Size selectivity, fractionation characterization

A more complicated, but flexible, system has been reported by Blomberg et al. (46). Here, size exclusion chromatography (SEC), normal phase EC (NPLC) and GC were coupled for the characterization of restricted (according to size) and selected (according to polarity) fractions of long residues. The seemingly incompatible separation modes, i.e. SEC and NPLC, are coupled by using an on-line solvent-evaporation step. 

The Xenopus oocyte expression system can be used for characterization of products of specific mRNAs transcribed in vitro from cDNA isolates. Expression cloning of novel cDNAs, whose function can be assayed following expression, can also be performed using oocytes. In the latter case, as shown in Eig. 4.3, total mRNA is fractionated by size, and expression in oocytes is used to identify mRNA fractions capable of producing the protein of interest. The enriched mRNA fractions are used for cDNA preparation. The cDNA of interest is identified by its ability to select (by hybridization) its mRNA from the total mRNA pool. The selected mRNA is assayed again in oocytes, as shown in Fig. 4.3. Repeated rounds of this procedure with further enrichment of the desired cDNA from the pool leads to the isolation of the cDNA. Alternatively, in vitro transcription of the cDNA pool can also be used to generate mRNA for injection and assay. Component cDNAs of the pool that... 

Three new methods to characterize the pore structure and pore size distribution in the top layer of asymmetric membranes have been developed or refined in our laboratory during the past few years a) the gas ad-sorption/desorption method, b) thermoporometry and o) selective permeation (fractional rejection). 

For a system consisting of C components, the phase rule indicates that, in the two-phase region, there are F=C-2 + 2 = C degrees of freedom. That is, it takes C independent variables to define the thermodynamic state of the system. The independent variables may be selected from a total of 2C intensive variables (i.e., variables that do not relate to the size of the system) that characterize the system the temperature, pressure, C - 1 vapor-component mole fractions, and C - 1 liquid-component mole fractions. The number of degrees of freedom is the number of intensive variables minus the number of equations that relate them to each other. These are the C vapor-liquid equilibrium relations, Yj = K,X, i=l,. .., C. The equilibrium distribution coefficients, AT, are themselves functions of the temperature, pressure, and vapor and liquid compositions. The number of degrees of freedom is, thus, 2C - C = C, which is the same as that determined by the phase rule. 

Uitrafiltration membranes have a range of pore sizes in the selective layer, and they are often characterized by a molecular weight cutoff based on measurements of fraction rejected versus molecular weight.Molecules larger than the... 

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