Calibration graphs have been obtained for mercury, zinc and copper ions using buffered (pH 7.4) stream water and centrifuged stream water samples. Two hypothesis explaining the shape of the obtained calibration graphs have been suggested; one refers the shape to the adsorption of-trace elements on the suspended particles and on the container surfaces, the other hypotheses refers the shape of calibration graphs to the presence of chemical interference. The strength of the two hypotheses has been compared. The effect of time on the shape of the calibration graphs has also been investigated.