glucose oxidase, enteropeptidase) ; fluorophores (eg. fluorescein, rhodamine, dansyl, phycobiliproteins, Nile blue, Texas red, umbelliferone) ; luminescers or luminescent source materials; transition metal chelates; enzyme substrates, cofactors, or inhibitors; particles (eg. magnetic, dye, high refractive index); and zymogens. These are exemplified in part by the following publications: U.S. patent 4,181,636; U.S patent 4,401,765; U.S. patent 3,646,346; U.S. patent 4,201,763; U.S. patent 3,992,631; U.S. patent 4,160,016, U.S. patent application 486016 (EP 0123265A.1) , all of which are herein incorporated by reference. The various functional configurations in which specific binding assays can be performed are well known in the art and are described extensively in, for example, Maggio (ed.), Enzyme Immunoassay, CRC Press, Boca Raton, Fl. , 1980, herein incorporated by reference. Several representative examples employing the methods of the present invention are described below. These methods may be used to detect the presence and/or amount of a wide variety of analytes. Representative analytes are listed in EP 123,265. Briefly, in a competitive immunoassay, sample suspected of containing antigen (analyte) and a known £ . amount of labeled antigen (tracer) compete for a limited amount of analyte-specific antibody. In heterogeneous competitive immunoassays, anti-immunoglobulin antibody or Staphylococcal protein A immobilized on a solid phase to form a binding component is added at the same time or in a subsequent step. Following incubation during which specific binding occurs, the binding component is passed through the layer(s) of the cushion, thereby separating bound label from unbound label. In homogeneous competitive enzyme-labeled assays, the assay mixture can pass through the cushion to mix with enzyme substrate color developer in a secondary layer. The binding component (in a heterogeneous assay) or the assay mixture (in a homogeneous assay) can pas through the cushion due to gravity (for large or very dens particles) or the assay vessel can be subjected to centrifugal force. If the binding component i magnetizable or magnetic, the assay vessel can be subjecte to a magnetic field to move the binding component throug the cushion or for mixing. The presence or amount of boun label is then determined by means appropriate to the label and is related to the presence or amount of analyt initially present in the sample, by comparison to a serie of known standards. For instance, gamma counters o scintillation counters are appropriate for detectin radioisotopes, spectrophoto eters are appropriate fo detecting substances or solutions which absorb light, etc. All the reagents comprising the reagent mixtur (including binding components and label) can be premixe and the assay initiated by the addition of sample. In thi case, the reaction typically will be allowed t substantially or completely equilibrate before th binding component or assay mixture is caused to pas through the primary layer. In such an embodiment, precis timing of the incubation period is not required Alternatively, sample and label can be premixed and adde simultaneously to the reagent mixture and incubated for 47 fixed interval to form a non-equilibrium assay mixture, then the binding component (for heterogeneous assays) or the entire mixture can be caused to pass through the primary layer. A preferred alternative protocol for a competitive immunoassay is to predispense binding components comprising antibody-capture particles, as well as label, to form a reaction mixture, with analyte-specific antibody isolated in a reagent reservoir in the assay vessel. Antibody can be delivered to the assay mixture to initiate the binding reaction using, for example, low speed centrifugation. Very high precision can be expected where all reagents are factory dispensed, and where simultaneous delivery of antibody to all assay vessels in a centrifuge initiates the reaction, and where the reaction is terminated simultaneously in all assay vessels when the binding components penetrate the primary layer. When standards and controls are included in such assays, critical timing and temperature control are not necessary, a run size is limited only by the centrifuge capacity (which can exceed 200 for some microcentrifuges) . As an alternative to competitive binding assays, a heterogeneous sandwich assay can be performed. For sandwich immunoassays, analyte is incubated with two antibodies which can be present in excess, one being immobilized, or capable of being immobilized (being the binding component), and the other conjugated to a label. The antibodies can be directed against two different, non-competing determinants (epitopes) on the analyte or, if there is a multiply repeated determinant on the analyte, they can be directed to the same determinant. Sandwich immunoassays can be carried out in simultaneous, forward, or reverse configurations (as described in U.S. patent 4,376,110, herein incorporated by reference), depending upon the order in which the analyte and the antibodies are added. Labeled antibody which is bound via analyte to the solid phase is then separated from 40 unbound labeled antibody by passage through the cushion, as described above, and the amount of bound label determined using means appropriate to the label. Some sandwich assays require addition of binding component, followed by separation of bound and unbound analyte, then followed by addition of label (labelled antibody) . In the present invention, the addition of label to the binding component could occur in a secondary layer. This has the advantage of eliminating a manual user step in such an assay method, adding convenience and reducing the opportunity for error. Selective movement of the binding component to a specific secondary layer prior to its movement to a more distal secondary layer can be achieved using an appropriate sequence of applied forces and selection of primary and secondary layer materials to have appropriate densities. For example, low speed and high speed centrifugation could be employed to cause ' the binding component to pass first to an intermediate secondary layer, then to pass through more distal, denser layers. Alternatively, a water-immiscible secondary layer could be employed with a melting temperature higher than the temperature maintained during the first separation step. The temperature could be raised above the melting point of this solid secondary layer in order to complete the assay. Sandwich assays offer the advantage that both antibodies can be present in excess, hence the sensitivity of the assay is not strictly limited by the affinity constant of the antibody(s). In one special application of the present invention, a noncompetitive sandwich binding assay is used to detect antibody in a sample, and thus is useful in clinical serology and in screening hybridoma cultures. For example, either anti-mouse IgG or antigen can be coated on the solid phase as described above. Substantial reduction in manipulations can be achieved using the present invention compared to standard procedures used in hybridoma screening. An added advantage is that where antibody is bound to the solid phase, rapid selection of high affinity antibodies is possible by detecting binding to subnano olar levels of labelled antigen. E. ASSAY VESSELS FOR INCUBATIONS AND SEPARATIONS The vessel in which the cushion (primary and any secondary layers) is contained is referred to herein as the "assay vessel". The assay vessel may also contain one or more components of the reaction mixture. Numerous geometric configurations using different sizes and shapes of assay vessels are possible within the scope of the present invention. Referring now to figure 1A (as preferred RIA vessel), in most applications the cushion, here comprising a primary layer 12, is contained within an assay vessel 10 which is closed at its distal or bottom end 13. The assay vessel has a substantially cylindrical body 14 which defines an elongated chamber 16. The primary layer 12 extends generally transversely within the chamber to form a barrier therein, typically filling approximately 1/3 to 7/8, and preferably filling 15/24 to 3/4 of the volume of the chamber. The optimal volume of the primary layer will be determined in part by the geometry of the assay vessel, the nature of the label, the detection method and device, if any, and the shield, if any. Where both primary and secondary layers are utilized, typically the volume of the secondary layer will be equal to or greater than the volume of the primary layer. When more than two layers are used, the distal layer is typically the largest. It will be evident to one skilled in the art that the ratio of the volumes of primary to secondary layers used will be influenced by the nature of the particular layer materials used, and the nature of the label and binding components used. For example, where an enzyme is used as the label and an enzyme substrate is an additive to a secondary layer, the ratio of primary to secondary layers will be low (typically as low as 1:10) in 50 order to achieve maximal sensitivity. In contrast, in the case where the label is a fluorescent material and a secondary layer is utilized to provide the optimum solvent environment for detection, the ratio can be high (typically as high as 5:1) . Suitable assay vessels include test tubes and wells, or strings of wells in a multiwell plate. It is preferred that the assay vessel be resealable at the top or proximal end 17, to protect the user and the environment from biohazards or chemical hazards in the sample or assay reagents. It is also preferred to provide the assay vessel with a penetrable septum 18. While a simple metal foil or polyethylene film is sufficient for this purpose, a seal with elastic properties such as, for example, a septum made from rubber (e.g. silicon, neoprene, or EPDM) or from a