CA1250231A - Multilayer element for analysis - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A multilayer element for the analysis of a liquid containing an analyte comprising a reaction layer which retains a predetermined amount of a diffusible reagent containing a detectable species; said reagent adapted to be rendered substantially non-diffusible by said analyte as a function of the concentration of said analyte; a registration layer adapted to receive reagent diffusing thereto; means for detecting said reagent in said registration layer and an insulating layer intermediate said reaction layer adapted to provide discrimination between detectable species in said reagent layer and said registration layer.

Description

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BACKGROUND OF THE INVENTION
A nwnber of devices are Xnown for the chemical analysis of substances in liquids, especially biological liquids. One type of device requires expensive equipment, 5 highly trained operators and a large volume of samples for analysis.
Another type of device is a relatively simple, portable, multilayer element for carrying out a single test at a time without expensive equipment.
U.S. Patent No. 3,723,064, issued March 23, 1973, is directed to a layered testing device comprising a layer impreynated with a chemical reagent for a reaction with the substance under test comprising a permeable member having first and second regions of different permeabilities to the 15 test fluid. A transmission layer is placed adjacent the permeable member to aid in drawing the reaction products resulting from the reaction of the chemical reagent and the substance under test through to the next layer, which is an indicator layer, impregnated with a chemical adapted to 20 react with the above-mentioned reaction products to produce a visual reaction indicative of the presence and concentration of the substance under test.
U.S. Patent ~o. 3,992,158, issued ~ovember 16, 1976, discloses an integral analytical element containing a 25 spreading layer and a reagent layer. The spreading layer is an isotropically, non-fibrous porous layer which spreads t~,g~ ~l within itsel-f at least a component of the applied liquid sample to obtain a uniform concentra-tion which then passes into the adjacent reagent layer where it can react to form a detectable species.
U.S. Patent No. ~,042,335, issued ~ugust 16, 1977, is directed to a multilayer element for the analysis of liquids such as biochemical and biological liquids. The element comprises a reagent layer which includes a composition that is interactive with the substance to be 10 analyzed to yrovide a diffusible, detectable species; and a registration layer that is permeable -to the detectable species and within which such species can be detected. The element can also include a spreader layer and a radiation blocking layer to en~ance detection of the detectable 15 species-U.S. Patent ~o. 4,050,898, issued September 27,1977, is directed to an analytical elemen-t comprising at least two superposed layers, a spreading layer and reagent layer, wherein the spreading layer is a non-Eibrous, 20 isotropically porous spreading layer of water-resis-tant material which includes a non-ionic surfactant to normalize transport in the spreading layer.
U.S. Patent No. 4,066,403, issued January 3, 1976 is directed to an integral analytical element for analyzing 25 complex fluids which comprises a first reagent for reacting with an analyte to produce a decomposition product and a second reagent adapted to react with said decomposition product to produce a detectable change wherein a barrier to said decom~osition product i8 disposed between the first and 30second reagents which is substantially impermeable to interferants.
U.S. Patent No. 4,069,016, issued January 17, 1978, is directed to a method for determining bilirubin in an aqueous liquid sample which comprises contacting said 35 sample with a composition containing a diffusible, bilirubin-displaceable, detectable ligand bound to a carrier which can also bind bilirubin, and detecting the thus-displaced detectable ligand.
U.S. Patent ~o. 4,144,306, issued March 13, 1979, 5 is directed to a displacement/release method employing a multilayer analytical element comprising at least a reagent layer and a registration layer wherein the reagent layer contains an interactive composition including a non-diffusible ma-terial having a preformed detectable moiety 10 bound thereto~ which composi-tion, in the presence of a liquid containing an analyte, provides a di~usible product comprising the preformed detectable moiety by displacement by the analyte. The registration layer receives the diffusible product. Other layers may also be employed, 15 including supports, spreader layers, radiation blocking layers, and the like. As an example of said in~eractive composition, mention is made of a tagged antigen-antibody complex wherein the analyte displaces the tagged member which migrates to the registration layer.
U.S. Patent No. 4,166,093, issued August 28, 1979, is directed to an elemen-t for the analysis of liquids which include a radiation-transmissive reagent layer containing a composition in-terac-tive wi-th an analyte to provide a radiometrically detectable species, a porous radiation 25 blocXing layer permeable to the analyte and a radiation-transmissive, detectable species migration inhibiting layer interposed between the reagent layer and the radiation-blocking layer, said detectable species migration-inhibiting layer being permeable to said analyte 30 an~ inhibiting the migration of said radiometrically detectable species to said porous radiation blocking layer upon contact of said element with the liquid under analysis.
U.S. Patent No. 4,258,001, issued March 24, 1981, is directed to a multilayer element ~or compe-titive 35 immunoassay which includes a zone having a particulate .~5~ 3~

