Carbon historical measurements

The radiocarbon age from Equation (16.2) is expressed in years BP, i.e. Before Present, where Present, by convention, is 1950 AD. Actually, 1950 is considered the reference year in radiocarbon dating, especially due to historical reasons. Following the same convention, in Equation (16.2), when measuring 14/f(t), its value is usually normalized to the concentration of a standard material in 1950 the normalized value is expressed in units of pMC (per cent of Modem Carbon), the standard material being 100 pMC. 

In this way, it is possible to reach an extremely high selective sensitivity down to 1 part in 1015, which in 14C dating corresponds to being able to date samples about 50 000 years old. Moreover, modern systems can measure isotopic ratios in modern carbon, both C/ C and C/ C, with an ultimate precision as good as 2%o and l%o, respectively. The former value corresponds to determining the conventional radiocarbon age with an absolute error, smaller than in the past, better than 20 years, while the l%o precision for the 13C/12C allows an adequate correction for isotopic fractionation effects. Even in routine measurements, at least in the case of historical samples, a precision of 5%o in the 14C/12C measured value is standard, corresponding to an uncertainty in the radiocarbon age of 40 years.[27]... 

Most historical studies do not contain the detailed information needed to develop carbon budgets. They are also confounded by erosional losses, changes in the chemical methods to measure SOC, management-induced differences in bulk density, and different methods to calculate turnover kinetics. The consequences of these problems are that it is difficult to compare studies and calculate carbon turnover rates. To overcome these problems simplifying assumptions are often used (Clay et al. 2006 Johnson et al. 2006 Bolinder et al. 2007). Assumptions can reduce the usefulness of the findings. This chapter reviews non-isotopic and 13C isotopic approaches for determining SOC maintenance and implications of simplifying assumptions on SOC turnover calculations. 

Infrared (IR) spectroscopy offers many unique advantages for measurements within an industrial environment, whether they are for environmental or for production-based applications. Historically, the technique has been used for a broad range of applications ranging from the composition of gas and/or liquid mixtures to the analysis of trace components for gas purity or environmental analysis. The instrumentation used ranges in complexity from simple filter-based photometers to optomechanically complicated devices, such as Fourier transform infrared (FTIR) spectrometers. Simple nondispersive infrared (NDIR) insttuments are in common use for measurements that feature well-defined methods of analysis, such as the analysis of combustion gases for carbon oxides and hydrocarbons. For more complex measurements it is normally necessary to obtain a greater amount of spectral information, and so either Ml-spectrum or multiple wavelength analyzers are required. 

Air pollution control agencies in the Los Angeles area have accumulated 22 years of consecutive hourly samples of "particulate matter measured in Km units." It was shown that the historical Km data should be proportional to elemental carbon concentrations present, and the Km samplers were calibrated to read elemental... 

These delta values (read delta C-13 , or delta carbon-13 ) are typically given in parts per thousand ( permil , %o). For historical reasons, carbon stable isotopes are reported relative to the PDB (Pee Dee Belemnite, a fossil) or the equivalent VPDB (Vienna PDB) standard. Oxygen and hydrogen stable isotopes are reported relative to SMOW (Standard Mean Ocean Water) or the equivalent VSMOW (Vienna SMOW). The VPDB and VSMOW standards are preferred, not only because the original standards, PDB and SMOW, are no longer available, but also because they imply that the measurements have been calibrated according to international conventions. 

A historically important form of gravimetric analysis was combustion analysis, used to determine the carbon and hydrogen content of organic compounds burned in excess 02 (Figure 27-4). Instead of weighing combustion products, modem instruments use thermal conductivity, infrared absorption, or coulometiy (with electrochemically generated reagents) to measure the products. 

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