heat-meltable, moldable, rubber-like plastic (e.g. Kraton® thermoplastic rubber from Shell Oil Co.) is preferable. Even more preferable, for ease of manufacturing plus ease and safety in use, is a resealable septum which is penetrable by a blunt-ended instrument, such as a blunt needle or a disposable pipette tip. Particularly preferred is a resealable, elastic septum which has been molded with a thin region, or partially or completely precut with a slit, so that air can vent during the addition of liquid assay reactants. Such vessels are essentially permanently sealed at the time of manufacture, eliminate the handling of 'Caps by the user, yet allow safe and convenient addition of assay reactants and/or sample by the user. For radioisotopic applications, the assay vessel may be composed of polyethylene or, more preferably of polypropylene for its strength and solvent resistance. Non-isotopic methods typically benefit from maximum clarit of the assay vessel, which can be made from glass, polystyrene, polycarbonate, nitrocellulose, and optical grade polypropylene (produced with clarifying additive from Milliken Chemicals, Spartanburg, S.C.). A surprisin 3 1 feature of the present invention is that test tubes composed of clear plastic such as polystyrene, which are desirable for nonisotopic assays, can be used with several of the primary layer materials even though such plastics are known to be vulnerable to damage by organic solvents and hydrocarbon oils. Adhesion of rubber and other septum materials to plastic or glass tubes can be readily accomplished. In one embodiment, a tight fitting molded cap is used with an elastic septum containing a precut slit. in another embodiment, a disk of rubber, precut with a slit, is fastened permanently to the flange at the top of a tube using methods well known in the polymer industry. • For example, silicone adhesive will effectively bond silicone rubber to may kinds of tubes, including glass and some plastics. With appropriate chemical priming, polypropylene tubes can be glued to various rubbers, such as EPDM polymer blends. Cyanσacrylate adhesive will bond EPDM rubbers to polypropylene even without priming. In one preferred embodiment especially suited for isotopic binding assays, the assay vessel is a 0.4 milliliter microcentrifuge tube (approximate dimensions 5x45 mm) composed of polypropylene, such as is commercially available from Sarstedt (Princeton, NJ) , West Coast Scientific (Emeryville, CA) , and from numerous other manufacturers and distributors. As shown in Figure 1A, during use of the assay vessel, an assay mixture 24 including specific binding components 26, is placed into contact with the primary layer 12. Substantially concurrent with separation of the binding components from the unbound label in the assay mixture, the binding components will enter the primary layer and will typically continue to the distal end 13. Referring now to figure IB, an alternative larger (2 mL) embodiment of the assay vessel 10 is shown. Other similar embodiments employ test tubes with external dimensions such as 8 by 55, 10 by 55, 10 by 75, 12 by 55, and 12 by 75 millimeters. Within this embodiment, the 52 chamber 16 defined by the assay vessel is of a size sufficient to receive one or more preformed beads which are initially positioned on the upper surface of the primary layer 12, which is in a solid form. Specific binding agents are attached to the beads to form binding components 26. As shown in figure IB, the cushion comprises a primary layer 12 and a secondary layer 28. The primary layer 12 will be the only layer to contact the assay mixture 24. Following incubation and conversion of the primary layer to a liquid form, the binding components with bound label pass through the primary layer, enter the secondary layer, and settle to the distal end 13 of the assay vessel. As an alternative to the cap 20 shown in Figure 1A, the assay vessel may be provided with a threaded portion 30 which is mateable with a suitable cap (not shown). In an embodiment related to that shown in Figure IB, employing a liquid primary layer and typically lacking secondary layers, the binding components 24 are initially positioned at the distal end of the assay vessel, and are then incubated with the other components of an assa mixture. Finally a primary layer material is poured o otherwise dispensed into the assay vessel, leaving th washed binding components and bound label at the bottom o the assay vessel, with the other components of the assa mixture (including free label) displaced to the top of th primary layer. This embodiment is also effective where th distal inner surface of the assay vessel has been coated t form the binding components. Referring now to Figure 2, another preferre embodiment is shown which is similar to that shown i Figure IB, with the use of a well 32 within a multiwel plate. An alternative embodiment which is preferable fo some applications uses strips of 1 or 1.4 mL tubes ( millimeter outside diameter, Skatron A.S., Lier, Norway which fit into a standard 96 well plate array. Thes embodiments can be sealed with penetrable septa, ar typically used with non-isotopic labels, and ar 53 appropriate for separations achieved by centrifugal, gravitational, or magnetic forces. The well 32 generally comprises a body 34 defining an open space 36. The well is of a size sufficient to receive one or more preformed beads, plus a predispensed primary layer, and some cases one or more predispensed secondary layers. In the embodiment shown in Figure 2, the beads are initially positioned on the upper surface of the primary layer 12. The beads have specific binding agents attached thereto, thus forming binding components 26. Positioned below the primary layer is a secondary layer 28. Following incubation and conversion of the primary layer to a liquid form, the binding component(s) with bound label pass through the primary layer, enter the secondary layer, and settle to the bottom of the well. Because of the short distance from the top of the primary layer to bottom of the well, this embodiment is especially appropriate for separations employing magnetic forces. Shielding is typically not needed in the embodiment shown in Figure 2 because signal generation occurs only in a layer separated from the secondary components containing free label. A preferred embodiment utilizes an enzyme label, a primary layer which is in a solid form during incubation and which is converted to a liquid form prior to separation, and a secondary layer which includes an enzyme substrate which produces detectable signal in the presence of label. In another preferred embodiment wherein the label is fluorescent and the detector "looks" up through the bottom of the well, side-excitation in the bottom region of the cushion can be used to prevent excitation of free label. Alternatively in such cases, a quenching agent (such as a resonance energy transfer receptor like rhodamine where fluorescein is the label) can be added to binding assay. The use of fluorescent quenching compounds has been described for homogeneous binding assays (Ullman and Schwarzberg, U.S. 3,996,345, herein incorporated by reference) . Such a quencher may be useful in a heterogeneous binding assay because it will quench fluorescence of unbound label but not that of bound label, since it will be removed from the binding components by the cushion. In certain cases where the aqueous compartment of a particulate solid phase is not removed by passage through a cushion, inclusion in the assay mixture of such a quencher would be particulary useful to reduce non-specific signal. Referring now to Figure 4, preferred embodiments are shown for performing non-isotopic heterogenous (A) and homogenous (B) assays with whole blood or serum samples, or other assay mixtures which include insoluble components. Suitable tubes can be made of "k-resin" resilient polystyrene, or more preferably of polypropylene. As shown in Figure 4A (a preferred enzyme-labeled assay vessel) , a cap reservoir 56 is loaded with reaction components 58 and provided with a seal 60 prior to insertion into the main body of the assay vessel. Sonic welding can be employed to permanently attach and seal the plastic parts. An incomplete reaction mixture 62 is in contact with a solid or gel barrier layer 64. A side port 54 is used to introduce the . sample into the incomplete reaction mixture. The primary layer 66 is water immiscible, and is contacted above by the barrier layer and below by an immiscible secondary oil layer 68 of greater density. Both the primary layer 66 and the secondary oil layer 68 are immiscible with each other. The aqueous substrate solution 70 contains 60% w/v sorbitol and thus has a greater density than the secondary oil layer 68. The particle spreading layer 72 is denser than the substrate solution 70. All cushion layers are generally immiscible with each other. Referring to Figure 4A (within a preferred embodiment) , upon addition of sample through the port 54 into the incomplete reaction mixture 62, the mixture is briefly agitated, and subsequently subjected to conditions -) D sufficient (usually cen ifugation) to separate any insoluble components from the incomplete reaction mixture. The insoluble components pass through the selectively penetrable barrier layer 64 and, within certain embodiments, substantially to the lower portion of the primary layer 66. At the discretion of the operator, the incomplete assay mixture may be analyzed to detect the presence of interferences or the absorbence of a reference dye, in order to determine dilution of a reference dye and therefrom calculate liquid sample volume. Subsequently, the seal 60 is opened (for example, by melting) to allow additional reaction components 58 within the . cap reservoir 56 to combine with the incomplete assay mixture, usually upon the addition of low speed centrifugal force, thereby forming a complete assay mixture. The complete assay