structure which comprises heat-stable, non-swellable organo-polymeric particles and an adhesive. An immunoreagent, such as a fluorescent-labelled antigen, may be bound to the particles. In Example 50, a competitive 5 immunoassay element is described wherein an antibody is immobilized in a spreading/reagent zone dispersion and labeled antigen is premixed before application to the test element. It is also stated that the labelled antigen may be incorporated into the element. However, it is stated (col.
10 30, lines 2 to 7) that where the labelled antigen is incorporated in the element, care should be taken to maintain the labelled antigen apart from the immobilized antibody which is also in the element to avoid premature binding of labelled antigen to antibody.
Immunochemistry, D. M. Weir, Third Edition, Blackwell Scientific Publication, Oxford, England, Vol. 1, Chapter 19, describes a radial, quantita-tive immunoprecipitation technique wherein an analyte containing an antigen is placed in the center of a layer of colloid 20 containing an antibody. The distance the antigen radiates from the center is measured to provide a quantitative determiation of the antigen in the analyte.
European Patent Application 0 051 183, published June 12th, 198~ discloses a multilayer analysis element 25 comprising a porous medium layer to which an antigen (or antibody) is immobilized and a layer through which substances which did not participate in the immunoprecipitation in the porous medium layer can diffuse, whereupon detection means can be employed to determine the 30 quantity of said substances. In operation, an antigen (or antibody) to be analyzed for is labelled and then applied to the porous medium layer. The labeled antigen (or antibody) reacts with the immobilized an-tibody (or antigen) to form a complex. Exce~s, unreacted antigen (or antibody) is 35 inherently separated from the complex and diffused into the 3~

reagent layer. ~nreacted labelled an~igen (or antibody) is optically measured to provide a quantitative determination.
This patent application demonstrates the quantitative nature of gel-mediated precipitation reactions.
SUMMARY OF THE INVENTION
A multilayer element for the analysis o~ a liquid containing an analyte comprising a reaction layer containing a predetermined amount of a diffusible reagent containing a detectable species; said reagent adapted to be rendered .
substantially non-diffusible by said analyte as a function of the concentration of said analyte, a registration layer adapted to receive reagent diffusing thereto; means for detecting said reagent in said registration layer and an insulating layer intermediate said reaction layer adapted to provide discrimination between detectable species in said reagent layer and said registration layer.
BRIEF DESCRIPTION OF THF. DRAWINGS
Figure 1 is a schematic, cross-sectional view of a preferred multilayer element of the present invention;
Fig. 2 is a schematic, cross-sectional view of the element of Fig. 1 indicating the location of some of the components in the element prior to contact with the analyte;
and Fig. 3 is a schematic, cross-sectional view of the element of Fig. 2 indicating the location of some of the components in the element subsequent to contact with the analyte.
DETAILED DESCRIPTION OF THE INVENTIO~
The present invention contemplates an analytical technique which involves a predetermined concentration of a reagent in a layer of the analytical elemen-t which is adapted to react with an analyte. Thu~, the analyte will react with and immobilize a quantity of reagent, which is normally diffusible, in a relationship to the quantity of analyte. The unreacted reagent which also contains a ~.~~ Z3~

