An Atmospheric Pressure Plasma Setup to Investigate the Reactive Species Formation

The overall goal of this procedure is to demonstrate an atmospheric pressure plasma setup that excludes air from the reaction environment. Our set-up, helps answer key questions in the field of bio-medical and thermal plasma such as the origin of reactive species that define the efficacy of plasma treatments of biological samples. The main advantage of this set-up is that it excludes the uncontrolled ambient environment.

The atmosphere inside the reactor consists only of the feed-gas components and the evaporated sample. The significance of this technique extends to our clinical applications of cold plasma because it provides fundamental understanding of physical chemical processes that take place when low-temperature plasma interacts with liquids. This method not only provides an insight in the effect of the bio-medical plasma, it can also be used for other purposes.

For instance studying chemical reactions in a controlled environment. Demonstrating the procedure will be Philip Groves, a post-graduate student from our laboratory. First, design and build a Faraday cage from wire-mesh to shield the equipment.

The cage must be large enough that the live electrode, the ground electrode, and the electrode cables will not contact each other or the wire-mesh when all equipment is in place. Plasma jets operate at the kilohertz frequencies are extremely susceptible to external fields. The Faraday cage, creates a stable, electro-magnetic environment, eliminating the influence of the surrounding equipments and plasma.

And protecting the operating equipment from plasma generated electro-magnetic fields. Ground the cage in all-metal supports inside the cage using a device appropriate for the electrical socket type. Affix a warning placard to the cage.

Connect a helium gas cylinder to two mass flow controllers or MFC's by a T-connector. Direct the output of one MFC to a Drexel flask. Join the output of the Drexel flask to the output of the other MFC with a T-connector.

Equip the cage with a stainless-steel gas-line. Direct the output of the second T-connector to the cage. To supply the feed-gas to the plasma jet insert the end of a quartz tube into the plastic tubing connected to the gas line.

Secure the quartz tube with a clamp stand. Set up an interlock to stop electrical connection to the plasma power-supply when the cage door is open to avoid the risk of electric shock from the high-voltage electrode during plasma operation. To make the high-voltage electrode connect the plasma power-supply to an electrode and a voltage probe.

Make a ground electrode wired via circular current probe. Attach the ground electrode to the plasma jet nozzle about 40 millimeters above the mouth of the nozzle. Then, attach the high-voltage electrode to the plasma jet nozzle 20 millimeters below the ground electrode.

Connect the voltage and current probes to an oscilloscope. Place the voltage and frequency controls outside the cage. Prepare a glass reactor containing a hollow, perforated glass stand with a reservoir placed on top.

Affix the upper-part of the reactor to a metal stand using an insulated clamp. Fit the top of the glass reactor with a rubber grommet. Insert the nozzle into the top of the reactor.

Connect to the reactor exhaust, on the lower part of the reactor, to the appropriate exhaust system with plastic tubing. Then attach the tubing to the extraction. Fill the Drexel flask with O16 water.

Set the dry gas MFC to 1, 800 standard cubic centimeters-per-minute and the other MFC to 200 standard cubic centimeters-per-minute and flush the system to achieve a 10%saturation of the feed-gas with O16 water-vapor. Then, turn off the gas system. Place a 100-millimeter solution of DMPO in O17 water in the reactor reservoir.

Then set up the rest of the reactor on a lab jack. Raise the lower section of the reactor, adjusting the nozzle as needed, until the reactor is sealed at the ground glass joint and the nozzle is about 10-milliliters above the surface of the sample in the reservoir. The connection between the two parts of the reactor and between the grommet and the quartz tube must be air-tight to exclude the diffusion of ambient air into the reactor.

Flush the reactor with the 10%O16 water saturated feed-gas for 30 seconds or as long as needed to replace the volume of air in the reactor with feed-gas. Flushing time strongly depends on the reactor size. With discharge parameters set to 18 kilovolts and 25 kilohertz, ignite the plasma.

Expose the DMPO solution to the plasma effluent for 60 seconds. Then, turn off the power supply and remove the sample. Prepare the sample and analyze it by electron paramagnetic resonance spectroscopy.

The DMPO solution in O17 water was exposed to plasma with and without O16 water-vapor in the feed-gas. With dry helium, the atmosphere inside the reactor contained only O16 and O17 water-vapor from the sample. Wet helium was the additional source of O16 water.

EPR analysis of the sample exposed to plasma with vapor-added feed-gas showed the presence of DMPO16OH, DMPO16OH and DMPOH adducts. Comparison with EPR intensities of samples with known Nitroxide concentration provided the absolute concentrations of the hydroxyl-radical adducts. Without water-vapor in the feed-gas the absolute concentrations were approximately the same.

With water-vapor, the concentrations of O16 hydroxyl-radical adduct was more than twice that of the O17. Mass spectrometry following a hydrolysis reaction gave the isotopic distribution of Oxygen in the water in the liquid sample. The relative amount of O16 water was only slightly higher in the sample exposed to plasma with vapor suggesting that the DMPO16OH species originates in the gas phase.

The presented procedure employs a simple and inexpensive plasma set-up that requires no modification to the actual plasma jet. Thus, being an instructive alternative to gas-shielding devices for plasma studies. While attempting the procedure it's important to ensure complete absence of ambient air, which contains impurities and random amounts of water-vapor from the reactor.

This technique paved the way for the resurgence in the fields of bio-medical, low-temperature plasma, to explore the reactive regions of the plasma systems.