An evaluation of the NOVA I ion selective electrode analyser for sodium and potassium determination

Introduction In the NOVA sodium and potassium analyser*, ion-selective electrodes measure sodium and potassium by direct potentiometry in separated plasma or serum or in the plasma phase of whole blood and in urine after dilution. Sodium is measured using a glass electrode and potassium using a valinomycin electrode. The sodium, potassium and reference electrodes all have a straight tube design and these are connected to a sampling probe, peristaltic pump and reagent pack via tubing and a pinch valve system. Samples can be aspirated from sample cups, capillary tubes or syringes. The sample volume required is 300 tl for blood, plasma or serum, or 100 ktl for urine. The NOVA is controlled by a microprocessor which initiates a two point calibration of the electrodes at 2 hourly intervals. A single point calibration with an aqueous standard is automatically carried out with the analysis of each specimen and the test result is displayed 58 seconds after the start of the sample aspiration cycle. The microprocessor also monitors the performance of the system and if a system fault is detected this is identified and a fault status code displayed. Urine diluent, standards, reference solution and a waste container, are all supplied in a disposable reagent pack within the instrument. The main operating controls consist of two push buttons labelled ’calibrate’ and ’analyze’, and auxiliary controls are also present. These comprise of a series of push buttons for other functions such as purging the system using a fast pump speed, testing the system display, displaying output from electrodes in millivolts and setting up the instrument for analysis in either ’blood’ or ’urine’ mode. The measurement of sodium and potassium in blood, serum or plasma is made on an undiluted sample, whereas urine samples are automatically diluted before analysis with magnesium acetate solution. After the recommended daily calibration of the instrument, a sample can be analysed with the push of a single control button and results are displayed in approximately one minute.