detectable species, diffuses to a registration layer where suitable detection means are employed to asce~tain the quantity of reagent in thc registration layer by virtue of the detectable species. Preferably, an insulating layer is 5 employed between the reaction layer and registration layer to limit detection of the detectable species to that which has diffused to the registration layer.
Although the present invention is suitable for use in analyzing substantially any substance that can react with 10 a second diffusible substance with a detectable species to immobilize said second substance, the invention is particularly suited for use employing immunoprecipitation techniques. For simplicity, the present invention will be described in detail in terms of immunoprecipitation.
Analytical techniques may be classified as heterogeneous or homogeneous methods. Heterogeneous methods require physical separation of free and bound reactants but are considered to be more sensitive. Homogeneous methods are preferred for their ease and simplicity of operation but 20 suffer Erom high bac~ground levels caused by incomplete discrimination between bound and free species. In a homogeneous method physical separation does not occur but rather a modulation of one of the reactant labels based on the carrier's state of complexation. The diminished level 25 of sensitivity is due, at least in part, to incomplete discrimination.
The present invention employs a heterogeneous immunoprecipitation analytical technique in an integral, one-step format, thus obtaining the simplicity of operation 30 o~ a homogeneous technique with the sensitivity of the heterogeneous technique in a thin film format.
The novel element and procedure of the present invention employs immunoprecipitation techniques in a single step and in a preformed thin film format to effect 35 separation of bound/unbound species and detection is . .

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accomplished by differential diffusion of a detectable, labelled antibody, thereby obviating the need for competitive reaction with additional antigen species. Thus, by means of the present invention any substance that 5 produces an immune complex can be analyzed without the employment of reactants other than those producing the immune complex.
The term "immune complex", as used herein, is intended to refer to an aggregate of immunoglobin with its 10 associated ligand in any proportion from antibody excess to antigen excess, either soluble or insoluble, including haptenic o~ multivalent antigen, and heterogeneous or monoclonal antibody.
In operation, a predetermined amount of sample 15 liquid containing the analyte is brought into contact with a measured amount of a diffusible antibody disposed in a preformed thin film wherein the diffusible antibody contains a detectable species bonded thereto. The antigen analyte associates with the antibody forming a complex species which 20 is substantially less diffusible than the uncomplexed antibody, quantitatively in proportion to the concentration of antigen in the sample liquid. Separation of the complexed and uncomplexed antibody occurs by virtue of the differential in diffusiblity of the uncomplexed antibody 25 which is in excess of that required to complex with the antigen. Determination of the quantity of uncomplexed antibody is carried out by measuring the quantity of antibody which diffuses out of the layer in which it is initially disposed, preferably to a registration layer where 30 it is concentrated by immobilization to prevent any back-diffusion or wandering of the antibody in the element.
Measurement is carried out by a procedure that determines the quantity of detectable species in the registration layer which indicates the quantity of antibody-antigen complex 3~

species and, therefore, the concentration of antigen analyte in the saZnple liquid.
Although the present invention is described primarily in terms of analyzing for an antigen employing a 5 diffusible antibody having a detectable species bonded thereto, it should be understood that since immunoprecipitation techniques are employed, an antibody can be analyzed for employing a diffusibLe antigen with a detectable species bonded thereto. For simplicity, the 10 present invention is described in terms of the antigen as the anal~te but it should be understood that the invention encompasses both procedures.
The following example illustrates the principle of thin-film irnmunoprecipitation.
~xample 1 The following materials were coated on 3 mil polystyrene base (sold under the trademark TRYCITE by Dow Chemical Co., Midland, MI), 10 mg/ft2-Eluorescent immunoglobulin (FITC-anti-human albumin sold by Atlantic 20 Antibodies, Inc., Scarborough, ME), 1000 mg/ft2 agarose (SEAKEM LE, sold by FMC Marine Colloids, Rockland, ME) and 0.1 mg/ft2 of surfactant (p-nonylphenoxy polyglycidol sold under the tradename OLIN 10G, by Olin ChemicaL Corp., Stamford, CT). The thus-coated base was dried and cut into 25 10 X 1 cm strips. The strips were exposed to antigen (globulin-free human albumln, sold by Sigma Chemical Co., St. Louis, MO), by imbibing part of the strip in a solution of the antigen for a period of 10 min then rinsing for 10 sec in distilled water followed by air drying at room 30 temperature. An antigen concentration series was prepared consisting of 0, 0.5, 1 and 2 mg/ml antigen in phosphate buffered saline. The relative flourescence intensities of the samples were measured from exposed and unexposed portions of individual sample strips using a FLOUROLOG
35 instrument employing 450 nm excitation, 5 nm slits, 470 to ? ~

