A Six-Electrode, Direct-Current, Variable-Length Plasma Source for Atomic Emission Spectroscopy

Abstract

A variable-length plasma source has been developed for analytical atomic emission spectroscopy. The length of the plasma, adjustable during plasma operation, provides a means of adjusting the sample residence time in the plasma. The plasma operates from three compact, simple, and inexpensive direct-current power supplies. Three concentric quartz tubes supply argon and sample aerosol to the plasma. The modular design allows the quartz tubing to be easily replaced and a variety of electrode distances to be established. The argon consumption, typically 7.4 L/min, is comparable to, or less than, that for commercial DCP and ICP systems. Many of the vertical spatial characteristics of the plasma described here are similar to those documented for the ICP. Changes in the nebulizer gas flow rate produced spatial shifts in the maximum of the vertical, spatial analyte emission profiles of Mg. Increases in the signal-to-background ratios at common analytical wavelengths were observed with increasing plasma length. The movement of the region of maximum emission to positions below the top of the outer quartz tube, which occurs as a result of increasing the plasma length, is thought to be the main reason for the worse detection limits. The addition of 10% nitrogen to the outer argon gas flow caused a 35% enhancement in the Ca ion and a 28% decrease in the Ca atom signal-to-background ratios.