METHOD FOR CALIBRINGING POTENTIOMETRIC MAGNESIUM ION SELECTIVE ELECTRODE

Description

[0001] The subject application claims benefit under 35 USC § 119(e) of US provisional Application No. 62 / 011,069, filed June 12, 2014.BACKGROUND

[0002] The use of ion selective electrodes (ISEs) to determine the presence and quantity of various analytes in biological samples has become a useful diagnostic technique. Indeed, ISEs have been used to detect analytes such as magnesium, sodium, potassium, calcium, and chloride, among others. Some of these ISEs are often housed within clinical diagnostic instruments for simultaneous analysis of a large number of analytes.

[0003] One such use of the ISEs is for the determination of the amount of magnesium ions in a biological sample, specifically blood. Blood comprises many ions; the main ions present are magnesium ions (Mg 2+< ), calcium ions (Ca 2+< ), and sodium ions (Na +< ). For each type of ion, ISEs have a different response kinetic pattern, which causes the data to be greatly skewed if the ISEs are not calibrated to take into account the different selectivities of the ions. Currently, the calibration of potentiometric ISEs for measuring ionized magnesium ("Mg ISE") generally encompasses calibrating the Mg ISE with three calibration reagents which characterize the slope, intercept, and selectivity of the magnesium ions against the calcium ions.DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0004] Figure 1 illustrates the quality control recovery of a planar Mg ISE sensor that was calibrated with the three calibration reagents that have decreased target amounts of Mg 2+< . AQC1 (diamonds), AQC2 (squares), and AQC3 (triangles) have target Mg 2+< concentrations of 0.9 mmol / L, 0.6 mmol / L, and 0.3 mmol / L, respectively. Each of the three calibrators has a Ca 2+< :Mg 2+< ratio of 1.14 to 1.34. As illustrated in Figure 1, the planar Mg ISE had an unstable recovery period for the first 3 to 5 days after coming in contact with each of these quality control reagents. Figure 2 illustrates the quality control recovery of a planar Mg ISE calibrated with three calibration reagents constructed in accordance with the presently disclosed and / or claimed inventive concept(s). All three calibration reagents have a Ca 2+< :Mg 2+< ratio greater than 1.5. Like in Figure 1, AQC1 (diamonds), AQC2 (squares), and AQC3 (triangles) have target Mg 2+< concentrations of 0.9 mmol / L, 0.6 mmol / L, and 0.3 mmol / L, respectively. However, in this Example, AQC1 (diamonds), AQC2 (squares), and AQC3 (triangles) have Ca 2+< :Mg 2+< ratios of 1.57, 1.92, and 2.89, respectively. As can be seen, the planar Mg ISE has a stable initial recovery period that lasts throughout the lifespan of the magnesium sensing membrane of the ISE. Figure 3 illustrates the variation in response kinetics for a planar Mg ISE in solutions with varying Ca 2+< :Mg 2+< ratios. Normalized Delta mV = mV (t32) - mV (t0), which stands for mV difference between the end point at 32 seconds and initial point at 0 seconds. DETAILED DESCRIPTION

[0005] Before explaining at least one embodiment of the inventive concept(s) in detail by way of exemplary drawings, experimentation, results, and laboratory procedures, it is to be understood that the inventive concept(s) is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings, experimentation and / or results. The inventive concept(s) is capable of other embodiments or of being practiced or carried out in various ways. As such, the language used herein is intended to be given the broadest possible scope and meaning; and the embodiments are meant to be exemplary - not exhaustive. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

[0006] Unless otherwise defined herein, scientific and technical terms used in connection with the presently disclosed and / or claimed inventive concept(s) shall have the meanings that are commonly understood by those of ordinary skill in the art. Further, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular. Enzymatic reactions and purification techniques are performed according to manufacturer's specifications or as commonly accomplished in the art or as described herein. The foregoing techniques and procedures are generally performed according to conventional methods well known in the art and as described In various general and more specific references that are cited and discussed throughout the present specification. The nomenclatures utilized in connection with, and the laboratory procedures and techniques of, analytical chemistry, synthetic organic chemistry, and medicinal and pharmaceutical chemistry described herein are those well-known and commonly used in the art.

