Contamination from glass

This formulation is particularly effective at removing oil, greases and other atmospheric contaminants from glass surfaces. 

The same sequence of desorption experiments was performed on silane-treated glass surfaces. Desorption of organic contaminants from glass fiber treated with methyltrimethoxysilane is shown by the dashed curve in Fig. 4A. The desorption envelope obtained from silane-treated glass with no probe on the column is... 

It goes without saying that, although absolutely pure water has a pH value of 7, the pH value is subject to a variation of 1 or 2 units through almost unavoidable contamination from glass or from NHa, H2S, or C02 from the air, and that to maintain an unfluctuating value of 7 a buffer must be used. 

Halocarbons, a class of polar solvents, are hydrocarbons with an attached halogen. There are commonly three types of halocarbon solvents those based on chlorine, fluorine, and a combination of the two. They are all powerful degreasing materials and can be particularly effective in removing polar contaminants from glass. The chlorofluorocarbons are currently under review because they cause environmental damage to the ozone layer.1... 

Viable glass fibers for optical communication are made from glass of an extremely high purity as well as a precise refractive index stmcture. The first fibers produced for this purpose in the 1960s attempted to improve on the quahty of traditional optical glasses, which at that time exhibited losses on the order of 1000 dB/km. To achieve optical transmission over sufficient distance to be competitive with existing systems, the optical losses had to be reduced to below 20 dB/km. It was realized that impurities such as transition-metal ion contamination in this glass must be reduced to unprecedented levels (see Fig. 

The high-purity water thus produced typically has a conductance of about 0.5 x 10-6fi-1cm-1 (0.5juScm-1) and is suitable for use under the most stringent requirements. It will meet the purity required for trace-element determinations and for operations such as ion chromatography. It must however be borne in mind that such water can readily become contaminated from the vessels in which it is stored, and also by exposure to the atmosphere. For the determination of organic compounds the water should be stored in containers made of resistant glass (e.g. Pyrex), or ideally of fused silica, whereas for inorganic determinations the water is best stored in containers made from polythene or from polypropylene. 

All preparative PLC precoated layers have glass plates as support. They are offered by a variety of manufacturers with or without fluorescent indicators and different binders. The layer thicknesses range uniformly between 0.5 and 2 mm or exhibit a gradient. Furthermore, in some cases the precoated silica gel plates are prescored to avoid cross-contamination from track to track. 

Portions of the sun visor arms used in automobiles can be manufactured from glass-filled thermoplastic polymers. Several visor arms contained visible blemishes on the surface. Typically, the steel insert of the visor arm is overmolded with the glass-filled thermoplastic polymer. Manufacturers sometimes utilize striations on the steel insert to provide a mechanical interlock with the over-molded thermoplastic polymer. However, in an improperly controlled environment, the mechanical process that produces the striations can also be the source of contamination and cause surface blemishes in the final product. 

An interesting example of the contamination risks which may be caused by a laboratory vessel is that of boron. Determination of very low boron concentrations, involves a prior separation by distillation and subsequent analysis by spectrometry, with a suitable reagent such a curcumin or carminic acid. The use of laboratory vessels made of borosilicate glass (such as Duran or Pyrex) could lead to very large errors in the boron content found. Such errors are caused by sample contamination from the boron present in the glassware. 

Cleaning with chromic acid may not always be necessary and depends largely on the previous contamination of the sample container However, it is advisable to use chromic acid in the case of protein tensiometry, since proteins have a tendency to quickly adsorb and slowly desorb from glass surfaces. 

If a glass is held for a long period at an elevated temperature it may start to crystallize or devitrijy. Devitrification of fused quartz (silica glass) to cristabolite is slow. Nucleation is usually at a free surface and is often stimulated by contamination from alkali ions such as sodium. The rate of growth of cristabolite is increased by oxygen and water vapor. With surface contamination, devitrification of fused quartz may occur at temperatures as low as 1000 °C. However, if the surface is clean it rarely occurs below 1150 °C. 

Research on chemical migration from other types or components of packaging material - e.g. glass, wood, cork, coatings, adhesives - has been carried out sporadically. There is now a concerted UK programme to study chemical migration from these so that problems can be identified and dealt with in a consistent way (Working Party on Chemical Contaminants from Food Contact Materials and Articles, 1999). 

---