Isotope response

Dearman, R.J., Caddick, H., Basketter, D.A., and Kimber, I. 2000. Divergent antibody isotope responses induced in mice by systemic exposure to proteins A comparison of ovalbumin with bovine serum albumin. Food Chem Toxicol 38 351-360. 

McDonnell J. J., Stewart M. K., and Owens I. F. (1991) Effect of catchment-scale subsurface mixing on stream isotopic response. Water Resour. Res. 27, 3065-3073. 

Culture studies of planktic foraminifera have shown that their 8 0 and 8 C values decrease as the carbonate ion concentration and/or pH of their growth medium increases (Spero et al. 1997 Lea et al. 1999 Bjima et al. 1999). The slopes of these isotopic responses were species-dependent, and were approximately three times steeper for 8 C than for 8 0 in the two planktic species studied (d(8 C)/d([CO ]) was -0.006%c/ p,mol/kg for Orbulina universa and approximately... 

Although Equations (8) and (9) fail to provide timescales for transformation of 5c and require that the oceans are well-mixed, they accurately provide the seme (positive or negative) of long-term temporal trends in 5c due to variation in other factors, and this is often the more interesting result. First-order approximations of the expected isotopic response of marine 5 C values to change in carbon input and output functions can be... 

A Relation between Reaction Mechanism and the Shape of Isotope Response Curves... 

In essence, Equation 51.2 is the model of differential reactor. This model clearly reveals the features of the isotope response caused by the reaction mechanism itself. Due to linearity of the right sides of Equation 51.2, the solution may be represented by an algebraic sum of exponents ... 

Thus, the qualitative (visual) analysis gives ground to make preliminary conclusions concerning the most probable pathways of reagent conversion to reaction products. However, the specificity of isotope response is dependent on the reaction mechanism and may be distorted by the action of gas dynamic processes in the reactor as well as by the nonideality of the input signal. 

Effect of Gas Flow Regime in the Reactor on the Isotope Response... 

Thus, in most cases, the isotope responses in plug-flow reactor are close to those calculated by the differential reactor model (Figure 51.4A,C) even in the range of rather high conversions (up to ca. 40%). As a consequence, discrimination of the various reaction mechanisms in a wide range of Tg values is possible. 

If the stabilization time of the input signal is far less compared with the characteristic time of isotope response, then the effect of nonideality of the step input can be ignored. If these times are comparable, then the effect of input function on the shape of isotope response curve should be taken into account. In some cases, where Finput(0 is approximated by a relatively simple function (e.g., exponent), the response curve can be corrected by making it closer to response to the step... 

Numerical analysis includes the solution of inverse problem within a specified reaction mechanism to estimate the concentration of intermediate species and the rates of reaction steps in order to fit the experimental isotope responses. 

The logarithmic time dependence of fraction in ethylene oxide, ln(l-ac2H4o) (see inset on Figure 51.6), is convex down, which is characteristic for the mechanism with a buffer step or with parallel ones. Note that isotope responses OCco2(0 and OCc2H4o(0 are close in the character of approaching a new steady state. Thus, slow relaxations for both reaction products are likely determined by the presence of a common buffer step. The observed isotope responses allowed the most probable mechanism to be presented as follows ... 

The readsorption of olefins is an important reaction in the Fischer-Tropsch synthesis that reverses the overall termination and increases the chain growth probability. Thus, readsorption results in heavier products. A larger time delay of the transient for the isotopic responses of olefin as compared with corresponding paraffin with increase in residence time (see Figure 51.10 for an example data on the ethane-ethene pair) is due to olefin readsorption. Transient experiments indicated that 1-olefins are the major candidates for readsorption on the catalyst surface, while the internal and iso-olefins readsorbed to a much less extent. 

These conclusions were the base for the numerical modeling of isotope responses. 

Numerical Modeling of Transient Isotope Responses On the first step, the authors analyzed in detail five possible heterogeneous methanation models based on two gas phase (CO, CH4) and three surface components (COads, Ca,ads, and Cp,ads) that follow from qualitative analysis of CO labeling data [19,21]. These models contained either a buffer step or parallel routes of methane formation. The homogeneous model having one type of methane intermediate was also considered. A methanation reaction was modeled separately from the entire set of reactions included in the Fischer-Tropsch ... 

The isotope responses obtained after H2 0 replacement with labeled H2 0 are presented in Figure 51.12. Fiberglass materials with and without platinum demonstrate nearly identical 0 responses. The simplest model of H2O transfer into the fiberglass bulk plus isotope exchange with hydroxyls (ri) was proposed ... 

Similar experiments on the replacement with in the feed gas that contained both H2O and NO were carried out (Figure 51.13). In this case, the isotope responses were obtained on catalyst with and without Pt differ substantially, and indicated that NO interacts with a platinum species, which effectively participates in isotope exchange between NO and H2O. 

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