Plasma Oxidation

Plasma is a fourth state of matter, one in which gaseous atoms become either completely or partially ionized such that the plasma is overall electrically neutral yet is capable of conducting electrical current. Neon lights are an example of a plasma gas discharge contained within a glass tube and excited by an applied current, while lightning and auroras are naturally occurring plasmas. Diatomic oxygen gas, when partially ionized by radio frequency (RF) energy, produces atomic oxygen that reacts readily with organic matter. Oxygen and argon plasmas have been used routinely for many years in the semiconductor industry to etch surfaces and remove traces of organic carbon adhering to silicon components.

In the 1980s, NASA researchers developed an atomic oxygen chamber to simulate the conditions that spacecraft would experience in the upper atmosphere in order to study the effects of such reactions on delicate equipment; this approach has been used to remove organic coatings from artists’ paintings.

The Rowe group at Texas A&M University developed plasma oxidation (or plasma-chemical oxidation, PCO) to selectively react excited oxygen atoms with organic carbon from a presumed binder/vehicle in rock painting samples. The process occurs at low temperature and pressure, and leaves the inorganic carbonates and oxalates that may be present intact. The resulting carbon dioxide is then dated using accelerator mass spectrometry. This was applied to samples of Pecos River rock paintings as a way to date the images directly.

Charcoal and other organic materials can also be prepared for AMS radiocarbon dating in this way, though PCO is generally reserved for small samples that would be difficult or impossible to date otherwise.