METHOD OF PREPARING INTEGRAL MULTILAYER ANALYTICAL ELEMENT

Abstract: A method of preparing an integral multilayer analytical element such as for analysis of calcium comprising a water-impermeable light-transmissive support, a reagent layer containing a water-soluble indicator capable of reacting with an analyte to produce an optically detectable change, and a porous spreading layer containing a spreading action controller and/or an acid capable of decomposing the calcium compounds in a sample, superposed in this order, which is characterized in that dissolving the spreading action controller and/or the acid in an organic solvent which does not dissolve the above water-soluble indicator, incorporating this solution into the porous spreading layer, and drying this superposed material.

Description: This invention relates to a method of preparing an integral multilayer analytical element for the analysis of calcium. More particularly, this invention relates to an improvement of a method of preparing a dry-type integral multilayer analytical element for analysis of calcium in a liquid sample such as a biological body fluid including blood (whole blood, blood plasma, blood serum), cerebrospinal fluid, lymph, saliva and urine, useful for diagnosis in clinical field. Various integral multilayer analytical elements have been known. However, in every analytical element, when a water-soluble substance is used as the indicator reacting with the analyte in a sample to generate optical change, the indicator diffuses into upper porous spreading layer to result in a lowering of analytical accuracy. (This diffusion is called "migration".) In the case that indicator is well soluble in water, such as in the case of the analytical element for analysis of calcium, analytical accuracy is remarkably lowered by the migration. While, since change of calcium concentration in blood is not so wide, this defect is fatal to this analytical element. Therefore, various investigations have been made in order to eliminate the defect. For example, a migration-inhibiting layer is introduced into the analytical element disclosed in US-A-4,166,093. This migration-inhibiting layer inhibits the migration of a water-soluble indicator by immobilizing it with a polymer mordant. However, the mordant often inhibits coloration of the indicator, and accordingly, it is not a fundamental solution to this problem. Besides, preparation of this layer is not so easy, and cost of the analytical element is raised. It is also a problem that an aqueous liquid sample spotted on a porous spreading layer spreads too broad in this spreading layer. And, in EP-A-0,162,302, hydrophilic cellulose derivatives and nonionic surfactants having a HLB value of more than 10 are disclosed to be effective as spreading action controller. The hydrophilic cellulose derivative is dissolved in water, and applied on spreading layer. Since solubility of the nonionic surfactant in water is low, it is dissolved in a mixture solvent of water and acetone. On the other hand, colorimetry using an indicator is widely employed in the field of clinical analysis. The indicator is usually o-Cresolphthalein Complexone of which the optimum pH for coloration is in alkaline, particularly higher than, pH 10 where binding to calcium is stable. By the way, it is known that the samples for clinical diagnosis such as blood contain the calcium in a bound state such as protein-bound calcium and acid-bound calcium in addition to the calcium in an ion state. Then all calcium in the sample is first ionized by acid treatment, and thereafter color reaction is carried out (Clin. Chem. Vol. 29, p 1497 (1983)). This method is, for example, described on pages 148 to 150 of "Jissen Rinsho Kagaku-Zoho Ban (Practical Clinical Chemistry-Revised and Enlarged edition)" (Kitamura et at., Ishiyaku Shuppan Kabushiki Kaisha, Japan, in 1982). Outline of this method is as follows. o-Cresolphthalein Complexone is dissolved in a small amount IN KOH, and glacial acetic acid is added to this solution to prepare an acidic solution 1. An alkaline solution 2 containing potassium acetate-HCl-diethylamine is separately prepared. The solution 1 is added to a sample, and allowed to react. Subsequently, the solution 2 is added to the reaction mixture, and color reaction proceeds. Recently, one step-type reagent kit has also been available ("Agent Calcium", OINABBAT, USA). It has also been reported that a linear relation between absorbance and concentration is obtained by allowing to coexist citric acid, its salt or mixture thereof through the reaction (Japanese Patent KOKAI No. 57-154058). However, in this patent, citric acid is used in a concentration of 0.01 to 0.05 M/l in an alkaline condition such as pH 10 to 11, but this acid is not utilized for acidification of sample nor color reaction. In the case of the analytical element for analysis of calcium, in order