There is now a consensus that Ca2+ measurements are more physiologically and clinically meaningful than CaT measurements. Ca2+ in serum, plasma, whole blood, and other biological fluids can be measured by direct potentiometry with ion-selective electrodes and a number of reliable instruments are commercially available for this measurement. Several factors affect the Ca2+ concentration and must be carefully controlled for the results to be meaningful. The most important of these considerations are the anaerobicity of the sample, the need to concurrently measure pH, and the concentration of heparin, if whole blood or plasma samples are used. The calibration of the Ca2+ ISE is critical to the accuracy of the measurement. The matrix of the calibrator should match that of the sample as closely as possible, particularly in regard to ionic strength and liquid junction potential. The measurement of Ca2+ in urine is complicated by the wide variation in ionic strength encountered in this type of sample; thus, it is more meaningful to standardize the ISE in terms of Ca2+ activity instead of concentration. Instrumentally, the measure of copper in biological samples can be achieved with high accuracy, high precision, without background correction, and with minimum sample pretreatment if care is taken to carefully plan and implement all the critical steps in the analysis procedure.(ABSTRACT TRUNCATED AT 250 WORDS)