Bulk material preparation

The samples to be distributed must be generally similar in matrix to the unknown samples that are routinely analysed (in respect of matrix composition and analyte concentration range). It is essential they are of acceptable homogeneity and stability. The bulk material prepared must be effectively homogeneous so that all laboratories will receive samples that do not differ significantly in analyte concentration. The co-ordinating laboratory should also show the bulk sample is sufficiently stable to ensure it will not undergo... 

Whilst it may not always be possible to use IDMS to assess the homogeneity of all materials for all analytes of interest, because of limited IDMS resource availability, the approach may be applied selectively to verify the suitability of bulk material preparation and subdivision procedures. Once IDMS has demonstrated that a particular bulk material preparation procedure is capable of producing material of sufficient homogeneity, the preparation procedure may be applied to similar material types without further IDMS measurements, but perhaps using more routine methodology to confirm that no gross inhomogeneity effects have unexpectedly occurred. 

Weighing is the operation of determining the mass of any material as represented by one or more objects or by a quantity of bulk material. Proportioning is the control, by weighing, of relative quantities of two or more ingredients according to a specific recipe in order to make a mixed product, or to prepare the ingredients for use in a chemical process. 

In the early days of TEM, sample preparation was divided into two categories, one for thin films and one for bulk materials. Thin-films, particularly metal layers, were often deposited on substrates and later removed by some sort of technique involving dissolution of the substrate. Bulk materials were cut and polished into thin slabs, which were then either electropolished (metals) or ion-milled (ceramics). The latter technique uses a focused ion beam (typically Ar+) of high-energy, which sputters the surface of the thinned slab. These techniques produce so-called plan-view thin foils. 

Cell cultures provide infeeted fluids that eontain little debris and can generally be satisfactorily clarified by filtration. Beeause most viral vaccines made fiom cell cultures consist of live attenuated vims, there is no inaetivation stage in their manufacture. There are, however, two important exeeptions inaetivated poliomyelitis vims vaccine is inactivated with dilute formalin or /3-propiolaotone and rabies vaccine is inactivated with /3-propiolactone. The preparation of these inaetivated vaccines also involves a concentration stage, by adsorption and elution of the vims in the case of poliomyelitis vaccine and by ultrafiltration in the case of rabies vaceine. When processing is complete the bulk materials may be stored until needed for blending into final vaccine. Because of the lability of many vimses, however, it is necessary to store most purified materials at temperatures of-70°C. 

CdjPj and CdjAs are low band gap semiconductors (0.5 and 0.1 eV, respectively). The bulk materials are black and start to absorb in the infrared. These materials have been prepared as colloids in alkaline solution by precipitation of Cd with phosphine and arsine Depending on the conditions of preparation, particles of different sizes (between about 2 and 10 nm) were obtained, which could also be recovered in the solid state after evaporation of the solvent. The color of these materials ranged from black to colorless with decreasing particle size, with all kinds of intermediate colors in the visible. 

In our early work with bimetallic systems, we noticed that, depending on the preparation procedure in UHV, different surface compositions could be produced over the same bulk material owing to the phenomenon of surface segregation [Stamenkovic et al., 2002]. It was essential, then, to establish a methodology for transferring a well-defined bimetallic system into an electrochemical environment for further electrochemical characterization. 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

The idea of 9 having a maximum value of unity is simplistic. Try this quick experiment take the pan used to prepare a curry and try to wash it clean. There will be a stain adjacent to the metal, above which is a layer of curry sauce. The stain is thin - possibly even transparent - which we call the adsorbed layer. The layer of stain has a minimal thickness and it is difficult to remove. Conversely, we call the overlayers of curry sauce bulk material , which can take any thickness and are relatively easy to remove, even with a fingernail. 

The major disadvantages in application of the AEM method relative to AAS is the time required for specimen preparation and analysis. Care must be taken that the particles analyzed are characteristic of the bulk material, that they are thin enough to meet the "thin-film criteria" of Equations (2) and (3), and that enough particles are analyzed to reduce the random error to acceptable levels. The AEM analyses shown in Figure 6 took several days to collect and involved considerble operator attention. 

Experiments using the DCC approach aimed at the discovery of improved phosphor materials have also been described. [9] In this case, samples are evaluated optically, an approach well suited to direct comparisons of large numbers of samples, although it is somewhat difficult to compare the results to the optical properties of bulk materials. Further spectroscopic evaluations of individual elements of the sample array are also easily accomplished by a variety of approaches including scanning fiber techniques. One concern in studies of phosphors is the sensitivity of the optical behavior including fluorescence intensity to processing effects such as details of the microstructure or surface preparation. 

---