[0007] All patents, published patent applications, and non-patent publications mentioned in the specification are indicative of the level of skill of those skilled in the art to which this presently disclosed and / or claimed inventive concept(s) pertains.

[0008] As utilized in accordance with the present disclosure, the following terms, unless otherwise indicated, shall be understood to have the following meanings:

[0009] The use of the word "a" or "an" when used in conjunction with the term "comprising" In the claims and / or the specification may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one." The singular forms "a," "an," and "the" include plural referents unless the context clearly indicates otherwise. Thus, for example, reference to "a compound" may refer to 1 or more, 2 or more, 3 or more, 4 or more, or greater numbers of compounds. The term "plurality" refers to "two or more." The use of the term "or" in the claims is used to mean "and / or" unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and "and / or." Throughout this application, the term "about" is used to indicate that a value includes the inherent variation of error for the device, the method being employed to determine the value, or the variation that exists among the study subjects. For example but not by way of limitation, when the term "about" is utilized, the designated value may vary by ± 20%, or ± 10%, or ± 5%, or ± 1%, or ± 0.1% from the specified value, as such variations are appropriate to perform the disclosed methods and as understood by persons having ordinary skill in the art. The use of the term "at least one" will be understood to include one as well as any quantity more than one, including but not limited to, 2, 3, 4, 5, 10, 15, 20, 30, 40, 50, 100, etc. The term "at least one" may extend up to 100 or 1000 or more, depending on the term to which it is attached; in addition, the quantities of 100 / 1000 are not to be considered limiting, as higher limits may also produce satisfactory results. In addition, the use of the term "at least one of X, Y and Z" will be understood to include X alone, Y alone, and Z alone, as well as any combination of X, Y and Z. The use of ordinal number terminology (i.e., "first", "second", "third", "fourth", etc.) is solely for the purpose of differentiating between two or more items and is not meant to imply any sequence or order or importance to one item over another or any order of addition, for example.

[0010] As used in this specification and claim(s), the terms "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0011] The term "or combinations thereof" as used herein refers to all permutations and combinations of the listed items preceding the term. For example, "A, B, C, or combinations thereof" is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, AAB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context.

[0012] As used herein, the term "substantially" means that the subsequently described event or circumstance completely occurs or that the subsequently described event or circumstance occurs to a great extent or degree. For example, the term "substantially" means that the subsequently described event or circumstance occurs at least 90% of the time, or at least 95% of the time, or at least 98% of the time.

[0013] As used herein, the phrase "associated with" includes both direct association of two moieties to one another as well as indirect association of two moieties to one another. Non-limiting examples of associations include covalent binding of one moiety to another moiety either by a direct bond or through a spacer group, non-covalent binding of one moiety to another moiety either directly or by means of specific binding pair members bound to the moieties, incorporation of one moiety into another moiety such as by dissolving one moiety in another moiety or by synthesis, and coating one moiety on another moiety.

[0014] The term "purified" as used herein means at least one order of magnitude of purification is achieved compared to the starting material or of the natural material, for example but not by way of limitation, two, three, four, or five orders of magnitude of purification of the starting material or of the natural material. Thus, the term "purified" as utilized herein does not necessarily mean that the material is 100% purified, and therefore such term does not exclude the presence of other material(s) present in the purified composition.

[0015] The term "sample" as used herein will be understood to include any type of biological sample that may be utilized in accordance with the presently disclosed and / or claimed inventive concept(s). Examples of biological samples that may be utilized include, but are not limited to, whole blood or any portion thereof (i.e., plasma or serum), saliva, sputum, cerebrospinal fluid (CSF), skin, interstitial fluid, tears, mucus, urine, swabs, combinations, and the like.

