MATERIAL PURITY ISSUES

Tunable filters in the form of AOTF devices, liquid crystal tunable filter (LCTF) and also tunable cavity Fabry-Perot etalon (FPE) devices have been considered in non-moving part instrument designs for many years. Today, the AOTF and the LCTF devices are used in the NIR spectral region.9,10 Originally, designs were also proposed for mid-IR AOTF devices, but these have not become available, mainly because of fabrication issues (cost and material purity). Tunable FPE devices, which are really just variable cavity interference filters, have been developed for the telecommunications industry. While these have been primarily used in the NIR, in most cases they can be fabricated to work also in the mid-IR, the latter being only an issue of material/substrate selection. 

The first step is to describe the basic scenario for the purification. General considerations answer questions such as What is the intended use of the product What kind of starting material is available and how should it be handled What are the purity issues in relation to the source material and intended use of the final product What has to be removed What must be removed completely What will be the final scale of purification If there is a need for scale-up, what consequences will this have on the chosen purification techniques What are the economical constraints and what resources and equipment are available ... 

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

Owing to the light and air sensitivity of the carotenoids and retinoids, sample handling is a critical issue. It is recommended to conduct extraction of these materials with peroxide-free solvents, to store biological samples at —70° C under argon and in the dark, to perform the analysis under yellow light, and to use reference compounds of high purity (57). 

In medicine the need for standards was just as acute. Beal (r95i) mentioned that the U.S. Pharmacopoeia VI, issued in 1880, took a big step forward by adding tests for purity and quality of the materials described in it, but the use of reference materials as an integral part of the pharmaceutical monographs for drugs did not start until the 1950 s. Another example of activities at the end of the 19th Century was associated with the introduction, by Ehrlich, of the first diphtheria antitoxin and his... 

However, pMBCl 42 has a thermal stability issue and is expensive (Aldrich price 25 g for 69.90 the largest bottle). On the other hand, pMBOH 43 is stable and economically viable (Aldrich price 500 g for 84.90 the largest bottle). It was found that mono-N-alkylation of 36 proceeded well by slow addition (over 3 h) of 43 to a solution of 36 in acetonitrile in the presence of a catalytic amount of acid (p-TsOH) at 70 °C, as shown in Scheme 1.16. Slow addition of alcohol 43 minimized the self-condensation of 43 to form symmetrical ether 44, which was an equally effective alkylating agent. The product 41 was then directly crystallized from the reaction mixture by addition of water and was isolated in 90% yield and in >99% purity. A toluene solution of 41 can be used for the next reaction without isolation but the yield and optical purity of the asymmetric addition product were more robust if isolated 41 was used. In general, the more complex the reaction, the purer the starting materials the better. 

The first move of the Methods section provides a description of chemicals, materials, and/or samples. Beginning writers often wonder what to include in this section (level of detail) and how these details should be presented (level of formality), both issues related to audience. With respect to detail, it is customary to report the name, purity, and vendor for all essential chemicals and materials used in the work. (Incidental chemicals, e.g., solvents used to clean glassware, need not be reported.) Similarly, for samples, both how and where the samples were collected should be described. With respect to formality, the journal article requires complete sentences. A common mistake is to use lists although commonplace in... 

Some of HTCVD s initial challenges have been solved. For example, using a coaxial injector solved the blocking of the inlet. Another issue is the purity of the graphite material, which may be overcome using properly coated graphite crucibles. 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

For many botanical preparations, there are often uncertainties in the identity of the plant species and/or consistency of botanical raw materials. Complicated manufacturing processes add further possible variation to the drug substance and final products. While most contamination problems unique to botanicals are resolvable, purity/potency and stability are more difficult technical issues, without knowing the identity of active ingredients. 

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