Retention factors controlling

According to Equation 3, the resolution of two peaks in column separation is controlled by three major variables retention defined in terms of the retention factor k column efficiency expressed as the number of theoretical plates N and selectivity characterized by the selectivity factor a [48] ... 

The graph in Fig. 10.12 shows that the purity decreases very quickly below acceptable levels as retention factor of the more retained enantiomer decreases. Flowever, with minor adjustment of the SMB internal flow rates, a variation of more than 10 % of the retention factor of the more retained enantiomer still meets required purity, productivity, and eluent consumption. Control of critical parameters such as retention factors can be made without modification of the feed and eluent flowrates. 

The above results only demonstrate the chromatographic behaviour of compounds with logP = 3. Their relative retention factors thus are influenced by the properties of solvents in the eluent. The relative values cannot be controlled by only one property of the solvents and chromatographic behaviour therefore also depends on the properties of the analytes. 

Increasing the column temperature reduces the retention factor. The ion-pair formation is based on a chemical equilibrium therefore, temperature control is important to obtain reproducible results. 

The silanol induced peak tailing is also a function of the pH of the mobile phase. It is much less pronounced at acidic pH than at neutral pH. Therefore many of the older HPLC methods use acidified mobile phases. However, pH is an important and very valuable tool in methods development. The selectivity of a separation of ionizable compounds is best adjusted by a manipulation of the pH value. The retention factor of the non-ionized form of an analyte is often by a factor of 30 larger than the one of the ionized form, and it can be adjusted to any value in between by careful control of the mobile phase pH. This control must include a good buffering capacity of the buffer to avoid random fluctuations of retention times. 

N is the average column isocratic theoretical plate number is the retention factor at the point of elution controlling the bandwidths in gradient elution— Equation 5.5... 

In cases where the various factors controlling regiospecificity are in conflict, mixtures of products may arise. Thus 1,3,5-trihydroxyxanthone affords a mixture of the chromenes (79) and (80) <70JCS(C)1662). The transition state leading to the linear chromene allows retention of the chelate stabilization with loss of pyranone resonance, whilst the reverse situation obtains in the formation of the angular molecule. 

Factors Controlling Speciation of S. Speciation of S in sediments is influenced by many of the same factors that control S retention. However, S speciation may be a more sensitive indicator of many of these variables. 

In MLC, the mobile phase consists of surfactants at concentrations above their critical micelle concentration (CMC) in an aqueous solvent with an alkyl-bonded phase (52). Retention behavior in MLC is controlled by solute partitioning from the bulk solvent into micelles and into stationary phase as well as on direct transfer from the micelles in the mobile phase into the stationary phase. Eluent strength in MLC is inversely related to micelle concentration. A linear relationship exists between the inverse of retention factor and micelle concentration. Similar to what is observed in RPLC, a linear relationship exists between retention in MLC and , the volume fraction of the organic modifier. Modeling retention in MLC is much more complicated than in RPLC. The number of parameters is important. Micelles are obviously a new domain in both liquid chromatography and electrophoresis. Readers interested in the topic will appreciate Ref. 53, a special volume on it. 

Timmermans, J., and Moes, A. J. Factors controlling the buoyancy and gastric retention capabilities of floating matrix capsules New data for reconsidering the controversy. J. Pharm. Sci. 83(l) 18-24, 1994. 

There are two classes of stationary phases in SEC, one type for GFC and the other for GPC. Stationary phases for GFC are hydrophilic and include polydextrans, polyvinyl alcohol gel, and silica gel those for GPC are hydrophobic, typically cross-linked, rigid polystyrene-divinylbenzene gels. Generally, columns of 15 to 50 cm length are used, packed with 7- to 10-/t,m particles and with an internal diameter between 0.6 and 0.8 cm. In SEC, unlike in other chromatographic modes, the stationary phase is the primary factor controlling retention. 

The experimental apparatus and technique has been described in detail elsewhere.(20>21) The retention factors of naphthalene and biphenyl under isothermal conditions at various pressures were obtained using capillary columns coated with a cross-linked phenyl polymethylphenyl-slloxane stationary phase with carbon dioxide as the fluid mobile phase. A Varian 8500 syringe pump was operated under computer control providing accurate, pulsefree control of the fluid pressure. 

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