Laboratory Methods and Quality Control

Facilities and Equipment

Radiometric and AMS graphitization laboratories are located in a building specifically designed for efficient radiocarbon dating analyses. The Radiometric division presently has 26 chemical synthesis lines and more than 65 state of the art Liquid Scintillation Counters. Beta Analytic employs the benzene method of analysis, which was in part pioneered by Dr. Tamers. The AMS division presently has 63 chemical combustion and graphitization lines. The capability of large volume analysis in both the radiometric and AMS divisions was through the design of Mr. Hood. Both techniques have a practical dating range of a few hundred years to as much as 45,000 years B.R (before present, 1950 A.D.).

Conventional Radiometric Technique

Standard Service* is for samples containing at least 1.0 to 4.0 grams of final carbon (carbon remaining after all necessary pretreatments and chemical syntheses have been performed). Typical quantities needed for sample types: Sending Samples & Quantities

Complex/Non-Standard Services* are for samples containing 0.3 to 1.0 grams of final carbon or those which require extensive specialized handling, such as organic sediments weighing more than 200 grams on arrival and bones.

Quoted precision (the "+/- value" reported with the age of the sample) for optimal size samples generally ranges from 40-80 years for samples less than 10,000 years old. Less than optimal size samples will produce larger than normal associated standard deviations that will vary with sample size and antiquity.

In many cases precision may be enhanced by using the Extended Counting Service*. This service may be authorized for less than recommended or minimal size samples at any time or for any sample where enhanced precision is required.

Accelerator Mass Spectrometry (AMS)

Accuracy and precision is similar to Radiometric analysis. However, much less material is necessary for the analysis. The AMS technique is uniquely suited for very small samples containing 0.00025 to 0.3 grams of final carbon. Typical quantities needed for sample types: Sending Samples & Quantities

Quoted precision generally ranges from 0.5 to 3 percent of the sample age and is independent of sample size. Precision will be better than Radiometric analysis for samples more than 10,000 years old.

Stable Isotope Ratios (C13/12)

Measurement of the 13C/12C ratio allows for correction of the measured 14C age based on the amount of isotopic fractionation (enrichment or depletion) in the individual sample as compared to the modern standard. If best accuracy is needed, this ratio should be requested along with the radiocarbon dating. If the measurement is not made, one is assumed in the age calculation. For identified materials, this estimate can be very close to the measured value. However, it is especially important for unidentified plant materials which may contain a mixture of C3 (e.g. typical hardwood trees) and C4 (e.g. corn) pathway plants, and CAMS pathway plants (e.g. yucca), which could result in a 250 year error in accuracy without the measurement. It is important for bones since it will give insight into the purity of the protein extracted for analysis.

Sample Quantities Required for Analyses

It is important to remember that the "General Sample Size Requirements" are estimates. Normal pretreatment procedures can remove 30 to 70 percent of the original material sent. Water, adhering mineral matter and carbon losses from essential chemical pretreatment are basic factors to consider and to allow for when determining the amount of material required. As an example, typically only 25% of the weight of a submitted, clean charcoal sample is available for analysis (1 gram final carbon from 4 grams submitted charcoal). Typical quantities needed for sample types: Sending Samples & Quantities

Quality Assurance

Beta Analytic is unequalled in its commitment to the quality of the results reported to our clients and colleagues. Our "Experience Factor", with over 120,000 dates reported on samples of all types from around the world, cannot be duplicated. All accepted international conventions for radiocarbon dating are carefully followed.

The modern reference used is the NBS Oxalic Acid Carbon-14 Standard. Extensive routine inter-calibrations as well as participation in ongoing International Radiocarbon lntercalibrations such as IAEA (International Atomic Energy Agency) and TIRI (Third International Radiocarbon Intercalibrations) assures reliable results. Daily monitoring of instrumentation and chemical purity is performed in addition to extensive computer cross checks of statistical analyses and final age calculations. (*Results of international Radiocarbon Intercalibrations and QA/QC documents are available upon request.)

Potential Anomalous Results

Experimentation has determined that the rate of production of radiocarbon in the earth's atmosphere is not constant. This is due to variations in the heliomagnetic modulation of the galactic cosmic radiation. Differences of several percent have been observed in the radiocarbon concentrations of contemporary plants. Dating errors, independent of statistics or laboratory procedures, caused by these variations in the sun's magnetic field, can be as much as several hundred of years. It is generally considered that four radiocarbon dates on a specific feature is sufficient to identify the occurrence of this error.

Report of Analyses

Upon receipt of samples at the laboratory, an acknowledgement is sent notifying you of the sample's safe arrival. The acknowledgement will also inform you of the date by which your results will be reported. When the analyses have been completed, you will receive by first class mail a written report (reports are also sent by E-mail or FAX) that details the results of the material analyzed. Additionally, where applicable, you will receive a publication quality print-out of the Calibration of Radiocarbon Age to Calendar Years (dendro-calibration) for each sample.

Note: Please be sure to indicate the geographic location of any sample. Additionally, if the sample is shell or carbonate, please indicate if the origin is marine or freshwater. This information is essential in determining the necessary reservoir corrections (D R values), that will be used in the final calculations for the Calibration of Radiocarbon Age to Calendar Years.

See:   Analytical Procedures and Final Report