Ion-selective sensors applied to the analysis of blood electrolytes
Direct potentiometric techniques are becoming an invaluable analytical resource in the clinical laboratory. One important aspect of their use is determining or monitoring the activities of ions present in physiological media such as blood, urine, cerebral fluid, serum etc. This work is based, mainly, on the measurement of the blood electrolytes sodium, potassium, calcium and pH. Various automatic, multi-electrolyte, direct potentiometric, ion-selective analysers are available. The problem, to date, has been twofold - firstly different instruments give different results with the same sample and secondly, direct potentiometry senses the activity of the free (hydrated) ions - an entity unfamiliar to clinicians who have been used to flame photometric concentrations. To resolve these problems it is necessary to establish an operational pION scale by international consensus. This study was aimed at contributing towards achieving this end. The first requirement for the establishment of a pION scale is a set of universally accepted standards. Multi-electrolyte calibration standards using "Good" buffers have been proposed. Analyte binding to these buffers have been evaluated using two non-linear least squares programs SCOGS2 and SUPERQUAD . Commercial and home made ion-selective electrodes and ion-selective field effect transistors were calibrated and tested in the calibration solutions by measuring transfer potentials. A flow through rig was set up using several different ion-selective electrodes from manufacturers' instruments but with a common reference electrode and this system was tested with the aqueous calibration solutions, plasma and serum. The second requirement for the establishment of a pION scale is, resolution of the problem of residual liquid junction potentials. There are two aspects of this problem. The first aspect is the variations arising from the difference between the 'junction' potential with the calibration solution and the junction potential with plasma. The second aspect is the 'inter' instrument differences arising because of choice of ions in the salt bridge solutions and different liquid junction geometries. To minimise the first aspect, the ionic composition of the calibration solutions were formulated to reflect the ionic composition of plasma. Residual liquid junction potentials of the calibration solutions against the NBS blood phosphate buffer were determined. The second aspect has been discussed. Other aspects of standardisation such as sample pre-treatment and measurement protocol have been studied briefly. These include heparin binding and carbon-dioxide contamination. Reporting results in activities based on activity co-efficients calculated by Covington and Ferris (Pitzer and Hydration models) rather than the conventional flame photometric concentrations, has also been discussed.