14C, measurement

However, the most significant development in recent years has been represented by the trend towards smaller facilities based on smaller accelerators. The work done at ETH in Zurich on the study of stripping yields, molecular dissociation and detection efficiency at low energies has led to the development of systems based on low voltage tandem machines they have demonstrated that 14C measurement is also possible using 500 kV or even 200 kV accelerators.[61,62] The main advantage in the use of dedicated 14C smaller accelerators is the reduced space requirements [63] for example, while the 3 MV VERA facility covers an area of 192 m2, the compact 500 kV AMS system at the Poznan Radiocarbon Laboratory in Poland covers an area of 30 m2 and the 200 kV MICADAS system in Zurich only about 7 m2. 

There would be, however, a considerable discrepancy between the model-calculated dilution factor and the dilution factor required if the biospheric C02 input were of comparable size as the fossil C02 input as stated by biologists [43]. For a discussion of this question see also Oeschger et al., [39]. Thirdly, we calculate the 14C dilution corresponding to the CO increase. In 1950, before the nuclear weapon tests, the integrated C02 production amounted to about 10 percent of the preindustrial atmospheric C02 content. If there had been no exchange with other reservoirs, a decrease of the 14C/C ratio by 10 percent would have resulted. Tree-ring 14C measurements indicated, however, a decrease by only about 2 percent. Again we calculate the system dilution. In a first approximation 4 and s are set equal to one and we obtain... 

Several laboratories therefore intend to measure profiles of these isotopes on ice cores drilled in polar ice caps. The new technique also makes possible measurements of the 14C in the C02 occluded in about 30 kg of ice. From the 14C measurements again twofold information is expected ... 

The second serious problem of a high transmission instrument relates to the presence of molecular fragments. These molecules, which originate in the source, have nearly the same mass as the wanted ions. For 14C measurements, the mass difference between this atom and the molecule 12CH2 is approximately 0.1 percent. The resolution needed to separate these components is about 6,000, because there may be 109 12CH2 molecules for each 14C atom. 

A spectrum that was derived during 14C measurement from the Rochester multiparameter heavy ion detector is shown in figure... 

All of the above particulate investigations were based on mini-radiocarbon measurement techniques, with sample masses typically in the range of 5-10 mg-carbon. This constituted a major advantage, because it was practicable to select special samples (given region, source impact, sediment depth) and to further subject such samples to physical (size) or chemical separation before 14C measurement. This type of "serial selectivity" provides maximum information content about the samples and in fact it is essential when information is sought for the sources or atmospheric distributions of pure chemical species, such as methane or elemental carbon. 

Amino acid composition data and stable isotope ratios are being evaluated as sources of information to indicate the presence of non-indigenous organics in bone samples intended for radiocarbon analyses. The study is being conducted in the context of the planned 14C measurement of Pleistocene bone samples by high energy mass spectrometric methods. 

The measurement is always relative, that is, in 14C measurements the amount of 14C is always related to that of 13C or 12C. The more abundant isotope is detected by a Faraday detector while the less abundant isotope is detected by a specialized device... 

SAQ8.19 To return to the example of the Turin Shroud. Suppose a sloppy technique caused the precision of the 14C measurement to decrease from 92.23 per cent to 90 2 percent. Calculate the range of ages for the shroud. [Hint perform two calculations, one for either of the extreme values of percentage.]... 

Ann McNichol is a Research Specialist at the National Ocean Sciences Accelerator Mass Spectrometry Facility at the Woods Hole Oceanographic Institution, which produces high-precision 14C measurements from small-volume samples. Dr. McNichol s research interests include the study and use of carbon, nitrogen, and oxygen isotope techniques to quantify bio-geochemical processes, the study of the fate of organic matter (both natu-... 

In contrast to the atmosphere, where much can be learned from a relatively small number of 14C sampling locations, the sea presents a formidable sampling problem. The sea does not achieve the nice vertical and zonal uniformity characteristic of the troposphere. The well-mixed surface layer of the sea, generally 50-100 meters deep but occasionally deeper, appears to achieve a uniform concentration but only vertically the levels of excess 14C in the mixed layer varies considerably from place to place. This arises partly from the upwelling of subsurface water of lower 14C content which perturbs the concentration of surface water in many areas of the ocean, adding to the difficulty of interpreting sea water 14C measurements. 

Despite the difficulty of interpreting 14C measurements on surface ocean water such measurements are of great interest. The net transport of excess 14C from the atmosphere to the sea depends on the difference between the 14C concentration in atmospheric C02 and that in the carbonate system at the sea surface. The decline in the atmospheric reservoir of excess 14C is therefore controlled by the 14C concentration at the sea surface. This in turn depends upon diffusion and advection into the deep sea. As the levels of excess 14C in the troposphere and the mixed layer of the sea begin to approach each other, mixing from the mixed layer of the sea into the deep sea will be the factor controlling the levels of excess 14C in the atmosphere. 

Measurements of 14C have rarely been used as tracers to study sources of individual DOC fractions. Aiken et al. (1996) used 14C measurements of fulvic acid in a permanently ice-covered closed basin lake in Antarctica to investigate sources of DOC in the lake. Sampling along a depth profile in the lake showed that the fraction of modern carbon decreased with depth and confirmed that the high concentrations of fulvic acid in the bottom waters represented an old source, with an age of around 3000 years, which was diffusing into the upper water column (Fig. 3). 