[0016] The term "wetup" as used herein will be understood to refer to the hydration process from the installation of a sensor in an analyzer to a point at which a stable signal is obtained out of calibration reagents.

[0017] The term "recovery" as used herein, either alone or in connection with another term (for example but without limitation, "quality control recovery," "recovery period," and "recovery elevation"), is understood to mean the yield of an analytical process with comparison to an assigned value(s) or reference value(s).

[0018] Issues with the current methods of calibrating ISEs for measuring magnesium ions stem from the calibration reagents that are currently used. These calibration reagents have been found to cause planar Mg ISE sensors to undergo a rapid wetup during the first few days after calibration, resulting in heightened response kinetics, and sometimes taking a week or more until the sensor is capable of providing reproducible measurements. Such a long wetup time significantly shortens the useable lifespan of the sensor and prevents new and / or recently calibrated sensors from being readily useable,

[0019] It has previously been determined that the pattern of rapid wetup and the resulting change in response kinetics for Mg ISEs is associated with the ratio of calcium ions to magnesium ions in the calibration reagents. This is due to the Mg ISEs having different response kinetics for calcium ions and magnesium ions. Specifically, the ionophores in Mg ISEs have previously been found to have an ionophore:Mg 2+< and iononophore:Ca 2+< stoichiometry of 1:1 and 2:1, respectively. See W. Zhang et al. (Analytical Sciences (2000) 16:11-18). This difference in stoichiometry values suggests that the ionophores for Mg ISEs may have a higher selectivity for calcium ions than magnesium ions and, therefore, the ratio of Ca 2+< to Mg 2+< should be calibrated to take into account this selectivity.

[0020] It was reported that the ratio of Ca 2+< to Mg 2+< in solution affects the response kinetics of Mg selective electrodes (i.e., the time to steady state). In general, the effect is produced by different rates of dehydration of the magnesium and calcium ions, the latter rate apparently being higher [see, for example, Maj-Zurawska et al. (Analytica Chimica Acta (1990) 236:331-335); Mikhelson et al. (Electroanalysis (2001) 13:876-881); Marsoner et al. (Scand J Clin Lab Invest (1994) 54(suppl 217):45-51); and Zhang et al. (Analytical Sciences, (2000) 16:11-18)]. The kinetic discrimination of calcium and magnesium ions may be associated with the role of these ions in all processes at the membrane / solution interface, including the calcium / magnesium biological pump or ion channel. During the initial wetup period, the Mg 2+< ISE membrane / solution interface has a very dynamic dehydration process for the Mg 2+< and Ca 2+< ions, along with different kinetics for each ion. When the Ca 2+< :Mg 2+< ratio in solution is relatively low, the net rate of dehydration for Ca 2+< and Mg 2+< at the membrane / solution interface may lead to unstable recovery evaluation of aqueous samples (e.g., QC) and calibration response signals (see Figure 1), which could thus lead to significantly skewed measurements.

[0021] As described, it was presently discovered that increasing the amount of Ca 2+< in the calibration reagents to a level wherein the ratio of Ca 2+< :Mg 2+< is similar to that of whole blood minimizes the impact of the initial wetup period. W. Zhang (Am. J. Biomed. Sci. 2011, 3(4), 301-312) determined that, when the calibration reagent has the same (or substantially similar) kinetic pattern as the whole blood samples to be tested, the Mg ISE can be effectively calibrated for measuring the magnesium ion content in blood. Additionally, applying the same Ca 2+< :Mg 2+< ratio to the quality control reagents and wash reagents likewise minimizes deviations in measurements taken using the quality control reagents or after washing the magnesium sensing membranes in the Mg ISEs.

[0022] It is to methods of calibrating magnesium ion selective electrode using such reagents, that the presently disclosed and / or claimed inventive concept(s) is directed. The invention is set out in the appended claims.