mixture is then generally agitated and incubated, and subsequently the complete assay mixture is subjected to conditions sufficient to cause binding components present within the complete assay mixture to penetrate the barrier layer (if present) and the primary and secondary layers. The binding components (not shown) pass to the bottom of the substrate solution 70 and redistribute as a relatively uniform layer adjacent to the particle spreading layer 72. Subsequently, the label bound to the binding components is detected, and the presence or amount of analyte determined. Referring now to figure 4B, a cap reservoir 76 is loaded with additional reaction components 78 and provided with a seal 80. An incomplete reaction mixture 82 is in contact with a primary layer 84. Upon addition of sample through a port 74 into the incomplete reaction mixture 62, the mixture is briefly agitated, and subsequently subjected to conditions (usually centrifugation) sufficient to separate any insoluble components from the incomplete reaction mixture. The insoluble components pass into the primary layer 84. Subsequently, the seal 80 is opened (for example, by melting) to allow the additional reaction components 78 with the cap reservoir 76 to combine with the 56 incomplete assay mixture, usually upon the addition of low speed centrifugal force, thereby forming a complete assay mixture. The complete assay mixture is then generally agitated and incubated under conditions and for a time sufficient to generate signal in proportion to the amount of analyte present in the sample. The signal within the complete assay mixture is then detected, and the presence or amount of analyte determined. F. SHIELDS Depending upon the nature of the signal emitted or produced by the label and the height of the cushion, it may or may not be desireable to physically shield the portion of the vessel containing the secondary components (with free label) in such a way that only signal emitted from the binding components is detected. Referring now to Figure 3, a reusable detection vessel is shown with an assay vessel place therein. The detection vessel 38 generally comprises a body 40 defining an interior chamber 42. If, for example, the label is a gamma emitting isotope the upper portion (and in some cases the extreme distal end) of the detection vessel 38 could be provided with a metallic or metallized shield 44, composed preferably of lead or leaded plastic, or of copper. If the label is a fluorophore or a luminescer, the upper portion of the detection vessel could be provided with a shield composed of a light-impenetrable material. It will be apparent that in certain applications, different assay vessels and different shields will be preferable. If shielding is desireable, the shield 44 can be either integral in the detection vessel body or it can be a separate shield, enclosed by the body of the detection vessel, into which the assay vessel 10 fits slideably. The latter configuration is generally preferred for its durability and superior geometry for shielding. For best shielding performance, the bore of the shield will 57 typically be cylindrical and of the minimum size required for convenient insertion and removal of the assay vessel . Referring again to Figure 3, the assay vessel 10 fits slideably into a shield composed of a radiation shielding material. The shield is open at both ends and has an inner diameter which is sufficiently greater than the outside diameter of the assay vessel to allow the assay vessel to slide easily into the shield. A particularly convenient configuration is one in which the assay vessel is a test tube which has a lip which engages the top of the shield and supports the tube within the shield. Microcentrifuge tubes having an approximate volume of 0.4 mL are commercially available from a number of sources and will slide easily into and out of a shield of inner diameter approximately 1/8 inch in diameter. Tubes with a similar outside diameter, but which are longer than 0.4 mL tubes, would be advantageous in certain applications. Because in this embodiment the assay vessel is small in diameter, the shield can also be small in diameter; hence, there is comparatively little scattered radiation detected from the supernatant or the cushion. Therefore the detection of bound radioisotopic label is essentially unimpeded by the inadvertent simultaneous detection of unbound label, unlike with prior art devices and methods. The composition of the shield will vary, depending upon the nature of the signal emitted or produced by the label, but its design and material will typically be sufficient to block detection of at least 90%, and more typically greater than 99%, and optimally greater than 99.7% of the label in the unbound fraction after separation of bound (solid) and unbound (supernatant) components of the assay mixture. For example, if the label is a gamma-emitting isotope such as 125-Iodine, the shield might be composed of lead, leaded plastic, copper, or other suitable material. For detection instruments comprising gamma counters with annular crystals (including Micromedjc 58 Systems, Horsham, PA, LKB Instruments, Gaithersburg, MD, and Beckman Instruments, Brea, CA) , a sleeve of 1/8 inch thick lead (3/8 inch O.D., 1/4 inch I.D.) provides an excellent combination of strength (to withstand manufacturing manipulations and centrifugation at least up to 3000 x g in use) and radiation shielding. However, MacKenzie (J. Immυnological Methods, 6J7:201-203, 1984, herein incorporated by reference) has calculated that a much thinner (1 mm) sheet of lead blocks 99.999964% of adose of 125-Iodine. Thus to achieve 99.0% shielding theoretically requires a lead foil only 36 millionths of an inch thick. High-integrity lead foils (.006 inch and .012 inch thick) are commercially available (Nuclear Associates, Carle Place, NY) and provide essentially complete radiation shielding with much less weight than a 1/8 inch thick sleeve. Lead foil could be used to form a shield in applications where the 1/8 inch thick sleeve is undesirably heavy. Lead-coated or lead-containing composite plastics or fabrics, produced from molded lead or lead foil are also effective lightweight shield materials. For such foils and thin films, strength is provided, by a plastic support sleeve. Other materials including non-lead metals such as brass can be used as shields for radiation including that emitted by 125-Iodine. If the label is a beta emitting isotope such as tritium or 32-Phosphorus, the shield might be composed of an opaque plastic. If the label is a fluorophore or a luminescer, the shield might be black plastic. However, in most applications, labels such as fluorophores and low energy beta-emitting radioisotopes will not require shields. Where required, the shield is designed to mask approximately the upper seventy-five percent of the assay vessel and usually not more than approximately the upper ninety percent of the assay vessel. A general purpose shield will typically be as long as possible without 59 significantly reducing the detectable label in the unshielded portion 46. For gamma counters with annular crystals and assay vessels such as shown in Figure 1A, a 3/8 inch O.D, 1/8 inch I.D. lead sleeve approximately 1 3/8 inch long is preferred. Such an assay vessel typically contains approximately 250 microliter of cushion liquid and 10 micro-liters or less of binding components. However, for certain detection instruments and for different cushion heights, modifications in shield length or in the volume of cushion and/or binding components will be desirable. In cases where the detector is centered near the bottom tip of the assay tube, part or all of the assay mixture may not need to be shielded laterally because the shield below will block undesirable radiation. This form of shield is effectively a skirt, and has the added advantage that the assay mixture can be directly observed (as during reagent additions to the top of the cushion) even while shielding is in effect and the assay vessel is in its final position. In addition to accommodating the assay vessel within itself, the shield should fit inside the body of the detection vessel, as shown in Figure 3. The detection vessel is closed at the bottom and may or may not be sealable at the top as well. Typically, the body of the detection vessel is a test tube, the inner diameter of which is sufficiently greater than the outside diameter of the shield to allow the shield to slide tightly thereinto for purposes of semi-permanent assembly. As shown in Figure 3, the shield may be provided with a shim 48, preferably composed of an adhesive paper label, glue, or a suitable resilient material, in order to maintain the position of the shield within the detection vessel. Suitable for use as detection vessel bodies are test tubes of polypropylene, polyethylene, or glass, typically having approximate outer dimensions 12x75 or 12x55 mm. Such tubes are commercially available from a variety of sources and are advantageous in that they fit 60 readily into gamma counters and/or scintillation counters. Where 0.4 mL assay vessels have tethered caps which bind on the inner walls of the detection vessel , a simple tool (e.g. Model 61-008 from the Stanley Tool Company) can be used to insert and withdraw assay vessels. Alternatively, a shorter (12x55 cm) detection vessel can be used with such assay vessels because assay vessels with tethered caps can be easily inserted and removed without a tool.