Impurity detection

The distribution of impurities over a flat sihcon surface can be measured by autoradiography or by scanning the surface using any of the methods appropriate for trace impurity detection (see Trace and residue analysis). Depth measurements can be made by combining any of the above measurements with the repeated removal of thin layers of sihcon, either by wet etching, plasma etching, or sputtering. Care must be taken, however, to ensure that the material removal method does not contaminate the sihcon surface. 

Figure 4.42. Trend analysis over 46 batches of a bulk chemical produced according to the same manufacturing procedure Small and scaled-up batch size [kg], HPLC and Titration assays [%], resp. individual HPLC impurity levels [%], versus batch number. The lack of full correlation between assays indicates that the titration is insensitive to some impurities detected by HPLC. The mass balance, where available, suggests that all relevant impurities are quantified. Impurities B and C, for instance, are highly correlated (r = 0.884, p = 0.0002).

Uniformly labeled 2,4-dichlorophenol- C (purchased from New England Nuclear Corp, Boston, Mass.) was used in the tracer preparation. This provided a label at all carbon positions in the dibenzo-dioxin structure. 2,7-Dichlorodibenzo-p-dioxin- C after initial cleanup by fractional sublimation, contained approximately 5% of an impurity, detected by thin layer chromatography (TLC) which gave mass peaks at 288, 290, 292, and 294 in the mass spectrometer, consistent with a trichloro-hydroxydiphenyl oxide. This is probably the initial condensation product of the Ullman reaction and is most likely 2-(2,4-dichlorophenoxy)-4-chlorophenol. It was removed easily by extractions with aqueous... 

Precursor Impurities detected by plasma emission (ppm) Material grown and temperature (°C) Carrier concentration, 77K (cm-3)a Mobility, /i77[c (cm2 V-1 s f... 

The lead precursor in PbSe film deposition was lead acetate complexed with triethanolamine. The growth rate for PbSe was 0.18-0.16nm/cycle. The films were polycrystalline (i.e., cubic) without preferred orientation. The stoichiometry of SILAR-grown PbSe was found to be 1 1 within the limits of the RBS technique. Impurities detected were 5 at.% of oxygen and 8 at.% of hydrogen.103... 

EtN02 was fractionally distilled and the last 15%, which was free from impurities detectable by gas liquid chromatography (GLC), was stirred with baked Na2S04 in a reservoir attached to the vacuum line. 

Materials 1,3-Dioxolane (1) and 1,3-dioxepane (5) were prepared and purified conventionally. Compound 1 contained no impurities detectable by GLC, but 5 contained a trace of tetrahydrofuran (THF) which could not be removed even by distillation on a Fenske column with a reflux ratio of 50 1 4-methyl-l,3-dioxolane (4) was prepared by Astle s method [10]. All monomers were dried preliminarily by storage over LiAlH4 in reservoirs attached to a conventional high-vacuum line fitted only with all-metal valves, and then stored with liquid Na-K alloy until used. Methylene dichloride was purified conventionally, distilled on a Normatron 1.5 m column, dried i.vac. over LiAlH4 on a conventional high-vacuum line, and then stored for 24 h over a fresh sodium film immediately before use, in a reservoir attached to the vacuum line. 

P. Rousselot-Pailley, D. Maux, J. M. Wieruszeski, J. L. Aubagnac, J. Martinez and G. Lippens, Impurity detection in solid-phase organic chemistry Scope and limits of HR MAS NMR,... 

The only radionuclidic impurity detected in the 1-122 is less than 0.1% radioxenons and other radioiodines, which neither interfere with scintigraphic imaging nor result in a high radiation exposure to the patient. Further improvement of the radioiodine contamination could be attained with an iodine trap between the storage reservoir and the growth chamber. The milking efficiency is about 40%. We consider this generator assembly to be a preliminary version that can be refined considerably. Further details may be obtained from Richards and Ku (8). 

All solvents used are either spectrograde or analytical grade. The analytical grade solvents are further purified by fractional distillation whenever blank determinations suggest the presence of impurities detectable by flame ionization GC. 