[0023] Turning now to embodiments of the presently disclosed inventive concept(s), methods of calibrating a solid-state planar magnesium sensing electrode using new and improved reagents are provided such that the Mg sensors exhibit increased stability and improved response kinetics over prior art sensor / reagent combinations. The sensor / reagent combination exhibits improved response kinetics and recovery stability during the initial period after calibrating the potentiometric ISEs for measuring ionized magnesium. The new and improved reagents are used with a solid-state planar magnesium sensing membrane for a potentiometric ion selective electrode that detects ionized magnesium in a biological sample.

[0024] The reagents are provided with a Ca 2+< to Mg 2+< distribution ratio that is substantially similar to the ratio found in the biological sample to be tested.

[0025] In certain embodiments, the reagent may have a pH in a range of from about 6 to about 8, or from about 6.5 to about 7.8, or from about 6.8 to about 7.2. In addition, the reagent may include additional components. For example but not by way of limitation, the reagent may further include one or more additional ions. Any other ion known in the art or otherwise contemplated herein may be present in the reagent and at any concentration, so long as the potentiometric ion selective electrode can function in accordance with the presently disclosed and / or claimed inventive concept(s). For example but not by way of limitation, the reagent may further include sodium ions. In one particular embodiment, the reagent may further comprise at least 50 mmol / L of sodium ions.

[0026] In other embodiments, the reagent may further comprise one or more surfactants. Any surfactant(s) known in the art or otherwise contemplated herein may be present in the reagent and at any concentration, so long as the potentiometric ion selective electrode can function in accordance with the presently disclosed and / or claimed inventive concept(s). In certain embodiments, the surfactant present in the reagent may be a poly(ethylene oxide) surfactant, wherein the poly(ethylene oxide) surfactant may be utilized at any concentration that allows the potentiometric ion selective electrode to function in accordance with the presently disclosed and / or claimed inventive concept(s). A non-limiting example of a poly(ethylene oxide) surfactant concentration that falls within the scope of the presently disclosed and / or claimed inventive concept(s) is less than about 100 mg / L.

[0027] Any poly(ethylene) surfactants known or otherwise contemplated within the art are capable of functioning as described herein and may be utilized in accordance with the presently disclosed and / or claimed inventive concept(s). Non-limiting examples of poly(ethylene oxide) surfactants that may be utilized in accordance with the presently disclosed and / or claimed inventive concept(s) are represented by the structures of formulas I-III, as shown below.         HO-(CH 2 -CH 2 -O-) 23 -C 12 H 25      Formula II

[0028] In Formula I, n is in a range of from about 9 to about 10; in Formula III, n is about 100. One non-limiting example of a surfactant represented by the structure of Formula I (for example, t-octylphenoxypolyethoxyethanol) is sold under the trade name TRITON ™< X-100 (Sigma-Aldrich, St. Louis, MO). One non-limiting example of a surfactant represented by the structure of Formula II (for example, polyoxyethylene 23 lauryl ether) is known in the art by the product designation BRIJ ®< 35 (CAS No. 9002-92-0). A non-limiting example of a surfactant represented by the structure of Formula III (wherein n is about 100) is polyoxyethylene(100) stearyl ether nonionic surfactant, which is known in the art by the product designation BRIJ ®< 700 (CAS No. 9005-00-9). Particular non-limiting examples of the surfactants represented by the structure of Formula III are disclosed in US Patent No. 8,496,900, issued to Zhang et al. on July 30, 2013.EXAMPLE

[0029] An Example is provided hereinbelow.

[0030] In this Example, the instability of QC (AQC) recovery of a potentiometric magnesium ion selective electrode was studied over a uselife of four weeks. As shown herein below, reformulation of the calibration reagents utilized in the method of calibrating the ISE with the desired Ca 2+< :Mg 2+< ratio improved the performance stability of the Mg sensor.

