Adventitious metal impurities

The sample surface is contaminated, e.g., with adventitious carbon (small signal in the figure with a height below the line thickness at 290 eV), with nitrogen compounds from templates, or with transition-metal impurities. Residual alkali ions from incomplete cation exchange or residual anions, e.g., from soUd-state ion exchange, are other sources of unexpected elements in the sample. 

Hydrogen peroxide as supplied is normally stabilised with phosphates and sometimes tin(IV) compounds, the latter being effective at the product s natural (weakly acid) pH by hydrocolloid formation, which occludes adventitious transition metal ions and reduces their catalytic activity. In many cases, extra stabilisation is not required when H2O2 or its derivatives are used in synthesis. However, elevated temperature, high alkalinity and increased metal impurities all tend to destabilise peroxy-gens, and where these conditions are unavoidable, additional stabilisers may be employed added either to the peroxide (subject to reactivity) or separately to the reaction mixture. Such stabilisers fall into two categories. 

A common problem of this synthesis is the decomposition of the product to a black insoluble and pyrophoric powder (presumably metallic Mo), which is possibly caused by the use of an impure starting material, to the adventitious introduction of air, to excessive exposure to light, or to a combination of those factors. If a black powder is formed, this can be removed at the end of the reaction by filtration through a glass frit the subsequent workup is as described above. Such decomposition reduces the yield but does not lead to a less pure product. If larger amounts of Mo(CO)6 are used, the sublimation of larger quantities of Mo(CO)6 to the condenser occurs, which may require interruption of the procedure and manual scraping of the sublimed solid back into the flask with a spatula under a flow of N2. 

Combined EPR and 57Fe Mossbauer spectroscopic studies of the MoFe protein in various overall oxidation states39 40,42,44 48,49) have provided strong evidence for the presence of six metal clusters two M centers that are the protein-bound form of the FeMo-cofactor (a novel Mo-Fe-S cluster) and four unusual tetranuclear iron clusters referred to as the P clusters. These will be discussed separately below. In addition, Mossbauer spectra of MoFe proteins from Azotobacter vinelandii, Clostridium pas-teurianum, and Klebsiella pneumoniae all show an additional component termed S39 40 42, which accounts for 6% of the total Fe present (2 Fe per molecule) and which has Mossbauer parameters (AEq = 1.35 mm/s d = 0.60 mm/s) different from those expected for likely impurities such as high-spin Fe2+ or Fe3+ ( adventitious iron). At present, it is difficult to decide whether species S is an unusual and persistent impurity or an integral part of the MoFe protein. 

Initiation is not due to the presence of adventitious impurities, such as peroxides, which might form a redox couple with the metal compound. 

The model satisfactorily described the cure behavior for the entire range as experimentally monitored by FTIR, DSC, and torsional braid analysis (TBA). This model satisfactorily explained the cure behavior of both catalyzed and uncatalyzed systems over a wide range of temperature and throughout the curing process. The authors proposing the kinetic model considered the reaction to be triggered by the adventitious water and phenol impurities (whose reactions with the cyanate ester is considered as an equilibrium reaction). Catalysis by the added metal ions, which stabilizes the imino carbonate intermediates by complex-ing, is also considered. The model has considered all possible reaction paths and intermediates as detailed in Sect. 4 and depicted in Scheme 14. Considering the various reactions, expressions could be obtained for the individual apparent empirical rate constants of the second order auto catalytic model in terms of the actual rate constants and equilibrium constants. 

The above outlined scheme leads to the conclusion that completely ionized thiols would give exclusively sulphinic and sulphonic acids nevertheless, the experimental results indicate formation of ca. 5% of disulphide in the oxidation of potassium benzenethiolate even with base in large excess. Since formation of disulphide would require the presence of undissociated thiol, other mechanisms must be operative. Again it is possible that the intervention of trace metal catalysis in the oxidation reaction has to be taken into account. Cullis, Hopton and Trimm reported that copper ions in concentrations as low as 10 M are still active as catalysts and indeed it is very hard to detect metal ions at such low concentrations and to exclude adventitious impurities of this order of magnitude. 

Polymer manufacture Defects imsaturation Impurities transition metal ions present as catalyst residues Impurities hydroperoxides and carbonyl compoimds formed by adventitious oxidation... 

Many polymers which are intrinsically transparent to the shorter wavelengths of the sun s spectrum (<290 nm), become sensitized by the presence of light absorbing impurities and trace level of metals and adventitious species produced during manufacture and fabrication. Hence the susceptibility of commercial polymers (e.g. PE and PP) to outdoor weathering. The outdoor performance of polymers can, however, be markedly improved by photoantioxidants and UV-stabilizers. 

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