Spatially resolved measurements and plasma tomography with respect to the rotational temperatures for a microwave plasma torch

Abstract

To study the analyte evaporation and desolvation capacity of the microwave plasma torch (MPT), as a new source for atomic spectrometry, the radially-resolved rotational temperatures as a good approximation for the gas-kinetic temperatures were determined. An atmospheric pressure microwave discharge in argon at a frequency of 2.45 GHz at a power of 100 W and a gas flow rate of 0.6 l min^–1 was studied. The procedure makes use of the rotational fine structure of the (A²Σ⁺→X²Π_i) OH band at 306.4 nm and the temperatures were obtained from the slope of a Boltzmann plot. To obtain spatially-resolved intensities and for simultaneous detection of the different rotational lines, a charge coupled device (CCD) combinend with a Czerny–Turner monochromator was used. The image of the axially-symmetric plasma was rotated with the aid of a three-mirror arrangement by 90° and imaged onto the entrance slit. Radially-resolved intensities were calculated by means of an Abel inversion and measurements at different observation heights allowed complete tomography of the plasma. For the Abel inversion and temperature determination, an Interactive Data Language (IDL) program was developed, which computes the results in a short time and allows the presentation of the results as colour contour-plots. A mean temperature of about 3600 K with an error below 10% was found under the conditions mentioned above. Also the influence of power and water-loading of the carrier gas was investigated. Both were found to affect the temperature distribution but no significant changes in the mean temperature could be observed in the range 70–170 W and at a water-loading of between 0.6 and 9.0 mg min^–1 of argon.