[0031] Figure 1 shows an AQC recovery plot of the response data obtained using a potentiometric ISE with a solid-state planar magnesium sensing membrane that was calibrated with calibration reagents of the prior art. One of three prior art calibration reagents had a Ca 2+< :Mg 2+< ratio of about 1.34. Three aqueous QC samples were tested at three different target Mg 2+< concentrations (AQC1: 0.9 mmol / L; AQC2: 0.6 mmol / L; and AQC3: 0.3 mmol / L). The sensor had blood contact since installation.

[0032] As can be seen, unstable recovery plots were observed at all three levels of AQCs tested, especially during the first week of uselife. The "humps" observed over the first 5-10 days of uselife indicate that the Mg 2+< concentrations of the reagents were incorrectly detected at a level as much as 2x-3x above their actual concentration during this time, This observation of "hump" QC recovery is explained by slow dynamic complexation between the magnesium ion ionophore in the sensor membrane and the Ca 2+< and Mg 2+< ions in the calibration reagents. The stoichiometry number between ionophore:Mg 2+< is believed to be 1:1, and the stoichiometry number between ionophore and Ca 2+< is believed to be 2:1 (see Zhang et al., Analytical Sciences, incorporated supra).

[0033] In contrast, Figure 2 illustrates an AQC recovery plot of the response data obtained using a potentiometric ISE with a solid-state planar magnesium sensing membrane that was calibrated with calibration reagents that possessed a Ca 2+< :Mg 2+< ratio above the prior art level of <1.5. These calibration reagents had Ca 2+< :Mg 2+< ratios in a range of from about 1.7 to about 3.25 (i.e., 1.57 for AQC1, 1.92 for AQC2, and 2.89 for AQC3), and the reagents were tested at the same three target Mg 2+< concentrations (AQC1: 0.9 mmol / L; AQC2: 0.6 mmol / L; and AQC3: 0.3 mmol / L). As above, the sensor had blood contact since installation.

[0034] Unlike the prior art reagents, the substantially linear lines of Figure 2 indicate stable recovery plots over the entire uselife window for all three levels of AQCs tested. These stable response data measurements for the useable life of the magnesium sensor were observed when the ratio of Ca 2+< :Mg 2+< is increased to be above 1.5 and closer to (or above) 2, which is the approximate Ca 2+< : Mg 2+< ratio for whole blood samples. It is believed that a calibration reagent setting of a Ca 2+< :Mg 2+< ratio in a range of from about 1.7 to about 3.25 helps build up dynamic equilibria between lonophore in the solid state sensor and the Ca 2+< and Mg 2+< ions in the calibration reagents.

[0035] It has been determined herein that, if the ratio of Ca 2+< :Mg 2+< is too far below 2:1 (i.e., 1.5:1 or lower) during the initial contact period between the sensor membrane and the calibration reagents, it leads to a slow procedure to reach complexation equilibrium between the ionophore and the Mg 2+< and Ca 2+< ions in the magnesium sensing membrane of the ISE (i.e., at the membrane interface and in the membrane bulk), as seen in Figure 1 and described herein above. When the sensor contacts a blood sample during the initial period, the adsorbed protein layer functions as an ion-exchanger that responds to electrolytes following the normal distribution ratio in blood for Ca 2+< and Mg 2+< (Ca 2+< = 1.2 mmol / L to Mg 2+< = 0.5 mmol / L).

[0036] In a calibration setting containing two to three calibration reagents, if one reagent has a Ca 2+< :Mg 2+< ratio that is well below 2:1 (e.g., <1.5), the thermodynamic complexation process between ionophore and Mg 2+< and Ca 2+< ions will be sluggish (Figure 3). It will take a long time to buildup a steady-state membrane potential response in all reagents when they possess different Ca 2+< :Mg 2+< ratios (membrane potential = phase boundary potential + membrane diffusion potential), ultimately leading to an unstable initial response period. Adjusting the Ca 2+< :Mg 2+< ratio in all of the reagents (i.e., all calibration, wash, and QC reagents utilized with a particular ISE) to a ratio in the range of from about 1.5 to about 3.25 will accelerate such thermodynamic process. Thus, the Mg 2+< sensor can reach a steady-state potential response in all reagents.

