MULTILAYER ANALYTICAL ELEMENT AND METHOD FOR DETERMINATION OF ANALYTES IN FLUIDS CONTAINING INTERFERING SUBSTANCES

Abstract: A method and device is provided for determination of an analyte in a fluid sample in the presence of substances interfering with an assay for the analyte. The methods and device comprise an absorbent pad between the site of application of the sample fluid and the reagent impregnated test pad(s) or layer(s) to selectively remove interfering substances. A preferred embodiment comprises a support element (11) having affixed thereto several layers including an adhesive tape layer (15), a reagent layer (12) and an interference removing layer (13). An aperture (16) may be incorporated into the multilayered device to allowing viewing. Preferred interference removing absorbent pads include polyamide, carbon-impregnated and ion-exchange resin impregnated membranes.

Description: TITLE OF THE INVENTION Multilayer Analytical Element and Method for Determination of Analytes in Fluids Containing Interfering Substances. TECHNICAL FIELD The present invention relates to a device and method for detecting analytes in materials containing interfering substances such as wines, musts, and other agricultural and food products. BACKGROUND OF THE INVENTION The present invention relates to a unitized dry reagent test device for the colorimetric determination of analytes and/or measurement of pH in samples containing interfering substances such as in the chemical analysis of wine and fruit juice samples. Although the methods and devices of the present invention are ideally suited for field or on-site testing, they can also be advantageously used in the laboratory. Reagent test papers have been in use for many years. Simple indicator papers for pH, so called litmus papers, have been in use for over a hundred years. Beginning in the 1 960's the first patent references appeared for reagent test strips for medical applications. These early devices consisted of papers or other fibrous or bibulous materials that were impregnated with a chemical composition and dried. In use, these papers were dipped into the sample to be tested or the sample to be tested was applied to the paper, a color was developed, and the color was compared against a precalibrated reference chart to obtain an analytical result. Alternatively, the developed color could be examined with a measuring device, such as a reflectance photometer, and the signal obtained compared to a previously determined calibration curve. One of the problems affecting the accuracy of these devices has been the presence of interfering substances in the sample to be tested. In some tests this problem was addressed by a separate sample treatment regimen, but the extra sample handling required reduced the convenience of the test strip and often introduced its own errors. On-device methods for eliminating the influence of interfering substances have also been previously investigated and applied. Some methods of interference elimination are listed below. 1 . "Filtration" of paniculate materials has been employed for the removal of interfering substances. U.S. Pat. No. 4,543,338 describes a test element with a semi-porous membrane as a top layer of the test element that can separate red blood cells and large molecular weight proteinaceous materials, but the separated materials must be wiped off of the device before a test can be read. U.S. Pat. No. 3, 1 58,532 teaches the use of an asymmetric membrane that traps cellular materials yet allows dissolved components to pass to a reaction area. A separate wipe-off step is not required with this device. U.S. Pat. No. 5,801 ,061 describes a dry reagent test device capable of removing interfering particulate materials from soil test samples. While satisfactory for their intended applications, these devices and methods are not effective in removing soluble colored interfering substances, such as tannins and anthocyanins, from food samples and especially from wine and grape juice samples. 2. Chemical elimination of interfering substances has been described for several different test elements and test systems. U.S. Pat. No. 4, 1 68,205 describes the use of the enzyme ascorbate oxidase and U.S. Pat. No 4,288,541 teaches the use of a mercury-ligand complex to eliminate the interfering effect of ascorbic acid. Japanese Pat. No. 4, 1 57,365 A2 describes the use of chelation for the removal of ionic interferences. U. S. Pat. Nos. 5 ,61 0,025 and 5 ,861 ,269 teach the use of inhibitors that prevent the action of enzymes capable of catalyzing color development other than that related to the desired analyte. U.S. Pat. No. 5,902,731 describes the use of nitrite salts to chemically suppress color formation caused by the presence of hemoglobin in a test sample. These chemical techniques prevented the need for a separate processing step to eliminate an identified interference and are appropriate for their intended purposes, but no chemical "neutralization" method has yet been identified for the broad families of colored and chemically interfering substances present in food samples. 3. Immunological elimination of interfering substances has been employed in a disposable reaction device. U. S. Pat. Nos. 5,403,745 and 6,01 0,866 teach the use of antibodies to separate interfering moieties from the target analyte. This approach is appropriate when the number of interfering materials is small, but does not apply in the broad range of colored interferences found in foods. In addition, no antibody to anthocyanins has yet been reported. 