Harkness, D. D., Harrison, A. F., and Bacon, P. J. (1991). The potential of bomb-14C measurements for estimating soil organic matter turnover. In Advances in Soil Organic Matter Research The Impact on Agriculture and the Environment, Wilson W. S., ed., Proceedings of a Symposium, Colchester, 3-4 September, 1990, pp. 240-251. 

Despite its importance in ecosystem C fluxes, soil respiration has limitations as a constraint on SOM turnover, for two main reasons. First, it is difficult to partition soil respiration into its two sources (1) decomposition of SOM by microbes (heterotrophic respiration) and (2) respiration from live plant roots (autotrophic respiration) (Kuzyakov, 2006). As a result, an increase in soil respiration may indicate not only an increase in SOM decomposition but also an increase in root respiration. Second, it is likely that in most soils only a small fraction of total SOM contributes to heterotrophic respiration. As a result, respiration measurements provide information about the dynamic fraction of SOM (particularly when combined with 14C measurements of respiration) but do not provide information about the large, stable pools unless they are destabilized and contribute to respiration (detectable with 14C02 respiration measurements). Attributing the sources of respiration from different SOM reservoirs, which may respond differently to climatic variables, is not... 

Note that this approach assumes Fl4C = 1.0 and is constant prior to 1950. Actually the 14C/12C of atmospheric C02 did vary with time prior to 1900, mostly reflecting changes in the rate of 14C production in the upper atmosphere. During the Holocene, these variations were less than 10%, and they are documented in the calibration data sets based on 14C measured in known-age wood. Between 1900 and 1950, Eatm declined due to the addition of 14C-free C02 derived from fossil fuels, known as the Suess effect. Modeling of turnover times should use the actual atmospheric 14C inputs to photosynthesis, although it is not as important before 1959 as after. 

The average relative abundances of isotopes in the Earth s crust, oceans, and atmosphere, commonly expressed as stable isotope ratios, are shown in table 7.5. Small differences in the ratios of a particular element in natural samples can be detected using mass spectrometry, however, it cannot be achieved with high precision and accuracy (Nier, 1947). The solution to this problem, as explained earlier for 14C measurements, is measuring isotope ratios in a sample concurrently with the standard this does allow for adequate precision and accuracy. The equation used to describe this relative difference or del (<5) value is as follows ... 

Trumbore, S., Vogel, J.S., and Southon, J. (1989) AMS 14C measurements of fractionated soil organic matter an approach to deciphering the soil carbon cycle. Radiocarbon 31, 644-654. 

Fig. 4.17 Tritium and 14C measurements during a pumping test conducted in a confined aquifer at the Aravaipa Valley, Arizona (Adar, 1984). Recent water of high tritium and 14C concentrations intruded from the overlying phreatic aquifer.

Groundwater samples that are recent in terms of the half-life of 36C1 (i.e., 36C1 decay being negligible). Samples that comply with this criterion are best selected with the aid of 14C measurements. They should be lower than pre-bomb concentrations in ground-... 

The number of laboratories that measure noble gases in water samples is slowly but steadily increasing. The know-how is actually available in most laboratories that do potassium argon dating of rocks. Because of increased demand from field hydrochemists, more laboratories are willing to enter the domain of noble gas hydrology. Eventually, noble gas measurements will become as routine as <5D, <5lsO, tritium, or 14C measurements. 

Since its introduction approximately 40 years ago, radiocarbon dating has assumed extraordinary importance in archaeology because of the widespread occurrence of carbon in archaeological contexts. 14C measurement techniques have been refined to the point where dates can now be determined for milligram-scale samples and with materials up to 80,000 years old. 

Sets of three samples each would be distributed to each of seven laboratories for 14C measurement. They were to be coded by number. 

As of October 21, 1988, the 14C measurements have been made, and the results sent in to the British Museum Research Laboratory. It has been announced that the date obtained falls within the 14th century. The agreement of dates from the three laboratories is an impressive demonstration of the new 14C dating technology. 

AMS does not determine the absolute concentration of isotopes rather an isotope ratio of a rare isotope to an abundant stable isotope of the same element is measured. There is no commercially available AMS instrument that measures all isotopes, such that most instruments are built to detect one or two elements only. Highly specialized, complex, and time-consuming sample preparation is often required prior to AMS analysis. AMS instruments for various radioisotope analyses have been reviewed in a recent publication [14], Here we would only discuss AMS instrument for 14C analysis, the most important radioisotope for biomedical applications. For 14C measurement, an AMS instrument usually consists of an ion source to generate negative ions, a low magnetic field for initial ion separation, a high-voltage... 

Summary This American test method establishes the procedures, equipment, materials, and condihons to measure the carbon content of the plastic sample through radiocarbon analysis. This test method is applicable to carbon-based plastic products that can undergo combushon in the presence of oxygen to produce carbon dioxide gas (CO2). This test method can be useful for companies and US federal agencies to promote the use of biobased products in commercial, non-food products. The test method directly discriminates between 14C/12C content of plastic samples made from contemporary carbon and those plastic samples made from petroleum sources that do not have 14C. Measurement of a plastic product s 14C/12C content is determined relative to the modem carbon-based oxahc acid radiocarbon Standard Reference Material 4990c. 

---