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60~ nm emission scan and integrated photon counting mode (Spex Industries, Inc. Metuchen, NJ). Comparison between samples was carried out by calculation of -the per cent diminution in flourescence caused by exposure to the antigen 5 solution. Referring to Table 1, below, it well be seen that higher levels of antigen fix higher levels of flourescent antibody.

Sample ~o. [Ag](mg/ml) Flourescence Intensity (%) 1 0 29.5

2 0.125 26~5

3 0.250 23.0

4 0-375 21.2 0.500 18.5 This example demonstrates that antigen was effectively and measurably reacted with pre-coated antibody in a thin-film layer of preformed colloid and that diffusion of antibody out of the layer, occurs differentially and inversely propor~ional to the quantity of antigen applied to 20 the layer, thus providing a mechanism or antigen determination.
The novel element of -the present invention employs, in i-ts simplest form:
a sample spreading layer;
a reaction layer comprising a diffusible antibody having a detectable species bonded thereto;
a porous layer substantially radiation opaque;
a registration layer adapted to receive and substantially immobilize said antibody diffusing thereto; 0 and a radiation transparent base carrying said layers.
The spreader layer possesses a void volume which retains a constant volume per unit area of liquid after uptaXe of sample is complete and provides for the precise 35 delivery to the reaction layer of a measured quantity of sample li~uid, thus eliminating -the need for any dispensing or measuring device. As examples of materials suitable for use as the spreader layer, mention may be made of a fibrous layer having a specific, predetermined void volume, the

5 isotropically porous non-fibrous material disclosed in U.S.
Patent No. 3,992,158, and the fabric of U.S. Patent ~o.
4,292,272, issued September 29, 1981.
In an alternative embodiment, the spreader layer may comprise a ~ibrous coating, of, e.g., organic or 10 inorganic fibers, such as paper, asbestos or potassium titanate.
The reaction layer comprises a hydrophilic colloid which is permeable to the liquid sample, inert to protein, swellable but insoluble to maintain wet-s-trength integrity 15 and in which the antibody having the detectable species bonded thereto is initially disposed and which is non-reac-tive and non-binding to said antibody. Suitable materials for the reaction layer include hydrophilic gelable materials including both naturally occurring substances such 20 as gelatin derivatives, hydrophilic cellulose derivatives, polysaccharides such as dextran, gum arabic, agarose and the lika, and also synthetic substances such as water-soluble polyvinyl compounds like polyvinyl alcohol and polyvinyl pyrrolidone, acrylamide polymers, etc. Thermoreversable 25 gels are preferable for their simplicity of fabrication.
The polymers employed as the reaction layer may be cross-linked, if desired, but care should be taken to avoid deactivation of the antibody by the cross-linker.
The detectable species may comprise conventional 30 labelling means known to the art. Among suitable detectable species mention may be made of radioactive labels, e.g., gamma detection, fluorescent labels, enzyme labels (to provide a colormetric signal), and luminescent labels. In a preferred embodiment, fluorescent material is employed as 3~L