4. Shifting the wavelength of detection has been applied as a means of overcoming the influence of interfering substances. U.S. Pat. Nos. 5 , 1 26,275 and 6,063,587 and Japanese Pat. No. 8,053,444 A2 describe chemical compounds that were synthesized to allow visual or instrumental detection at 550 nm or higher, a color/wavelength region that does not overlap the colors/wavelengths intrinsic to bilirubm and hemoglobin, two usual components of human blood. This is an advantageous improvement when performing analyses of human blood 75 and human serum, but is not of benefit when testing samples such as grape juice and wine. These materials have broad absorption spectra at acidic pH in the range 500 - 550 nm, and at alkaline pH, the pH used for detection of some key wine and must analytes, the absorption maxima of tannins and anthocyanins shifts to the 550 - 690 nm range Shifting of the wavelength of detection is not a 80 suitable solution to the problem of colored substances interfering in the analysis of foods containing tannins and anthocyanins. 5. Attempts have also been made to optically eliminate the influence of colored interfering substances. U S. Pat. No. 4, 1 25,372 teaches the use of a system consisting of a test piece and a reflectance compensation piece, the latter 85 having properties substantially identical to the test piece with respect to the liquid to be tested except that it does not have the capability of giving a color indication in connection with a given substance in the test liquid. By comparing the color difference, if any, between the two pieces after they are removed from the test liquid it is possible to determine the amount of color change caused by the 90 desired substance. This method is an improvement over simple visual comparison to a color chart, but it requires an instrument for making the optical correction and does not provide a correction if the interfering material reacts with the test piece U.S. Pat. No. 4, 1 60,646 improves upon the color correction method of the above-mentioned patent by employing measurements at several 95 wavelengths to effect the interfering color correction, but it requires an even more complicated instrument and is still susceptible to interfering chemical reactions and to pH effects that can affect the spectral characteristics of the primary chromophore different from the interfering chromophore U.S Pat. No. 5,31 2,591 addresses this problem by modifying the correction pad to include a 100 substance that adjusts its pH to the pH of the test system. While overcoming one unintended cause of increased imprecision, the system described still allows interfering substances to come into contact with the primary reactants and still requires a complex instrument to effect the described corrections and still leaves the primary reactants susceptible to reactions with the interfering substances. 105 Known on-device non-instrumental methods for removing the influence of interfering materials from test samples intended for use with reagent test strip devices are numerous and generally have served their intended purposes, but none has proven appropriate or satisfactory for the removal of tannins, anthocyanins, and other colored or chemically interfering substances from food 1 10 samples such as fruit juice and wine. DISCLOSURE OF THE INVENTION The present invention provides an integrated method for the removal of interfering substances from samples to be tested using reagent test strip devices, comprising an absorbent interference-removing membrane layer in addition to 1 15 the conventional test reagent-containing layer(s) or pad(s). In its simplest configuration a reasonably sized segment of absorbent membrane of known absorption characteristics is affixed to a reagent test pad impregnated with a test reagent composition specifically formulated to give a colorimetric response with the analyte being detected. This assembly is attached to one end of a longer strip 120 of semi rigid fluid-impervious plastic for ease of handling and sample impregnation BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a description of the interference removal structure mounted on a commercial reagent test strip. 125 Figure 2 is a depiction of an interference removal structure assembled as an integral part of a reagent test strip for determining analyte concentration. DETAILED DESCRIPTION OF THE INVENTION In the testing of fruit juices or extracts of plants it is frequently necessary to remove colored materials and/or tannins and anthocyanins in order to prevent 130 these materials from interfering with an assay for an analyte in the fruit juice or extract. One method of effecting the removal of the indicated interfering materials is a separate pretreatment step, consisting of adding a slurry of polyamide powder or polyvinylpolypyrrolidone, waiting for adsorption to occur, the filtering the mixture so that the filtrate can be tested (Boehringer Mannheim, 135 directional insert for L-Lactic Acid, product 1 39 084, 1 997 and Randox Laboratories Ltd., directional insert for Glucose/Fructose, product GF 2635, 1 999). Alternately, interfering tannins have been removed by applying plant extracts to a polyamide column and testing the tannin-free eluate (M. E. Wall, et al., 7. Pharm. Sci. 58: 839 (1 969) and M. E. Wall, et al., Phytomedicine 3: 281 140 (1 996)). Seeking to apply to above sample pretreatment techniques to reagent test strip devices, a series of materials and chemical additives were studied to identify an "on board" technique for removal of interfering substances from test samples. It was discovered that certain membranes and impregnated papers had sufficient 145 