The major thrusts of this work have been: 1) To develop a high-efficiency low-power TM-010 microwave cavity for nitrogen support gas at atmospheric pressure, 2) To discover and physically characterize potential laser and emission spectroscopic applications of this atom source, with a particular emphasis on laser-induced fluorescence.

The result is the most efficient microwave-induced plasma cavity for nitrogen at one atmosphere that exists to date, giving stable and analytically useful molecular plasmas with only 50 Watts applied power. It is called the “High-Efficiency Molecular Microwave Plasma" (HEMMP) cavity. The HEMMP possesses excellent vaporization and atomization properties. It can handle aqueous sample flows of around 1 mL/min, introduced as an aerosol from a nebulizer. A detection system and sampling system were designed and an analytical instrument was built around the HEMMP cavity. Details of construction, operating conditions and operation of the instrument are described.

Applications investigated include laser-induced fluorescence (LIF), atomic emission spectroscopy (AES), and laser-enhanced ionization (LEI) [also known as the opto-galvanic effect (OGE)]. The major emphasis of the application work has been physical characterization of the low-power nitrogen plasma as an atom source for LIF.

This is the first time that either laser-induced fluorescence or laser-enhanced ionization have been observed and extensively characterized in any microwave-induced plasma (MIP). This is also the first time that atomic emission has been studied in a low-power N₂-MIP. LIF, AES, and LEI signal intensities were studied as a function of applied microwave power, support gas flow rate, signal observation height, and support gas composition using nitrogen and argon mixtures. Results for LIF yielded detection limits in the very low parts per billion range, and for AES in the low parts per billion range. Limit of detection (LOD) and background noise studies were done for all 3 techniques. Signal intensities were measured as a function of laser light intensity for LIF and LEI. Laser saturation was not observed with 300 mW power from the CW dye laser. The effects of electrode geometry and applied electrode voltage on LEI signals were also studied. Extensive background spectral studies were done for the nitrogen plasma.

Analytical feasibility has been demonstrated for AES, LIF, and LEI in the low-power nitrogen MIP. The results presented provide the background physical investigations required for a full-scale development of these techniques for chemical analysis.