How Does Plasma Oxidation Work?

How Does Plasma Oxidation Work?

Step 1:

Place the sample into the glass chamber

The Shumla plasma oxidation system.
Intern Assyl Arykbayeva removing the glass chamber, so that a sample can be loaded for oxidation.

Steps 2 and 3:

Fill chamber with argon or oxygen (depending on the process). Apply electricity to the gas inside the chamber. It is called a low-temperature plasma because it is an ionized gas…much like a neon sign that glows.
Here you can see the two copper electrodes capacitively coupled on the outside of the glass sample chamber. Samples are loaded into this glass portion of the instrument. Depending upon our process, argon or oxygen is filled into the chamber at 1 torr of pressure and electricity in the form of RF power is applied to the gas to create a glow discharge. We are using electrically excited oxygen to convert organic material in the paint sample to carbon dioxide, which we collect in the glass tubes by freezing with liquid nitrogen. This glass tube is what we send to CAMS for radiocarbon measurement.

Step 4:

Electrically excited oxygen is used to oxidize or convert organic material in the paint sample to carbon dioxide (CO2) and water.
It’s called a low-temperature plasma or glow discharge because it is an ionized gas…much like a neon sign that glows.

Step 5:

The glass tube is immersed in liquid nitrogen in order to freeze the gas COto dry ice.
Shumla intern, Marie Desrochers, filling a dewar with liquid nitrogen. Organic material in paint samples in converted to carbon dioxide and water, which is frozen in a glass tube immersed in liquid nitrogen.
Liquid nitrogen is cool!

Step 6:

Seal off the tube of CO2 using a blow torch.
Karen using an oxygen/propane torch to seal a glass a carbon dioxide sample for AMS radiocarbon dating.
Karen using an oxygen/propane torch to seal a glass a carbon dioxide sample for AMS radiocarbon dating.

Step 7:

Send glass tube of CO2 off for radiocarbon dating at an Accelerator Mass Spectrometry (AMS) laboratory.
Accelerator mass spectrometry schematic for radiocarbon measurement (courtesy of LLNL).

The Plasma Oxidation Advantage

One beneficial aspect of plasma oxidation over other sample preparation techniques is that only the organic carbon is extracted, leaving intact the inorganic portion of the paint sample because the system remains below the decomposition temperature of these minerals. This allows us to avoid the extensive acid treatments that traditional sample preparation and combustion use. And, we are able to analyze much smaller samples with high mineral content, which is ideal for dating paint that is still adhering to rock! Once the carbon dioxide is extracted, it is sent for radiocarbon dating using AMS. I have collaborated with Lawrence Livermore National Laboratory’s Center for Accelerator Mass Spectrometry (CAMS) for over 20 years.

This sample was collected from a pictograph panel and was radiocarbon dated after plasma oxidation. The results of this assay are reported in Bates et al. (2015)

Ongoing Shumla Radiocarbon Research

Since the 1990s, the focus of rock art dating in the Lower Pecos has been on dating Pecos River Style paintings, with 33 radiocarbon assays from 9 different sites. The radiocarbon dates for the Pecos River Style range from approximately 2700 B.C. to A.D. 600 (Bates et al. 2015). With this data set, the Lower Pecos Canyonlands is considered one of the best-dated rock art provinces in the world. However, most of these samples were collected in the 1990s, and in many ways this early work was experimental — Marvin and his students were demonstrating that plasma oxidation could be utilized to directly date rock paintings. But, because the Lower Pecos Canyonlands was the first place that plasma oxidation was used to radiocarbon date rock paintings, and the focus was on developing the technique, there was very little focus on answering archaeological research questions. For most of the dated samples, we know nothing more about the sample than the radiocarbon date itself. In other words, we have no information on the rock figures which were dated. This is a crucial piece of missing information because one of the research projects I and other Shumla researchers are most interested in pursuing is determining the chronological distribution of specific rock art images and motifs across the Lower Pecos landscape.


The Shumla Chemistry Lab Part 2: Plasma Oxidation and Eagle Cave Pictographs

Stay tuned for Part 2 of the Shumla Chemistry Lab blog which will detail how we used plasma oxidation to date the rock art at Eagle Cave, and how we plan to use radiocarbon dating alongside The Alexandria Project to enhance our knowledge of Lower Pecos chronology.

Read more about radiocarbon dating and plasma oxidation in part 2 of the Shumla Chemistry Lab blog!

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