Ratios of Stable Isotopes

Lead has four stable isotopes, Pb, Pb, Pb and 2 Pb. The stability of Pb has been debated, but the half-life is 10 y and, with respect to the age of the earth and the universe, Pb can be considered to be stable. In the course of the genesis of the elements certain amounts of the above-mentioned four stable isotopes of Pb have been formed (primordial Pb). Additional amounts of Pb, Pb and Pb (radiogenic Pb) have been produced by the decay of and Th, respectively. 

On average, the relative percentages of the Pb isotopes in the earth s crust are 2 Pb 1.4%, 206pb 24.1%, 207pb 22.1%, ospb 52.4%. These ratios vary 

However, measurement of the ratios of the Pb isotopes alone (Pb/Pb method) offers the possibilities of dating without knowledge of the contents of U and Th and of taking into account possible losses of U due to oxidation of to TJOj and leaching of the latter, by simultaneous application of two or even three of the chronometers U/ Pb, 2 U/ Pb, Th/ Pb. The basis for the Pb/Pb method is given by eqs. (16.6), (16.7) and (16.8), knowledge of the ratio as a func- 

The Ar/ °Ar method is a variant on the °K/ °Ar method. For deteimination of the amount of K present in the sample, neutron activation is applied. The sample and a standard of known age (i.e. containing a known ratio °Ar °K) are irradiated under the same conditions at a neutron flux density of about 10 cm s for about 1 day. Ar is produced by the nuclear reaction K(n,p) Ar and the ratio 39Ar 40Ar is measured by MS. Because the half-life of Ar is rather long (h /2 = 269 y), its decay after the end of irradiation can be neglected. From the relation 

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The ratios of stable isotopes (red dots) fall within a narrow range, referred to as the "belt of stability." For light isotopes of small atomic number the stable ratio is 1 1. For heavier isotopes the ratio gradually increases to about 1.5 1. Isotopes outside the band of stability are unstable and radioactive. There are no stable isotopes for elements of atomic number greater than 83 (Bi). 

Unlike the photoplate, the Faraday detector (or Faraday cup) is still very much in use today. The main reasons for its lasting popularity are accuracy, reliability, and mgged construction. The simplest form of Faraday detector is a metal (conductive) cup that collects charged particles and is electrically connected to an instrument that measures the produced current (Fig. 2.21b). Faraday cups are not particularly sensitive and the signal produced must in most applications be significantly amplified. An important application for Faraday detectors is precise measurements of ratios of stable isotopes [278]. See, for example, Section 2.2.7 and Chapter 11 for examples of applications and methods in which Faraday detectors are utilized. 

Ratio of stable isotopes may be measured in a mass spectrometer it is advisable to use relatively simple molecules (for example from degradation of one under study) so that the parent peaks are isolated from other possible ones. For example COj and C02 have parent masses at 44 and 45 and the only nearby peaks are those from small amounts of O and 0 in the oxygen. A complex organic molecule will normally yield many smaller peaks immediately around the parent due to normal deuterium, C and 0 and determination of isotopic enrichments is only possible in uncomplicated mass spectra. Results good to 0.1-0.2% are easily obtained but simultaneous collection of ions from the two peaks of interest using a dual collector instrument gives precision to 0.01%. 

Recent patterns of Pb deposition can be monitored through sediment analysis as well. Sample ages within the past 100-200 years can be determined by Pb dating (ty2 = 22.26 years) (31). The source of the lead can be determined either by ratios of stable isotopes as described above (33), or by comparing the level of °Pb to the levels of and " Pb, with which it should be in secular equilibrium (419). The excess °Pb is assumed to be due to atmospheric... 

Isotopic composition of carbon C is usually measured by the ratio of stable isotopes and i C. As the standard is used the PDB standard... 

Stable isotope analysis (SIA) method uses the stable isotope ratios of various elements as biological tracers. Human and animal body tissues are composed of elements that are ingested through food or water. The isotope ratios of stable isotopes of some of these elements (e.g., C, N, S, O, H, Sr, and Pb) from food and water are preserved in bone and teeth. Therefore, we can measure the isotope ratios of those elements in bone and teeth and then, by comparing those values to known isotope values of possible foods and water sources it is possible to determine aspects of past diets (palaeodiet) as well as the location where that human or animal consumed the food and water (migration). 

The ratios of stable isotopes in biological materials provide a valuable tool for the analyst in detecting adulteration. 

Stable isotope ratio (SIR) The ratio of stable isotopes ratios such as that are determined by mass spec-... 

The first approach is based on the early work of Dubach et al. [38], who used radiotracers to determine iron bioavailability. In healthy individuals it takes about 14 days until freshly absorbed iron is incorporated into reticulocytes, that is, freshly matured red blood cells. The amount ratio of stable isotopic label to natural iron, that is, the tracer to tracee ratio, can be determined using IDMS concepts in a... 

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