the detectable species, particularly because of sensitivity of detection.
In a preferred embodiment, the hydrophilic colloid in the reaction layer also contains a permeator to enhance 5 the diffusion constant of the labelled antibody in the pol~ner matrix, i.e., the permeator effectively increases the pore size of the hydrophilic colloid medium.
Preferably, the permeator is a hydrophilic, non-gelling polymer such as polyvinyl pyrrolidone or sorbitol. While 10 not intending to be bound by theory, it is believed that the permeator reduces the degree of hydrogen bonding in the gelled polymer, providing less resistance to the diffusion of the antibody with the detectable species.
If fluorometric detection techniques are employed 15 care should also be taXen to avoid the use of a material which can lower the quantum efficiency of flourescence, thus rendering the antibody substantially less detectable.
To illustrate the effectiveness of permeators in the polymer matrix comparative radial diffusion measurements 20 were performed on a wide variety of coated polymer matrices.
Example 2 Agarose (SEA~EM LE)*solutions were prepared with the below-indicated permeator additives and coated by extrusion on a polystyrene base (TRYCITE~*which was 25 subcoated to promote adhesion with a composition comprising 68.3g of adhesive polymer (~HOPLEX,*Rhom and Haas Co., Philadelphia, PA), 5.6g of a 10~ solution of dioctyl malonate, 2607g of a 10% colloidal silica dispersion tLUDOX,*
E.I. DuPont de Namours Company, Wilmington, DE), 40 g of a 30 10% solution of phthalated gelatin, 3g of a 50~ solu-tion of polynonylphenoxy glycidol (OLIN lOG)*and 862g of water. A
5% solution of sodium fluoresceinate was used as a tracer dye. The coated samples to be tested were mounted by double stick tape to a glass plate and the tests were carried out 35 in a Selectrasol*TLC-Solvent Selector System (Schleicher &
* Trade Mark 3~L

Schell, Inc. ~eene, NH). The tracer dye solution was applied through a porous wick to the center o~ each test sample for 24 hours in a humid environment to prevent premature evaporation of the solvent. Table 2 summarizes 5 the results.

Dye Spot Diameter Sample No. Material/Permeator Coverage mg/ft2 (Micrometers) 1 agarose 2000 8 2 agarose 1500 8 3 agarose 1000 8 4 agarose/sorbitol 1000/500 13 agarose/carbowax 1000/lO00 11

6 agarose/polysucrose 1000/1000 10 The carbowax employed was polyethylene glycol, M.W. 3000-37,000 Fisher Scientific Co., Fairhaven, NJ.
The polysucrose was FICOLL 400*sold by Pharmacos Fine Chemicals, Uppsala, Sweden.
Table 2 illustrates the effectiveness of various permeators to increase the rate of diffusion through the agarose layer.
The porous opaque layer is permeable to the diffusing antibody with the detectable species ancl 25 provides opacity to prevent a false reading by eliminating any possibility of the detection of detectable species o-ther than in the registration layer. In other words, the porous opaque layer prevents detection of the detectable species retained in the reaction layer. For example, in the case of 30 a fluorescent detectable species, the porous opaque layer will absorb in the region of the exciting radiation, e.g., the ultraviolet region, while maintaining reflectivity in the reyion of the flourescing radiation, e~g., the visible region. Thus, the exciting radiation will be effectively 35 re~trained from reaching the bound label while the emitted * Trade Mark ; - ~ ..

