[en] Noise power spectra of the ⁸⁵Rb⁺ signal and the ⁹³Nb⁺ signal from an inductively coupled plasma-mass spectrometer were compared to the noise power spectrum of Sr 2 emission from the plasma. Discrete frequency noise in the emission at the mass spectrometer sampling orifice was found to be nearly identical to that in the mass spectrometer signal. However, discrete frequency noise in emission from the plasma alone differed substantially in frequency from that in the mass spectrometer signal. The dependence of noise frequencies on plasma operating conditions was generally the same for the mass spectrometric measurements and both emission measurements. These results indicate that the plasma was the source of discrete frequency noise in the mass spectrometric signal. The major source of signal instability in this particular ICP-mass spectrometer was found to be 1/f noise. A new, home-made ICP-mass spectrometer was used to study the influence of Na, K and U on ion signals from Y, Co, and As. In general, analyte signals were suppressed in the presence of excess matrix element. The extent of suppression was greatest for the element with high ionization energy and least for the element with the smallest ionization energy. These trends in the interference effect could be altered by applying a positive voltage to the first element of the ion optics. When the skimmer orifice diameter was made smaller than the sampling orifice diameter, analyte signal suppression was uniform and more extensive. Under these conditions, the heavy matrix element induced more suppression than did the light matrix element