METHOD FOR THE QUANTITATIVE DETERMINATION OF CALCIUM IN BIOLOGICAL FLUIDS THROUGH DIRECT POTENTIOMETRY

Abstract: Description of a method for the potentiometric determi­ nation of the total calcium (II) content in biological fluids (serum, plasma and urine) based on dilution of the sample. The results obtained, when compared to the usual methods in current use, show the validity of the proposed method; furthermore, the pH of the buffer solution used for diluting the sample guarantees optimum operating condi­ tions as well as long working life of the potentiometric sensing elements and maximum reliability when compared to other methods employing acidification of the sample.

Description: Studying the method for the potentiometric determination of the total calcium in biological fluids forming the matter of the present invention the following documents have been considered: a) Patents : United States Letters Patents 3957607 4/1976 Simon et al. 204/195 M 3932233 1/1976 Ruzicka et al. 204/195 M b) Publications 1) MOORE W.E. "Studies with ion-exchange calcium electrodes in biological fluids; some applications in biomedical research and clinical medicine" in R.A. Durst - Ion selective electrodes. NBS special publications 314 pages 215-285. 2) SCOTT J.B. and BRADWELL A.R. Identification of the serum binding proteins for iron, zinc, cadmium, nickel and calcium. Clin chem. 29, 629-633 (1983). 3) D. AMMAN, H.B. JENNY, P.C. MEIER and W. SIMON New ion selective electrodes and their clinical and biological application in Electroanalysis in Hygiene, environmental, clinical and pharmaceutical chemistry. Proceedings of a conference, organized by the electroanalytical group of the Chemical Society of London, held at Chelsea College, University of London, April 17th - 20th 1979. Edited by W. Franklin Smith Elsevier Scientific Publishing Company. The teaching of the above cited prior documents has been duly considered, modified and improved, and in the end the present method was arrived at. Above all, the present invention is directed to a method for determination of total calcium content in a serum, a plasma or a urine sample based on a potentiometric system that does not employ acidification of the sample using: a calcium ion selective electrode a reference electrode a measuring cell of insulating material with temperature stabilization; a volumetric diluter; buffer for dilution of the sample or standard solution containing a calcium-protein complexing agent in form of metals, other than calcium exhibiting binding affinity towards such proteins and silicon oil emulsion; buffered calibration calcium standard solution containing a complexing or chelating agent to calcium. The measuring system adopted entails the use of a calcium (II) ion selective potentiometric sensing element coupled to a reference electrode (saturated calomel/potassium chloride in this case). The Calcium (II) ion selective electrode consists of a solid film of plastic material, preferably polyvinylchloride, containing the electroactive material ("neutral carrier" type described by Simon et al. - in U.S. Patent 3,957,607 or of the "ion-exchanger" type described by Ruzicka et al. in U.S. Patent 3,932,233), in a appropriate plasticizer. For the automatic determination of total calcium content in serum, plasma or urine samples a buffer solution for dilution is used which is based on Tris 0.1 M pH 7,5 at 25 °C. The method in accordance with the invention represents a valid alternative to techniques such as atomic absorption spectroscopy or colorimetry and fluorimetry which are widely used in determining total calcium content in serum or other biological fluids. When this method is employed in conjunction with the particular automatic system chosen, it gives the total calcium concentration in the serum by measuring the electromotive force following the electrochemical system as reported below or other equivalent system: Hg; Hg₂Cl₂ ; sat. KCl//sample/Ca⁺⁺ membrane/10⁻² CaCl ₂ . AgCl; Ag To the present specification of the method according to this invention, for better understanding, and only for example, we annex drawings, in which more precisely: Fig. 1 is a block diagram of the apparatus which can be used to carry out the method in accordance with the invention. In fig. 2 it has been shown schematically the calcium ion selective electrode for showing the construction details of it. As shown in the drawings: Measuring electrode 12 and reference electrode 26 are contained within an anodized aluminium block 10 for temperature stability purposes. The measuring cell or chamber consists of a block of transparent acrylic material 14 incorporating sample flow passages with inlet at 18 and outlet at 20. All measurements are carried out at ambient temperature. The sample is aspirated preferably by means of a peristaltic pump 16 with flow rate 1 ml/min. The electromotive force is measured in the first fifteen seconds, this being the time necessary for stable measurement. 