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radiation from the registration Layer is reflected rather than absorbed. The porous opaque layer may comprise a suitable reflecting material in a binder. Preferably, titantium dioxide in agarose is coated at various coverages 5 to provide the desired degree of porosit~. As desired, the porous opaque layer may also include suitable additives to improve opacity and reflectance.
Since many pigments process high surface binding coefficients for non-specific absorption of 10 immunoglobulins, it is desirable to pretreat such pigments with another polymer which is strongly absorbed to the surface of the pigment but which has no affinity itself for the immunoreagent.
The following example illustrates the 15 effectiveness of a surface treatment to inhibit undesireable i~munoqlobulinbinding to particulate materials.
Example 3 Surface Treatment Process lOOg of titanium dioxide (TiPure R-931, E.I.
20 DuPont de Namours Company, Wilmington, DE) was mixed with 400g of a 1% solu,ion of photographic grade gelatin Type 9168 (Rousselot Corp., Paris, France) and stirred at 40~C
for 1 hour. At the end of that period the titanium dioxide was separated by low-speed centrifugation. Excess gelatin 25 was removed, the titanium dioxide resuspended in distilled water followed by contrifugation. This gelatin removal step was repeated three times.
Example 4 Surface Activity Determination Samples of pigment (0.3g) treated according to the procedure of Example 3 were stirred for 1 hour with 3.0g of labeled immunoglobulin (FITC-antihuman albumin, Atlantic An-tibodies, Inc., Scarborough, ME), diluted 1:9 with phosphate buffered saline and then diluted with water to a 35 total of 6.89 g. The mixture was loaded in an : , ultrafil~ration cell with an AMICON XM-3000 membrane filter (AMICO~, Inc., Cambridge, MA) and centrifuged at low spped for several minutes. 1 ml of the ultrafil-trate was diluted 1:9 with phosphate buffered saline and the flourescence 5 intensity was measured with a Beckman 5864 Spectrophotometer Fluorescence Accessory MODEL 196776 (Beckman Instruments, Inc. Fullterton, CA) set up for flourescence readout.
The control sample was the immunoglobulin solution at the same dilution as the test samples and 10 ultracentrifuged the same as the test samples. Table 3 contains the flourescence intensity (FI) measurements from the samples normalized to 100% for the control.

Sample F.I.
Untreated Tio2 ~.4 Treated Tio2 (Ex. 3) 54.4 Control (immunoglobulin solution) 100.00 Table 3 il lustrates the effectiveness of pigment treatment in inhibiting immunoglobulin binding.
The regis-tration layer acts as a location to concentrate by immobilization the diffusing antibody with the detectable species. ~etection means are employed to measure the quantity of detectable species in -this layer~
Materials adapted to non-specifically bind, immobilize or 25 "mordan-t" antibodies are well known in the art. See, for example, Journal of the Reticuloendothelial Society, Vol. 3, p. 29-40 (1966).
While many of the materials known to the art to immobilize or mordant antibodies will function in solution, 30 they appear to be inactivated in a gelled environment such as agarose and, therefore, re~uire special atten-tion to their selection for use in the present invention.
The following non-limiting example illustrates the effectiveness of non-specific adsorption characteristics in S;~

the registratlon layer ~or the fixation o~ labeled immunoglobulin.
Example 5 A series of coatings were prepared employing 5 agarose as the gelled environment with the below-indicated materials incorporated therein as a discontinuous phase. A
surfactant, p-nonylphenoxypolyglycidol, (OLIN 10G) was added as a coating acid. The coatings were evaluated by allowing each coating to contact a fixed volume of labeled antibody 10 solution for exactly 10 min. The antibody solution employed was the same as the one set ~orth in Example 3. At the end of the contact period the coatings were rinsed in distilled water and air dried at ambient temperatures. Fluorescence tin arbitrary relative units1 was measured by the same 5 technique as described in Example 4.
Table Coating No. Materials Coverage (mg/~t2) F.I.(X104) l agarose/starch 1000/1000 1.7 2 agarose/polyethylene oxide 1000/500 3.6 3 agarose/TiO2 1000/500 6.7 4 agarose/Polymer A/
polyvinyl pyrrolidone 1000/1000/1000 8.3 agarose/aluminum oxide 1000/500 13.3 Polymer ~ was a graft of 4-vinyl pyridine (4~P) and ben7.yl trimethyl ammonium chloride (TM2) on hydroxyethyl cellulose (HEC) at a ratio o~ 2.2 HEC/2.2 4VP/l TM~. The starch was soluble starch sold by Mallinkrodt, Inc., Paris, KY. The 30 polyethylene oxide was an emulsion sold under the trade name POLYOX WSR-N-80 by Union Carbide Corp., ~ew York, NY. The titanium dioxide was the same as that described in Example 3. The polyvinyl pyrrolidone was Type NP-Kl5, sold by GAF
Corp., New YorX, NY. The aluminum oxide (0.3 micrometers) 35 was sold by Fluka Chemical Co., Hauppauge, NY.

