Heterogenous, problems

Small sample size (<100 mg for on-column) heterogeneity problems... 

ID-ICP-MS [425,426], Certified values for the 4 PE/Cd materials (VDA-001 to -004) with nominal Cd concentrations of 0.35 to 3.6mmolkg 1 were established by IDMS, using an enriched mCd spike [427]. The results did not show significant differences between bottles at the 300-mg level. For the sample with the highest Cd content (VDA-004 407 ppm) the measurement reproducibility was worse, which stands in relation to heterogeneity problems, as confirmed by GF-ZAAS results. 

As before, it is appropriate to divide the discussion of the outer boundary condition into two cases, one for survival probability (the heterogeneous problem) and one for reaction of a reactant in a vast (homogeneous) excess of the other reactant. 

In this section we develop rate expressions for the mechanistic schemes presented in Section 5.1. The goal is to determine the rate in terms of measurable quantities, such as the concentrations of the reactants and products, and the temperature. Heterogeneous problems are considered in Section 5.6. The approach taken for homogeneous and heterogeneous reactions is similar, but additional constraints are placed on heterogeneous problems so the synthesis reaction example is presented after discussing these constraints. 

In this section we study the smooth heterogeneous problem... 

Disadvantages Heterogeneity problem can be large results are very sensitive to surface preparation method used no standard can be added identical standards are not always available. 

Disadvantages Can have severe heterogeneity problems trace-level grinding... 

D.iii.c. Hydrophilic Phosphine-Assisted Method. The use of hydrophilic phosphines allows either the resolution of the heterogeneity problem of the reaction in the presence of inorganic salts or the design of a biphasic phase-separable technique. 

The low yields of covalent reaction dictate that one focus experimentally on the probe molecules that actually label receptor components. But at the same time, to resolve the heterogeneity problem, one must determine the functional status of the pertinent probe molecules. Hence, the basic feature of the optimal experimental design involves selective observation of the ligand analog molecules, which act both as affinity labels and as receptor substrates. 

Crabtree RH (2012) Resolving heterogeneity problems and impurity artifacts in opoationally homogeneous transition metal catalysts. Chem Rev 112 1536-1554... 

Figure 7-17 (Kretzchmar et al., 1995) illustrates one part of the dynamic heterogeneity problem deposition site blockage by deposited colloids. In this example (Fig. 7-17a), natural soil colloids are injected until breakthrough occurs. Water is injected and no colloids are released. A second pulse of injected colloids is eluted coincident with a conservative tracer suggesting that sites for colloid deposition are already occupied. A lower concentration of injected colloids (Fig. 7-17b) results in a slower rate of breakthrough as it takes longer in the experimental runs with a lower colloid concentration for the colloids to occupy favorable colloid deposition sites.

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