Low Energy Plasma Radiocarbon Sampling (LEPRS) Laboratory

Lab Directors: Shelby Jones and Marvin Rowe (OAS Research Associate)

Radiocarbon dating is an incredibly marvelous archaeological dating tool developed in the 1950s. It has been refined ever since, both in its techniques and how archeologists interpret results. It uses the decay of a naturally produced radioactive isotope, radiocarbon (¹⁴C), and all living organisms incorporate ¹⁴C while they are alive – and almost without exception at almost the same level of ¹⁴C/¹²C. As soon as an organism dies, no longer breathing in air, the ratio ¹⁴C to non-radioactive ¹²C changes with a slow decay of the radiocarbon. Half of the ¹⁴C atoms decay and disappear from a body in 5,730 years. That’s perfect for archeology because that means half is gone in 5,730 years, another half is gone in 11,460 years, etc., and we’ve got a very nice measure of time scale going back easily to about 15,000 years, and further with larger uncertainty. And <15,000 years is the era American archaeologists are generally most interested.

The plasma laboratory is housed in the Office of Archaeological Studies Analytical Laboratory at the new Center for New Mexico Archaeology in Santa Fe. The aim of the laboratory is to produce and extract carbon dioxide gas (CO₂) using a plasma oxidation technique developed by chemistry Professor Marvin W. Rowe while working at Texas A&M University in College Station, Texas. The idea was germinated by a Texas A&M University archaeology professor asking Rowe’s group to radiocarbon date a rock painting sample. Having recently come across a chemically reducing hydrogen plasma technique for restoring metal artifacts, Dr. Rowe thought an analogous technique could be used to oxidize organic carbon remaining from vehicle/binder materials in the rock art using oxygen plasmas. After several incarnations of plasma machines, the most elaborate system developed so far is located at the Center for New Mexico Archaeology facilities.

Advantages of Low Energy Plasma ¹⁴C Sampling (LEPRS) Laboratory

At the Office of Archaeological Studies, Center for New Mexico Archaeology, Museum of New Mexico, Department of Cultural Affairs, Santa Fe, NM, we built and use an advanced, low energy plasma sampling apparatus to permit extraction of ¹⁴C for radiocarbon dating. The principle advantages of the plasma technique for ¹⁴C sampling are:

(1) It can extract very small samples for dating, as little as 30-100 millionths of a gram of carbon, sufficient for a radiocarbon date at the ETH-Zürich accelerator mass spectrometry laboratory with whom we collaborate.

(2) There is no necessity to remove carbonates or oxalates from artifacts, alleviating the need for stringent strong acid treatments at elevated temperatures that conventional ¹⁴C dating techniques are forced to use.

(3) The sampling occurs on the surface of the artifact using plasmas, i.e., slightly ionized and energized gases, at vacuum pressures approximately 1/1000th of atmospheric pressure, subjected to radio frequency power. That allows virtually nondestructive sampling for most artifacts.

(4) The technique removes surface layers of carbon so that it is possible to remove carbon in a stepwise fashion soot layers in shelters, yielding chronological information about the times of deposition of the various fires.

(5) Masking is possible to isolate carbon-bearing material from the rest of a sample using alumina powder.

(6) It has the ability to collect multiple ¹⁴C dates from a single, small sample. Tow samples are routinely taken, but 65 have been taken from a single soot sample.

Experimental Procedure

Radio frequency induced plasmas produce excited atomic and ionic species that are useful.

(1) We start with oxygen plasmas in an empty chamber to remove any organic contamination that may be in the chamber before the sample is introduced. This and the following step are routinely done with four chambers operating simultaneously.

Four chambers with the blue glow of Ar plasmas being operated simultaneously.

(2) After the chamber is deemed to be clean (negligible CO₂ released), we then insert the sample into the chamber and run chemically inert argon plasmas to energetically knock off, through elastic and inelastic collisions, any atmospheric CO₂ adsorbed on the sample and chamber surfaces; the similarity of the atomic/molecular masses of argon (40) and CO₂ (44) make the elastic collisions relatively efficient.

(3) Then we run oxygen plasmas to oxidize the organic material present in the sample to CO₂.

(4) The CO₂ collected is frozen with liquid nitrogen into a 4 mm glass tubing that is then sealed to send for radiocarbon analysis at the ETH-Zürich accelerator mass spectrometry laboratory. The technique is potentially applicable to any artifact that contains organic material that relates to the archaeological date of the material.

Significant improvements to previous plasma systems have been instituted here, not only in the system itself, but also in experimental procedures followed. Multiple (up to five), simultaneously operated chambers have been demonstrated for extracting CO₂ for dating (a substantial increase in operational efficiency). Dual internal argon and oxygen storage chambers were added for quick refilling of chambers. Masking procedures are currently being tested to isolate specific carbon-bearing material from the rest of a sample using aluminum foil or aluminum oxide. The addition of a residual gas analyzer has greatly increased our capacity for understanding the complicated gas reactions occurring during the plasma reactions. LEPRS permits virtually nondestructive radiocarbon dating of organic archaeological artifacts. We have used the current system dating rock paintings and many other archaeological artifacts.

Five chambers with the blue glow of Ar plasmas being operated simultaneously. The nearest chamber is one of our two large chambers for non-routine samples that are larger than normal.

The plasma oxidation technique is a “nondestructive” method for collection of ¹⁴C material for dating. Traditional pretreatments for radiocarbon dating rely on washes with a strong acid, followed by strong base washes and finally another strong acid treatment, usually at 50°C. This pretreatment destroys a portion of the material submitted for analysis (1/3 to 2/3 of the sample). Contrarily, we utilize a more non-destructive process of pretreatment rinsing the artifact in a pH8 buffer to remove humic acids.

In traditional ¹⁴C dating, the CO₂ is finally extracted by total combustion of the artifact sample to convert organic material, resulting in total destruction of a sample. That CO₂ is routinely converted into graphite for measurement at a radiocarbon laboratory. In our technique, we rely on direct radiocarbon analysis of the CO₂ gas directly.

Our ability to use lower temperatures allows fragile objects to be analyzed. Our current system can operate plasmas at near human body temperature and still collect enough material for dating purposes. Since the plasma only interacts with the surface of the material being oxidized, only a microscopically thin layer of material is “destroyed”, i.e., converted to CO₂. So although technically it can never be 100% non-destructive, the visible effects are not easily discernible even under microscopy. This may be thought of as an analogy to exfoliating the dead skin cells off of one’s arm with a loofah.

The amount of material needed for processing is also less than traditional radiocarbon dating due to no significant loss of material in our pretreatment or plasma processing, and because the requirement of very little amount of material being extracted from the sample. The accelerator mass spectrometry laboratory we collaborate with for the ¹⁴C analysis measures only 30-100 millionths of a gram of CO₂. Any organic material can be used in the plasma process, with previously processed materials ranging from eggshells to peyote to rock art to a mummified infant burial set to ceramics to soot.

Lab Directors: Shelby Jones and Marvin Rowe (OAS Research Associate)

Further reading: