Problem of isolation

Preferably, high pressure Hquid chromatography (hplc) is used to separate the active pre- and cis-isomers of vitamin D from other isomers and allows their analysis by comparison with the chromatograph of a sample of pure reference i j -vitainin D, which is equiUbrated to a mixture of pre- and cis-isomers (82,84,85). This method is more sensitive and provides information on isomer distribution as well as the active pre- and cis-isomer content of a vitamin D sample. It is appHcable to most forms of vitamin D, including the more dilute formulations, ie, multivitamin preparations containing at least 1 lU/g (AOAC Methods 979.24 980.26 981.17 982.29 985.27) (82). The practical problem of isolation of the vitamin material from interfering and extraneous components is the limiting factor in the assay of low level formulations. 

In the semi-dilute regime, the rate of shear degradation was found to decrease with the polymer concentration [132, 170]. By extrapolation to the dilute regime, it is frequently argued that chain scission should be nonexistent in the absence of entanglements under laminar conditions. No definite proof for this statement has been reported yet and the problem of isolated polymer chain degradation in simple shear flow remains open to further investigation. 

Now the failure detection problem consists of three tasks alarm, isolation, and estimation. The alarm task consists of making a binary decision either something has gone wrong or everything is fine. The problem of isolation is that of identifying... 

They applied this newly-found chemical distinction to the problem of isolating both isomers. On treatment of the (Z)[ 10.8] precursor 64a with H2S04-Ac0H in benzene for 16 h, they found that the resulting mixture contained a 70 30 mixture of ( )[10.8] and unchanged (Z)[10.8] compounds, and an identical mixture was found to result from the ( )[10.8] isomer 64a upon similar treatment. Pure ( )[10.8] was reported to be obtained from these reaction mixtures by selectively converting the (Z)[10.8] precursor to the epoxide. 

Aside from the question of the precise model by which relaxation times are interpreted there is the more practical problem of isolating that part of the relaxation specifically caused by diffusion. The contributions of exchange processes (see below), spin-rotation interaction (9), and spin diffusion (9) can be identified by temperature dependences different from that which is solely the result of the motionally modulated nuclear dipolar interaction as sketched above, and corrections can be made. The molecular rotation contributions to dipolar relaxation can be removed or corrected for by (a) isotopic substitution methods (19), (b) the fact that rotation is in some cases much faster than diffusion, and its relaxation effects are shifted to much lower temperatures (7, 20), and (c) doping with paramagnetic impurities as outlined above. The last method has been used in almost all cases reported thus far, more by default than by design, because commercial zeolites are thus doped by their method of preparation this... 

The use of tertiary amines as acid acceptors tends to stabilize some of the intermediate products and to simplify some of the problems of isolation. However, there are as yet few published reports on this phase of the problem. 

A variety of extraction separation techniques has been applied to problems of isolation and radiochemical purification of nuclides. Most of the extractions employed in radiochemistry are based on previously developed conventional analytical extractions, and general Information of the technique can be found in the literature (89,185) ... 

For these deprotonation procedures, the solubility of the ionic species present in the reaction mixture is of crucial importance. Although their solubility in water and in organic solvents might vary to some extent with their structure, the problem of isolation of pure target compounds of type 1 may sometimes be serious. In this connection, two examples of mesomeric betaines 10 reported by Dorofeenko and co-workers (78KG944) have been rechecked (87JOC5009, Table V). 

Consider the problem of isolating a low-molecular weight substance from a mixture of similar compounds. This problem can be solved in a very elegant manner with Sephadex if some suitable polymer can be combined with the substance under study to form a specific soluble and stable... 

The problem of isolating pure blood pigments from timicates is very difficult owing to the extreme air-sensitivity and reactivity of the native compounds. Several steps and techniques proved to be necessary to produce a few milligrams of purified material. Structure elucidation following isolation of the purified pigment is also a complex procedure. The main steps leading to structure determination are as follows. 

Problems of isolation which may arise for higher boiling or temperature sensitive acid halides can be circumvented by the use of polymer-bound phosphorus halides. Thus, a suspension of trisubstituted phosphine dichlorides, chemically connected to cross-linked polystyrene, reacted with carboxylic acids in methylene chloride almost quantitatively according to equation (3). ... 

Certainly one of the major experimental difficulties of organic chemistry is the separation and isolation of individual compounds from mixtures. The term active principles has been placed in the title with the thought that if it were only called Fractionation Theory or The Art of Separating Mixtures , little interest would be aroused. Nonetheless, to the organic chemist the problem of isolating an active principle is basically no different than any other problem of fractionation with the exception that the presence of a particular compound or group of compounds has been indicated by a specific biological response. 

Adsorption or chromatography is one of the best known methods of approach to the problem of isolating an active principle. Once a suitable adsorbent has been found, the problem is not difficult. The approach used for countercurrent distribution can aid in understanding the process (solid phase which takes the place of the heavier phase in the series of separatory funnels. If an adsorbent and solvent is chosen so that about half the solute is adsorbed from the solution, the conditions are met for easy application... 

Fibromyalgia is a disease characterized by diffuse musculoskeletal pain and generalized tender points. It affects approximately 2% of the general population, and women are more susceptible than men [18]. Studies show FM to be more prevalent within families than in the general population [19,20] however, the problem of isolating environmental and genetic factors continues. Some evidence exists that FM is autosomal dominant [21], An... 

First, the general method for calculating G (t) for any system for which the possible sites of the chromophores are nonequivalent will be outlined. Then, the necessary equations for treating the problem of isolated polymer chains will be given. 

The synthesis of transition metal alkyls usually involves the interaction of a very reactive metal alkyl, with a transition metal halide or alkoxide. This approach necessarily involves having two metals in the system which considerably complicates the problem of isolation of single pure compounds. In addition, low-valence transition metal compounds are electron-deficient molecules, and, for this reason, they will attempt to expand their coordination number by sharing ligands (halogen, alkyl, etc.) between two metal centers with the formation of bimetallic complexes. 

In forming artificial radioactive isotopes, problems of isolation are often encountered. For example, a product may decay quickly with the result that the initial product is contaminated with the daughter nuclide. 

Tung et al. (1991) devised another approach to the problem of isolating S-ibuprofen. They determined that the formation of a diasteriomeric salt yielded favorable solubility characteristics. They reacted 5-lysine with racemic ibuprofen to form the (5,5) and (R, S) salts. This afforded a 50-50 mixture of the two salts, from... 

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