Microplasmas: An enabling technology for chemical analysis in the field

Abstract

ABSTRACT: The field of microplasma research has certainly had a large impact in the plasma television sector. After a quick overview of plasma displays, however, some new applications for microplasmas as sensors will be discussed in detail. Plasmas are simply gases to which energy has been added until some of the atoms and molecules have been stripped of an electron. These free electrons cause the plasma to become an electrical conductor, so the plasma can be thought of as a gas where electromagnetic properties dominate the behavior. One ubiquitous example of plasma is the common fluorescent light. The fluorescent tube is filled with argon gas and mercury. As power is applied to the tube through two electrodes, the gas will transition to the plasma state. The characteristic glow of the fluorescent light is caused by excited states of the argon and mercury atoms. In analytical chemistry, plasmas are used to determine the composition of unknown substances. A sample of the substance is injected into a plasma, and the wavelength and intensity of the light emitted by the plasma allows one to determine the type and concentration of each chemical element. Typically, chemical analysis systems are very large, expensive, and costly. Therefore, chemical analysis has been relegated to the laboratory. The goal of this microplasma research is to miniaturize the chemical analysis system such that low cost, battery-operated sensors can be deployed in the field. We have reduced the power requirements from 2kW to as little as 250 mW. In addition the weight and cost have been decreased by three orders of magnitude. Examples of a commercial product made under a license agreement will be presented. The basic technology used to generate a robust microplasma for chemical analysis is a cell phone power amplifier chip (f=900 or 1800 MHz) and a microstrip transmission line circuit. The microstrip line concentrates the output power of the cell phone power amp into an area of approximately 25 ƒÝm by 50 ƒÝm. This method allows us to produce power densities of 107 W/cm3 and electric field intensities exceeding 107 V/m. A circuit model of the microstrip line device and microplasma demonstrates that the electric field intensity exists almost entirely within the microplasma, rather than at the boundary of the microplasma. We will show that the microplasma efficiency is greatly enhanced by semi-ballistic electron heating due to the high electric field that is impressed within the microplasma. Finally, a novel application of microplasma for the trapping and classification of nanoparticles will be discussed.