Compound-specific coefficients

Methods Based on the Frost-Kalkwarf Equation The Frost-Kalkwarf equation relies on four compound-specific coefficients to correlate vapor pressure-temperature data ... 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

Where is the initial analyte concentration in the liquid phase, C( the concentration of analyte in the gas phase, K the gas-liquid partition coefficient for the analyte at the analysis temperature, V, the volume of liquid phase, and V, the volume of gas phase (318-321,324,325). From equation (8.3) it can be seen that the concentration of the analyte in the headspace above a liquid in equilibrium with a vapor phase will depend on the volume ratio of the geis and liquid phases and the compound-specific partition coefficient which, in turn, is matrix dependent. The sensitivity 1 of the headspace sampling method can be increased in some instances adjusting the pH, salting out or raising the... 

Because neither nor are strong functions of the physical-chemical properties of the analytes (see below), the issue of which phase controls the uptake rate is primarily governed by the membrane-water partition coefficient, which varies between compounds by many orders of magnitude (Reynolds et al., 1990 Lefkovitz et al., 1996 Booij et al., 2003a). With increasing log Ko , there always will be a critical log Kov, value where the uptake rates will be controlled by the WBL instead of by the membrane. Next to /fmw (which is a compound specific property) it is important to note that rate control is also dependent on the magnitude of kw, which is determined by the hydrodynamical conditions prevailing at the membrane-water... 

Most QPPRs, QSPRs, or GCMs yield properties at a reference temperature. Temperature-dependent property estimation is often restricted to compounds for which the compound-specific temperature coefficients in thermodynamic or empirical relationships have been evaluated. Compound-specific temperature functions have the following general form ... 

Selected compound-specific functions, property-temperature-property relationships, or structure-temperature-property relationships are supplied and discussed in this book for density (Section 3.5), refractive index (Section 4.5), surface tension (Section 5.4), viscosity (Section 6.4), vapor pressure (Section 7.4), enthalpy of vaporization (Section 8.5), aqueous solubility (Section 11.8), and air-water partition coefficients (Section 12.5). 

It may be deduced from KP = Koc x foc that partition coefficients of hydro-phobic organic compounds in general are dependent upon the chemical of interest (compound-specific properties affect the value of Koc) and the matrix properties of the medium in which it resides. In addition to the fraction of organic carbon present in the sorption phase, additional environmental factors affect partitioning. These factors include temperature, particle size distribution, the surface area of the sorbent, pH, ionic strength, the presence of suspended material or colloidal material, and the presence of surfactants. In addition, clay minerals may act as additional sorption phases for organic compounds. Nevertheless, organic carbon-normalized partition... 

The second set of parameters are compound specific and will determine its transport over barriers (e.g., between gut and blood, blood and tissues), its biotransformation, and its excretion. Most of the transport parameters can be described as partition coefficients [20] and are strongly determined by the compound s physicochemical parameters, lipophilicity and volatility being the most important ones. However, the role of transporter proteins present in cellular membranes and responsible for the partitioning over specific barriers needs special attention, e.g., in the gut, the kidney, and at the blood-brain barrier [22], the blood placenta barrier, and the liver-bile interface [23]. 

In constructing a PBBK model, the physiological parameters for a specific organism or a population can be found in the literature [24], The chemical-specific parameters can be either measured or modelled. For many compounds partition coefficients have been determined... 

The needed compound-specific Inputs Include molar extinction coefficients, indicating the efficiency with which incident light is absorbed, and a disappearance quantum yield ( ) which gauges the efficiency for conversion of absorbed light energy to pho-... 

The organic carbon-water partition coefficient is a compound-specific term that allows the user to estimate the mobility of a solvent in saturated-soil water systems if the amount or organic carbon is known. For hydrophilic solvents, values have been measured for many compounds. Other values were derived from empirical relationships drawn between water solubility or octanol-water partition coefficients. ... 

The oxidation of alkanes, alkenes and simple aromatics at 293 K under NOx rich tropospheric conditions has been studied using laser pulse initiation combined with cw laser long path absorption/LIF for the detection of OH and NO2. In the case of aliphatic hydrocarbons the absolute yield and the kinetics of the formation of these products have been found to be sensitive indicators for the reaction behaviour of the oxy radicals RO. In combination with mechanistic simulations rate constants for individual reactions as well as branching ratios have been derived, which permit the evaluation of the compound specific NO/NO2 conversion factors (NOCON - factors) for the first oxidation steps. In the case of benzene and toluene oxidation the results indicate that reaction of the primary formed X cyclohexa-dienyl radical (X = Cl, OH) with O2 is the dominant pathway, although the rate coefficients were found to be lower than 2 x 10" cmVs. 

The compound-specific data required for exposure assessments comprise the 1-octanol/water partition coefficient (log water solubility (S ), vapour pressure (p ), Henry s law constant (H, H ), soil sorption coefficient hydrolysis half-life time, photolysis half-life time and information on biodegradability (OECD, 1993c). These parameters generally relate to steady-state conditions - conditions that are rarely met in the real environment. The experimental data underlying the QSAR models are preferably determined by standardized protocols, but, even then, the absolute values are of variable reliability and precision, which clearly affects the accuracy of the predictions based on the acquired QSARs. The endpoints discussed in the following sections were selected because of their consideration in regulatory evaluation schemes in, for example, the EU (EEC, 1990). 

However, even at low partial pressures, carboxylic acids may be extensively dimerized in the vapour phase, and corrections are necessary at higher pressures the fugacities of components must be obtained from published data or by applying a fugacity coefficient , = //p, derived from a generalized or compound-specific chart.)... 

The coefficients of extinction in acetone for each pigment are calculated from those given in the bibliography for chlorophylls and pheophytins in ethyl ether [194] by using a pigment solution of known concentration. It is assumed that the coefficients of extinction of compounds with different chemical structure but identical electron absorption spectrum (e.g., chlorophyll/chlorophyllide) do not differ significantly [251]. Thus, the specific coefficient of extinction (e ) can be calculated from a known molar coefficient (st) of another compound (i) with an identical spectrum ... 

Indicators There are certain compounds that are suitable as indicators for sensitive and specific clinical analysis. Nicotinamide adenine dinucleotide (NAD) occurs in oxidized (NAD" ) and reduced (NADH) forms. Nicotinamide adenine dinucleotide phosphate (NADP) also has two states, NADP" and NADPH. NADH has a very high uv—vis absorption at 339 nm, extinction coefficient = 6300 (M cm) , but NAD" does not. Similarly, NADPH absorbs light very strongly whereas NADP" does not. 

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