Silicon, sediment sample analysis

This somewhat lengthy experiment provides a thorough introduction to the use of GG for the analysis of trace-level environmental pollutants. Sediment samples are extracted by sonicating with 3 X 100-mL portions of 1 1 acetone hexane. The extracts are then filtered and concentrated before bringing to a final volume of 10 mL. Samples are analyzed with a capillary column using a stationary phase of 5% phenylmethyl silicone, a splitless injection, and an EGD detector. 

Clearly some form of sample pretreatment is required for soils and sediments. Total levels may be obtained following sodium carbonate-boric acid fusion and the dissolution in hydrochloric acid employing lanthanum as a buffer and releasing agent. If the determination of silicon is not required, it may be volatilized as silicon tetrafluoride using hydrofluoric acid, although some calcium may also be lost as calcium fluoride. For many samples, however, it may be more appropriate to determine the exchangeable cation content of the sample. Here, the sample may be shaken with an extractant solution, for example, 1 mol 1 ammonium chloride, ammonium acetate, or disodium EDTA, prior to filtration and analysis. Where final solutions contain more than - 0.5% of dissolved material, the standards should also contain the major constituents, even where no chemical interference is expected, in order to match the viscosity and surface tension and avoid matrix effects. 

The shape of the average specimen is such that only rarely can it be directly placed into the diffractometer for analysis, so that special care has to be taken to achieve the random distribution of the crystallites to have meaningful peak intensities. The problems arising in sample preparation are (1) Particle size. The powdered sample must consist of particles smaller than 5 pm. Collection and separation of particles of these dimensions can be effected by sieving, sedimentation, or elutriation. (2) Surface flatness. Special precautions are needed to make the surface smooth and flat, with its plane including the diffractometer axis. If required, common binders, for instance, collodion, paraffin wax, or silicone grease, are applicable. (3) Preferred orientation. When it is necessary to ensure that the particles do not show preferred orientation, mix crushed glass or other amorphous medium with the powder or coat the plane surface of the sample carrier with a film of adhesive that dries at a moderate rate and then dust a layer of powder on the adhesive, polyfvinyl chloride), after it has become tacky. 

Silicones have been extracted from environmental samples with solvents such as hexane, diethyl ether, methyl isobutylketone, ethyl acetate, and THF, using either sequential or Soxhlet techniques (690-695). Silicones of a wide range of molecular weights and polarities are soluble in THF. This feature, coupled with its volatility and miscibility with water, makes THF an excellent solvent for the extraction of silicones from wet samples, ie, soils and sediments. Trace levels of silicones extracted from environmental samples have been measured by a number of techniques, including atomic absorption spectroscopy (AA), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), pyrolysis GC-MS, as well as H and Si NMR spectroscopy (674,684,692,696-700). The use of separation techniques, such as gel permeation and high pressure liquid chromatography interfaced with sensitive, silicon-specific AA or ICP detectors, has been particularly advantageous for the analysis of silicones in environmental extracts (685,701-704). 

One aluminum strip was removed each month and the amount of " C-labled oil in the RTVll was measured by thermal oxidation, based on 3-5 samples. This process is described in a report by J. Carpenter (9). The total amount of oil in each coupon was calculated based on the total weight of the silicone topcoat for a specified aluminum coupon. In addition, the amount of in the water and sediment was determined by liquid scintillation analysis and thermal oxidation/ liquid scintillation analysis, respectively. 

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