Alte~nati~ely, an antibody may be precoate,l in the registration layer to provlde the mordanting characteristics. See, for example, U. S. Patent No.
4,2~8,001, supra. However, this is not necessary employing 5 the registration layers described in Table 4 since those registration layers are not only effec~ive mordants for the antibody but are also non-specific, i.e., they will bind to substantially all immunoglobulin G's regardless of their source.
The radiation transparent base employed in the present invention is transmissive to the radiation emitted by the detector as well as the detectable species. For example, in a preferred embodiment, the detector will emit exciting radiation and detect fluorescence emitted by the 15 detectable species. As examples of suitable film bases for use as the radiation transparent base mention may be ~ade of materials such as cellulose acetate, polyethylene terephthala-te, polycarbonates, and polyvinyl compounds such as polystyrenes. To promote adhesion, it may be preferable 20 to subcoat the ilm base.
It should also be understood that the novel analytical element of the present invention may include optional layers such as filter layers, timing layers, and the like. For example, a filtering layer may be employed 25 over the spreader layer or reaction layer to remove micromolecules and particulates tha-t might interfere with the analysis. It may also be desirable to provide a timing layer to regulate -the delay between antigen contact with the reaction layer and diffusion of the antibody with the 30 detectable species to avoid any false reading~ as a result of antibody diffusion before the antigen has had an opportunity to bind thereto in the reaction layer. A
filtering layer may also be employed to prevent extraneous materials from entering the registration layer.

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Turning now to the drawings, Fig. 1 is a cross-sectional view of a preferred element of the present invention. Element 10 is composed of the following layers, in order; spreader layer 12, to which the analyte is 5 applied, reaction layer 14, which contains the antibody with detectable species, interlayer 16, which, as described above, may include timing layers and/or filtering layers;
porous opaque layer 18, adapted to isolate the detectable species remaining in the reaction layer from the detectable 10 species in registration layer 20 which registration layer includesa concentration or immobilizing element for the diffusing antibody with detectable species, and radiation transparent layer 22 which functions as a support for element 10 and through which the detector determines the 15 emitted radiation from the detectable species in registration layer 20.
Figs. 2 and 3 show the operation of element 10 of Fig. 1. Sample fluid containing analyte, in this case antigen which is the substance under analysis, is applied to 20 spreader layer 12 where a uniform predetermined amount is received and which then diffuses to reaction layer 14. The antigen will associate with a like quantity of antibody located in layer 14 forming a substantially non-diffusing complex. The e~cess of antibody over antigen, which is 25 normally diffusible through the element in the sample liquid, will diffuse through interlayer 16 and porous opaque layer 18 and become immobilized in registration layer 20 where the detectable species will be activated by incident radiation through the radiation transparent layer 22 and the 30 quantity of detectable species ascertained.
The following non-limiting example illustrates the operation of the element of the present invention.