[0037] Additionally, ionic strength in all of the reagents used in the claimed method of calibration is between at least 50 - 160 mmol / L so that the concentration values of Mg 2+< and Ca 2+< are not significantly affected by activity coefficient differences among the reagents. As shown in Table 1, ionic strength (IS) has a significant impact on the molal activity variation of divalent cations. If one of the calibration reagents has an IS of 50 mmol / L (Na +< ) and the other calibration reagent(s) has an IS of 125 mmol / L, the molal activity variation of the divalent cation cannot be neglected. For Mg 2+< and Ca 2+< concentrations of 0.5 mmol / L and 1.2 mmol / L, respectively, the molal activities of the cations are 0.21 mmol / L (Mg 2+< ) and 0.44 mmol / L (Ca 2+< ) in 125 mmol / L Na +< solution. However, in 25 mmol / L Na +< solution, the molal activities of the cations become 0.29 mmol / L (Mg 2+< ) and 0.66 mmol / L (Ca 2+< ). Since a potentiometric sensor responds only to the molal activity (as opposed to the mass concentration), molal activity variation induced by ionic strength can lead to biased calibration results (slope, selectivity, and intercept). With such biased calibration parameters, blood sample recovery can be wrongly calculated. Table 1. Impact of Ionic Strength on Activity Coefficient and Molal Activity of Cations in Aqueous Solution Ionic Strength (Na +< , mmol / L)K +< (4 mmol / L)Ca 2+< (1.2 mmol / L)Mg 2+< (0.5 mmol / L)Activity coefficient1500.7130.3480.3981250.7290.3670.4141000.7470.3910.436750.7700.4240.464500.8000.4710.505250.8440.5530.578100.8890.6520.667Molal activity (mmol / L)1502.850.420.201252.920.440.211002.990.470.22753.080.510.23503.200.570.25253.380.660.29103.560.780.33

Claims

1. A method of calibrating a potentiometric ion selective electrode configured to detect ionized magnesium in a biological sample, said electrode comprising a solid-state planar magnesium sensing membrane, comprising the steps of: (a) contacting said magnesium sensing membrane with a first solution comprising calcium ions and magnesium ions having a calcium : magnesium molar ratio of 1.57 and a target magnesium ion concentration of 0.9 mmol / L; (b) contacting said magnesium sensing membrane with a second solution comprising calcium ions and magnesium ions having a calcium : magnesium molar ratio of 1.92 and a target magnesium ion concentration of 0.6 mmol / L; and (c) contacting said magnesium sensing membrane with a third solution comprising calcium ions and magnesium ions having a calcium : magnesium molar ratio of 2.89 and a target magnesium ion concentration of 0.3 mmol / L wherein the ionic strength in the first, second and third solution is between 50 and 160 mmol / L.

2. The method of claim 1, wherein the solutions have a pH in a range of from 6 to 8, preferably of from 6.5 to 7.8.

3. The method of claim 1 or 2, wherein the solutions further comprise sodium ions at a concentration of at least 50 mmol / L.

4. The method of any one of claims 1-3, wherein the solutions further comprise a surfactant.

5. The method of claim 4, wherein the surfactant comprises a poly(ethylene oxide) surfactant.

6. The method of claim 5, wherein the poly(ethylene oxide) surfactant is represented by the structure of formula I: wherein n is in the range of from 9 to 10.

7. The method of claim 6, wherein the poly(ethylene oxide) surfactant is represented by the structure of formula II:         HO-(CH2-CH2-O-)23-C12H25     Formula II or wherein the poly(ethylene oxide) surfactant is represented by the structure of formula III: wherein n is 100.

8. The method of any one of claims 5-7, wherein the concentration of the poly(ethylene oxide) surfactant is less than 100 mg / L.