Analyte concentration gradient

This equation states that the formation of the analyte concentrational gradient in a fixed moment of time in any place of the column should be equal to the time-dependent variation of the analyte amount. 

In flow analysis, the Schlieren effect tends to be more pronounced [28] because a perfectly mixed flowing sample is not practically achievable, and an analyte concentration gradient is always present. Undesirable concentration gradients and/or discontinuities along the monitored sample can give rise to the formation of relatively steady liquid lenses as well as a myriad of randomly distributed transient mirrors. These optical artefacts lead to fluctuations in the emergent radiation that can alter the measured signal. 

Conventional or passive dialysis. The driving force is the analyte concentration gradient across the membrane, which reflects the concentration difference between the donor and acceptor solutions. Ions and low-molecular-weight compounds are transferred, whereas dissolved and suspended material with high molecular mass is retained. 

Sections, 10-12 p thick, were cut from flash-frozen tissue, thaw-mounted on MALDI plates, and dried in a desiccator prior to deposition of the matrix. Sublimation of 2,5-dihydroxy-benzoic acid over -4 min resulted in a homogenous 5-10 p coating. Alternatively, the tissue was dry-coated with matrix that was passed through a 20 p sieve for 20 min. Both methods were stable in the vacuum for 24 h, although back-sublimation is a risk that can create artificial analyte concentration gradients. 

Concentration gradients for the analyte in the absence of convection, showing the time-dependent change in diffusion as a method of mass transport. 

Concentration gradient for the analyte showing the effects of diffusion and convection as methods of mass transport. 

An ion-selective electrode contains a semipermeable membrane in contact with a reference solution on one side and a sample solution on the other side. The membrane will be permeable to either cations or anions and the transport of counter ions will be restricted by the membrane, and thus a separation of charge occurs at the interface. This is the Donnan potential (Fig. 5 a) and contains the analytically useful information. A concentration gradient will promote diffusion of ions within the membrane. If the ionic mobilities vary greatly, a charge separation occurs (Fig. 5 b) giving rise to what is called a diffusion potential. 

Volume of analyte injected As the volume of sample and standard solution injected increases, the peak height and duration increase. Nevertheless, if the volume increases too much, dispersion of the carrier into the analyte is limited and the concentration gradient within the flow is distorted resulting in very high dispersion values at the edges and very low values in the center. As a result, double peaks are recorded that are not appropriate for analytical measurement. 

The understanding of the nature of transient current after the imposition of a potential pulse is fundamental to the development of voltammetry and its analytical applications. Consider the same reaction, O+ne- = R, taking place in a quiet solution at a potential such that the reaction is diffusion controlled. Figure 18b.6a shows the pulse and Fig. 18b.6b shows the concentration gradient O as a function of time and distance from the electrode surface. 

Water and hydrocarbons occurring together, in shallow aquifer systems, may be considered immiscible for flow calculation purposes however, each is somewhat soluble in the other. Since groundwater cleanup is the purpose behind restorations, it receives greater attention. Definition of water quality based on samples retrieved from monitoring wells relies heavily upon the concentration of individual chemical components found dissolved in those samples. An understanding of the processes that cause concentration gradients is important for the proper interpretation of analytical results. 

In any chromatographic analysis the method of detection is determined by the nature of the analyte and the mobile phase used must not interfere with this system. The use of ultraviolet absorption detection systems is very common but the solvents used must not absorb significantly at the wavelength used. For instance, absorption at 280 nm is frequently used to detect protein but some solvents, e.g. acetone, absorb at this wavelength. Similarly the use of concentration gradients in the mobile phase may present problems with refractive index and electrochemical detection systems. 

After a period of time, a steep concentration gradient forms around the electrode since the solution immediately adjacent to the HMDE is entirely depleted of Cu. In response, (solvated) Cu analyte ions from the bulk of the solution will diffuse toward the HMDE and themselves be reduced. After a further period of time, all of the copper ions will have been removed from solution and accumulate on the surface of the drop (Figure 5.6(b)). Here, we say that we have exhausted the solution. ... 

When the fast reactions occurring in the system have stoichiometries different from the simple one shown by Eq. (5.78), analytical solutions of the diffusion equations are difficult to obtain. Nevertheless, numerical solutions can be obtained by iterative routines, and the results are conceptually similar to those described. The additional complications introduced by non-steady-state diffusion and nonlinear concentration gradients can be similarly handled. 

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