Selected Methodological Problems

Some major methodological problems are present in all uptake systems. They are described in this section. 

Loss of Amino Acids by Cold-Shock Treatment 

Several authors concluded that the high-affinity uptake system might represent a homoexchange process (Levi and Raiteri, 1974 Simon et al., 1974 Raiteri et al., 1975 Levi et al., 1976). Synaptosome preparations preloaded with [ H]-GABA or [ H]-glycine released them in a dose-dependent manner when 

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Differential tissue analysis is not frequent in studies involving small invertebrates, such as bivalves, polychaetes, and gastropods, since, as discussed earlier, their diminutive size poses challenging methodological problems. Nonetheless, examples of the usefulness of this approach can be found in the literature. One study compared the specific tissue distribution of cadmium and two forms of mercury in the clam Corbicula fluminea when exposed to contaminated water or sediments 85 the metal distributions in five selected organs were reported to display strong specificities, in accordance... 

Due to methodological problems it is presently not possible to quantifying those taxa which are determined to be present in clone libraries these problems, that are associated with the extraction of nucleic acids, PCR-primer sensitivity and selectivity, cloning steps, and the dependence of PCR amplificate amount on undeterminable genomic properties, have been summarized [21]. Thus, it is also not possible to decide whether the identified clones belong to a majority or a minority population of the naturally occurring prokaryotes. 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

The intention of this chapter is to provide a general survey on the preparative methodologies for the size- and shape-selective synthesis of metallic nanoparticles that have emerged from the benches of chemical basic research during the last few decades and become established as practical standard protocols. Industrial scale-up, however, has only just started to test the economic viability of these procedures and to determine whether they can meet the challenges of a number of very specific applications. The commercial manufacture of such thermodynamically extremely unstable nanoparticles in defined sizes and shapes on the kilo-scale is still confronted by a number of major problems and it remains to be seen how these can be solved. 

The main obstacles to increasing our knowledge in this field are methodological (103). Microbial cell enumeration techniques and identification procedures are often difficult or tedious, and the collection of relevant samples or the simulation of natural conditions in the laboratory can be problematical. However, the development of molecular approaches for the study of microbial populations can contribute to solving these problems. Considering the vast array of techniques used and presented in the literature, only a selection of these methods is discussed in this review. 

Nowadays, MS plays an important role in many fields of scientific and technological research for identification, structural characterization and quantitative determination of wide classes of compounds. Its unique capabilities, such as high sensitivity, high selectivity, accuracy in molecular weight determination, and ability to analyze complex mixtures, give this methodology great importance in the solution of problems not easily handled by other techniques. 

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