Magnetically tailored direct current plasma sources for atomic emission spectroscopy.

Abstract

The most important problem associated with direct current plasma excitation sources is aerosol sample rejection by the plasma due to the presence of intense thermal gradients. This problem limits the efficiency of the direct current plasma as a useful tool for trace elemental analysis. A novel direct current plasma called a magnetron rotating direct-current plasma (DCP) which eliminates this problem has been investigated. The magnetron rotating DCP uses a concentric electrode configuration and a magnetic field parallel to the electrode axis to obtain rotation of the arc current channel at a frequency in the 1-3 kHz range. The anode is a hollow graphite cylinder, and the cathode is a thoriated tungsten wire. A magnetic field of several hundred gauss is obtained from a ring-shaped ceramic ring magnet. For a 5-mm anode diameter, the plasma is diffuse, and the current channel has the form of a thin radial sheet which undergoes relatively small periodic perturbations at the rotational frequency. Sample vapor from a graphite furnace is introduced into the plasma by passing the vapor through the anode tube. This ensures adequate sample-plasma interaction and results in detection limits generally in the parts per billion (ppb) range. Solid powder samples deposited in the graphite furnace can also be vaporized and introduced in the magnetron rotating DCP. Analytical data for a number of biological reference materials showed percent error values for the determination of various metallic elements ranged from 1.2 to 19.6%. A hybrid arc-furnace system using a magnetically-rotated, concentric-electrode arc connected to a pyrolytic graphite tube furnace was also developed. Rotation around a horizontal axis allows the system to quickly convert between a sample loading position and a plasma discharge position. Detection limits for various metallic elements are in the ppb and sub-ppb range.