to decompose the bound state of calcium, pH of the upper layer is made lower than pH 5, usually pH 1 to 2. On the other hand, optimum reaction pH of preferable indicators such as o-Cresolphthalein is usually higher than pH 10. Since it is difficult for such a high pH layer to coexist with the low pH layer in a thin analytical element, the coloring reaction was set to proceed at about pH 5.5. Chlorophosphonazo-III and Arsenazo-III were used as the indicator. However, in this analytical element, absorbance of background is high, such as higher than 2 and measurement is carried out at a long wave length (near 680 nm). Accordingly, analytical accuracy becomes worse. EP-A-0 158 993 and DE-A-3 332 315 describe the manufacture of analytical elements having at least a support, a reagent layer and a spreading layer. The object of the invention is to provide a method of preparing an integral multilayer analytical element capable of determinating total calcium concentration easily without pretreatment by treating with acid in the analytical element and subsequently allowing to react to form color. Another object of the invention is to provide a method of preparing an integral multilayer analytical element using a water-soluble indicator capable of determinating total calcium concentration in high accuracy. Another object of the invention is to provide a method of easily preparing an integral multilayer analytical element capable of determinating total calcium concentration having the above characteristics. Another object of the invention is to provide an integral multilayer analytical element capable of determinating total calcium concentration having the above characteristics. Still another object of the invention is to provide a method of measuring total calcium concentration of an aqueous sample easily and rapidly by using such an integral multilayer analytical element. The present inventors have investigated in order to achieve such objects, and found that the migration of a water-soluble indicator can be inhibited by dissolving a an acid and optionally a spreading action controller in an organic solvent which does not dissolve the water-soluble indicator, and incorporating the acid and optionally the spreading action controller into the porous spreading layer such as by application of the above acid/spreading action controller solution on it. Thus, the present invention provides a method of preparing an integral multilayer analytical element for the analysis of calcium comprising a water-impermeable light-transmissive support, a reagent layer containing a water-soluble indicator capable of reacting with calcium to produce an optically detectable change, and a porous spreading layer containing an acid capable of decomposing the calcium compounds in a sample, superposed in this order, which is characterized by dissolving said acid in an organic solvent which does not dissolve said water-soluble indicator, incorporating this solution into said porous spreading layer, and drying this superposed material. As the water-impermeable light-transmissive support, a known support employed in an usual multilayer analytical element may be employed. Such a support is a film, a sheet or a flat plate having a thickness of 50 µm to 1 mm, preferably 80 µm to 0.3 mm and capable of transmitting the object light being in the wave length range of 200 nm to 900 nm. Such a support may be made from a polyester (for example, polyethylene terephthalate or polycarbonate of bisphenol A), a cellulose ester (for example, cellulose diacetate, cellulose triacetate or cellulose acetate propionate), or polystyrene. Optical property of the support may be controlled by suspending light-reflective or light-absorptive particles such as titanium dioxide particles, barium sulfate particles or carbon black. A known undercoating layer or a known adhesive layer may be provided on the surface of the support in order to secure the adhesion of the support to the reagent layer, a water-absorption layer or the like superposed on the support. The reagent layer is water-absorptive and water-permeable, and it comprises a hydrophilic polymer as a polymer binder and a reagent composition containing at least an indicator capable of reacting with the calcium in an aqueous sample to produce an optically detectable change dispersed therein uniformly. The hydrophilic polymer used in the reagent layer has a swelling ratio being in the range from 150 % to 2000 %, preferably 250 % to 1500 % at water absorption at 30°C. Examples of the hydrophilic polymer are gelatins including acid treated gelatin and deionized gelatin, gelatin delivatives such as phthalated gelatin and hydroxyalkyl acrylate grafted gelatin, agarose, pullulan, pullulan derivatives, polyacrylamide, polyvinyl alcohol and polyvinyl pyrrolidone. They are disclosed in EP-A-0,119,861 and EP-A-0,142,849. Preferable hydrophilic polymers are