- In general, plastic tubes (especially polypropylene) are preferred over glass tubes for use as detection vessel bodies because there is less risk of breakage and they can typically withstand greater centrifugal force. In general, too, it is preferred that the detection vessels be reuseable. In a preferred embodiment for centrifugation of assay vessels directly in shields made with the 3/8 inch outside diameter lead cylinders described above, the detection vessel would contain the shield therein, supported by acylindrical member 50. Such a cylindrical member 50 is preferably composed of plastic such as polystyrene, and may be closed at a level distal from the shield so as to support assay vessels at a constant height. While most commercially available gamma counters exhibit good shielding using the 3/8 inch outside diameter lead cylinder described above, some with well-type crystals (especially many gamma counters having more than four crystals) require a modification in the shield design. The support cylinder closed at one end as described above can contain a shielding disk 52, made from a suitable shielding material such as lead. This disk is positioned at the bottom of the well formed by the member 50 to shield the gamma counter from unbound label radiation which is traveling generally parallel to the long axis of the assay vessel. A surprising advantage of this design is that improved shielding is obtained with all gamma counters, while causing only a slight decrease in detectability of 61 the bound label located in the distal end of the assay vessel. Configured in the manner described above, namely, an inner assay vessel prefilled with a cushion as described herein and slideably fitted into a shield within the body of a detection vessel, where the shield is supported by a cylindrical member, a specific binding assay can be rapidly and conveniently performed in a self-contained microtube with as little as one liquid addition (sample) step and one brief centrifugation step prior to detection of bound label. In certain instances, the centrifuga ion step can be eliminated. For example, one such situation is where gravity separation is employed using dense particles and a meltable primary layer. These binding assay methods can use equipment currently available in most laboratories which perform such assays. These assays can be accomplished with considerable reduction in time, skilled labor, and radioactive waste volume over specific binding assay methods as currently practiced. Comparable advantages will be experienced for both isotopic and non-isotopic applications. G. COMBINED USE OF CUSHIONS AND SHIELDS Surprising and valuable features are inherent in the combined use of water-immiscible primary layers and radiation shields. Even the most convenient currently available isotopic assays using antibody-coated tubes or large, antibody-coated beads must be processed both before and after incubation by skilled persons or by sophisticated liquid handling equipment. Such processing includes post-incubation addition of wash solution, aspiration or decanting to remove free label, and usually a repeat of these steps. Not only are these steps inconvenient, they risk spills and contamination from both biohazards in the sample and radioactivity from the components of radioisotopic assays. 62 Unless carefully controlled, this washing can be disadvantageous for several reasons. First, assay precision and accuracy can suffer from dissociation of antibody-antigen complexes which occurs during the washing step, potentially reducing signal. This is especially significant with monoepitopic assays (such as with small antigens or in many assays using monoclonal antibodies), where a single attachment between antibody and antigen is formed. Furthermore, the wash liquid volume in conventional heterogeneous binding assays must be significantly larger than the volume of the assay mixture, and the larger the wash volume, the more effective the washing procedure. When this wash solution is removed, typically by decanting onto an adsorbent pad, a significant increase in radioactive waste volume is produced compared to the initial mixture volume. A valuable and surprising feature of the present invention is that the above described wash solution can be eliminated and the reagents can be kept totally contained in the assay vessel. This feature provides improved safety compared to conventional methods because potentially hazardous materials (for example, radioactivity and/or Infectious material) is totally contained for safe and convenient disposal. Subsequent to loading the assay mixture, the need for special skills or care is eliminated. Another surprising feature is that the water-immiscible layer can be small relative to the volume of the mixture, and much smaller than the typical wash volume used in traditional heterogeneous assays. As the binding components pass through the cushion, they continuously encounter fresh cushion medium and thus are effectively washed in a small volume. The above configuration also represents a significant improvement over prior art shielding methods because the introduction of an immiscible phase between the assay mixture and the binding component dramatically increases the preciseness and completeness of separation of 63 the bound from unbound label fractions. This immiscible phase coupled with the shielding features described above allow one to effectively perform self-contained binding assays such as radioimmunoassays. Separation of bound and unbound label in such assays is virtually instantaneous and can produce equilibrium binding assay data for applications in characterizing the tightness of interaction for binding pairs. The geometry of the assay vessel and shield, both being elongated and relatively small in diameter, virtually eliminates the contribution of scattered radiation to the total signal measured, hence practically no mathematical correction of the data is required. Because the assay mixture and its components are immiscible in the primary layer, neither dilution nor dissociation occur during incubation of the mixture mixture in contact with this layer, and no dissociation of binding pairs occurs as is observed in the prior art using sucrose and related materials as barriers. Thus the entire assay including mixing and incubation steps can occur in contact with the primary layer, eliminating the need to transfer the incubated mixture mixture onto a cushion, or to controllably inject a washing solution of material such as sucrose under the incubated mixture mixture, as in the prior art. A further feature of the present invention is evident with the use of binding components attached to the assay vessel or to large, dense beads. A water-immiscible cushion denser than the secondary components but less dense than the large beads (if any) can be added at the end of the assay if desired, achieving separation of bound from free label without requiring the removal of unbound label and other assay mixture components from the assay vessel. A further attribute of binding assays employing incubation of an assay mixture on a water-immiscible liquid is the dramatic reduction in the volume of the mixture. Manipulation of a visible pellet is not required and the 64 assay mixture components can be predispensed onto the top of the primary layer. Such predispensed assay mixture component can be stored as liquids, or concentrated and/or stabilized by lyophilization, then rehydrated or diluted for use by the addition of a small (e.g. 10 microliters) sample. Thus the assay can be miniaturized, waste dramatically reduced, and safety significantly increased, while simultaneously saving labor and reducing error-producing steps in the performance of specific binding assays. H. UNSHIELDED CUSHION EMBODIMENTS For unshielded applications, especially using enzyme or fluorescent labels in multiwell plates, the use of an immiscible primary layer and an aqueous secondary layer makes possible effective separation of binding components from free aqueous label (by gravity, centrifugation, or magnetic forces) over a distance too small to be effective with wholly aqueous cushions. Especially useful in such applications are primary layers which are readily solidified by cooling, or which are solid at storage and/or incubation temperatures in the range of 15-50 C, and can be liquified (typically melted) for the separation step in this temperature range. Very dense binding component solid phase particles (e.g. glass or metal spheres) can be used which will sink through the primary layer when it is liquified by warming. It will be apparent that methods using the present invention are compatible with existing automated clinical analyzers designed for colorimetric and fluorometric clinical assays. The following examples are offered by way of illustration, not by way of limitation. 65 EXAMPLES Abbreviations used in the examples include PBS (phosphate buffered saline) , BSA (bovine serum albumin) , TGF-a (transforming growth factor alpha) , hEGF (humanepidermal growth factor), RIA (radioimmunoassay), DTT (dithiothreitol) , and CPM (counts per minute). EXAMPLE I MINIATURIZED COMPETITIVE RADIOIMMUNOASSAYS FOR TRANSFORMING GROWTH FACTOR ALPHA All assays employed as assay vessels 0. 