Trace impurity content

In Total Reflection X-Ray Fluorescence Analysis (TXRF), the sutface of a solid specimen is exposed to an X-ray beam in grazing geometry. The angle of incidence is kept below the critical angle for total reflection, which is determined by the electron density in the specimen surface layer, and is on the order of mrad. With total reflection, only a few nm of the surface layer are penetrated by the X rays, and the surface is excited to emit characteristic X-ray fluorescence radiation. The energy spectrum recorded by the detector contains quantitative information about the elemental composition and, especially, the trace impurity content of the surface, e.g., semiconductor wafers. TXRF requires a specular surface of the specimen with regard to the primary X-ray light. 

In Secondary Ion Mass Spectrometry (SIMS), a solid specimen, placed in a vacuum, is bombarded with a narrow beam of ions, called primary ions, that are suffi-ciendy energedc to cause ejection (sputtering) of atoms and small clusters of atoms from the bombarded region. Some of the atoms and atomic clusters are ejected as ions, called secondary ions. The secondary ions are subsequently accelerated into a mass spectrometer, where they are separated according to their mass-to-charge ratio and counted. The relative quantities of the measured secondary ions are converted to concentrations, by comparison with standards, to reveal the composition and trace impurity content of the specimen as a function of sputtering dme (depth). 

To prepare a USP-grade Epsom salt, higher purity MgO or Mg(OH)2 is used. USP and food grades require low chloride levels, limiting allowable chloride content of the MgO to 0.08 wt %. Trace impurities including iron and aluminum are precipitated using excess MgO. EoUowing crystallization, the Epsom salt is washed free of mother Hquor. 

Chemical Properties. Elemental analysis, impurity content, and stoichiometry are determined by chemical or iastmmental analysis. The use of iastmmental analytical methods (qv) is increasing because these ate usually faster, can be automated, and can be used to determine very small concentrations of elements (see Trace AND RESIDUE ANALYSIS). Atomic absorption spectroscopy and x-ray fluorescence methods are the most useful iastmmental techniques ia determining chemical compositions of inorganic pigments. Chemical analysis of principal components is carried out to determine pigment stoichiometry. Analysis of trace elements is important. The presence of undesirable elements, such as heavy metals, even in small amounts, can make the pigment unusable for environmental reasons. 

Aluminum obtained by electrolysis of cryoHte baths contains iron [7439-89-6] and siUcon [7440-21-3] as impurities. Iron content may vary from 0.05 to 0.4% and siUcon from 0.05 to 0.15% depending on the raw materials and the age and condition of the reduction cell. Primary aluminum metal also contains small, usually not to exceed 0.05% in total, amounts of many other elements. Some of these trace impurities are Cu, Mn, Ni, Zn, V, Na, Ti, Mg, and Ga, most of which are present in quantities substantially below 100 ppm. 

Citric acid is also commercially available as a 50% w/w solution made either by dissolving crystalline citric acid in water, or a combination or crystalline citric acid, and one of the citric acid process streams. There are several grades of citric acid solutions available, each made according to quaUty which is measured by color and trace impurities. The citric acid content of each grade can be identical, 50% w/w, which is near the solubiUty limit. 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

FBAs applied in combination may show a synergistic effect. Synergism is, at present, only of commercial interest with polyester brighteners. This phenomenon is discussed further in section 11.10. Conversely, the presence of a trace impurity in an FBA formulation may greatly reduce its effectiveness. In industrial laboratories much time and effort is expended in developing processes to minimise the content of such impurities or even to eliminate them completely. 

We also made a few measurements as a function of ionic strength at pH = 3 and 13. The results at pH = 13 gave log k = 1.33 0.01 min 1 M-1 for four measurements between I = 0 to 3m. At a pH = 3 in dilute solutions below 0.04M, no ionic strength dependence was found however, at I = 3.0m, the rate was ten times faster that at I = 0. We attribute this increase in rate to the presence of trace metals. All of our runs at pH = 8 to 13 were made with enough borax to complex these trace metals and suppress the catalytic effect. An experiment at pH = 11 without borax was completed within 5 minutes compared to 1.5 hours with 0.01 M borax. These results support our contention that the effect of ionic strength on the rates of oxidation are independent of pH if the catalytic effects of trace impurities are avoided. 

The ionic composition of dilute sodium silicate is a very complex problem involving Na20/SiC>2 ratio, water content, and even trace impurities. Equilibration seems to be very slow at ordinary temperatures. As shown in Figure 5, Harris, et.al, were able to identify a wide variety of structures in potassium silicate solutions which bear a striking resemblence to the secondary building units proposed for zeolite... 

It is interesting to note the effect of chromium content on reaction rate at high pressures (,—500 p.s.i.g.). Experiments (5) were carried out with normal air-activated catalysts (Figure 4). Catalysts were used with chromium contents ranging from 0.7 to 0.0005 wt. % of the total catalyst. Results of one-hour ethylene polymerization tests at 132°C. and 450 p.s.i.g. with these catalysts, activated at 500°C., are given. As the concentration of chromium was decreased, catalyst charge was increased to compensate for poisoning of catalyst sites by trace impurities and to keep total rate of production about constant. 

In a number of cases, geochemically similar elements are well correlated to each other in the sand fractions from each unit (e.g., Sm and Eu in Figure 7). This correlation is not surprising if the trace elements are associated with the heavy minerals. Small variations in the amount of a particular heavy mineral will cause large differences in the trace element content measured, if the sand fraction is mostly quartz and feldspar, which contain so few trace impurities. Figure 7 shows the relationship between Sm and Eu. There is a correlation between these elements in the samples from each of the two levels. Figure 7 suggests that there is a different Sm/Eu ratio for the sands from these two Nile sediment deposits. 

CONTENTS OF TRACE IMPURITIES AS DETERMINED IN NIOBIUM PENTOXIDE BY DIRECT SOLID SAMPLING ETA-AAS AND COMPARISON WITH THE RESULTS OBTAINED BY SLURRY SAMPLING ETA-AAS (SIS-ETA-AAS) AND SOLUTION ETA-AAS (SoI-ETA-AAS)... 

As for many solid state reactions, the properties of any particular oxide preparation may be influenced by its method of synthesis [11]. Oxides are often products of thermal treatment. Such heating may influence the surface area, impurity content (e.g. strongly-retained traces of water from hydroxide dehydration, oxidized species retained from the decomposition of a nitrate, carbonate etc.) and concentrations and distribution of defects (e.g. vacancies and non-stoichiometry arising during oxidation of a metal). Thus the preparative method exerts significant control over the numbers... 

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