usually gelatins, gelatin derivatives, polyacrylamide and polyvinyl alcohol, and deionized gelatin is the most preferable. Thickness of the reagent layer in dry state is 5 µm to 50 µm, preferably 7 µm to 30 µm. The coating weight of the reagent layer itself is 5 g/m² to 50 g/m², preferably 7 g/m² to 30 g/m². The indicator reacts with calcium to produce an optically detectable change. The wave length used for detection is not limited to visible region, and includes ultraviolet region and infrared region. This indicator is water-soluble, and the method of the invention is particularly effective when solubility of the indicator is higher than 5 mg/100 g water at 25°C. On the other hand, it is also necessary not to dissolve in the organic solvent used for the spreading action controller. Such an indicator is selected from known ones, and it may form a complex with an analyte. Calcium exists in a state of ion, a salt of a fatty acid, a protein conjugate or the like. The liquid sample containing this analyte includes various biological fluids, foods, drinks, liquors and medicines. The indicators for analysis of calcium are, for example, described in "Dotite Reagents Catalog" (Dojindo Laboratories, Kumamoto, Japan, 1980). Examples of the indicator are o-Cresolphthalein Complexone (3,3′-bis[[di(carboxymethyl)amino] methyl]-o-cresolphthalien [2411-89-4], optimum pH; about 10.5), Eriochrome Black T (monosodium salt of 1-(1-hydroxy-2-naphthylazo)-6-nitro-2-hydroxynaphthalene-4-sulfonic acid [1787-61-7], Methylthymol Blue Complexone (tetrasodium salt of 3,3′-bis[[di(carboxymethyl)amino]methyl]thymolphthalein [1945-77-31]), Thymolphthalein Complexone (3,3′-bis[[di(carboxymethyl)amino]methyl]thymolphthalein [1913-93-5]), Arsenazo-III(2,7-bis[2-arsonophenyl)azo]-1,8-dihydroxynaphthalene-3,6-disulfonic acid [1668-00-4]) and Chlorophosphonazo-III(2,7-bis[(4-chloro-2-phosphonophenyl)-azo]-1,8-dihydroxynaphthalene-3,6-disulfonic acid [1914-99-4]). Figures in the above angular brackets represent Chemical Abstracts Registry Number. Among these, o-Cresolphthalein Complexone and Arsenazo-III are preferable due to the most accurate determination of calcium. In the multilayer analytical element for analysis of calcium, a masking agent for masking magnesium ion is preferably incorporated. Such a masking agent includes 8-hydroxyquinoline, 8-hydroxyquinoline-5-sulfonic acid, 8-hydroxyquinoline sulfate and the like. Suitable layers for incorporating the masking agent are the reagent layer and/or its upper (far from the support) adhesive layer or the spreading layer. Content of the masking agent is about 1.5 to about 10 times, preferably about 2 to about 5 times, the content of indicator. The reagent composition may be incorporated into two or more layers, for example, the reagent layer and a water-absorption layer. In the reagent layer, a pH buffer may also be incorporated in order to maintain the reaction with the indicator at its optimum. This pH is different according to each reaction, and in the case of analyzing calcium using o-Cresolphthalein as the indicator, the optimum pH is at 9 to 10.5. The pH buffers suitable for integral multilayer analytical elements are described in "Kagaku Benran Kiso-Hen", pp 1312-1320, Maruzen, Tokyo 1966, R.M.C. Dawson et al., "Data for Biochemical Research", 2nd Ed., pp 476-508, Oxford at the Clarendon Press, 1969, "Biochemistry", vol. 5, from p 467, 1966 and "Analytical Biochemistry", vol 104, pp 300-310, 1980. The pH buffer in the range of pH 8.0 to 11.0, particularly pH 9.0 to 10.5 includes buffers containing tris(hydroxymethyl)aminomethane, buffers containing phosphate, buffers containing borate, buffers containing carbonate, buffers containing glycine and the like. Examples are N,N-bis(2-hydroxyethyl)glycine (Bicine), sodium or potassium salt of N-2-hydroxyethyl-piperazine-N′-2-hydroxypropane-3-sulfonic acid (HEPPS), sodium or potassium salt of N-2-hydroxyethylpiperazine-N′-3-sulfonic acid (EPPS), sodium or potassium salt of 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS), sodium or potassim salt of N-[tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid (TAPS), sodium or potassium salt of N-2-hydroxethylpiperazine-N′-2-ethanesulfonic acid (HEPES) and a combination of any of them and an acid, an alkali or a salt. Preferable buffers include Tris-sodium borate, Bicine, HEPPS, sodium salt of HEPPS, EPPS, sodium salt of EPPS, CAPS, sodium salt of CAPS, TAPS and sodium salt of TAPS. When gelatin or its derivative is used as the hydrophilic polymer, the pH buffer containing boric acid or sodium borate is preferable in view that this layer can suitably be hardened by cross-linking using a cross-linking agent having vinyl sulfone structure. This gelatin or its derivative layer can stably be provided by application, and quantitative analysis can be performed in high accuracy. A known base polymer is also usable as pH