22 denotes the inlet for the reverse flow rising fluid and 24 its outlet. The calcium ion selective electrode, comprises (fig. 2) a silver wire 28, a body in two sections 30, 32, as well as O-ring 34, membrane 44 and Tygon® seal ring 42. 32 denotes a housing 36 containing the filling solution and 30' the narrower diameter section of closing piece 30. Housing 36 runs into another housing of narrower diameter 40 which besides holding the filling solution, also contains silver wire 28. All that being stated, the method of the present invention has considered more in detail. The sample to be analyzed (serum, plasma, urine, or the standard solution for calibration) is diluted in the ratio 1:20 with a buffer (0.1M) of pH 7.5 at 25°C. Buffers used at this pH include Tris(hydroxymethyl)aminoethane(Tris) and Tris derivatives. A silicone oil/water emulsion is added to this buffer in the ratio 1 part emulsion to 9-19 parts buffer. The purpose of this is to improve electrode performance by almost totally eliminating the memory effect, thereby allowing maximum optimum surface involvement of the calcium ion sensitive membrane. Moreover, this system differs from the other ones currently available on the market, in that it does not employ acidification of the sample, therefore it guarantees stability and long life of the electrodes. The concentrated emulsion was prepared by homogenizing silicon oil (of type DC 200 Dow Corning of AK 10 Waker) with non ionic surfactants (of type Brij 35 I.C.I. or Antarox Co 530 GAF). The calcium is contained in the serum in both ionic form and complexed form with certain proteins (Scott J.B. and Bradwell A.R.). Therefore addition of a protein complexing agent (D. Amman, H.B. Jenny et al.) to the buffer displaces the calcium-protein complexing equilibrium towards the ionic form. The complexing agent was chosen from metals exhibiting binding affinity towards proteins (Scott J.B. and Bradwell A.R.) such as iron, zinc, cadmium, nickel and copper. The best experimental results were obtained by adding 0.2 mmol/l zinc to the buffer solution used in dilution. The considerations arrived at and the results obtained in the experimental phase proved of fundamental importance in the formulation of the system calibration solutions. Experiments carried out on the automatic system in question showed that by appropriately diluting the serum, plasma and urine in the buffer, the quantity of complexed calcium is constantly kept below 4 to 5% (reference value is determined by atomic absorption spectroscopy). This percentage of inaccuracy is of the same order as that found with electrodes for bivalent ions under normal operating conditions. At the same time it was found that this difference is independent of protein concentration in the serum within the range 4.5 to 8.5 g/dl. Hence calibration solutions were formulated as close as possible (as regards calcium content, either ionic or complexed) to the solutions to be measured. The best results were obtained when using 2,2-Bis-(hydroxymethyl)-2,2', 2"-nitrilotriethanol (BIS-TRIS) in the preparation of standards used for determinig suitable calibration curves. As the "Bis-Tris" exhibits a chelating (or complexing) affinity towards calcium, it corrects the calibration curve without any alteration of the electrode's electrochemical properties. Furthermore, it was found that addition of "Bis-Tris" between 300 and 400 mg/dl was not critical. The basic composition of the calibration solutions employed was the same as that of the buffer used for dilution, without, however, the ionic agent (Zn⁺⁺ ), and with the "Bis-Tris" and calcium chloride. Lastly, the method in accordance with the invention, also provides, after each calibration or analysis, the automatic rinsing in counterflow direction to that of the normal sampling cycle, using a buffered solution of known calcium composition. Various experimental tests have been made on the method according to the invention. They make the matter of the following examples. Example 1 The influence of the buffer solution used for dilution was assessed by comparing the results obtained (percentage recovery) with those of a reference analytical technique (Atomic Absorption Spectroscopy A.A.S.). The buffer solution was prepared by dissolving 2.73 mg ZnCl₂ in 100 ml Tris buffer 0.1M and adjusting to required pH with hydrochloric acid. Calibration was carried out by using solutions of 10⁻³ and 3.10⁻³ M of calcium in Tris