Neutral carrier calcium electrodes

High-quality calcium ISEs can also be fabricated with either acylic (6, 7, 23, 28-31) or cyclic (32-34) neutral carrier compounds (Table 3.12). Thus N, N -di[-(ll-ethoxycarbonyl)undecyl]N, N -4, 5-tetramethyl 3, 6-dioxa-octane diamide (ETH 1001) (5) now constitutes the basis of many commercial cocktails (Table 3.12). As demonstrated for a new pH-buffered calcium ion calibrant based on a mixed ligand calcium ion-buffered system, it may be 

Such neutral carriers (also called ionophores), which carry no net charge until complexed with a cation, have led to ISEs with a wide range of selectivities (7). Three fundamental requirements are mandatory in order to realize such potentiometric sensors (6)  

Calcium phosphate sensors are commonly based on solvents with low permittivities, e.g. DOPP (s = 6.2), whereas neutral carrier types may require highly polar solvents, typically 2-nitrophenyl octylether (2-NPOE), e = 23.6 (28). Another characteristic feature of neutral carrier systems (Table 3.12) is the presence of lipophilic anions, e.g. tetraphenylborate (KTPB) which was first incorporated to reduce the interference by lipophilic sample anions (36) and so dislodge interferents from the membrane phase. Today tetrakis(4-chlorophenyl)borate, KTpClPB, is generally preferred because its water solubility is smaller than that of KTPB by a factor of about 1000, and its 

N-tetracyclohexyl-3-oxapentane diamide (6), where the optimal sodium and potassium selectivities are observed for a concentration of about 

5 mass % KTpClPB, that is, about 50 mol % with respect to ionophore. The performance of both acyclic sensors is considered superior to the phosphate counterparts, particularly with respect to magnesium selectivity. It ipust be emphasized, however, that the excellent selectivity and low detection limits relate to evaluation using calcium-ion-buffered media with a high background of interferents (31). 

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Calcium Meat, sugar, milk, fruits, wine, seaweed Ca + neutral carrier polymer electrode... 

ORGANIC SENSOR MATERIALS FOR ION-SELECTIVE ELECTRODES Table 3.12 Some characteristics of PVC neutral carrier calcium ISEs... 

P. Anker, E. Wieland, D. Ammann, R.E. Dohner, R. Asper, and W. Simon, Neutral carrier based ion-selective electrode for the determination of total calcium in blood-serum. Anal. Chem. 53, 1970-1974... 

Ammann, D., Pretsch, E., Simon, W., Calcium ion-selective electrode based on a neutral carrier. Analytical Letters 1972, 5, 843-850. 

Many physiological anions, including protein, phosphate, citrate, lactate, sulfate, and oxalate, form complexes with calcium ions. Although these anions reduce the concentration of free calcium by complex formation, they do not directly interfere with the measurement of the calcium that is free. Protein deposits on the electrode may act as a divalent cation exchanger, resulting in positive interference with high concentrations of Mg. Older electrodes were sensitive to the concentration of protein in the sample. The newer electrodes use a dialysis membrane or neutral carrier to reduce or eliminate this protein effect. Investiga-... 

Instruments for the measurement of free magnesium in whole blood, plasma, or serum are available commercially. These instruments use ISEs with neutral carrier ionophores, including ETH 5220, ETH 7025, or a proprietary ionophore. Current ionophores or electrodes have insufficient selectivity for magnesium over calcium. Free calcium is simultaneously determined and used chemometrically with the signal from the magnesium electrode to calculate free magnesium concentrations. 

Ionized calcium and total calcium measurements are both performed with PVC-type electrodes. Membranes based on lipophilic alkyl phosphates with phosphonate plasticizers have only marginal selectivity over magnesium but have been used in the past to determine ionized calcium (i.e., free calcium) in undiluted blood samples (M8). Interference from protons at low pHs prevents such membranes from being employed for total calcium determinations on samples diluted with acid. Use of ionophore ETH 1001 (see Fig. 2) overcomes any concerns about selectivity, whether from magnesium or pH, and is now the neutral carrier system most often utilized within analyzers to detect ionized or total calcium. 

The first electrode of this type was based on the Ca-dodecylphos-phate/dioctylphenyl phosphonate system [71]. A mixture of 5% PVC in cyclohexanone and 0.1 M calcium dodecylphosphate in dioctylphenyl phosphonate was dried on the end of a platinum wire. This electrode exhibits greater selectivity for Ca-" over other divalent cations, as compared to traditional i.s.e.s, with the exception of Pb-" and. Its response relies upon the complexation of aqueous Ca by dodecylphosphate dispersed in the organic (membrane) phase. Anion-selective CWEs can be prepared in a similar manner, e.g., by the incorporation of methyltricaprylammonium salts into a polymer membrane placed on a copper wire [72]. Other mediators, including particularly neutral carriers, show promise for utilization in CWE construction. In some cases, polymethylmethacrylate or epoxy resin could be substituted for PVC with retention of response. 

The most widely used Ion selective electrode Is that which Is sensitive to the pH of a solution. This electrode is fabricated with a glass membrane which shows a selective permeation for hydronlum Ions. Glass electrodes are also available for many other cations) such as lithium) potassium) sodium) and calcium. Membranes which employ liquid Ion exchangers or neutral carriers) crystal membranes) and gas sensors) have been developed and used successfully for the determination of cations[3]. However) to be useful for in vivo applications) these electrodes must be constructed with micrometer dimensions. The liquid Ion-exchange membranes are particularly useful in this regard[4]. These electrodes are constructed from a glass pipette with a very small tip (1-2 jjm diameter)) and the liquid Ion exchanger Is placed In the tip. 

Using PVC-membrane electrodes a wide variety of alternative exchanger systems have been investigated and the results show that a solution of calcium bis[di-4-(l,1,3,3-tetramethylbutyl)phenyl phosphate] in dioctyl phenylphosphonate is preferred at present. However, calcivun ISE based upon a neutral carrier, N,N -dir(il-ethoxycarbonyl)-undecylJ-N,N -4,5-tetramethyl-3,6-dioxaoctane diamide, are gaining acceptance, particularly in the clinical field. 

Schefer U, Ammann D, Pretsch E, Oesch U, Simon W (1986) Neutral carrier based calcium(2 -i-)-selective electrode with detection limit in the sub-nanomolar range. Anal Chem 58 2282-... 

Ammann D, Giiggi M, Pretsch E, Simon W (1975) Improved calcium ion-selective electrode based on a neutral carrier. Anal Lett 8 709-720... 

Potentiometry ISEs for the determination of ionized (free) magnesium are now available. They use membranes with neutral ion carriers (e.g., modified ETH 5220), which, however, lack specificity with respect to calciiun. Therefore, magnesium and calcium must be measured simultaneously in the same analytical portion (with different electrodes) and magnesium concentration is calculated from this. 

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