buffer. A surfactant such as the nonionic surfactant described later may also be added to the reagent layer. The reagent layer is preferably transparent, but its optical property may be controlled by suspending a small amount of titanium dioxide particules, barium sulfate particles or carbon black. The porous spreading layer includes spreading layers of woven fabric disclosed in US-A-4,292,272 and GB-A-2,087,074, such as plain weaves including broad cloth and poplin, spreading layers of knitted fabric disclosed in EP-A-0,162,302 such as tricot fabric, double tricot fabric and milanese fabric, spreading layers composed of a paper containing fibrous pulp of an organic polymer disclosed in Japanese Patent KOKAI No. 57-148250, membrane filters (blushed polymer layer) disclosed in US-A-3,992,158, continuous microspaces-containing porous layers where polymer particulates, glass particulates or diatomaceous earth are dispersed in a hydrophilic polymer binder, continuous microspaces-containing porous layers where polymer particulates, glass particulates, etc. are joined so as to contact with each other at a point by using a polymer adhesive which does not swell in water (three-dimensional lattice structure layer) disclosed in US-A-4,258,001. In order to raise adhesive force, the above fibrous porous spreading layer such as woven fabrics, knitted fabrics and papers may be made hydrophilic by a physical activation treatment such as glow discharge or corona discharge disclosed in GB-A-2,087,074, a chemical treatment such as washing, degreasing and immersing in a hydrophilic polymer solution, disclosed in US-A-4,292,272 and GB-A-2,087,074, or a combination thereof. A spreading action controller can be incorporated in the porous spreading layer. The spreading action controller controls the spreading of an aqueous liquid sample in the spreading layer so as not to spread too broad, and it is selected from hydrophilic polymers or nonionic surfactants. The hydrophilic polymers are e.g. polyvinyl pyrrolidone, polyvinyl alcohol, polyacrylamide, polyacrylic acid, or hydrophilic cellulose derivatives. The hydrophilic cellulose derivatives are cellulose ethers in which a part of or all hydroxyl groups of cellulose are made ethers by introducing lower alkyl groups having a carbon number of 1 to 3 or hydroxyl group-substituted lower alkyl groups having a carbon number of 1 to 4. Examples of such a cellulose ether are water-soluble, and include methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose and hydroxybutyl methyl cellulose. Preferable hydrophilic polymers are polyvinyl pyrrolidone, polyvinyl alcohol and water-soluble cellulose ethers. Two or more hydrophilic polymers may be combined to use. Content of hydrophilic polymer in the porous spreading layer is 0.5 to 15 g/m², preferably 0.7 to 10 g/m². The nonionic surfactants are e.g. polyhydric alcohol ester ethylene oxide adducts (condensate), polyethylene glycol monoesters, polyethylene glycol diesters, higher alcohol ethylene oxide adducts (condensate), alkylphenol ethylene oxide adducts (condensate), or higher fatty acid alkanol amides. Examples of the nonionic surfactants are: POE (20) sorbitan monooleate POE (10) sorbitan monooleate POE (4) Sorbitan tristearate POE (4) trioleate POE (30) stearate POE (40) stearate POE (100) stearate PEG (400) monostearate PEG (400) monolaurate PEG (1000) dilaurate PEG (1540) distearate Lauryl alcohol EO 6 moles condensate Lauryl alcohol EO 10 moles condensate Lauryl alcohol EO 30 moles condensate Oleyl alcohol EO 20 moles condensate Cetyl alcohol EO 20 moles condensate POE (10) octylphenyl ether POE (15) octylphenyl ether POE (30) octylphenyl ether POE (12) nonylphenyl ether POE (20) nonylphenyl ether Triethanolamine oleate (Note) POE : Polyethylene oxide PEG : Polyethylene glycol EO : Ethylene oxide The number in parentheses represents the condensation number of ethylene oxide units Two or more nonionic surfactants may be combined to use. When a nonionic surfactant is combined with a hydrophilic polymer, the HLB value of the nonionic surfactant is preferably more than 10. Content of nonionic surfactant in the porous spreading layer is 0.1 to 3 g/m², preferably 0.2 to 2 g/m². In the analytical elements for the analysis of calcium, an acid for decomposing the calcium compounds in a sample is preferably dissolved in an organic solvent which does not dissolve the indicator selected from the mentioned previously. The calcium compounds include protein-bound (such as albumin-bound) calcium compounds and acid-bound calcium compounds such as calcium phosphate. Heretofore, the error caused by calcium phosphate has not been recognized as a problem. The present inventors have found that the analytical value of calcium concentration becomes lower with increasing phosphate concentration. In the analytical element of the invention, the exact analytical value can be obtained, though phosphate concentration is relatively high. In order to decompose protein-bound calcium, the pH is preferably lower than pH 4, while, in order to decompose calcium phosphate, the pH is preferably at about pH 1 to 2. Since a pH buffer is not incorporated in the porous spreading layer, the pH of this layer can be adjusted to such a low range by adding a small amount of an acid. The acid may be incorporated into the porous spreading layer in a form of suspension. Examples of the acid are fatty acids such as monocarboxylic acids including acetic acid and dicarboxylic acids including malonic acid, glutaric acid, adipic acid and pimelic acid, halogen-substituted fatty acids such as monochloroacetic acid and trichloracetic acid, aromatic carboxylic acids such as phthalic acid, aliphatic hydroxycarboxylic acids such as lactic acid, malic acid and citric acid, aromatic hydroxycarboxylic acids such as salicylic acid and sulfonic acids such as sulfosalicylic acid and 1,5-naphthalenedisulfonic acid. Two or more acids may be combined, if necessary. In the spreading layer of the analytical element for analysis of calcium, a quaternary amine polymer or the like may be incorporated, and inorganic phosphorus such as phosphate ions released by the acid treatment are trapped by this polymer. In such an analytical element, the bound state calcium such as protein-bound calcium and calcium phosphate compounds are decomposed by action of the acid incorporated in the spreading layer to generate calcium ions, while the calcium in an ion state passes through the spreading layer as it is. The acid accompanied with calcium ion is neutralized with the pH buffer incorporated in the reagent layer or other layers, and adjusted around the optimum pH of color reaction. Then, total calcium ion reacts with indicator to produce a detectable optical change, and the effective calcium content can be determined by detecting this optical change. Other layers may be incorporated in the analytical element. For example, a water-absorption layer may be provided between the support and the reagent layer. The water-absorption layer is mainly composed of a hydrophilic polymer which absorbs water to swell, and it absorbs the water of aqueous liquid sample which reaches the surface of this layer. In the case of whole blood sample, it accelerates permeation of blood plasma component into the reagent layer. The hydrophilic polymer is selected from the ones mentioned previously. In general, gelatin, a gelatin derivative, polyacrylamide and polyvinyl alcohol are preferable, and gelatin is the most preferable. Dry thickness of the water-absorption layer is 3 µm to 100 µm, preferably 5 µm to 30 µm. The coating weight of the water-absorption layer itself is 3 g/m² to 100 g/m², preferably 5 g/m² to 30 g/m². A pH buffer selected from the ones described previously or a base polymer may be incorporated in the water-absorption layer. A nonionic surfactant and a known mordant or polymer mordant may also be incorporated. A hydrophilic nonporous intermediate layer may be provided between the porous spreading layer and the reagent layer. This layer inhibits permeation of proteins such as albumin and globulin, and it is composed of a hydrophilic polymer mentioned previously or cross-linked thereof. Preferable hydrophilic polymers are gelatin, gelatin derivatives, polyacrylamide and polyvinyl alcohol, and gelatin, particularly deionized gelatin is the most preferable. Examples of cross-linking agent are vinylsulfonyl cross-linking agents such as 1,2-bis (vinylsulfonylacetamide)ethane and bis(vinylsulfonylmethyl) ether and aldehydes for gelatin, and aldehydes and two glycidyl groups containing epoxy compounds for methallyl alcohol copolymer. Dry thickness of the intermediate layer is 3 µm to 20 µm, preferably 5 µm to 15 µm. In this layer, a pH buffer mentioned previously or a know base polymer and a nonionic surfactant may be added. Light-reflecting particles such as titanium dioxide particles or barium sulfate particles may be suspended,and it is also functioned as a light-reflecting layer. The light-reflecting layer may separately be provided. Dry thickness of the light-reflecting layer is 5 µm to 50 µm, preferably 7 µm to 30 µm. The coating weight of the light-reflecting layer itself is 5 g/m² 50 g/m², preferably 7 g/m² to 30 g/m². An adhesive layer may be provided in order to fortify adhesive force to the spreading layer. This layer is composed of the hydrophilic polymer mentioned previously such as gelatin, and its dry thickness is 0.5 µm to 5 µm. The method of the invention is characterized by incorporating into the porous spreading layer the acid and optionally the spreading action controller mentioned previously using an organic solvent which does not dissolve the water-soluble indicator incorporated in the reagent layer. Such a solvent is usually a polar solvent having a boiling point