Mass-Transport Limitation

Mass transport limitation occurs when the association of an analyte (protein) to the ligand (DNA) is limited by the diffusion of analyte to the surface of the chip. 

Most protein-DNA interactions are characterized by very fast association rates. If a protein has an association rate constant (feon) above 1 x 10 M s it will likely be limited by mass transport [6]. This can be reduced by several means  

1) The amount of DNA immobilized should be as low as possible. As a rule of thumb the response of the protein should not exceed 100 RU—the range of 30-50 RU still works well. 

3) New BIAcore systems such as BIAcore 3000 or TlOO use a different flow cell geometry than the older systems BIAcore 2000 or BIAcore X, further reducing mass transport effects. 

4) No glycerol or sucrose should be present in the buffer or sample since this reduces the diffusion rate. 

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The expression for the mass-transport-limiting current density may be employed together with the Nemst equation to deduce the complete current-potential response in a solution containing only oxidized or reduced species... 

Skovborg, P. and Rasmussen, P., 1994. A mass transport limited model for the growth of methane and ethane gas hydrates. Chemical Engineering Science, 49(8), 1131-1143. 

Fig. 22. Mass transport limitation and CO breakthrough in a monolith with high intrinsic catalystic activity.

Each concentration can be evaluated assuming no mass transport limitation and the usual priorities for the distribution of the emulsifier polymer particles > aqueous phase > micelles (12). The... 

Phase transfer catalysts these have been around for about 40 years and were developed as a means of increasing the rates and yields of reactions in which the reactants are in two separate phases. In these cases poor mass transport often limits the reaction. Phase transfer catalysts act by transporting the reactants from one phase into another, thus overcoming mass-transport limitations. 

We shall now introduce an efficiency factor, which is again defined as the ratio of the conversion in the pore with and without mass transport limitation ... 

The second mechanism often invoked to explain the increase in n y of simple Fe porphyrins at potentials more reducing than that of the Fe couple (under anaerobic conditions) is based on the fact that at such potentials the fraction of the catalyst in the 5 -coordinate ferrous state is maximal because (i) the equilibrium (18.9) is shifted completely to the ferrous form and (ii) the concentration of O2 in the catalytic film is low owing to mass transport limitations. The higher the concentration of the 5-coor-dinate ferrous porphyrin in the catalytic film, the greater the probability that any released H2O2 will re-enter the catalytic cycle by coordinating to a molecule of ferrous porphyrin and decay according to (18.13b) instead of (18.17). 

The majority of RDC studies have concentrated on the measurement of solute transfer resistances, in particular, focusing on their relevance as model systems for drug transfer across skin [14,39-41]. In these studies, isopropyl myristate is commonly used as a solvent, since it is considered to serve as a model compound for skin lipids. However, it has since been reported that the true interfacial kinetics cannot be resolved with the RDC due to the severe mass transport limitations inherent in the technique [15]. The RDC has also been used to study more complicated interfacial processes such as kinetics in a microemulsion system [42], where one of the compartments contains an emulsion. 

Wilson, D.J., Gomez-Lahoz, C. and Rodriquez-Maroto, J.M., Soil cleanup by in-situ aeration. XVI. Solution and diffusion in mass-transport-limited operation and calculation of Darcy s constants. Sep. Sci. Technol., 29, 1133-1163, 1994. 

The solutions for moisture uptake presented in this section are based on the experimental condition of a pure water vapor atmosphere. In the next section a derivation of moisture uptake equations is based on both heat and mass transport that are characteristic of moisture uptake in air. The final section of this chapter presents the results of studies where heat transport is unimportant and mass transport dominates the process. Thus, we will have a collection of solutions covering models that are (1) heat transport limited, (2) mass transport limited, (3) heat and mass transport limited, and (4) mass transport limited with a moving boundary for the uptake of water by water-soluble substances. 

The basic assumption for a mass transport limited model is that diffusion of water vapor thorugh air provides the major resistance to moisture sorption on hygroscopic materials. The boundary conditions for the mass transport limited sorption model are that at the surface of the condensed film the partial pressure of water is given by the vapor pressure above a saturated solution of the salt (Ps) and at the edge of the diffusion boundary layer the vapor pressure is experimentally fixed to be Pc. The problem involves setting up a mass balance and solving the differential equation according to the boundary conditions (see Fig. 10). 

Figure 10 Moisture uptakes in rectangular coordinates for the mass transport limited model.

We now use Eq. (39) to build models for the mass transport limited uptake of water by hygroscopic materials. 

A. Mass Transport Limited Moisture Uptake in One Dimension... 

Consider first the corrosion of low alloy steel in HC1 per se, i.e. before the addition of organic inhibitors. As shown in Figures 1 and 2 for N80 steel in 15% and 28% HC1 at 65 C, Nyquist plots for steel in concentrated HC1 typically have only one distinct feature a single capacitance loop (a loop above the Z axis) with a hint of a second capacitance loop at lower frequencies. The low-frequency loop is more fully developed in 28% HC1 than in 15% HC1. Mass transport limitations are not evident except under extreme conditions, e.g. above 28% HC1 and 65 C. 

If kc is very large such that we are in the mass transport-limited region of the current and the second term in equation (2.162) — 0, then the observed current is its limiting value ... 

Moffat [80] reported the electrodeposition of Ni-Al alloy from solutions of Ni(II) in the 66.7 m/o AlCl3-NaCl melt at 150 °C. The results obtained in this melt system are very similar to those found in the AlCh-EtMcImCI melt. For example, Ni deposits at the mass-transport-limited rate during the co-deposition of Al, and the co-deposition of Al commences several hundred millivolts positive of the thermodynamic potential for the A1(III)/A1 couple. A significant difference between the voltammetric-derived compositions from the AlCl3-NaCl melt and AlCl3-EtMeImCl melt is that alloy composition is independent of Ni(II) concentration at the elevated temperature. Similar to what has been observed for room-temperature Cu-Al, the rate of the aluminum partial reaction is first order in the Ni(II) concentration. Moffat s... 

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