buffer 0.1M pH 7.5 containing 3.5 g/l of "Bis-Tris". Dilution ratio was 1:20 Table 1) sums up the results Example 2 The effect of protein concentration in the serum was assessed by using test serums from different sources. Formulation of the Tris buffer pH 7.5 is described in example 1) which also gives the composition of the calibration solutions. The samples were diluted in the ratio 1:20 and the results compared with those obtained through A.A.S. Table 2) sums up the results obtained. Example 3 The influence of Zn⁺⁺ concentration in the buffer solution used for dilution was assessed by adding a variable quantity from 0.05 to 0.5 mmol/l of Zn⁺⁺ to the buffer described in example 1) which also describes the composition of the calibration standards. Dilution ratio was 1:20 Example 4 Here the influence was assessed of adding a silicone oil/water emulsion to the buffer for improving level of accuracy of the method in accordance with the invention. The concentrated emulsion was prepared by homogenizing silicone oil -10% (DC 200 Dow Corning or AK 10 Waker) with 1% surfactant (Brij 35 I.C.I. or Antarox Co 530 GAF). The as-prepared solution was diluted with the buffer described in example 1) in the ratio one part emulsion to 9 parts buffer. Calibration and dilution were carried out as described in example 1). Table 4) sums up the results. Example 5 The influence was assessed of adding the emulsion described in example 4) as rinsing solution (1 part emulsion to 9 parts tris buffer) in order to improve level of accuracy of the method in accordance with the invention. Table 5 sums up the results. Example 6 The influence was assessed of the amount of Bis-Tris added to the calibration solution in the range 0.3 to 0.4 mg/dl. Calibration and dilution were carried out as described previously. Table 6 sums up the results obtained with three different concentrations of Bis-Tris. Dilution ratio was 1:20 and comparative method was through A.A.S.

Claims: Method for determination of total calcium content in a serum, a plasma or a urine sample based on a potentiometric system that does not employ acidification of the sample using: a calcium ion selective electrode; a reference electrode; a measuring cell of insulating material with temperature stabilization; a volumetric diluter; buffer for dilution of the sample or standard solution containing a calcium-protein complexing agent in form of metals, other than calcium exhibiting binding affinity towards such proteins and silicon oil emulsion; buffered calibration calcium standard solution containing a complexing or chelating agent to calcium. Method as claimed in claim 1 where the buffer solution for dilution is based on Tris 0.1 M pH 7.5 at 25 °C. Method as claimed in claim 2 whereby the buffer contains from 0.1 to 0.3 mM ZnCl₂ as calcium-protein complexing agent. Method as claimed in claims 1, 2, 3 using a buffer according to claim 2, which contains a silicone oil/water emulsion prepared by homogenizing said oil from 10 to 20% with non-ionic surfactant from 1 to 5% in water. Method as claimed in claim 1, using a buffer solution for dilution as according to claim 2, which contains the emulsion as described in claim 4 with concentration ranging from 5 to 20%. Method as claimed in claim 1, using a buffer solution for dilution as according to claims 2, 3 containing the emulsion described in claims 4, 5. Method as claimed in claim 1, which entails dilution of the sample or the standard with the buffer as claimed in claim 3 wherein the dilution ratio ranges from 1:10 to 1:50. Method as claimed in claim 1, which entails dilution of the sample or the standard with the buffer as described in claim 3 and with dilution ratio of 1:20. Method as claimed in claim 1, which entails dilution of the sample or standard with the buffer as described in claim 6 wherein the dilution ratio ranges from 1:10 to 1:50. Method as claimed in claim 1, which entails dilution of the sample or the standard with the buffer as described in claim 6 and with dilution ration 1:20. Method as claimed in claim 1 combined with calibration using a standard of known composition of Ca⁺⁺ buffered with Tris 0.1 M solution pH 7.5 at 25 °C, containing from 300 to 400 mg/dl of Bis-Tris. Method as claimed in claim 1 employing a rinsing cycle after each standardization or sampling. Rinsing solution of known composition of calcium in Tris 0.1 M buffer solution pH 7.5 at 25 °C containing the emulsion as claimed in claim 4 whose concentration ranges from 5 to 20% in the solution.

Inventors: CALZI, CLAUDIO, MANZONI, ANGELO, PREMOLI, PIETRO
Applicants: INSTRUMENTATION LABORATORY S.P.A.
Application year: 1985
Main classification: G01N33
IPC codes: H02K21, G01N27, H02K19, G01N33