4 mL microcentrifuge tubes containing 0.25-0.3 mL of cushion material. Labeled peptide was produced by chloramine- T iodination (specific activity ranged from 200-500 μCi/μg) . Unless otherwise specified, all solid phases were prepared using S. aureus suspensions. Cent ifugations were for 30-60 seconds in a microcentrifuge at approximately 10,000 x g. (A) RIA Using Peptide fragment and Butyl Phthalate Cushion: The synthetic peptide used for immunization of rabbits was a protein and glutaraldehyde conjugate of the c-terminus 17 amino acids of rat rTGF-a (Marquardt et al., Science 2_2^:1079-1082, 1984). This peptide (unconjugated) was also used as a reference standard and as the label (125-iodine labeled). Antiserum or normal rabbit serum (for nonspecific binding determinations) was adsorbed onto a commercial preparation of formalin-fixed S_^ aureus (I re Corp, Seattle, WA) to form an antibody solid phase suspension with 5% solids in PBS. Label cocktail was prepared by mixing, in 500 μL total volume, 100 μL (250,000 CPM) of labelled peptide, 100 μL of 10% (0.65 M) dithiothreitol, 30 μL of 10 mg/mL BSA, 5 μL of 10% sodium azide, 50 μL of lOx PBS, and 215 μL distilled water. Into each assay vessel was loaded 30 μL of label cocktail, 40 μL of sample, and 30 μL of antibody suspension. After mixing the assay mixture and incubating overnight at 66 4°C, the assay vessels were centrifuged, then placed in radiation shields (Figure 3) and counted in a Beckman LS-100C scintillation counter using Gammavials (Koch-Light Ltd, Suffolk, England; counting efficiency was ca. 40%). The data obtained using synthetic peptide calibrator is shown below. Bioactive synthetic rat and human TGF-alpha gave competition curves equivalent to peptide fragment on a molar basis, with 50% competition at approximately 0.6 nM peptide. TGF-a RIA: PEPTIDE FRAGMENT, BUTYL PHTHALATE CUSHIONS SAMPLE* ANTIBODY LABEL BOUND BUFFER ANTI-PEPTIDE FRAGMENT 43% 0.5 nM " 30 1.0 nM " 24 2.0 nM " 16 10 nM " 7 BUFFER NORMAL RABBIT SERUM 3 BUFFER MINUS ANTIBODY SUSPENSION 0.1 *final concentration in assay (B) RIA using anti-fragment antisera and bioactive syntheticpeptide as tracer and reference standard, with methylcinnamate cushions and assay reactants predispensed. Trans-methyl cinnamate (Table 1, item 11, Aldrich Chemical Co., St. Louis, MO) was melted by brief heating in a microwave oven just prior to dispensing into assay vessels. The cushion solidified spontaneously at room temperature. The solid phase was prepared as in (A) above in double strength assay buffer (including 4% NP-40 nonionic detergent) . This suspension was stable at 4°C for at least one year. Label cocktail (1.5 mL) was prepared using 0.3 mL lOx assay buffer (minus nonionic detergent), 0.6 mL 10% NP-40, 0.58 mL distilled water, and 30 μL label 67 concentrate (600,000 CPM) prepared from bioactive synthetic rat TGF-alpha (res. 1-50). Into each assay was loaded 50 μL of reference standard sample (bioactive, synthetic rat TGF-alpha, res. 1-50), 25 μL of label cocktail, followed by 25 μL of solid phase suspension. Where indicated, all assay reactants were predispensed and equilibrated for at least 3 days at 4°C prior to initiation of the assay by sample addition and mixing. After the indicated incubation periods, assay vessels were centrifuged and counted using radiation shields (Figure 3, lacking disk 52) in an gamma counter (Abbott Model 200) . Two temperature and mixing treatments were compared with four incubation times. One treatment consisted of incubating at 32°C, just below the melting temperature of the cushion, and mixing at 15 minute intervals. The second treatment consisted of incubating at 40°C, above the melting temperature of the cushion, with mixing only at initiation of Incubation (prior to warming). Both assays yielded low nonspecific binding, high specific binding, and high competition with reference standards, even after only 30 minutes of incubation. Detailed results are shown below: 68 TGF-a RIA: BIOACTIVE PEPTIDES & METHYL CINNAMATE CUSHIONS INCUBATION TIME TEMPERATURE 32°C 40°C 30 MINUTES Nonspecific binding Total bound % bound with 0.3 nM sample % bound with 10 nM sample 60 MINUTES Nonspecific binding Total bound % bound with 0.3 nM sample % bound with 10 nM sample 90 MINUTES Nonspecific binding Total bound % bound with 0.3 nM sample % bound with 10 nM sample 120 MINUTES Nonspecific binding 1.0% 1.1' Total bound 31.0 36.4 % bound with 0.3 nM sample 25.6 35.8 % bound with 10 nM sample 6.1 7.6 69 TGF-a RIA: PREDISPENSED REACTANTS & METH. CINNAMATE CUSHIONS Incubation=120 minutes, Temperature=37°C, Total CPM=7872 REACTANTS LABEL BOUND C.V.* %OF MAX.BOUND Buffer sample, 1.6% 18% N.A. Nonspecific binding (146 CPM) Buffer sample, 16.8 6% 100% Specific binding (1321 CPM) 2.5 nM sample 8.2 4% 48.6% competition (642 CPM) * Coefficient of variation (std deviation/average) , N=6 • c ) TGF-a RIA using human serum samples with anti-rabbit IgG-coated solid phase. The assay was performed using butyl phthalate cushions as described in (A) above except that the antibody-coated solid phase was prepared either with fixed S. aureus (Pansorbin, Behring Diagnostics, La Jolla, CA) or with glutaraldehyde cross linked, anti-rabbit IgG-coated S. aureus (Tachisorb, Behring Diagnostics). The concentration of solids in each case was the equivalent of 12.5 μL of a 10% w/v suspension per 100 μL assay mixture volume. All tubes were preparaed in duplicate and incubated for two hours at 37°C. A 50 μL sample of diluted normal human serum was added to each tube containing a predispensed cushion, followed immediately by 25 μL of labelled peptide and 25 μL of antibody solid phase suspension to initiate the reaction. The results indicate that even the highest concentration of human serum had no signficant effect on the Tachisorb assay, while with Pansorbin even the most dilute human serum sample caused 41% nonspecific competition, presumably by displacing rabbit antibodies bound to protein A on the solid phase. With Tachisorb, non-specific binding was lower, specific binding was greater, and competion with 1.25 nM standard was greater than with Pansorbin. Detailed results are shown in Table 3 below. 71 Table 3. COMPARISON OF PANSORBIN AND TACHISORB ASSAY LABEL BOUND AS BOUND AS % OF CONDITIONS % OF TOTAL ADDED* MAXIMUM BOUND Pansorbin, Normal Rabbit 1.9 N.A. serum, buffer sample Pansorbin, Rabbit anti- 27.9 100 serum, buffer sample Pansorbin, Rabbit anti- 16.4 58.9 serum, 1.25 nM Std in buffer Pansorbin, Rabbit anti- 14.3 51.3 serum, normal human serum sample (1:10 with buffer) Pansorbin, Rabbit anti- 18.1 64.9 serum, normal human serum sample (1:50 with buffer) Pansorbin, Rabbit anti- 16.5 59.4 serum, normal human serum sample (1:100 with buffer) Tachisorb, Normal Rabbit 1.4 N.A. serum, buffer sample Tachisorb, Rabbit anti- 31.7 100 serum, buffer sample Tachisorb, Rabbit anti- 18.2 57.4 serum, 1.25 nM Std in buffer Tachisorb, Rabbit anti- 31.2 98.4 serum, normal human serum sample (1:10 with buffer) Tachisorb, Rabbit anti- 31.6 99.6 serum, normal human serum sample (1:50 with buffer) Tachisorb, Rabbit anti- 30.8 98.5 serum, normal human serum sample (1:100 with buffer) *Total CPM added = 8080 72 ( D ) RIA for human TGF-a using antisera recognizing the complete, bioactive s_y n_ t h e t i c hormone Synthesis of hTGF-a (1-50) peptide and immunization: The sequence of human TGF-a as determined by DeRynck et al. (Cell 3^:287-297, 1985) " was used to synthesize the low molecular weight form of the hormone (residues 1-50) using an automated instrument (Biosearch) . The resultant peptide was used to immunize rabbits repeatedly using 0.5 mg of peptide at multiple sites. Immunoassay procedure: The assay used reference standards and radio-iodinated tracer prepared from purified, bioactive synthetic rat TGF-alpha (Peninsula Laboratories, Belmont, California). Label cocktail was prepared by mixing, in 1.5 ml. total volume, 300 μL lOx buffer (0.5 M Hepes, 2 mg/mL BSA, 0.2% sodium azide) , 600 μL 10% nonidet P-40 (Shell Oil Co.), 580 μL distilled water, and 30 μL of labeled peptide (rTGF-a, 800,000 CPM). The antibody suspension was prepared essentially as described in (A) above. To each 0.4 mL tube containg 0.25-0.3 L cushions of butyl phthalate was added 25 μL of label cocktail, 50 μL of sample, and 25 μL of antibody suspension. Where indicated, 10 μL of 1M DTT (freshly dissolved in 0.5M sodium bicarbonate) was added to each assay mixture. After mixing, the tubes were incubated overnight at 4°C, then processed as described in (A) except that the detection instrument was a gamma counter (Abbott Model 200). The assay detected rat and human synthetic TGF-a (res.1-50) equivalentl , whether or not the peptides were unfolded by reduction with DTT. Further, the assay detected authentic biological human TGF-a from cell culture media conditioned by A375 cells (Marquardt et al . , PNAS 80:4684-4688, 1983). Detailed results are shown below: Percentage table here 74 EXAMPLE II ENZYME-LABELLED QUALITATIVE CENTRIFUGAL COMPETITIVE BINDING ASSAY IN 0.4 ML TUBES TO DETECT RABBIT IGG IN A SAMPLE USING ENZYME-LABELLED RABBIT IMMUNOGLOBULIN AND CUSHIONS CONTAINNG ENZYME SUBSTRATE IN THE BOTTOM LAYER (A) Reagents: Labelled antibody was affinity purified rabbit anti-goat immunoglobulin coupled to horseradish peroxidase (Zymed), diluted 1:3000 in phosphate buffered saline containing 1 mg/ml bovine serum albumin. The solid phase was a 10% suspension of heat-killed, formalin-fixed S. aureus (Imre Corp, Seattle, Washington) . The sorbitol substrate cushion solution was prepared by dissolving 22 grams of sorbitol in 50 mL of distilled water, then dissolving 100 mg of chromogenic substrate (OPD, from Zymed, So. San Francisco, CA) in one mL of water and adding 0.1 mL of the OPD stock solution and 0.1 mL of 3% hydrogen peroxide to 9.8 mL of the sorbitol. (B) Assay: The assay vessels (0.4 mL polyethylene micro-centrifuge tubes. West Coast Scientific, Emeryville, CA) were then loaded with 0.1 mL of the sorbitol substrate solution, then overlaid with 0.2 mL of dibutyl phthalate. Another set of assay vessels was loaded with 0.3 mL of sorbitol substrate solution. On top of the butyl phthalate cushion was pipetted 0.05 mL of 10% pansorbin in phosphate buffered saline containing 0.1% sodium azide. To one tube was added .005 mL of rabbit serum, then .05 L of rabbit anti-goat IgG, affinity purified and labelled with horseradish peroxidase (RAG-HRP from Zymed, diluted 1:3000 in PBS containing 1 mg/mL BSA). To the other tube was added .005 mL dilution buffer and .05 L of RAG-HRP. After two minutes, tubes were spun for one minute in a high-speed 75 microcentrifuge (Fisher model 235B) and examined for signal development. The control pellet was immediately "negative" (dark brown or black) on its upper surface, while the side contacting the tube remained light amber. The pellet treated with sample was "positive" (light amber in color). No color developed in the sample layer or in the separate, clearly visible primary cushion layer, where substrate was absent. Surprisingly, only