Inadvertent trace impurities

One issue, however, requires addressing before the pigments community can fully welcome PIC as a way of doing business. At this time, three of the inadvertent trace impurities, which may be present in some HPPs, are polychlorinated biphenyls, polychlorinated dibenzodioxins, and polychlorinated dibenzofurans. While... 

One issue, however, requires addressing before the pigments community can fuUy welcome PIC as a way of doing business. At this time, three of the inadvertent trace impurities, which may be present in some HPPs, are polychlorinated biphenyls, polychlorinated dibenzodioxins, and polychlorinated dibenzofurans. While these impurities are fuUy compliant with TSCA, and may only be present in low parts per million or billion, USEPA s Enforcement Division currently requires that, if analytically detectable, any listed trace impurities must stQl be reportable when exported out of the United States. To compound the difficulty,... 

With most transition metals, eg, Cu, Co, and Mn, both valence states react with hydroperoxides via one electron transfer (eqs. 11 andl2). Thus, a small amount of transition-metal ion can decompose a large amount of hydroperoxide and, consequendy, inadvertent contamination of hydroperoxides with traces of transition-metal impurities should be avoided. 

The materials used, including the solvent, should be as pure as possible. There are several instances recorded (and doubtless a number unrecognized) in which traces of impurities introduced inadvertantly into a system have had catastrophic consequences. 

In general liquid standards are prepared in a solvent matrix which should be the same as the matrix of the unknown. In many cases the liquid may be an extraction solvent or simply a dilution solvent, depending on the type of analysis and the form of the original sample. If the solvent choice is left to the analyst (as opposed to being prescribed by a procedure), the solvent has to be chosen such that it does not interfere with any of the potential sample components. For trace analysis it is important that the solvent be checked for impurities and that these impurities will not be confused with sample components. Chromatographing the solvent at the maximum sensitivity to be used in the analysis is referred to as "blanking the solvent." It is very important to blank the solvent each time it is used to be sure it has not been inadvertently contaminated. Also in trace analysis it is preferred to have a solvent elute from the column following the sample components of interest rather than ahead of the sample. 

It may be that trace quantities of materials inadvertently present as impurities in the monomers or solvents used, or contaminating the walls of experimental vessels and the like, can have a major catalytic influence on the polyamidation reaction in the melt. In the case of poly(ethylene terephthalate), Stevenson [72] studied catalysis on plates of various materials in order to find a surface with substantially no catalytic activity. The use of an inert plate allowed a study of catalysis by antimony trioxide to be made in thin polyester films (see Section 5.3.2b). A systematic study of the catalysis of polyamidation in the melt would seem to be a worthwhile study, both for commercial reasons, where catalysts could increase productivity, and in the area of kinetics, where such a study might permit more precise and meaningful measurements to be made. 

Moisture and oxygen must be rigorously excluded from the system if the synthesis is to be successful. Inadvertant introduction of trace protic impurities of metal oxides during the synthesis leads to the formation of polydimethylsilioxane homopolymer. This material, present in even small amounts, is detrimental to the mechanical properties of the final block copolymer. This effect can be illustrated by deliberately including known amounts of polydimethylsiloxane in a block copolymer of demonstrated mechanical strength. These effects are summarized in Table I. 

Finally a note of caution on the use of supporting electrolytes when you are working at trace or lower levels of analyte concentration. When AnalaR grade supporting electrolytes are used at a concentration level of 0.1 mol dm - the concentrations of impurities added inadvertently, eg heavy metals, is significant at the ppb level. This is not a problem with dc polarography but is in the more advanced forms of polarography discussed in later parts of this unit. 

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