Magnetically tailored atmospheric pressure plasmas for atomic emission spectroscopy.

Abstract

Solid powder samples deposited on Ag thin films are electrically vaporized by capacitive discharge. The aerosol produced by this method is introduced into the ICP. A low volume vaporization chamber has been designed for these experiments. A magnetic field of a few kG normal to the electric field in the thin film plasma is used to improve the plasma-sample interaction. A continuous flow of Ar or Ar/O$\sb2$ through the chamber carries the aerosol from the capacitive discharge plasma into the ICP. Analytical data will be presented for a number of NBS reference materials emphasizing biological and refractory inorganic matrices, for Mn, V and Ni. Aerosol generated by electrical vaporization for subsequent introduction into the ICP has been examined. Of the Ag ablated during vaporization, less than 10% is trapped on 0.1 $\mu$m pore size filters placed at the end of transfer tube. Much larger masses of carbonaceous material, up to 250 $\mu$g are introduced into the ICP torch, possibly accounting for intense time-dependent band and continuum emission. On examination with scanning electron microscopy, trapped particles of exploded film material show evidence of melting. Particles of Mn and VC collected after vaporization show evidence of size reductions. A magnetic field is used to alter the properties of an Ar two electrode dc arc plasma. A pair of colinear electromagnets oriented normal to the electric field in the plasma and parallel to the optical axis provides an adjustible cw or oscillatory (60 Hz) magnetic field of less than 0.5 kG. This configuration results in an E x B drift motion causing a displacement in the arc current channel. With cw fields displacements of several mm are obtained with magnetic fields of 200 G. Stronger fields blow the arc off the electrodes and extinguish it. With oscillating fields, the current channel moves up and down at the oscillation frequency. This is used to improve plasma-sample interaction. Spatially resolved radiation intensity data using both cw and oscillating magnetic fields is discussed for continuum background, Ar and W neutral atom lines. (Abstract shortened with permission of author.)