C-14 Sample Selection and Prep Protocol
The protocols described here apply to dating of samples by the liquid scintillation counting method. Sample sizes and treatment steps are described for each of the types of material that are commonly dated using this method. The analysis involves three major steps:
- pretreatment (to remove contaminants)
- chemical preparation (to convert the sample's carbon to a datable form)
- and the actual counting process.
To determine how old a sample is.
Datable materials include:
The sample sizes listed below are the minimum needed to produce 3 ml of benzene equivalent to 2.41 gm of carbon), which is the minimum volume of benzene required by the counter. Samples yielding less than 3 ml of benzene can still be counted, but must be diluted first with dead benzene to bring the total volume up to 3 ml, which tends to increase the magnitude of the counting deviation (the ± figure in the date).
Please note: Except in the case of charcoal, these are minimum sizes. A larger quantity of material may be required if the carbon content of the material is unusually low.
- Charcoal: 5-6 gm of clean charcoal (actual pieces of charcoal, not counting any included dirt).
- Wood: 6-7 gm of hardwood or 10 gm of softwood.
- Peat: 15-20 gm if silt-free, or proportionally more if there is a significant silt content.
- Other organic materials: Varies depending on the proportion of carbon to other constituents.
- Shell: 20-100 gm of clean shell, depending on the degree of weathering. In the case of badly weathered shell as much as 90% of the outer part of the shell may have to be discarded.
- Other carbonates (including mortar): 20-30 gm if silt-free, or proportionally more if there is a significant silt content.
- Bone: 1 kg of clean bone, or proportionally more if the bone is badly weathered. As with shell, as much as 90% of the original sample may have to be discarded if the bone is badly weathered. It is desirable to date both the apatite and collagen fractions, if possible, although it frequently is not possible to extract enough collagen to obtain meaningful results.
- Soil: Two one-gallon ziplock bags (or the equivalent) of soil if only a total humate date is required; twice that amount if humin or humic acid dates also are to be run.
- Ground water: To date ground water, carbon suspended in the water must be precipitated out as strontium carbonate by a separate chemical process, which the Radiocarbon Lab is not equipped to perform. 60-70 gm of clean, dry precipitate are required.
Charcoal & Wood
- All adhering dirt and other foreign matter is removed by washing the sample with hot water through a USGS screen of appropriate mesh. For charcoal samples, any large chunks are broken up with a knife (preferred size is ca. 1 cm.); wood samples are cut into shavings.
- The dirt-free sample is placed in a 1-liter beaker, immersed in 2% HCl, and boiled for 10-60 minutes, depending on the size of the sample. This step removes any CaCO3 in the sample. The sample is left in the cold HCl solution to digest overnight. It is then washed repeatedly with distilled H2O until the pH is 5.5 (near neutral).
- The sample is washed into a clean beaker. 2% NaOH is added and the sample is boiled to remove possible humic acid contaminants. The sample is again left standing overnight and then is washed repeatedly with distilled H2O until the pH is 5.5 (near neutral). The NaOH is decanted, and saved if the humic acid fraction is to be dated. Otherwise, it is discarded.
- Because NaOH tends to imbibe modern and ultra-modern CO2 from the atmosphere during the removal of the humic acid fraction, to form Na2CO3, the sample is again immersed in cold 2% HCl to ensure the removal of atmospheric CO2. When the sample is again freed of HCl by washing with distilled H2O, it is dried in an oven overnight at approximately 100°C. The dried sample is then visually inspected under low magnification to remove any rootlets and other contaminating foreign matter.
Normally, no pretreatment is required except to break the sample into small pieces, and remove any obvious concentrations of silt.
However, pretreatment may be required if there is evidence of significant carbonate deposits
Soil Total Humate
- About one gallon of the sample is placed in a 5-gallon stainless steel pot. Deionized water is added, the sample is stirred well, and it is allowed to stand overnight to disaggregate.
- The following day, the contents of the pot are stirred well a second time and any floating particles are skimmed off with a 60-mesh or more appropriate size USGS screen. The stirring and skimming is repeated until no floating material remains.
- The material remaining in the pot is passed through a 230-mesh sieve into a second 5-gallon pot, stirring the original pot and adding deionized water as needed to keep fine material in suspension. Two full pots will result from this transferring and sieving. The water and sediment remaining in the bottom of the original pot is discarded.
- If any floating material is observed in the new pots, it is skimmed off with a 230-mesh or finer sieve. The sample is allowed to stand until settling has occurred, or centrifuged in several 1-liter batches to separate out the solids.
- The pots are decanted, and the sediment is flushed into 3-liter beakers using distilled water.
- Steps 1 through 5 are repeated for each gallon of the sample until all have been processed.
- A small amount (1-2 ml) of concentrated HCl is added to one of the beakers to check for carbonates. If the material is not reactive, it is acidified to a pH of about 5. If there is a reaction, HCl is added about 5 ml at a time, alternating with distilled water, until the reaction appears to have stopped. The acidified sample is allowed to stand overnight to ensure neutralization of any dolomite that may be present.
- The sample is stirred, transferred to wide-mouth, 1-liter centrifuge bottles, centrifuged, and decanted, discarding the supernatant. Distilled water is added, and the sample is agitated and then centrifuged; this is repeated several times to rinse the sediment. During this process the sample is kept at a pH of about 5 by the addition of a few drops of HCl as needed.
- The sediment is transferred into clean beakers, and allowed to stand until all solids have settled out. The supernatant is siphoned off, and the solid residue is dried in an oven at approximately 100°C.
- The solid residue is pulverized in a mill. The resulting powder is the total humate fraction, which can be dated as is, or given additional processing to separate out the humic acid and humin fractions.