of lower than 100°C, and includes aliphatic alcohols such as methanol, ethanol, propanol, butanol and isopropanol, dialkylketones such as acetone and methyl ethyl ketone, dialkyl ethers such as diethyl ether, aliphatic cyclic ethers such as tetrahydrofuran and dioxane. Some nonpolar solvent such as acetonitrile, benzene and hexane are also included. Among these, aliphatic alcohols are preferable, and low toxic alcohols such as ethanol, propanol, butanol and isopropanol are particularly preferable in view of working circumstances. For example, when o-Cresolphthalein is employed as the indicator for analysis of calcium, the above alcohols are suitable. This compound itself is slightly soluble in water but readily soluble in alcohols. However, near its optimum pH (pH 10.5), it is readily soluble in water but insoluble in alcohols to the contrary. Concentration of the spreading action controller is preferably high, such as 0.2 % to 10 %, particularly 0.3 % to 7 %. Small amounts of water may be added to the spreading action controller solution to the extent not to cause migration of the water-soluble indicator to the spreading layer. The spreading action controller solution may be applied or sprayed uniformly on the spreading layer. When both of a hydrophilic polymer and a nonionic surfactant are incorporated, the mixture solution of them may be used. They may also be incorporated separately. In the case of the acid, a suspension of the acid may be used instead of its solution. The application or spraying is, in general, preferably carried out after the spreading layer is incorporated into the analytical element. When the spreading layer is membrane filter (blushed polymer layer), knitted fabric or woven fabric, the solution or suspension may be added to the spreading layer prior to its lamination. However, in this case, it is difficult to adjust the content of the spreading action controller or the acid to a prescribed value. After the application or spraying, the analytical element is dried under reduced pressure, air-drying or the like. Other known processes for preparing the analytical element of the invention may be used. The integral multilayer analytical element of the invention is preferably cut into square or circular pieces having a side or diameter of 15 mm to 30 mm, and put in a slide frame disclosed in US-A-4,169,751, JP-C-57-63452, US-A-4,387,990 and Japanese Utility Model KOKAI No. 58-32350, US-A-4,169,751, US-A-4,387,990, PCT application WO 83/00391, to use. The measurement is carried out, for example, according to the manner disclosed in the specifications of the foregoing patents. 5 µl to 30 µl, preferably 8 µl to 15 µl of an aqueous sample are spotted on the spreading layer, and incubated at a definite temperature in the range of 20°C to 45°C for 1 to 10 minutes. Thereafter, detectable change such as color change or coloring in the multilayer analytical element is measured from the side of the support through reflection photometry, and the subject component in the sample is determined by the principle of colorimetry. When this measurement is carried out by using the chemical-analytical apparatus disclosed in US-A-4,488,810, US-A-4,424,191 and US-A-4,424,191, highly accurate results can easily be obtained by a simple operation. In the method of the invention, the migration of the water-soluble indicator in the reagent layer at the time of incorporating the acid and optionally the spreading action controller into the spreading layer is inhibited by using a particular organic solvent. Migrations of the indicator and colored material to the spreading layer are also lowered by the spreading action controller, and by these, analytical accuracy of the analytical element is improved. In the analytical element for the analysis of calcium, permeation of the acid into other layers than the spreading layer is inhibited, and permeation of a pH buffer in other layer(s) into the spreading layer is also inhibited by using a particular organic solvent. Thereby, effective decomposition of calcium compounds in a sample and effective coloration can be achieved. EXAMPLES Example 1 (without acid in the porous spreading layer) The support employed was a colorless transparent polyethylene terephthalate (PET) film having a thickness of 180 µm. The following layers were successively coated and dried to form a laminate. Water-absorption layer: Deionized gelatin 4.8 g/m² Nonylphenoxypolyethoxyethanol 0.11 g/m² (Containing 10 hydroxyethylene units on average) 1,2-Bis(vinylsulfonylacetamide)ethane 0.5 g/m² The aqueous solution was adjusted to pH 6.5 by using NaOH. Reagent layer: Deionized gelatin 23.9 g/m² Polyoxyethylene nonyl phenyl ether 0.41 g/m² (Containing 10 hydroxyethylene units on average) CAPS 3.81 g/m² o-Cresolphthalein Complexone 0.15 g/m² 8-Hydroxyquinolin-5-sulfonic acid 0.54 g/m² The aqueous solution was adjusted to pH 10.6 by using