10 ml of 0.1% human serum albumin (antigen) (Sigma 35 Chemical Co., St. Louis, M0) and 100 ml of goat anti-human serum albumin IgG fraction labeled with fluorescein isothiocyanate (antibody) (sold by Atlantic Antibodies, Inc., Scarborough, ME) diluted 1:5 in phosphate buffered saline solution were mixed in a test tube. A whitish 5 precipitate was immediately formed. The contents of the test tube was spotted onto an element comprising:
a) a reaction layer comprising 1000 mg/ft2 of agarose and 1000 mg~ft2 of polyethylene glycol, ~.W.3000-3700 (Fisher Chemical Co., Fasilawa, NJ), b) a porous layer substantially opaque to radiation comprising 520 mg/ft2 of titanium dioxide (sold by E. I. DuPont de Namours, Inc., Wilmington DE under the tradename TiPure R-933) pre-treated with gelatin and 500 mg/ft2 of agarose;
c) a registration layer comprising 1000 mg/ft2 of agarose and 100 mg/ft~ polyethylene glycol M.W. 4000, and d) a 3 mil subcoated polystyrene base (TRICITE
sold by Dow Chemical Co., Midland, MI).
After a two hour imbibition period ultraviolet 20 light was shined through the base of the above element and a control element to which no human serum albumin was applied with the goat anti-human serum albumin. In the case of the control a considerable amount of fluorescence was detected indicating a substantial amount of -the fluorescein 25 labeled antibody reaching the registration layer while the test sample showed significantly less fluorescence indicating that a substantial amount o~ fluorescein labeled antibody was immobilized by the antigen and did not reach the registration layer.
Contrary to the kinetic techiques set forth in the prior art described above, the present invention employs an endpoint technique. The term "endpoint technique" as used herein is intended to refer to a delineation based on a chemical reaction yielding a detectable product, the total 35 amount of which remains substantially fixed after some desi~nated perlod of time over which the reac-tants are substantially exhausted. An endpoint technique operates with ambient levels of analyte and stoichiometric quantities of reagent and is inherently less sensitive to variations in 5 temperature than kinetic techniques.
The dispacement release mechanisms described ln U.S. Patent Nos. 3,g92,158, 4,422,335, and 4,144,306, for example, are inherently kinetic assay systems because a large excess of reagents are necessary to accelerate the 10 reaction to a measurable rate. Kinetic techniques are temperature and time sensitive and require complex instrumentation to determine rate constants.
The novel method and element of the present invention provides inherently faster reaction time since the 15 association reaction rate is orders of magnitude faster than the dissociation which is a necessary prerequisite for displacement release as employed in the above-mentioned patents.

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A multilayer element for the analysis of a liquid con-taining an analyte comprising a reaction layer which retains a predetermined amount of a diffusible reagent containing a detectable species, said diffusible reagent being present in an amount in excess of that reactable with an analyte to be deter-mined; said reagent adapted to be rendered substantially non-diffusible by said analyte as a function of the concentration of said analyte; a registration layer adapted to receive reagents diffusing thereto; and an insulating layer permeable to said diffusible reagent intermediate said reaction layer and said registration layer, said insulating layer preventing detection of the detectable species retained in the reaction layer.

2. The element of claim 1 which includes a sample spreading layer for receiving the liquid to be analyzed.

3. The element of claim 2 wherein said spreading layer is an isotropically non-fibrous spreading layer.

4. The element of claim 2 wherein said spreading layer is fibrous.

5. The element of claim 1 wherein said detectable species is a fluorescent material.

6. The element of claim 1 wherein said detectable species is a radioactive material.

7. The element of claim 1 wherein said insulating layer is opaque to the radiation from said detectable species.

8. The element of claim 1 wherein said liquid contains antigen (antibody) and said reaction layer contains diffusible antibody (antigen) with a detectable species attached thereto.

9. The element of claim 2 wherein said spreading layer is a fabric.

10. The element of claim 2 wherein said reaction layer com-prises agarose.

11. The element of claim 1 wherein said insulating layer comprises titanium dioxide.

12. The element of claim 1 wherein said registration layer comprises agarose.

13. The element of claim 1 wherein said registration layer includes polymeric emulsion.

14. The element of claim 1 including a radiation trans-parent base carrying said layers.

The element of claim 14 wherein said base is poly-styrene.

16. A method for analyzing an analyte in a liquid which comprises adding said liquid to an element comprising a plurality of layers including a reaction layer and a registration layer;
said reaction layer containing a predetermined amount of a diffusible reagent containing a detectable species, said diffu-sible reagent being present in an amount in excess of that reac-table with an analyte to be determined; said reagent being rendered non-diffusible by said analyte as a function of the concentration of said analyte; said registration layer adapted to receive reagent diffusing thereto; and measuring the quantity of reagent diffusion to said registration layer.