a little color developed in the lower substrate solution, but as expected the sample tube was nonetheless visibly positive (light yellow) compared to the control tube (amber) . The unexpected concentration of the substrate on the surface of the solid phase itself provided a dramatic concentrating effect, amplifying the difference between positive and negative samples. While differences in the substrate solutions were apparent with careful visual examination, the pellets were easily distinguished at a glance. No further changes were seen over the next 30 minutes while the samples were kept at 25 C, but over the next 2 hours the almost black control pellet became somewhat lighter (dark brown) , while the light amber sample pellet became somewhat darker (light brown or orange in color). No obvious further changes occurred, and the two pellets were easily distinguished after more than one week storage at room temperature (18-15°C). After extended storage, the butyl phthalate layer became amber, as if extracting the chromophore from the aqueous lower phase. The oil layer in the control tube was darker amber, distinguishable by eye from the oil layer from the sample tube. An analogous experiment using a sorbitol substrate cushion without the intervening oil layer, and using an air space between the sample and the substrate cushion also gave visually distinguishable results. After centrifugation, no demarkation of the sample and cushion layers was visible. Washing of the solid phase was not as effective since a colored streak traced the path of the 76 solids down the wail of the assay vessel. However, the control vessel streak, and pellet, were clearly darker amber than those in the sample vessel. With time, the entire solution (sample and cushion) became amber, though after one week the control vessel was overall still darker amber than the sample vessel. EXAMPLE III MINIATURIZED IMMUNOASSAY USING REAGENTS LYOPHILIZED ONTO TOP OF PRIMARY CUSHION The reactants are prepared as in example one, except that the sample is omitted and the oil is methyl cinnamate, which is a solid below 36°C. The assay vessels are frozen and subjected to lyophilization in a Speed Vac (Savant) under low speed centrifugation. When the reactants are dry, tubes are stored at room temperature. When sample is added (0.05 mL) , the reactants are rehydrated, and after two hours at room temperature, the tubes are warmed to 37-40°C and spun as in example 1 above and signal measured. EXAMPLE IV DETECTION OF 32-P LABELLED DNA BOUND TO HYDROXYAPATITE BY CENTRIFUGATION THROUGH A DIBUTYL PHTHALATE CUSHION CONTAINING SCINTILLATION FLUORS (A) Reagents: 32p-iabelled double-stranded DNA was divided into two aliquots. One part was boiled for ten minutes, then placed on ice. Each aliquot (20 microliters, in 10 mM Tris buffer, pH 8.2) received 100 microliters of a 10% hydroxyapatite suspension in the same buffer. Cushions were prepared in 0.4 mL microcentrifuge tubes by pipetting 0.3 L of one of the following solutions: (1) butyl phthalate containing 40 mg/mL omnifluor (New England 77 Nuclear), (2) butyl phthalate containing 1.25 mg/mL omnifluor, (3) butyl phthalate alone. (B) Binding assay: Onto each cushion was pipetted 10 microliters of the suspension containing unheated labelled DNA and solid phase. This mixture was spun one minute in a Fisher microcentrifuge (model 235B) . Tubes were counted using a Beckman LS-100C liquid scintillation counter. CPM, EACH CHANNEL CUSHION 32-P 14-C BUTYL PHTHALATE(BPH) BPH + 12.5 mg/mL omnifluor BPH + 40.0 mg/mL omnifluor When counted using the 14-C channel, the 32-P was detected with or without fluor in the cushion. Counting on the 14-C channel in the presence of a 1/8 inch thick lead shield resulted in less than 10% reduction in counts, indicating that most of the DNA was bound to the solid phase under these low-salt conditions. These results indicate that, as tested here, the use of fluor-containing butyl phthalate eliminated the need for a shield, since using the 32-P channel, free label which had not entered the cushion would not be detected. These data also show that even on the 14-C channel, which gave somewhat higher signal than the 32-P channel, the inclusion of fluor in the cushion gave more than 50% greater signal compared to cushions lacking fluor. On this channel, however, a shield is required to mask the free label in the supernatant. Even using the 32-P channel, the background signal caused by radiation from supernatant entering the cushion can be greatly reduced or eliminated by using a shield, and that the shielding is more effective than when using the 14-C channel. This is demonstrated using the 78 heated DNA, which bound less completely to the solid phase in this ^ series of experiments. The heated DNA was processed on butyl phthalate cushions as described above. C M, EACH CHANNEL CUSHION 32-P 14-C BUTYL PHTHALATE(BPH) 110, 170 101355, 108850 BPH + shield 79860, 87160 BPH + 40.0 mg/mL omnifluor 38255, 41305 132310, 141855 BPH + omnifluor + shield 25825 108995, 117705 On the 32-P channel , the lead shield with fluorcontaining cushion gave almost 40% less signal the same cushion without the lead shield, indicating significant signal originating from the supernatant or upper portion of the cushion. Approximately 20% shielding of signal was obtained using the 14-C channel for the same samples. EXAMPLE V USE OF ANTIBODY-COATED TUBES WITH DISPLACEMENT OF FREE LABEL BY ADDITION OF WATER-IMMISCIBLE "CUSHION" Fifty microliters of either a BSA solution (1 mg/mL in PBS) or antibody against the rTGF-alpha c-terminal 17 residue fragment (prediluted 1:1000 in the same BSA solution) was added to 8x50 mm polypropylene tubes precoated with goat anti-rabbit IgG (Micromedic, Horsham, PA). To each of these tubes was added 50 microliters of 125-iodine labeled peptide fragment (1 nM in PBS with 0.2 mg/mL BSA) . After 5 minutes at room temperature, duplicate tubes received one milliliter of either dibutyl phthalate or a fluorocarbon oil, FC40 (both from Sigma Chemical Co., St. Louis). The dense oils displaced the aqueous assay mixtures from the bottoms of the tubes. Those with dibutyl 79 phthalate required some agitation to dislodge droplets of aqueous assay mixtures trapped near the bottom, and * a thin film of water appeared to persist between the oil and the tube inner surface. With FC40, the water floated immediately to the surface, without any apparent retention in the oil phase. All tubes were counted immediately in a scintillation counter, using 13x50 mm plastic tubes as holders for gam avials (Koch-Light) , after wrapping the supernatant and most of the oil layer in a 1.25 inch long cylinder of 0.006 inch lead foil which was supported 7/8 inch above the bottom by a plastic cylinder. RESULTS assay primary layer cpm %bound %bound mixture material (total) (specific) S/N* antibody butyl phthalate 14680 68.2 49.4 3.6 control 3580 18.8 antibody fluorocarbon oil 1430 8.3 5.8 3.3 control (FC-40) 400 2.5 *S/N = signal to noise ratio While both oils produced significant signal, they differed in performance. Butyl phthalate required some manipulation and yielded a high background, but quite high signal considering the short incubation and the relatively high antibody dilution (equilibrium binding at 1:2000 antibody dilution would be expected to yield approximately 35-40% specific binding). FC40 yielded a very low back-ground, and a signal closer to the expected value for a 5 minute incubation. In both cases, the signal to noise ratio was similar. 80 EXAMPLE VI URINE SAMPLES FROM CANCER PATIENTS TESTED WITH TGF-ALPHA(ANTI-FRAGMENT) AND HEGF RIAS For the TGF assay, 2.5 mL of urine was desalted through a G15 Sephadex column (PD-10, Pharmacia) which had been equilibrated with ammonium bicarbonate buffer. The void volume fractions containing urine peptides were lyophilized and reconstituted with 120 μL of water plus 12 μL of a reducing solution containing 1 M dithiothreitol and 0.5 M sodium bicarbonate. Myeloma samples received an extra 10 μL of reducing solution and only 110 μL of water. For the hEGF assay, urine was diluted five fold with buffer. A 0.050 L sample of each processed urine sample was mixed with 0.025 mL antibody suspension and 0.025 mL of radioiodinated tracer (full length TGF-a, residues 1-50, or hEGF, residues 1-53, 250-275 μCi/μG, approximatel 10,000 cpm) in incubation/separation vessels containing 0.25 L dibutyl phthalate. After incubating overnight at 4°C, vessels were centrifuged for 30 seconds at approximately 10,000 x g and were placed into radiation shields (42 in Figure 3) and were counted one minute in an LKB Rackgamma counter. Standards consisted full length TGF-a and hEGF in buffer containing 0.2 mg/mL bovine serum, albumin and treated in the same manner as urine samples. 