Obstruction factors

Elution volume, exclusion chromatography Flow rate, column Gas/liquid volume ratio Inner column volume Interstitial (outer) volume Kovats retention indices Matrix volume Net retention volume Obstruction factor Packing uniformity factor Particle diameter Partition coefficient Partition ratio Peak asymmetry factor Peak resolution Plate height Plate number Porosity, column Pressure, column inlet Presure, column outlet Pressure drop... 

FIG. 29 Obstruction factor from barrier theory of Boyack and Giddings for fibers. (Reprinted with permission Ref. 45, Copyright 1963, Academic Press.)... 

Obstruction factors, 29, 32 Open tubular (OT) columns, see Column, open tubular (OT)... 

An obstruction factor t i is included in the numerator of this term to account for the obstruction to free diffusion caused by packed beds. Some typical values of i r are given in Table 3. The faster the mobile phase moves, the less time the zone is in the column, the less time there is for diffusion, and the lower is the broadening due to diffusion. Consequently, this term... 

Volume fraction Flow resistance parameter Obstruction factor (rate equation)... 

During the diffusion of small molecules (with molecular weights less than about 1000 g/mol or molar volumes less than about 0.500 m3/kg) in protein solution, the diffusion may be blocked by the large molecules. In order to account for this effect, we need the diffusivity l)Mi of solute A in water alone, the water of hydration on the protein, and an obstruction factor. A semitheoretical relation to approximate the diffusivity of solute in a globular-type protein solution is... 

The B constant is a function of longitudinal diffusion that serves to spread analyte molecules apart and is linearly dependent on D, the diffusion coefficient of an analyte in the mobile phase according to an obstruction factor designated... 

The velocity-independent term A characterises the contribution of eddy (radial) diffusion to band broadening and is a function of the size and the distribution of interparticle channels and of possible non-uniformiiies in the packed bed (coefficient A.) it is directly proportional to the mean diameter of the column packing particles, dp. The term B describes the effect of the molecular (longitudinal) diffusion in the axial direction and is directly proportional to the solute diffusion coefficient in the mobile phase, D, . The obstruction factor y takes into account the hindrance to the rate of diffusion by the particle skeleton. 

The diffusion coefficient must be reduced in packed columns because there are obstructions that prevent simple linear diffusion. This is given a dimensionless measure as an obstructive factor, y, with a value of approximately 0.6. Simple substitution... 

B — longitudinal diffusion term = 2jDm y obstruction factor Dm = diffusion coefficient... 

When using a support with sufficiently large pores and/or a completely nonporous support, or in the case that the pores of a microporous support are completely filled with the appUed Uquid sorbent, the term al C ) can be omitted. When the liquid sorbent forms a completely continuous film on the support (a situation which may occur in an ideal case when using a macroporous support or when using a capillary column), then q in the term al(Cg)) has a value of 2/3, whereas in the case of a microporous support with the pores filled completely with the liquid sorbent, q in the term o (Cs,) equals l/SOyg and df = dp, where is the obstructive factor for diffusion of the solute in the liquid sorbent inside the pores. 

The factor y, appropriately called the obstruction factor, depends on the amount and the nature of the obstruction that is in the way of free movement of the molecule. 

Care must be taken with the definition of the velocity. S ne part of the obstruction factor can be explained by an inappropriate definition of the velocity. We must keep in mind that it refers to the residence time in the mobile phase of the sample compound whose diffusion is measured. This residence time is not necessarily identical to the residence time of an unretained sample compound, which is used to measure the linear velocity. Also, we implicitly assimied that the diffusion coefficient in the pores is the same as the diffusion coefficient in the interstitial mobile phase. This is also not necessarily the case. If me pore size is less than about 10 times larger than the size of the molecule, the diffusion coefficient depends on the ratio of the size of the sample molecule to w pore size of the packing. 

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