adsorptive capacity to remove interfering substances, yet possessed fluidic properties such that they did not detract form the reagent test strip concept of a one step, simple to use device. With this discovery, a range of assays that has heretofore been impractical for the reagent test strip format can now be pursued. The present invention will be more fully described in the following 150 examples: EXAMPLE 1 This example describes the identification of candidate absorbent membranes. A series of experimental test devices was assembled according to figure 1 . Item 1 1 is a polyester support element that has affixed to a portion of 155 its upper surface a piece of doubled-sided adhesive tape, item 1 5. Both of these items have a 3 / 1 6 inch viewing aperture, item 1 6. Arranged above these two items, and affixed to item 1 1 by item 1 6 are in order a reagent test pad, item 1 2, an interference removal pad, item 1 3. Approximately 1 2 microliters of a sample of '97 Cabernet sauvignon wine were applied to the top of the 160 interference removal pad, item 1 4. The amount of burgundy/red anthocyanin color reaching the lower surface of the reagent test pad was estimated visually on a scale of 0 (none) to 4 (deep burgundy) and by reflectance spectrometer (a reading of 0.00 corresponds to no red color reaching the lower surface of pad 1 2.) The non-impregnated reagent test pad for all test samples was either a 165 polyamide membrane or a polysulfone membrane. Results are given in Table 1 . Table 1 Interference removal Pad candidate Visual Rating Reflectance Ion exchange resin-impreg. Paper (Whatman) 2 0.030 170 Activated carbon-impreg. Paper (S&S) 0 0.000 Polyamide membrane (Cuno) 4 0.1 90 Polyamide membrane (S&S) 0 0.020 Quantitative filter paper (Whatman) 3 0.1 30 Polysulfone membrane (Pall) 4 n/a 175 Polyamide membrane ( Pall) 0 0.01 0 From these data it can be seen that polyamide membranes, activated carbon-impregnated paper membranes, and ion exchange resin-impregnated paper are effective at removing anthocyanins and other colored interfering 180 substances from food samples as part of an integrated sample conditioning element in a reagent test strip device, and are preferred embodiments. ("Pad," "membrane," and "layer" are used interchangeably in discussion of test devices in accordance with this invention to designate discrete functional elements of a reagent test strip device. Also, the interference removing "pad,", "membrane," or 185 "layer" may consist of more than one physical thickness of, e.g., a polyamide membrane.) EXAMPLE 2 A reagent strip test system to evaluate the functionality of the above discovery was prepared by assembling the interference removal material on top of 190 a commercial reagent test strip according to figure 2. The commercial test strip used was colorpHast Indicator Strips, pH 2.5 - 4.5, offered by EM Science of Gibbstown, NJ. These strips are designated as item 1 in figure 2, and include an integral reagent test pad, item 2. Assembled above the test pad was a polyamide membrane, Biodyne A, item 3, and above the polyamide membrane was 195 assembled a non-woven fabric, item 4, used to absorb sample in excess of that required to saturate the test pad and maintain the test pad moist. Items 3 and 4 were held to the commercial strip support material by double-sided adhesive tape, items 5. A solution of 0.1 g/L L-malic acid was prepared, adjusted to pH 3.4 and 200 applied to the reagent test strip as commercially available. The same solution was applied to the interference removal structure described above. Both test devices registered a color corresponding to pH 3.3, the test pad color nearest the actual pH of 3.4 on the product standard color interpretation chart. Next, a wine sample ('99 Merlot) of pH 3.48 was applied to a reagent test strip as commercially 205 obtained and to another commercial test strip which had an added interference removal structure as described above. The color of the test pad with the interference removal structure corresponded to a pH or 3.6, the nearest color on the product color interpretation chart. The color of the test pad on the reagent strip used as commercially available corresponded to a pH of 3.0 - 3.3, much 210 lower than actual. The red color of the wine prevented an accurate determination of pH without the use of the interference removal structure that is the subject of this disclosure. EXAMPLE 3 A reagent strip test system to further evaluate the functionality of the 215 initial observation was prepared by assembling the interference removal material on top of a commercial reagent test strip according to figure 2. The commercial test strip used for this example was Chemstrip uG for glucose, offered by Boehringer Mannheim Corp. of Indianapolis, IN. These strips also used the assembly structure detailed in figure 2. The interference removal membrane used 220 in this evaluation was again Biodyne A, available from Pall Corp. of Far Hills, NY. Two test solutions were prepared for this evaluation. One was an aqueous solution containing 500 mg/dL glucose. The second was a solution of 500 g/dL glucose in Merlot wine, a red wine. The test solutions and a sample of neat wine were applied to both interference removal test structures described above and to 225 reagent strips as obtained from the supplier. Note that the red wine had a residual glucose of < 20 mg/dL. The results are given in Table 2. Table 2 Interpretation per Standard Color Chart 1 Structure As received from w/ Interference 230 Strip Supplier Removal Structure Sample 500 mg/dL aqueous glucose 500 mg/dL 500 mg/dL 500 mg/dL glucose