Isotopes change

Figure 7.9 shows a schematic representation of this effect, in which the ratio of the two isotopes changes with time. To obtain an accurate estimate of the ratio of ion abundances, it is better if the relative ion yields decrease linearly (Figure 7.9) which can be achieved by adjusting the filament temperature continuously to obtain the desired linear response. An almost constant response for the isotope ratio can be obtained by slow evaporation of the sample, viz., by keeping the filament temperature as low as is consistent with sufficient sensitivity of detection (Figure 7.9). 

Schematic illustrations of the effect of temperature and surface density (time) on the ratio of two isotopes, (a) shows that, generally, there is a fractionation of the two isotopes as time and temperature change the ratio of the two isotopes changes throughout the experiment and makes difficult an assessment of their precise ratio in the original sample, (b) illustrates the effect of gradually changing the temperature of the filament to keep the ratio of ion yields linear, which simplifies the task of estimating the ratio in the original sample. The best method is one in which the rate of evaporation is low enough that the ratio of the isotopes is virtually constant this ratio then relates exactly to the ratio in the original sample.

In the last decades, several ice cores over 1,000 m depth have been recovered from Greenland and Antarctica. In these cores, seasonal variations are generally observed only for the uppermost portions. After a certain depth, which depends on accumulation rates, seasonal variations disappear completely and isotopic changes reflect long-term climatic variations. No matter how thin a sample one cuts from the ice core, its isotope composition will represent a mean value of several years of snow deposition. 

Fig. 3.36 Generalized scheme of hydrogen isotope changes in plants (White 1989)...

Because isotopic changes caused by chemical processes are so small, the isotopic data are typically reported as deviations in parts per thousand from a standard composition. For example, the shift in the 13C/12C ratio relative to a standard is given in delta notation by ... 

One means to assess the relative contributions of the various sources of sulfur to the atmosphere is through the use of sulfur isotope ratios (12-16). Isotopic ratios may be used as source tracers if 1) the isotopic composition of the sources as they enter the atmosphere are known, 2) the isotopic compositions of the various sources are different from one another, and 3) the isotopic changes that occur during biological, physical and/or chemical transformations are understood. Presently, isotopic data for sulfur compounds in the remote atmosphere (Table I) are limited. However, collection and analytical techniques are now available to make isotopic measurements of the critical species. In the text that follows, various aspects of sulfur isotope chemistry will be discussed. 

The present chapter is devoted exclusively to an analysis of the problems of isotopically mixed solvents. It will not concern itself, except in passing, with the measurement and interpretation of solvent effects on equilibrium and rate constants due to the isotopic change from pure H20 to pure D20. The aim is to show to what extent measurements of this type are of practical utility, especially as a tool in the investigation of reaction mechanisms. For this reason, the development of theory is mainly directed towards compromise solutions of a complex problem, i.e. solutions which enable the theory to be tested and applied but lay no claim to being theoretically unassailable. The guiding principle has been to cast the formulation in terms of parameters or types of measurement which are either known or at least known to be feasible. 

As might be anticipated from the mineralogical trends observed in Bermudian limestones, these rocks also exhibit bulk chemical and isotopic changes with increasing geologic age (Figure 7.28). The Mg, Sr, K, total Fe, Na and Cl... 

Figure 9.1 Freshwater-marine mixing ratios of dissolved inorganic carbon (DIC) and isotopic composition (DI13C) across three different salinity gradients. Bottom isotopic change between — 10%e at the freshwater end-member, and +2%c at the marine end-member, both end-member values are based on concentration-weighted averages (data sources Spiker and Schemel, 1979 Spiker, 1980). (Modified from Fry, 2002.)...

An isotopic change in the E-group that is transferred in the product-forming step will result in an overall isotope effect on the observed rate of reaction, the magnitude of which increases as the partitioning ratio decreases. Thus, a reaction mechanism of this type is characterized by an isotope effect that increases with increasing temperature. 

When Srinivasan and Noyes116 photooxidized diethyl ketone in the presence of 98% oxygen-18, they found the quantum yield of diethyl ketone-18 was 0.7 and, as in the case of the acetone-oxygen-18 photooxidation, no isotopic change was detected in the oxygen composition. 

Figure 34.3 Nitrogen isotopic fractionation processes. The vectors approximate the isotopic changes associated with the indicated addition or removal of nitrogen from an initial nitrate pool with a of 5%o (black filled circle). Note that the fractionating processes (water column deni-...

---