Neutral carrier based ion-selective electrode for the determination of total calcium in blood serum

Abstract

A simple potentiometric determination of the total calcium concentration in human blood serum using a liquid membrane electrode is described. The bound calcium Is displaced from its complexes through acidification of the serum sample with an acetate buffer to pH 3.5. A novel neutral carrier based Ca2+-seiective electrode showing a much higher rejection of H+ ions in comparison to previously described carrier membranes as well as classical ion-exchanger membranes permits the Ca2+ determination at such a low pH value. A correlation of the total calcium concentrations determined by ion-selective electrodes with those obtained by atomic absorption spectrometry gives a residual standard deviation of f 0.04 mM over the 1.48-3.00 mM CaZ+ range.Through the introduction of Ca2+-selective electrodes especially by Ross (1, 2) the determination of ionized Ca2+ in blood serum became possible. Today improved electrodes based on organic phosphates ( 3 , 4 ) and on neutral carriers (5, 6 ) are used in several routine instruments (7-10). Although the activity of the ionized Ca2+ is claimed to be physiologically more relevant (11)(12)(13), the determination of the total calcium concentration remains a useful diagnostic parameter (14)(15)(16). For the assessment of total calcium, e.g., the sum of the concentrations of the ionized Ca2+ and the calcium bound to proteins and various other complexing agents, atomic absorption spectrometry (AAS) and colorimetry predominate in daily use (see ref 13 and 14). AAS is rather complex for clinical environments and colorimetry necessitates an extended and involved sample treatment (1 7-21). Several components interfere with the colorimetric detection (13, 14, 20, 21). Potentiometric sensors would offer an attractive alternative if it became possible to displace the bound calcium by a simple procedure. It has been shown that calcium can be displaced by hydrogen ions (13,14,(22)(23)(24)(25). This requires ion-selective electrodes with sufficient preference of Ca2+ over H 3 0 ' .Neutral carrier based Ca2+-selective electrodes are unique in this respect (5). Here we report on the use of a newly developed membrane for the measurement of total Ca2+ in diluted, acidified human blood serum. EXPERIMENTAL SECTIONMacroelectrode Systems. Cells of the type Hg,Hg&&;KCl (satd.)l3 M KCllsample solution(lmembrane)lO.OOl M CaC1,; AgC1,Ag were used for macroelectrodes. The external macroreference electrode was a double junction calomel electrode Philips R 44/2 SD-1. Two different membranes were examined, one consisted of 1.0 wt % ligand ETH 1001, 0.7 wt % potassium tetrakisb-chloropheny1)borate (KTpClPB), 65.3 wt % o-nitrophenyl n-octyl ether (o-NPOE), and 33.0 wt % poly(viny1 chloride) (PVC) and the other of 3.4 wt % ligand ETH 1001, 2.0 wt % potassium tetrakisb-chlorophenyl)borate, 62.9 wt % bis(2-ethylhexyl) sebacate (DOS), and 31.7 w t % poly(viny1 chloride). Both membranes were prepared by dissolving the four membrane 'Permanent address: Medico-chemical Central Laboratory, University Hospital, CH-8091 Zurich...

Sensitivity and Working Range of Backside Calibration Potentiometry

“…This equation is written in general, both for the ions with the same or different charges. Here, we only consider the case with divalent primary ions and only one type of monovalent interfering ions so that eq 3 reduces to (4) For small levels of ion-exchange with interfering ions (less than about 10%), a less complex mixed ion response equation can be used, 18 and equation 4 is simplified to: (5) Eq. 5 is rewritten for the front side of the membrane: (6) And the backside of the membrane as: (7) At steady state, the ion concentrations are usually different in each aqueous phase boundary layer at both sides of the membrane, where c ifb , c ibb represent the bulk solution concentrations of I 2+ at the front and back side of the membrane, respectively (Figure 1).…”

Charge-Transfer Triiodide Ion-Selective Electrode Based on 7,16-Dibenzyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane

“…Since the nature of a plasticizer influences the dielectric constant of the membrane phase, the mobility of the ionophore and the state of the ligands, [38][39][40] it is expected to play an important role in determining the ion-selective characteristics. As can be seen from Table 1, among the different plasticizers examined, DBP resulted in the best sensitivity and the widest linear range.…”

A New PVC-Membrane Electrode Based on a Thia-Substituted Macrocyclic Diamide for Selective Potentiometric Determination of Silver Ion

“…It is reported that the response characteristics of ion-selective electrodes are largely affected by the nature of the plasticizer used. [41][42][43][44] This is due to the influence of the plasticizer on the dielectric constant of the membrane phase, the mobility of the ionophore molecules and the state of the ligands. 10 Thus, in the present work we examined DBS, DDP, BA and NPOE as plasticizer and the resulting emf-pAg plots are shown in Figure 1.…”