Field-flow fractionation factors

Schure, M.R. (1999). Limit of detection, dilution factors, and technique compatibility in multidimensional chromatography, capillary electrophoresis, and field-flow fractionation. Anal. Chem. 71, 1645-1657. 

The capacity factor k to be discussed shortly, is an alternate measure of retention. While k is used more often than R in chromatography, the use of R is advantageous because (i) it is directly proportional to peak migration velocity and is thus a more direct measure of retention than k (ii) most equations describing chromatography are simpler when expressed in terms of R rather than k and (iii) R is a more universal measure of retention R but not k applies to other perpendicular flow methods such as field-flow fractionation. 

Figure 2 gives some characteristics of the size separation techniques that can be used to study the distribution of trace elements associated with various constituents of natural waters. It is obvious that the dimensions given in the figure are tentative as various factors influence the association/dissociation and aggregation/dispersion processes. However, preservation of real equilibria and labile species of elements, especially at concentrations of less than 10 g 1 prior to analysis is a much more serious problem encountered with methods that are not based on a direct physical separation. From this point of view, membrane filtration as well as some variants of field-flow fractionation (FFF) have advantages, although some uncertainties connected with equilibria shifts always exist. 

Giddings JC (1997) Factors influencing accuracy of colloidal and macromolecular properties measured by field-flow fractionation. Analytical Chemistry 69 552-557. 

Relaxation in FFF is the main factor for the exponential layer to form under the influence of the external field. The minimizing of relaxation time could lead to reduction of the total analysis time for the separation and characterization of colloidal samples by field flow fractionation. Looking into the future, it is reasonable to expect more experimental and theoretical work for further investigation. 

The surface phenomena in SdFFF are the main factors influencing the accuracy of colloidal properties measured by field-flow fractionation. It is, therefore, important to point out the interactions between the colloidal particles and the SdFFF channel wall in order to correct the separation resolution and/or the analyte characterization. 

In electrochemical cells we often find convective transport of reaction components toward (or away from) the electrode surface. In this case the balance equation describing the supply and escape of the components should be written in the general form (1.38). However, this equation needs further explanation. At any current density during current flow, the migration and diffusion fluxes (or field strength and concentration gradients) will spontaneously settle at values such that condition (4.14) is satisfied. The convective flux, on the other hand, depends on the arbitrary values selected for the flow velocity v and for the component concentrations (i.e., is determined by factors independent of the values selected for the current density). Hence, in the balance equation (1.38), it is not the total convective flux that should appear, only the part that corresponds to the true consumption of reactants from the flux or true product release into the flux. This fraction is defined as tfie difference between the fluxes away from and to the electrode ... 

Specific conversion rates are calculated in the usual way for a flow reactor k = (F/S) ln[(Ceq - C0)/(Ce<, - Cx)], where F is the flow rate (mol s 1), S the total catalyst surface (m2), C, the ortho-para equilibrium ratio at the reactor temperature, C0 the ratio for hydrogen entering the reactor and Cx the ratio for hydrogen leaving the reactor. For different samples of the same catalyst the zero field conversion reproducibility is seldom better than by a factor of 5, but the fractional change AkH = (kH - k0)/ko may often be reproduced to 5%. In some cases a change of 0.5% is measurable. (kH is the specific rate in a field H, k0 that in zero or negligible field). 

For dilute suspensions, particle-particle interactions can be neglected. The extent of transfer of particles by the gradient in the particle phase density or volume fraction of particles is proportional to the diffusivity of particles Dp. Here Dp accounts for the random motion of particles in the flow field induced by various factors, including the diffusivity of the fluid whether laminar or turbulent, the wake of the particles in their relative motion to the fluid, the Brownian motion of particles, the particle-wall interaction, and the perturbation of the flow field by the particles. 

The volume fraction of the dispersed phase is the most important factor that affects the viscosity of emulsions. When particles are introduced into a given flow field, the flow field becomes distorted, and consequently the rate of energy dissipation increases, in turn leading to an increase in the viscosity of the system. Einstein (24, 25) showed that the increase in the viscosity of the system due to addition of particles is a function of the volume fraction of the dispersed particles. As the volume fraction of the particles increases, the viscosity of the system increases. Several viscosity equations have been proposed in the literature relating viscosity to volume fraction of the dispersed phase. We discuss these equations in a later section. 

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