NaOH. Adhensive layer: Deionized gelatin 1.46 g/m² Polyoxyethylene nonyl phenyl ether 0.10 g/m² (Containing 10 hydroxyethylene units on average) Titanium dioxide particles 0.85 g/m² The adhesive layer was uniformly dampened with water, and a PET tricot fabric cloth knitted from 100S PET spun yarn having a mean thickness of 250 µm was pressed to laminate thereon as the spreading layer. Subsequently, the following polymer ethanol solution was applied on the spreading layer, and dried to obtain an integral multilayer analytical element for the analysis of calcium without an acid in the spreading layer. Polymer ethanol solution: Polyvinyl pyrrolidone 1.01 g/m² (Mean molecular weight; 360,000) Polyoxyethylene nonyl phenyl ether 2.11 g/m² (Containing 40 hydroxyethylene units on average) Dissolved in ethanol. Comparative Example 1 An integral multilayer analytical element for the analysis of calcium was prepared in the same manner as Example 1 except the following points. Content of o-Cresolphthalein Complexone in the reagent layer was increased from 0.15 g/m² to 0.46 g/m², and content of 8-hydroxyquinoline-5-sulfonic acid in the reagent layer was increased from 0.54 g/m² to 1.65 g/m². Instead of the polymer ethanol solution, the following aqueous polymer solution was employed. Aqueous polymer solution: Methyl cellulose 5.75 g/m² (Viscosity of 2 % aqueous solution at 20° C = 0,1 Pa·s) Nonylphenoxypolyethoxyethanol 7.44 g/m² (Containing 40 hydroxyethylene units on average) Titanium dioxide particles 15.5 g/m² Dissolved and suspended in water, and applied. Example 2 The integral multilayer analytical elements prepared in Example 1 and Comparative Example 1 were evaluated by using a commercial control serum "Monitrol IX" (DADE, U.S.A.), and the results are shown in Table 1. As shown in the table, in the case of the analytical element of the Example, dispersion of the measured values is remarkably small. Example 1 Comparative 1 Run No. Measured Value mg/dl Run No. Measured Value mg/dl 1 7.7 1 7.9 2 7.7 2 7.6 3 7.7 3 7.9 4 7.8 4 7.7 5 7.9 5 7.7 7 7.7 7 7.6 8 7.8 8 7.6 9 7.8 9 7.4 10 7.6 10 7.6 11 7.8 11 7.6 12 7.7 12 7.9 13 7.7 13 7.6 14 7.7 14 7.7 15 7.8 15 7.7 16 7.8 16 7.7 17 7.8 17 7.4 18 7.7 18 7.6 19 7.7 19 7.4 20 7.6 20 7.5 21 7.6 21 7.5 22 7.7 22 7.7 23 7.8 23 7.6 24 7.8 24 7.5 25 7.7 25 7.7 Average 7.74 Average 7.63 SD 0.0757 SD 0.1406 CV 0.98 % CV 1.8 % Example 3 The following layers were successively applied on a colorless transparent PET film having a thickness of 180 µm, and dried to form a laminate. Reagent layer: Deionized gelatin 16.8 g/m² Nonylphenoxypolyethoxyethanol 1.1 g/m² (Containing 10 hydroxyethylene unit on average) CAPS 2.8 g/m² 0-Cresolphthalein Complexone 0.15 g/m² 8-Hydroxyquinoline-5-sulfonic acid 0.56 g/m² The aqueous solution was adjusted to pH 10.6 by using NaOH. Adhesive layer: Deionized gelatin 1.46 g/m² Nonylphenoxypolyethoxyethanol 0.10 g/m² (Containing 10 hydroxyethylene units on average) Titanium dioxide 0.85 g/m² The aqueous solution was adjusted to pH 10.6 by using NaOH. The adhesive layer was uniformly dampened with water, and a tricot knitted fabric cloth was pressed to laminate thereon as the spreading layer. Subsequently, the following polymer ethanol solution was applied on the spreading layer, and dried to obtain the integral multilayer analytical element for the analysis of calcium. Polymer ethanol solution: Polyvinyl pyrrolidone 4.1 g/m² (Mean molecular weight; 360,000) Nonylphenoxypolyethoxyethanol 8.6 g/m² (Containing 10 hydroxyethylene units on average) Trichloroacetic acid 0.9 g/m² dissolved in ethanol. Comparative Example 2 An integral multilayer analytical element was prepared in the same manner as Example 3 except trichloroacetic acid was not added to the polymer ethanol solution. Example 4 Calcium concentrations of the following Solution 1 and Solution 2 were measured by using the analytical elements prepared in Example 3 and Comparative Example 2. Solution 1: Aqueous solution containing 10 mg/ml of calcium (CaCl₂ was used). Solution 2: Aqueous solution containing 10 mg/ml of calcium (CaCl₂ was used) and 10 mg/ml of phosphorus (Na₂HPO₄ was used). Since the solution became turbid, it was strongly stirred before each sampling. The results are shown in Table 2. Example 3 Comparative 2 Solution 1 10.0 mg/dl 10.0 mg/dl Solution 2 9.4 mg/dl 6.2 mg/dl As shown in the table, negative error in the case of using the analytical element according to Example 3 in the presence of phosphorus is less than one sixth than that of Comparative Example 2. Example 5 An integral multilayer analytical element was prepared in the same manner as in Example 3 except 0.9 g/m² of trichloroacetic acid in the polymer ethanol solution was replaced by 0.6 g/m² of citric acid. Calcium concentrations of Solution 1 and Solution 2 were measured by using this analytical element, and similar results to Example 4 were obtained.

