Isotope Metrology     [ IRMS ... AMS ... LLC ]

Isotope metrology is the total system of isotope measurement. This system is the framework having a foundation of primary standards coupled to isotope measurement techniques through mechanisms that provide explicit measurement traceability and quality assurance. Our group is focused mainly on the isotope metrology of carbon, since carbon is one of the most ubiquitus and chemically diverse elements of relevance to our quality of life by way of industrial applications and environmental impacts.

There are three isotopes of importance in carbon: carbon-12, which is the predominate stable isotope and present at about 99%; carbon-13, the minor stable isotope present at about 1%; and carbon-14, a radioactive isotope naturally produced by cosmic ray interactions in the upper atmosphere. Carbon-14 is present in the biosphere at about twelve orders of magnitude below that of carbon-12.

The natural variation in the isotopic abundance of carbon is small with distributions dependent on a wide variety of natural processes and anthropogenic influences. If measured accurately and precisely, the carbon isotope signature in a sample can be used to help identify the source of its carbonaceous constituents, which has applications in industry and environmental protection. Isotopes are normally defined as conservative tracers, since the isotope signatures of elements in source materials are typically retained in derived materials despite chemical processing.

Carbon-13 abundance in nature varies by about 0.1% (absolute C-13/C-12). These variations, mainly a result of natural enzymatic bioprocessing (such as photosynthesis or bacterial diagenesis), are measurable by several techniques, and most precisely by gas isotope ratio mass spectrometry (IRMS), where the carbon is converted to and measured as carbon dioxide. With the proper metrological controls, IRMS measurements can resolve differences below 0.0001% (absolute C-13/C-12), which enables the discrimination of many chemically identical samples resulting from different industrial processes or environmental routes.

Carbon-14 is a definitive tracer for discriminating fossil (anthropogenic) sources from non-fossil (biogenic) sources. The half-life of carbon-14, 5730 years, is short enough to assure its complete decay and absence in fossil petroleum reserves so that carbon coming from this source, such as in refined petrochemicals and derived products, retains this characteristic.

Quantifying the low abundance of carbon-14 in samples is accomplished through either of two techniques: low-level counting (LLC), or accelerator mass spectrometry (AMS). With either technique, the quality of the results is highly dependent on the proper use of standard materials and data evaluation/reporting procedures.