The ampul is removed from the furnace and opened. Caution. Hydrogen selenide vapors are formed during the washing, and it is advisable that this operation also be performed in a hood. The crystals of cadmium chromium(III) selenide can be washed in water to remove the cadmium chloride. The crystals are black octahedra which vary in size up to about 3 mm. on an edge. The only impurities detected in these crystals by arc emission spectrographic analysis were Mg and Cu, and these are present only in amounts of less than 20 p.p.m. Anal. Calcd. for CdCr2 Se4 Cd, 21.1 Cr, 19.5 Se, 59.3. Found Cd, 20.8 Cr, 17.7 Se, 58.6. 

Extraction of neptunium, plutonium, and americium from simulated radioactive liquid waste was carried out in particular with tert-butyl and dealkylated tetramers, hexamers, and octamers of calixarene [ethoxy(diphenylphosphine oxide)]. Among these six calixarenes, the highest distribution ratios were obtained with the dealkylated calix[8]arene. Using a different sample of the dealkylated hexamer, the Strasbourg group concluded that this compound is the most efficient. This discrepancy can be explained by the presence of impurities, detected by NMR, which were probably responsible for the poor performances of the dealkylated hexamer tested at Cadarache. 

Table 3. Minimum mass fraction of impurity detectable by density measurement for a mixture. ...

Separations in Protocol 9 are comparable to those of SDS-PAGE but each sample run is less than 20 min and more accurate quantitation can be achieved. Limits of protein impurity detection in CGE systems with absorbance detection at a low UV wavelengths are approximately 0.1% (1000 ppm). This is about 10-fold lower than the detection limit of SDS-PAGE. 

TABLE 2 Approximate Protein Impurity Detection Limits for Common Bioanalytical Techniques... 

Figure 2. A comparison of the sensitivity of impurity detection in polyethylene cable insulation achieved with a) NAA of 1 mg samples, the solid points, b) NAA with 1 g samples, the open points and c) PIXE with 2.0 MeV protons, the two solid curves are for detection of the K and the L X-rays (data taken from ref. 2).

Figure 9. The radial concentration profiles for the impurities detected in two adjacent breakdown trees 5A-1 and 5A-2. Note the breaks in the logarithmic concentration scale and also the essentially constant concentration of sulphur in comparison with the large variations in the Na, Cl, K and Ca concentrations.

A comparative study of microsilicas from 18 sources showed considerable variation in composition and properties, one of those examined containing as little as 23% of SiOj and having a specific surface area of only 7.5 m g (A21). The same study showed that in most of the samples the diffuse XRD peak from the glass accounted for 98-99.5% of the total diffracted intensity and that it peaked at the value of 0.405 nm characteristic of vitreous silica. The commonest crystalline impurities detected were KCl, quartz, metallic iron and iron silicide, and pozzolanic reactivity was found to depend more on the chemical composition and nature of impurities than on the fineness or SiOj content. A surface layer of carbon, if present, greatly decreased reactivity. 

Tetrabenzyl pyrophosphate should be stored in a freezer as a dry solid. Stability data on TBPP are tabulated below [reported in Liquid Chromatography Area Percent (LCAP)]. The only impurity detected was dibenzyl phosphate. For this study, solid TBPP was sealed in scintillation vials with Parafilm, which were in turn enclosed in desiccators with indicating Drierite. 

An alternative method which avoids this problem is one in which a 200-fold dilution of the concentrated sample solution is prepared. This solution is then used as a reference solution for all the impurities detected in the concentrated solution. The content of each impurity in the sample is calculated as follows ... 

Identification of organic impurities detected and reported at or near the required thresholds with analytical methods based on HPLC and GC requires ... 

Direct and easy interface between the organic impurity detection/reporting analytical technique/method (HPLC or GC) and the compound specific detection technique(s). 

Besides the major features in electrolysis discussed here, there are smaller problems to be solved. It is, of course, easiest to consult an experienced electrochemist on such matters, but such a person may not always turn up when wanted, so the following items are briefly mentioned circuit, counterelectrode reaction, removal of oxygen before electrolysis, removal of impurities, detection of intermediates, integration of current, and workup. 

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