Matrix effect, kinetic

Dougherty and co-workers have conducted an especially detailed treatment of matrix effects in inhomogeneous solids, and have developed several valuable tools for modeling the kinetics of such systems. The details and applications of these methods on specific systems will be described later in this chapter. 

In subsequent experiments, it was found that rearrangement of 2 persisted in solid glasses at temperatures as low as 28 K, where the reaction rates became nearly temperature independent, as expected. The fact that similar kinetics were measured in solution and in low temperature solids indicated that matrix effects were minimal, at least in these intramolecular hydrogen abstractions. 

The interpretation of the elements of the matrix 0 is slightly more subtle, as they represent the derivatives of unknown functions fi(x) with respect to the variables x at the point x° = 1. Nevertheless, an interpretation of these parameters is possible and does not rely on the explicit knowledge of the detailed functional form of the rate equations. Note that the definition corresponds to the scaled elasticity coefficients of Metabolic Control Analysis, and the interpretation is reminiscent to the interpretation of the power-law coefficients of Section VII.C Each element 6% of the matrix measures the normalized degree of saturation, or likewise, the effective kinetic order, of a reaction v, with respect to a substrate Si at the metabolic state S°. Importantly, the interpretation of the elements of does again not hinge upon any specific mathematical representation of specific... 

Matrix effects in the analysis of nutrients in seawater are caused by differences in background electrolyte composition and concentration (salinity) between the standard solutions and samples. This effect causes several methodological difficulties. First, the effect of ionic strength on the kinetics of colorimetric reactions results in color intensity changes with matrix composition and electrolyte concentration. In practice, analytical sensitivity depends upon the actual sample matrix. This effect is most serious in silicate analysis using the molybdenum blue method. Second, matrix differences can also cause refractive index interference in automated continuous flow analysis, the most popular technique for routine nutrient measurement. To deal with these matrix effects, seawater of... 

In contrast with the sensors described elsewhere in this Chapter, the device proposed by the authors group uses no reagent, but photons, to induce a photochemical reaction, and involves electrochemical detection of the photochemical product, which allows one to continuously monitor the formation of the electroactive product. Kinetic monitoring increases the selectivity of determinations by eliminating matrix effects and the contribution of side reactions, whether slower or faster than the main reaction. The electrochemical system chosen for implementation of this special sensor was the Fe(II)/C204 couple, which was used for the kinetic determination of oxalate ion based on the following reaction ... 

The initial step of the reaction, Eq. (7), provides the individual metal sulfide molecules via reaction of the M2+ ions with H2S. In the case of films derived from fatty acids, the two carboxylate functions, associated with the M2+ ion, are the sink for the two protons released from the reaction. The diffusion and coalescence of the individual MS molecules to give MS particles are depicted in Eq. (8). Despite an abundance of literature concerning Q-state particle formation in LB films, there has been little discussion relating to mechanistic aspects of how the nature of the LB support matrix effects the processes depicted in Eq. (7) and (8). The remainder of this section outlines the mechanistic and kinetic insights gained into these processes over the course of study of metal chalcogenide formation in LB films. 

One serious limitation common to most small-amplitude techniques is the greatly reduced response for systems with slow charge-transfer kinetics. Due to the high activation energy of slow electron-transfer processes, they are particularly sensitive to the presence of other species in the solution. Real-world environmental samples are notoriously dirty and these matrix effects can be difficult to deal with. Applications notes from the instrument manufacturers are frequently an invaluable source of practical information for dealing with these problems for specific elements and matrices. 

In cases where well-defined pores ranging in sizes from a few hundredths to several hundred microns exist throughout the matrix, the kinetics of drug release can still be described by Equations (1)—(3) provided that an effective diffusion coefficient is used. When the drug diffusion only takes place through the solvent filled porous network, the effective diffusion coefficient is further related to the matrix structure by ... 

If the compound is present at a low concentration in the presence of many interfering compounds, cleanup and concentration steps become necessary [8]. Solvent extraction procedures are used often for sample preparation for drugs and many other small molecules in the chromatographic technique. Solid-phase methods are very popular because of the wide choice of the packing material. Sample extraction methods have two other main advantages in CE, namely sample concentration and elimination of both sample ions and proteins. Double-solvent extraction is very useful for elec-trokinetic injection, especially for basic compounds. It eliminates variability due to matrix effects in electro-kinetic injection. 

Accelerated solvent extraction is a closed system of extraction which utilises higher temperature and pressure. Closed systems are designed to minimise the loss of volatiles, improve the efficiency and increase the throughput. The elevated temperature improves analyte solubility. For example, anthracene, a PAH, is 15 times more soluble in methylene chloride at 150°C than at 50°C. High temperature also helps to overcome sample extraction matrix effects and gives faster desorption kinetics. The lower solvent viscosity allows diffusion of the solvent into the matrix to occur more quickly than other extraction techniques. Increased pressure also elevates the boiling point of the solvent. 

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