The Detection of Isotopes

By the late 1940s methods of ionization counting had been developed for the estimation of 14C. Thin layers of suitably prepared material could be counted in windowless counters, with the emissions being detected by the probe in a flow of inert gas. Otherwise 14C-containing material could be combusted to 14C02 and this drawn over the probe. Neither method was very convenient to use and the instruments were costly. Early experiments with 14C were mainly performed in the U.S. (see below). 

Autoradiography offered an alternative technique. The earliest experiments followed the uptake of radioiodine into the thyroid by placing the tissue sections in direct contact with photographic plates (Hamilton, Soley, and Eichom, 1940 Leblond, 1943). Belanger and Leblond introduced the use of liquid emulsion in 1946. Initially this was painted onto sections mounted on microscope slides. Later, slides were dipped into liquid emulsion (Joftes and Warren, 1955) or wrapped around with stripping film (Doniach and Pelc, 1950). Semiquantitative comparisons 

By 1930 experiences of the earliest workers with radioisotopes, especially a emitters, had offered dramatic evidence of their dangers. It 

Rates of reaction of urease with deuterated and especially tritiated urea were markedly reduced compared with the rate with the unlabeled ( H) substrate, but usually isotope effects are insignificant biochemically except in rigorous kinetic studies. 

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And so it happened that the detection of isotopic carbon from UC02 in the [i carboxyl group of a-ketoglutarate was celebrated as a discovery of CO 2 fixation in animal tissues, and was taken as evidence that there wasn t any citric acid in the citric acid cycle. A citric acid precursor of a-ketoglutarate would have to be labelled as in compound 1... 

Detailed accounts of the history of mass spectrometry and the detection of isotopes are given by Ewald and Hintenberger,70 de Bievre,10 Kienitz,71 Brunnee,72,73 Duckworth et al.,74 de Laeter1 and Budzikiewicz and Grigsby.75... 

The work of Brown and Russell is just one example of the way in which we can gain insight into a chemical reaction by using isotopically labeled compounds. We shall encounter many other examples in which isotopes, used cither as tracers, as in this case, or for the detection of isotope effects (Sec. 11.15), give us information about reaction mechanisms that we could not get in any other way. 

A sensitive method for the detection of isotopes depends on spectroscopic measurements. In the case of hydrogen and deuterium (or D), with very... 

Mass spectrometry is particularly useftil in the detection of isotopes. Due to the relatively high percentages of the heavy isotopes, the halogens chlorine and bromine are readily identified by the pattern of peaks in the molecular ion region of their mass spectra (Fig. 10.3). 

NMR isotope measured. For example, for the detection of phosphorus by NMR in a sample containing 3 wt.% phosphorus, approximately 10 mg of sample are required. By contrast, the corresponding detection limit for Si in a similar situation is 22 dmes higher, due to the much lower natural abundance (4.7%) of the Si isotope. 

Because the beam monitor allows accurate measurement of the total number of ions that are analyzed, a graded series of exposures (i.e., with varying numbers of ions impinging on the plate) is collected, resulting in the detection of a wide range of concentrations, from matrix elements to trace levels of impurities. In Figure 2, the values of the individual exposures have been replaced with the concentration range that can be expected for a mono-isotopic species just visible on that exposure. In this example, exposures from a known Pt sample have been added to determine the response curve of the emulsion. 

This technique is based upon the detection of corrosion products, in the form of dissolved metal ions, in the process stream. A thin layer of radioactive material is created on the process side of an item of plant. As corrosion occurs, radioactive isotopes of the elements in the construction material of the plant pass into the process stream and are detected. The rate of metal loss is quantified and local rates of corrosion are inferred. This monitoring technique is not yet in widespread use but it has been proven in several industries. 

Another important method for the detection of early life forms involves the ratio of the two carbon isotopes 12C and 13C. The lighter isotope 12C ( normal carbon ) is incorporated preferentially into biomolecules, so that the value of the ratio of 13C to 12C can indicate the presence of material which was formed in living things. It must of course be certain that other, non-biological processes can be excluded, though it is questionable whether this is always possible, in particular when we are unsure of the geological processes which occurred 3 1 billion years ago. 

Farkas and Farkas (32) examined the kinetics of the exchange and hydrogenation of benzene catalyzed by platinized platinum foil at room temperature. The occurrence of isotope exchange was detected by the thermal conductivity technique. They reported (i) that the exchange reaction was only a little faster than hydrogenation and (ii) that exchange... 

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