Claims: A method of preparing an integral multilayer analytical element for the analysis of calcium comprising a water-impermeable light-transmissive support, a reagent layer containing a water-soluble indicator capable of reacting with calcium to produce an optically detectable change, and a porous spreading layer containing an acid capable of decomposing the calcium compounds in a sample, superposed in this order, characterized by dissolving said acid in an organic solvent which does not dissolve said water-soluble indicator, incorporating this solution into said porous spreading layer, and drying this superposed material. The method of claim 1 wherein said integral multilayer analytical element contains a pH buffer capable of maintaining the pH of the reaction with said indicator at its optimum in said reagent layer or an intermediate layer between said reagent layer and said porous spreading layer. The method of claim 1 wherein said compounds are protein-bound calcium and calcium phosphate. The method of claim 1 wherein said acid is selected from monocarboxylic acids, dicarboxylic acids, halogen-substituted fatty acids, aromatic carboxylic acids, aliphatic hydroxycarboxylic acids, aromatic hydroxycarboxylic acids, and sulfonic acids. The method of claim 3 wherein said acid is selected from acetic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, monochloroacetic acid, trichloroacetic acid, phthalic acid, lactic acid, malic acid, citric acid, salicylic acid, sulfosalicylic acid and 1,5-naphthalenedisulfonic acid. The method of claim 1 wherein said solution additionally contains a hydrophilic polymer and/or a nonionic surfactant as a spreading action controller. The method of claim 1 wherein said hydrophilic polymer is selected from polyvinyl pyrrolidone, polyvinyl alcohol, polyacrylic acid, methyl cellulose and ethyl cellulose, and said nonionic surfactant is selected from polyhydric alcohol ester ethylene oxide adducts, polyethylene glycol monoesters, polyethylene glycol diesters, higher alcohol ethylene oxide adducts, alkylphenol ethylene oxide adducts and higher fatty acid alkanol amides. The method of claim 1 wherein said organic solvent is a polar organic solvent having a boiling point of lower than 100 °C. The method of claim 8 wherein said polar organic solvent is selected from aliphatic alcohols, dialkylketones, dialkyl ethers and aliphatic cyclic ethers. The method of claim 9 wherein said polar organic solvent is selected from methanol, ethanol, propanol, butanol and isopropanol. The method of claim 1 wherein said indicator is o-Cresolphthalein Complexone, Arsenazo-III or Chlorophosphonazo -III. An integral multilayer analytical element for the analysis of calcium prepared according to the method of claims 1 to 11 comprising a water-impermeable light-transmissive support, a reagent layer containing a water-soluble indicator capable of reacting with calcium to produce an optically detectable change, and a porous spreading layer containing an acid capable of decomposing the calcium compounds in a sample, superposed in this order, and a pH buffer capable of maintaining the reaction with said indicator at its optimum incorporated in said reagent layer or an intermediate layer between said reagent layer and said porous spreading layer. The integral multilayer analytical element of claim 12 wherein said porous spreading layer additionally contains a hydrophilic polymer and/or a nonionic surfactant as a spreading action controller. A method of analyzing calcium which comprises (a) using an integral multilayer analytical element for the analysis of calcium prepared according to the method of claims 1 to 11 comprising a water-impermeable light-transmissive support, a reagent layer containing a water-soluble indicator capable of reacting with calcium to produce an optically detectable change, and a porous spreading layer containing an acid capable of decomposing the calcium compounds in a sample, superposed in this order, and a pH buffer capable of maintaining the reaction with said indicator at its optimum incorporated in said reagent layer or an intermediate layer between said reagent layer and said porous spreading layer, (b) spotting an aqueous liquid sample on said spreading layer, thereby releasing calcium ions from said calcium compounds in the presence of said acid contained in said spreading layer, thereby producing said optically detectable change by supplying said acid treated sample to said reagent layer to interact with said indicator, and (c) detecting said change optically.

Inventors: FUMINORI, ARAI, C/O FUJI PHOTO FILM CO.LTD., KAORU, TERASHIMA, C/O FUJI PHOTO FILM CO.LTD., MITSUTOSHI, TANAKA, C/O FUJI PHOTO FILM CO.LTD., NAKATSUGU, YAGINUMA, C/O FUJI PHOTO FILM CO.LTD.
Applicants: FUJI PHOTO FILM CO., LTD.
Application year: 1987
Main classification: G01N33
IPC codes: G01N33