Sample submission

Sample type and size
Packing and sending of sample
Pre-treatment procedures
Conversion of sample carbon into graphite
Hot samples

 

Sample type and size for AMS 14C

A wide variety of organic samples (e.g. charcoal, seeds, leaves, wood, and sediments), carbonates, and bones can be dated. Identifiable samples (macrofossils) with high carbon contents are preferred over sediments and soils. For high precision we recommend at least 1 mg of carbon after chemical preparation (see: Pre-treatment procedures). Approximate sample sizes required for the AMS 14C analysis of different materials are given in the table below. Please provide the carbon content of organic materials if available.

 

Sample type Recommended sample size
Charcoal ≥ 20 mg
Wood ≥ 20 mg
Plant remains  ≥10 mg
Peat ≥ 50 mg*
Organic sediment, soil 200 to >1000 mg**
Fabrics, paper 20 mg
Carbonate (Mussel, Foraminifera) ≥ 10 mg
Bones 200 - 2000 mg**
Seawater 100 ml
Prepared graphite 1-2 mg

Please contact us before sending these samples.
* Sample size of dry peat; more material is required if the mineral content is high or the peat is decomposed.
** Sample size depending on the organic content.

 

The precision of radiocarbon dates for recent samples (younger than 2000 years) of “normal” sample size (1-2 mg of carbon) is better than  0.5% (typically 0.3 - 0.4%) which equals +/-40 years (25 - 30 years) for the 1-σ statistical uncertainty of the measured age. The precision decreases with increasing sample age.

Analysis of samples containing less than 1 mg of carbon is possible but requires special considerations. Please contact us if you want to analyze microgram samples or individual organic compounds and if you have questions regarding sample amount, sampling or packing.

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Packing and sending of samples

  • Samples should be handled in a dust-free environment, if possible wearing overall, to avoid contamination (see figure A).
  • The sample material should be packed into labelled polythene bags (see figure B). For packing of small samples sizes please use labelled plastic or glass bottles. Please do not use aluminium foil for moist samples as it may corrode.
  • Wet samples should be stored dark and cooled.
  • Dried samples (drying at <60°C) are preferred to avoid growth of bacteria, fungi or algae.
  • Please give a detailed description of each sample in the data sheet. When submitting larger sets of samples, please fill in one data sheet per site and a list of the individual samples.

Please email the data sheet and table to: ahamann-wilke@leibniz.uni-kiel.de

A: Contamination of a sediment sample with fibers B: Example of recommended packing, e.g. of bone fragments (left)
or sediment (right)

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Pre-treatment procedures

At the start of sample processing, each organic sample and most carbonate samples are inspected under a light microscope where recognizable organic fragments (charcoal, wood, seeds, leaves, pollen, etc.) are selected for dating and contaminants such as rootlets, clothing fiber or hair are removed. Special features of the sample are recorded via a digital microscope camera in the sample database.

Following the optical inspection, the material to be dated is "cleaned" chemically to remove contaminating carbon. The pre-treatment procedures, used for different sample types, are explained below.

Alkali-Acid-Alkali extraction of organic samples (e.g. charcoal, wood, sediments)
Our chemical pre-treatment of organic samples is based on the familiar acid-alkali-acid (AAA) cleaning with diluted hydrochloric acid and sodium hydroxide, which is commonly used in radiocarbon laboratories. This procedure is based on the premise that a likely way to introduce contaminants into sample material in the soil is via percolating water or groundwater. The AAA extraction often produces two fractions: (i) the alkali residue (humin), free of the mobile contaminants, which (often) will yield a reliable sample age upon radiocarbon dating and (ii) the humic acid fraction, which may contain contaminating, soluble organic components.

In addition to the alkali residue, the humic acid fraction of a sample can be dated to evaluate the degree of sample contamination and thus the reliability of the radiocarbon age. For sediments with carbon contents below 1% C, we recommend dating of both fractions.

Hydrolysis of carbonates
Carbonate samples are cleaned with hydrochloric peroxide in an ultrasonic bath. They are converted to CO2 by acidification of CaCO3 with 100% phosphoric acid at 90°C in an evacuated, flame sealed quartz tube.

Removal of conservation substances from art and archaeological samples
Art and archaeological objects are often conserved with non-polar chemicals, and thus potentially contaminated with respect to their 14C content. Such samples are treated with a sequence of five solvents with increasing polarity, which dissolve most common conservation chemicals, with the help of a computer-controlled "Soxhlet"-type extractor. This device uses a continuous procedure of boiling and condensation of different solvents for extraction and vacuum filtration under constant process conditions.

Collagen extraction from bones
Bones and bone-like materials like antler and ivory are gently demineralised with acid followed by a selective extraction and recovery of the collagen by freeze drying of a gelatin solution. The procedure yields two fractions: (i) the collagen, the fraction preferred for dating, and (ii) the acid- and water-insoluble organic bone material. In certain cases both fractions are dated. Again, the size of the age discrepancy between the two dates provides an indication of the reliability of the (collagen) sample age.

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Conversion of sample carbon into graphite


Multi port reduction system for graphitization of sample CO2

Both inorganic and organic materials are converted into CO2: (i) carbonates by reaction with phosphoric acid and (ii) organic materials via combustion in evacuated, flame sealed quartz tubes containing copper oxide for oxygen supply. The CO2 is then reduced to graphite with H2 on an iron catalyst in a modular reduction system with 28 units on three lines as shown in the figure on the left. The resulting graphite-iron powder is pressed into aluminium target holders for the ion sputter source with the help of a pneumatic press.

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Hot samples

Hot samples are samples with 14C concentrations above natural levels. Based on our experience, samples can be easily contaminated by radiocarbon used in tracer studies, e.g. for primary productivity. As tracer concentrations are typically a millionfold natural, minute quantities suffice to seriously contaminate samples with natural radiocarbon levels. Those quantities may remain for years in equipment, e.g. drying ovens or freezers, and rooms where tracer samples have been processed. Please avoid any contact of your samples for AMS 14C dating with such areas or equipment.

Since the introduction of hot samples in our laboratory may mean time out for cleaning and/or replacement of the apparatus used for sample pre-treatment and may result in the loss of possibly irreplaceable samples of other customers, we ask you to get information on the laboratory you preparing your samples in.

If you are not sure about a possible 14C contamination of your lab, we can provide help. Repeated submission of contaminated samples from one customer or substantial replacement of our equipment will be charged.