in red wine 3000 mg/dL 500 mg/dL 235 Red Wine 2000 mg/dL 2 negative i Based on the closest match in the standard color chart supplied by the reagent strip manufacturer 240 2 The first color interpretation pad was deeply colored blue-green, indicating a chemical interference with its reaction scheme The second color interpretation pad was colored burgundy and could not be matched to the standard color chart Neither the neat wine nor the wine sample with the added glucose could be 245 adequately quantitated with the glucose reagent test strips as provided by the manufacturer. With the addition of an interference removal structure that is the subject of this disclosure both samples could be correctly analyzed. EXAMPLE 4 This example illustrates a further application of the present invention. A 250 series of test devices for the determination of pH was fabricated according to figure 3 without reliance on commercially available reagent test strips. Item 21 is a polyester su pport element that has affixed to a portion of its u pper surface a piece of doubled-sided adhesive tape, item 25. Both of these items have a 3 / 1 6 i nch viewi ng aperture, item 26. Arranged above these two items, and affixed to 255 item 21 by item 26 are i n order a reagent test pad, item 22 , an i nterference removal pad, item 23 , and an absorbent pad, item 24 The i nterference removal pad is BioDyne A polyamide membrane, available from Pall Corp (A polyamide membrane is the most preferred embodiment of an interference removal device ) The reagent test pad is a polyamide membrane available from Cuno, Meriden, CT, which was impregnated with an aqueous solution comprising approximately 1 .32 g/L bromphenol blue, 1 .0 g/L Triton X-l 00 and 2.0 g/L gelatin. Following impregnation excess fluid was blotted off and the pads were air dried for at least 30 min. prior to use. Reagent test pads were 0.25 inch diameter circles. The interference removal pads were 5 / 1 6 inch squares. In practice the absorbent pad of this device, approximately 5 / 1 6 inches by 7/ 1 6 inches in size, is wetted with 1 5 - 20 microliters of test solution as shown in Figure 3. The test solution migrates through the interference removal pad in which the interfering substances are trapped. Target analytes migrate to the reagent test pad where appropriate chemical reactions are intended to occur. In the format employed the reagent test pad was allowed to dry, then any color change in the reagent test pad was observed through the viewing aperture either visually or by using a suitable reflectance measuring device, e.g., spectrometer PC2000-ISA assembled in the reflectance mode and available from Ocean Optics, Inc. of Dunedin, FL. A series of pH standards from pH 2.9 to pH 3.8 was prepared. These standards were applied to a series of test devices assembled as just described. The test devices were allowed to air dry for at least 30 min., their reflectance spectra were obtained, then they were set aside as visual reference standards. Two samples were next applied to additional test devices and the devices were allowed to dry. Determinations of pH were made visually and by reflectance spectrometer. The results are given in Table 3. Table 3 Observation by: Wine sample. '97 C. sauviqnon luice sample, red table grapes pH meter (reference) pH 3.52 pH 3.80 Visual (observer 1 ) pH 3.4 - 3.6 pH 3.8 Reflectance spectrometer pH 3.5 pH 3.8 The visual and instrumental methods both gave correct determinations of sample pH without influence from the deep red color of the test samples. While this invention and the practice thereof have been described with respect to various embodiments, it is recognized that one skilled in the art may practice further embodiments of the invention without departing from the spirit and scope of the invention set forth and claimed herein.

Claims: CLAIMS 1 . A multilayer analytical element for the determination of an analyte in a liquid containing substances interfering with an assay for the analyte, said multilayer analytical element comprising an interference removal layer and one or more detection layers. 2. A multilayer analytical element as claimed in claim 1 wherein the interference-removing layer is composed of a polyamide membrane. 3. A multilayer analytical element as claimed in claim 1 wherein the interference removing layer is composed of an activated carbon-impregnated paper. 4. A multilayer analytical element as claimed in claim 1 wherein the interference removing layer is composed of an ion exchange resin-impregnated paper. 5. A multilayer analytical element as claimed in claim 1 wherein the detection layer is affixed to a support member. 6. A method for determination of an analyte in a fluid sample in the presence of substances interfering with an assay for the analyte, comprising contacting said liquid to a multilayer analytical element, said multilayer analytical element comprising an interference removal layer and one or more detection layers. 7. A method as in claim 6 wherein the interference-removing layer is composed of a polyamide membrane. 8. A method as in claim 6 wherein the interference removing layer is composed of an activated carbon-impregnated paper. 9. A method as in claim 6 wherein the interference removing layer is composed of an ion exchange resin-impregnated paper. 1 0. A method as in claim 6 wherein the fluid sample contacts the interference removal layer prior to contacting a detection layer.

Inventors: MILLER MICHAEL M, MILLER, MICHAEL, M.
Applicants: MILLER MICHAEL M, MILLER, MICHAEL, M.
Application year: 2001
Main classification: G01N21
IPC codes: G01N33