81 SUMMARY OF TGF/EGF RESULTS USING HIGHEST NORMAL AS CUTOFF (NORMALS=10) POSITIVES POSTIVES FROM SAMPLE TYPE N FROM TGF TGF/EGF RATIO EXAMPLE VII USE OF MULTIPLE-LAYER CUSHIONS Different materials of potential use as primary or secondary cushion layers were tested for their ability to maintain discrete boundaries during formation of the cushion and subsequent centrifugation, and to allow the pelleting of S^ aureus particles in a brief spin. All potential cushion materials were tested for the ability of fixed S. aureus to pellet in 0.4 L polypropylene microcentrifuge tubes during a one minute centrifugation at full speed in a microcentrifuge (Savant, 10,000 RPM). Under these conditions, pelleting occurred equally well for sucrose solutions (10-40% w/v, d=l.0374-1.1758 at 22°C) and the water-immiscible materials listed below: diethyl succinate, ethyl cinnamate, dibutyl phthalate, methyl adipate, and diethyl aleate. EXAMPLE VIII COMPETITIVE RIA FOR THYROID STIMULATIONG HORMONE (TSH) A commercial 125-iodine RIA kit for determining TSH was obtained from American Bioclinical (Portland, OR) and adapted to the separation and detection methods of the present invention. All assay reactants were used according to the manufacturer's instructions except that reactant 82 volumes were decreased four-fold, and S. aureus (25 μL of a 10% w/v .suspension per test) was substituted for the "second antibody" precipitating solution. The adapted test was performed using 0.4 mL microcentrifuge tubes containing 0.25 mL butyl phthalate cushions. Even though the adapted test was only incubated for two hours (37°C) versus four hours (25°C) for the standard test, the adapted test exhibited significantly lower nonspecific binding with equivalent total bound and greater overall sensitivty. Detailed results are given below: 83 COMPARISON OF STANDARD RIA AND ADAPTED TSH RIA CONDITIONS STANDARD TEST ADAPTED TEST TIME: 4 HOURS 2 HOURS TEMPERATURE: ROOM TEMPERATURE 37°C ASSAY MIXTURE VOLUME: 0.50 mL + 1 L .15 mL PRECIPITATING SOLN USER STEPS: l.MIX SAMPLE+ l.MIX SAMPLE+ ANTIBODY ANTIBODY+TRACER 2.ADD TRACER 3.ADD 2ND ANTIBODY 4.SPIN 10 MINUTES 2.SPIN 0.5MINUTE 5.DRAIN SUPERNATANT 6.COUNT CPM 3.COUNT CPM RESULTS 84 EXAMPLE IX PERFORMANCE OF RIA COMPONENTS: PRECISION & SHIELDING EFFICIENCY (A) Shielding effectiveness of radiation shields. Radiation shields (42, figure ' 3) were tested for efficiency of shielding 125-iodine radiation, with and without shielding disks 52. Aliquots of 125-1 containing solutions were pipetted into 0.4 mL assay vessels. Total unshielded counts were determined using tubes without cushions, counted without shields. Detection efficiency was determined by counting these same tubes in the two types of shields. Shielding efficiency was determined by counting tubes containing cushions with two kinds of shields (Figure 3, with and without the disk 52). DETECTION OF BOUND LABEL DETECTION OF UNBOUND (IN DISTAL END OF ASSAY VESSEL) (IN ASSAY MIXTURE) Total bound tracer replicates were measured using the TGF assay (Example IA) . Four groups of 15 tubes each were counted on two different gamma counters. 85 MICROMEDIC FOUR-CHANNEL COUNTER (3 MINUTE COUNTS) SAMPLE SET AVERAGE STD DEVIATION %CV I 2222 69 3.1 II 2121 85 4.0 III 2114 89 4.2 IV 2113 104 4.9 BECKMAN ONE-CHANNEL COUNTER (ONE MINUTE COUNTS) <\TION %CV 4.0 4.0 4.5 5.7 EXAMPLE X RADIOIMMUNOASSAY FOR DIGOXIN A rapid assay using the present invention was compared with conventional double antibody assay methods. Commercially-available preparations of rabbit antiserum (Immunosearch, San Francisco, CA) , and 125-Iodine labelled digoxin and digoxin standards (from Cambridge Medical Diagnostics, Cambridge, MA), were used. The binding component was prepared as a 10:1 blend of 10% Tachisorb with 10% Woods strain of S. aureus (both from Behring Diagnostics, La Jolla, CA) . The anti-digoxin antibody concentration in each assay type was adjusted to bind approximately 60% of 50,000 CPM of labelled digoxin after a fifteen minute incubation. Centrifugation was for five minutes at ca. 10,000 x g in a Savant microcentri uge with a 36-tube, fixed angle rotor. Radioactivity was determined with one minute counts. 86 Method: To 0.4 mL polypropylene tubes containing 0.200 mL dibutyl phthalate was added 50 microliters containing labelled digoxin, 1% NGS, 5% w/v Tachisorb-R and 0.5% Sansorbin (Behring Diagnostics, San Diego, CA) , and 1% NP-40 (Sigma Chemicals, St. Louis MO). A 50 microliter sample was added to each tube, then the assay was inititated by rapidly adding 50 microliters of antiserum diluted in 1% NGS. Tubes were capped and mixed by partially inverting several times, then incubated stationary for 15 minutes. After centrifugation, assay tubes were transferred to radiation shields (Biotope Cat #AC-010, essentially as described in Figure 3) and radioactivity determined in the gamma counter. Results are described below: Results: PRECISION - N= Mean %CV 18 1.57 ng/mL 4.89 18 0.68 ng/mL 3.65 SENSITIVITY - .07 ng/mL (2 s.d. from "zero") CORRELATION WITH COMMERCIAL DIGOXIN ASSAYS: (42 patient samples) versus Clinical Assays RIA R = 0.957748 slope = .953 intercept = 0.119 ng/mL versus Cambridge Medical Diagnostics RIA R = 0.977 slope = 1.014 intercept = -.029 ng/mL A modification of the method employed carboxylated Bromostyrene latex particles (JSR America Corp.), which were covalently coated with affinity-puri ied 87 goat-anti-rabbit IgG. Final particle concentration in the assay was .07%. The primary layer material was Silicone oil (#35 from Table 1) and centrifugation was for one minute. Nonspecific binding was reduced from approximately 1-3% in the above method to less than 0.1% in the modified method and precision appeared to be improved. Equivalent assay performance was achieved with either a 15 minute incubation at room temperature or a 5 minute incubation at 37°C. These results are striking in that the assay of the present invention is completed in less than twenty minutes, yet correlates well with, and provides better precision than current clinical assays which take 1-2 hours to complete. EXAMPLE XI ENZYME IMMUNOASSAYS FOR DIGOXIN An enzyme-labelled digoxin assay using the method of the present invention is adapted from a commercial digoxin kit (Immunotech, Allston, MA). The only changes in reagents are the inclusion of 20% sorbitol in the color developer solution, and the substitution of 50 microliters of Tachisorb-R (Behring Diagnostics, La Jolla, CA) for the 500 microliters of precipitating solution (goat-anti rabbit IgG) provided with the kit. In two mL, screw-top microcentrifuge tubes (Sarstedt, Princeton, NJ) , 1 L of sorbitol-color developer is dispensed, then overlaid with 0.4 mL of dibutyl phthalate. A 200 microliter assay mixture is prepared by adding 100 microliters of Tachisorb-R, 25 microliters of digoxin-enzy e (alkaline phosphatase) conjugate, 25 microliters of sample (serum-based standards), and 50 microliters of antibody solution. The assay is incubated at room temperature for 15 minutes, centrifuged two minutes 88 at 10,000 x g, and further incubated one hour for color development. Color is measured at 400 nm in a Shimadzu Model UV-160 spectropho ometer. Because of the opacity of the polypropylene tubes, results for each tube are corrected for nonspecific absorbance at 500 nm. Results are shown below: In this assay, normal rabbit serum is present as a diluent for the anti-digoxin antibody, and the total IgG exceeds the capacity of the binding components added. Substantially greater signal can be obtained by optimizing the method to capture all of the analyte-speci ic antibody in the assay. A more rapid assay may be performed using fluorogenic substrate in which the incubation step is five minutes, the separation step is 0.5 minutes, and the signal development and detection steps are typically 1-5 minutes. The tables (Figure 4A) contain, from bottom to top: lower layer - 10 μl fluorinert FC40 - 20 μl enzyme substrate - 150 μl silicone oil (Dow 550 fluid) - 50 μl incomplete reaction mixture (5 μl conjugate) cap reservoir - 50 μl anti-digoxin bound to bromostyrene particles, sealed with 5 μl eicosane ( p 36°C) Enzyme substrate contains 0.5 mg/mL methyl umbelliferone, phosphate in 2M diethanolamine plus 50% w/v sorbitol, pH 9.6. The incomplete reaction mixture contains diluted 89 conjugate in 50 mM sodium bicarbonate with 0.1% BSA. The Cap reservoir contains latex particles coated with goat anti-rabbit antibody as described in method X. The sample (50 μl of serum) is added, and the temperature subsequently adjusted to 40°C to melt the cap reservoir seal. Brief (2 seconds) low speed cent ifugation delivers the antibody particles to initiate the reaction. After mixing, incubating, and centrifuging at high speed to separate bound from unbound label, particles are distributed in a uniform disk at the substrate/FC40 interface. Enzyme rate or end point fluoresence measurements are determined in a fluori eter (excitation approximately 350 nm, emission approximately 450 nm) . EXAMPLE XII SEMIQUANTITATIVE VISUAL ASSAY FOR DIGOXIN The enzyme-labeled cushion assay adapted from the Immunotech digoxin assay was performed as described in Example XI, except that 0.4 L assay tubes were used, wi h 100 microliters each of color developer and dibutylphthalate, 25 microliters of Tachisorb-R, 25 microliters of digoxin-enzyme conjugate, 25 microliter samples, and 50 microliters of antibody solution. After 15 minutes at room temperature, tubes were centrifuged at 10,000 x g for one minute. Color initially developed in the particle pellets and gradually migrated in the liquid color developer layer. Low standards (0,1,2 ng/mL) could be distinguished visually from high standards (4,8 ng/mL) after a ten minute color development at room temperature (22°C). 90 EXAMPLE XIII AFFINIeTY BINDING ASSAY FOR GLYCOSYLATED HEMOGLOBIN The principle of the test is that glycosylated hemoglobin binds to an "affinity resin", boronic acid particles (Glyco-Gel B ) which were obtained from the Pierce Chemical Company (Rockford, Illinois). These were used as binding components in a non-immunological assay of the present invention. After centrifuging the binding components through a primary layer, bound hemoglobin was eluted with a sugar solution (15% sorbitol) contained in a secondary layer. The color in the secondary layer was measured with a spectrophoto eter to provide quantified results. If the sorbitol solution was omitted, visual detection of hemoglobin bound to the solid phase was possible. Method: Into a two L microcentrifuge tube was pipetted 0.7 mL 15% sorbitol, then 0.3 L primary layer material (diethylmethylmalonate), followed by 0.3 mL of a 25% (v/v) aqueous suspension of binding components. To begin the assay, .05 mL of hemolysed blood (1:10 dilution) was added to the particle suspension and incubated 30 minutes at room temperature or 10 minutes at 37°C. After centrifugation at 5000 x g for five minutes, absorbance at 418 nm was determined using the assay vessel as a cuvette. Nonspecific absorbance at 500 nm was subtracted. Total hemoglobin or non-glycosylated hemoglobin was determined separately and the % glycosylated was calculated. Results: Normal and elevated standards provided with a Pierce clinical diagnostics kit for glycosylated hemoglobin were consistently distinguished using both quantitative and visual detection methods. Essentially all of the bound hemoglobin is released from the particles into the sorbitol secondary layer, allowing for accurate quantitation. 91 EXAMPLE XIV SELF-CONTAINED GLUCOSE ASSAY USING WHOLE BLOOD SAMPLES A quantitative, self-contained, glucose assay was performed with whole blood samples using the present invention. A commercial kit (GOD Glucose, Trace America, Miami, Florida) was reconstituted with one-fifth the normal volume of water. This concentra ion was dispensed (40 μl) into cap reservoirs (Figure 4B) , which were then sealed with 3 μl of molten eicosane. The cap reservoirs were assembled onto the main body of the polypropylene assay vessel which had been previously loaded with a cushion (200 μl of silicone oil # 39 from Table 1) and partial reaction mixture (260 μl) of saline containing one mg/mL of heparin. Either a liquid calibrator (2 μl) or a fresh whole blood sample (4 μl) was pipetted into the sample port. After briefly agitating the tubes to mix, they were centrifuged at 10,000 xg for 60 seconds to separate blood cells into the cushion. Tubes were then warmed to 40°C to melt the cap reservoir seal. Brief (2 second) centrifugation delivered the cap reservoirs contents to initiate the reaction. After a 10 minute incubation, the tubes were transferred to a spectrophotometer to measure the color intensity at 500 nm. Non-specific absorbance (650 nm) was subtracted. Over the range of 100 - 800 mg/dL, the standard curve was linear (R = 0.9998). Signal from the whole blood sample developed normally, while the red blood cells were visible as a small pellet at the bottom of the cushion. Results: Glucose Level Absorbance (500 - 650 nm) 100 mg/dL .134 300 mg/dL .320 800 mg/dL .932 sample .230

Claims: Claims 1. An assay vessel for separating bound label from unbound label within an assay mixture, wherein said assay mixture includes one or more binding components, label bound to at least some of said binding components, and a substantially aqueous solution containing unbound label, said binding components differing in apparent density from said aqueous solution, comprising: a vessel having a proximal end and a closed distal end, said vessel defining an elongated chamber therein; a primary layer extending generally transversely within the chamber to form a selective barrier therein, said primary layer being immiscible with both said unbound label and said binding components; and a barrier layer between said primary layer and said proximal end. 2. The assay vessel of claim 1 wherein said primary layer is selectively liquifiable. 3. The assay vessel of claim 1 wherein said primary layer is is a mixture of two or more water-immiscible substances that are miscible with one another. 4. The assay vessel of claim 3 wherein at least one of said water-immiscible substances is selectively liquifiable. 5. The assay vessel of claim 1 wherein said primary layer is a mixture of two or more water-immiscible substances, at least one of which has a greater density than water when in liquid form. 6. The assay vessel of claim 1 wherein said barrier layer is selectively liquifiable and is substantially miscible with the assay mixture. 7. The assay vessel of claim 6 wherein said barrier layer is a gel layer of agarose or collagen. 8. The assay vessel of claim 1, further including a reagent reservoir, said reservoir containing additional reaction components. 9. The assay vessel of claim 8 wherein said reagent reservoir is positioned between said primary layer and said distal end. 10. The assay vessel of claim 8 wherein said reservoir is positioned adjacent to said proximal end, said reservoir further including a reversibly sealed aperture. 11. A method for detecting the presence or amount of an analyte within a sample, comprising: combining a sample with a partial reaction mixture to form an incomplete assay mixture, said incomplete assay mixture being in contact with a primary layer; subjecting the incomplete assay mixture to conditions sufficient to cause any insoluble components to separate from the incomplete assay mixture; combining additional reaction components with the incomplete assay mixture to form a complete assay mixture in contact with said primary layer, said complete assay mixture containing one or more binding components and label, at least some of said label and some of said analyte binding, directly or indirectly, to said binding components, said binding components and unbound label being immiscible with and of different density than said primary layer; s ubj ecting s a id comp lete as s ay mixtu r e to conditions sufficient to cause said binding components and ___> said unbound label to separate; and detecting the label bound to said binding components and therefrom determining the presence or amount of said analyte. 12. The method of claim 11 wherein the primary layer is a mixture of two or more water-immiscible, dense oils that are immiscible with one another. 13. The method of claim 12 wherein the primary layer is a mixture of one or more silicone oils and one or more fluorosilicone oils. 14. The method of claim 11 wherein the binding components are magnetic or magnetizable binding components. 15. A method for detecting the presence or amount of an analyte within a sample, comprising: combining a sample with a partial reaction mixture to form an incomplete assay mixture, said incomplete assay mixture being in contact with a primary layer; subjecting the incomplete assay mixture to conditions sufficient to cause any insoluble components to separate from the incomplete assay mixture; combining additional reaction components with the incomplete assay mixture to form a complete assay mixture in contact with said primary layer; incubating said complete assay mixture under conditions and for a time sufficient to generate signal in proportion to the amount of analyte present in the sample; and detecting the signal within the complete assay mixture and therefrom determining the presence or amount of said analyte. 16. A method for detecting the presence or amount of an analyte within a sample, comprising: combining a sample with a reaction mixture to form a complete assay mixture in contact with a primary layer and containing a precipitate, said complete assay mixture further containing one or more binding components and label, at least some of said label and some of said analyte binding, directly or indirectly, to said binding components, said binding components and unbound label being immiscible with and of different density than said primary layer; subjecting said complete assay mixture to conditions sufficient to cause said binding components to separate from said unbound label and said precipitate; and detecting the label bound to said binding components and therefrom determining the presence or amount of said analyte. 17. The method of claim 16 wherein the primary layer is a mixture of two or more water-immiscible, dense oils that are immiscible with one another. 18. The method of claim 17 wherein the primary layer is a mixture of one or more silicone oils and one or more fluorosilicone oils. 19. The method of claim 16 wherein the binding components are magnetic or magnetizable binding components. 20. A method for detecting the presence or amount of an analyte within a sample, comprising: combining a sample with a reaction mixture to form a complete assay mixture in contact with a primary layer and containing a precipitate; incubating said complete assay mixture under conditions and for a time sufficient to generate signal in proportion to the amount of said analyte present in said sample; subjecting said complete assay mixture to conditions sufficient to cause said precipitate to separate from said complete assay mixture; and detecting the signal within the complete assay mixture and therefrom determining the presence or amount of said analyte. 21. A method of detecting the presence or amount of an analyte within a fluid sample, comprising: combining said fluid sample with a reaction mixture to form an assay mixture, said assay mixture formed within an assay vessel, wherein said assay mixture contains one or more magnetic or magnetizable binding components and label, at least some of said label and some of said analyte binding, directly or indirectly, to said binding components; contacting said assay mixture with a primary layer, said binding components and unbound label being immiscible with said primary layer; subjecting said assay mixture to conditions sufficient to cause said binding components and said unbound label to separate; and detecting the label bound to said binding components and therefrom determining the presence or amount of said analyte. 22. A method for detecting the presence or amount of an analyte within a fluid sample, comprising: combining said fluid sample with a reaction mixture to form an assay mixture, wherein said assay mixture contains one or more magnetic or magnetizable binding components and non-isotopic label, at least some of said label and some of said analyte binding, directly or indirectly, to said binding components; contacting said assay mixture with a primary layer, said binding components and unbound label being immiscible with said primary layer; subjecting said assay mixture to conditions sufficient to cause said binding components and said unbound label to separate; and detecting the label bound to said binding components and therefrom determining the presence or amount of said analyte.

Inventors: HARGREAVES WILLIAM R, HARGREAVES, WILLIAM, R.
Applicants: BIOTOPE INC, BIOTOPE, INC.
Application year: 1988
Main classification: G01N33
IPC codes: B01L3, G01N21, G01N33