Silicon Compounds—Other Significant

Silicon Compounds—Other Significant Hexamethyldisilicon Silver... 

Heterocyclic Sulfur Compounds with Two or More Rings Hexachlorobenzene under Ring-Substituted Aromatics Hexachlorobutadiene under Unsaturated Alkyl Halides Hexachlorocyclohexane Lindane under Saturated Alkyl Halides Hexachlorocyclopentadiene under Unsaturated Alkyl Halides Hexachloroethane under Saturated Alkyl Halides Hexadecane under Alkanes and Cyclic Alkanes Hexafluoroethane under Saturated Alkyl Halides Hexamethyldisihzane under Silicon Compounds—Other Significant Hexanes under Alkanes and Cyclic Alkanes Hexylamine under Primary Aliphatic Amines and Diamines... 

Contamination. Contamination of samples by external sources can be a serious source of error and may be extremely variable. An excellent example of how serious this can be has been documented in the analysis of samples for polychlorinated biphenyls (PCBs). PCBs are synthetic mixtures of organochlorine compounds that were first manufactured in 1929 and have become of concern as significant environmental pollutants. It has been demonstrated that samples archived since 1914, before PCBs were manufactured, picked up measurable amounts of PCBs in a few hours just sitting in a modem laboratory (Erickson). Aluminum levels in the dust in a normal laboratory are so high that dust prohibits the determination of low ppb levels of aluminum in samples. A special dust-free clean lab or clean bench with a filter to remove small dust particles may be required, similar to the clean rooms needed in the semiconductor industry, for determination of traces of aluminum, silicon, and other common elements such as iron. When trace (inorganic analysis is required, the laboratory environment can be a significant source of contamination. 

Unlike hydrated precipitated silica, fumed silica is used almost exclusively in silicone rubber compounds. Fumed silica is incorporated into silicone gum stock with the use of dough mixers. Fumed silica is unique to silicone rubber because it significantly improves the physical properties of the cured compound (better than hydrated precipitated silica). On the other hand, fumed silica will have a greater tendency than the hydrated precipitated silica to impart crepe hardening to the uncured silicone compound. Many times, the use of fumed silica in silicone rubber is extended with the less expensive precipitated silica. 

For Ge the dominant relaxation mechanism is quadrupolar and Tj is readily available from the linewidth however, no specially significant conclusions have been reached. For Si the (negative) proton NOE can often be significant and, when allied to Ti measurements, can be used to assess segmental motion in silicone polymers. Other work has tended to concentrate on small molecules and can be of diagnostic value for compound type. Relaxation studies of SnCL have yielded a value of ca. 27 Hz for 7( Cl- Si) in this molecule. 

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