Noise Characterization of the Microwave Plasma Torch (MPT) Source

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Abstract
Noise characteristics of emission signals from the Ar microwave plasma torch (MPT) discharge have been analyzed. Noise spectra are reported with the use of a nebulizer sample introduction system for desolvated and undesolvated aerosols, for varying analyte concentrations, and for background emission from argon and water vapor. The analytical system appears to be flicker-noise limited at frequencies below about 1 Hz. White noise was found to be dominant below 100 Hz, but discrete noise peaks were detected in the frequency region extending to 300 Hz. The noise frequency characteristics are independent of the plasma viewing position, analyte concentration, and sample introduction method but strongly dependent on the argon carrier flow rate and on the introduction of water vapor. The discrete-frequency peaks presumably have a fluid-dynamics origin, caused by flow fluctuations as argon exits the top of the torch. Noise from the measurement system and from external sources was not significant compared with the noise level observed from the plasma.