Separation by molecular size

Separation and concentration of high-molecular-weight solutes from low-molecular-weight solutes by application of hydrodynamic force over the solution above a semipermeable membrane (ultrafiltration). 

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Chemical separations may first be accomplished by partitioning on the basis of polarity into a series of solvents from non-polar hexane to very polar compounds like methanol. Compounds may also be separated by molecular size, charge, or adsorptive characteristics, etc. Various chromatography methods are utilized, including columns, thin layer (TLC) gas-liquid (GLC), and more recently, high pressure liquid (HPLC) systems. HPLC has proven particularly useful for separations of water soluble compounds from relatively crude plant extracts. Previously, the major effort toward compound identification involved chemical tests to detect specific functional groups, whereas characterization is now usually accomplished by using a... 

Gel filtration with Sephadex was used by Ghassemi and Christman [426] to make separations by molecular size on water samples concentrated by vacuum evaporation. Fluorescence was also used as one method for following the fractionation. Molecular size was also used by Gjessing [427] but with pressure dialysis as the method of separation. A similar method of concentration and separation was used by Brown [428] to follow the dispersion of these materials as fresh and salt water mixed in the Baltic Sea. 

The elution volumes of polystyrene and benzene in the size-exclusion mode were 0.98 and 1.78 ml, respectively (Figure 1.4A). This means that separations by molecular size can be achieved between 0.98 and 1.78 ml in this system. In the normal phase mode the elution volumes of octylbenzene and benzene were 1.98 and 2.08 ml, respectively, in n-hexane solution (Figure 1.4B). This type of chromatography is called adsorption or non-aqueous reversed-phase liquid chromatography. These are adsorption liquid chromatography and non-aqueous reversed-phase liquid chromatography. The elution order of the alkylbenzenes in the reversed-phase mode using acetonitrile was reversed... 

Packings based on silicas for separation by molecular size are also presently available. However, the molecular size range available is more limited and possibility of encountering adsorption effects is more likely. Such columns must be throughly evaluated for each new type of sample for which a separation and/or molecular weight distribution is desired. 

Synergistic Use of Mixed Separation Mechanisms. In OC the first SBC utilizes conventional separation by molecular size exclusion. 

Gel-permeation. This clean-up procedure is based on the separation by molecular size. 

Traditional adsorbents such as sihca [7631 -86-9] Si02 activated alumina [1318-23-6] AI2O2 and activated carbon [7440-44-0], C, exhibit large surface areas and micropore volumes. The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However, the micropore size distribution is fairly broad for these materials (45). This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited (see Aluminum compounds, aluminum oxide (ALUMINA) Silicon compounds, synthetic inorganic silicates). 

Species separated by molecular sieving effects when kinetic diameters fall iato different zeoHte aperture size categories (standard molecular sieve diameters = 300, 400, 500, 800, 1000,1300 pm. 

Nanoparticles can be separated by molecular exclusion chromatography just as proteins are separated. Figure 26-15 shows the relation between measured size and retention time of CdSe quantum dots. These are particles containing 2 000 CdSe units in a dense, crystalline core capped by alkyl thiol (RS) groups on Cd and trialkylphosphine (R3P) groups on Se. 

The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However-, the microporc size distribution is fairly broad for these materials. This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited. 

Exclusion or gel permeation, in which compounds are separated by molecular si/.e into a range of pore sizes in a polymeric gel. This method is useful for measuring Ihe molecular-weight distribution of polymers. 

LIQUID CHROMATOGRAPHY. An analytical method based on separation of the components of a mixture in solution by selective adsorption. All systems include a moving solvent, a means of producing solvent motion (such us gravity or a pump I, a means ol sample introduction, a fractionating column, and a detector. Innovations in functional systems provide the analytical capability for operating in three separation modes (1) liquid-liquid partition in which separations depend on relative solubilities of sample components in two immiscible solvents (one of which is usually water) 12) liquid-solid adsorption where the differences in polarities nf sample components and their relative adsorption on an active surface determine tile degree ol separation (2) molecular size separations which depend on the effective molecular size of sample components ill solution. 

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