A previously unrecognised source of noise in inductively coupled plasma (ICP) atomic emission spectrometry was identified with a combination of high-speed motion picture photography and noise spectrum analysis. The noise mechanism is a fluid mechanics phenomenon and involves axisymmetric oscillations of the plasma as the plasma gases flow from the torch into the surrounding static atmosphere. The oscillations develop into vortex rings with increasing height above the torch. As these plasma oscillations pass through the optical axis of the measurement system they produce periodic variations in the analytical signal, typically in the 100-600 Hz range. The frequencies of the oscillations observed in the films agreed with the frequencies of the -major noise peaks i n the noise power spectra. Knowledge of this noise phenomenon is relevant to studies of the fundamental properties of the ICP and its applications.

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