Steady-state absorbance

An alternative to derivatizing carbohydrates is the use of indirect photometric detection. In this method, a detectable co-ion in the electrolyte is added to the buffer system generating a steady state absorbance signal in the detector. As the analyte ions migrate in front of the detector window, they displace the detectable co-ion and cause a decrease or negative response in the detector signal. This method provides universal detection of all anions or cations. Since most carbohydrates are not ionized... 

In this paper, we present a preliminary analysis of the steady-state and time-resolved fluorescence of pyrene in supercritical C02. In addition, we employ steady-state absorbance spectroscopy to determine pyrene solubility and determine the ground-state interactions. Similarly, the steady-state excitation and emission spectra gives us qualitative insights into the excimer formation process. Finally, time-resolved fluorescence experiments yield the entire ensemble of rate coefficients associated with the observed pyrene emission (Figure 1). From these rates we can then determine if the excimer formation process is diffusion controlled in supercritical C02. 

Lowe, D.J., Fisher, K., and Thomeley, R.N.F. (1993) Pre-steady-state absorbance changes show that redox changes occur in the Klebsiella pneumonia MoFe-protein that depend on substrate and components ratio a role for P-centers in reducing dinitrogen, Biochem. J. 292, 93-. 

The absorbance of liquid crystalline solutions of PBLG as calculated from the relation, ) = Vs (On + 2 >x), dightly increases or decreases with time, depending on the solvent used when an electric field is applied to the solution, whereas, it should be constant for inddent light perpendicular to the field direction (and to the orientation direction), as in this case if wall effects are not involved. The steady-state absorbance, however, becomes independent of both temperature and external field strength in case the ratio of 730 is obtained, indicating that wall effects are then ne igible (25). 

By cyclic potential scan steady-state chronoabsorptometry (CPS/ SSCA) OTTLSET, a steady-state absorbance-cyclic potential scan (A-F) curve at a certain wavelength is recorded. The two inflection points for the oxidation and reduction processes in the A-E curve correspond to... 

As p-nitrophenylphosphate diffuses from the bulk solution to the biocatalytic layer, p-nitrophenoxide is formed at the surface of the fiber-optic bundle. A portion of the incident radiation is absorbed by this chromophore and a decrease in the amount of light that reaches the detector is recorded. A steady-state concentration of p-nitrophenoxide is established which corresponds to a steady-state absorbance value. A linear response curve is obtained when the resulting absorbance is plotted with respect to the concentration of p-nitrophenylphosphate in the bulk solution. Figure 2 shows an example of such a response curve. 

With reference to Fig. 1, A is the constant steady state absorbance (integrated signal amplitude) between d and 1. A and A are... 

Lin S H and Villaeys A A 1994 Theoretical description of steady-state sum-frequency generation in molecular absorbates Phys. Rev. A 50 5134-44... 

Medroxyprog esteroneAcetate. Accurate pharmacokinetic and metaboHsm studies on MPA have been difficult because the radioimmunoassays employed caimot differentiate between MPA and its metaboHtes (346). Comparison of MPA plasma levels assayed by hplc and radioimmunoassay show that radioimmunoassay may overestimate intact MPA concentrations by about fivefold (347). However, values of the mean elimination half-life of MPA were similar, being 33.8 and 39.7 h when measured by hplc and radioimmunoassay, respectively (347). Approximately 94% of MPA in the blood is bound to albumin (348). When taken orally, MPA is rapidly absorbed with Htde or no first-pass metaboHsm (13). Peak semm levels ate reached after 3 h. Steady state occurs after three days of daily adininistration (349). The pharmacokinetics of MPA when adininistered in a depot formulation have been described (350). 

Fluorescence. The fluorescence detection technique is often used in clinical chemistry analyzers for analyte concentrations that are too low for the simpler absorbance method to be appHed. Fluorescence measurements can be categorized into steady-state and dynamic techniques. Included in the former are the conventional simultaneous excitation-emission method and fluorescence polarization. 

Tocainide is rapidly and well absorbed from the GI tract and undergoes very fitde hepatic first-pass metabolism. Unlike lidocaine which is - 30% bioavailable, tocainide s availability approaches 100% of the administered dose. Eood delays absorption and decreases plasma levels but does not affect bio availability. Less than 10% of the dmg is bound to plasma proteins. Therapeutic plasma concentrations are 3—9 jig/mL. Toxic plasma levels are >10 fig/mL. Peak plasma concentrations are achieved in 0.5—2 h. About 30—40% of tocainide is metabolized in the fiver by deamination and glucuronidation to inactive metabolites. The metabolism is stereoselective and the steady-state plasma concentration of the (3)-(—) enantiomer is about four times that of the (R)-(+) enantiomer. About 50% of the tocainide dose is efirninated by the kidneys unchanged, and the rest is efirninated as metabolites. The elimination half-life of tocainide is about 15 h, and is prolonged in patients with renal disease (1,2,23). 

Elecainide is weU absorbed and 90% of the po dose is bioavailable. Binding to plasma protein is only 40% and peak plasma concentrations are attained in about 1—6 h. Three to five days may be requited to attain steady-state plasma concentrations when multiple doses are used. Therapeutic plasma concentrations are 0.2—1.0 lg/mL. Elecainide has an elimination half-life of 12—27 h, allowing twice a day dosing. The plasma half-life is increased in patients with renal failure or low cardiac outputs. About 70% of the flecainide in plasma is metabolized by the Hver to two principal metaboUtes. The antiarrhythmic potency of the meta-O-dealkylated metaboUte and the meta-O-dealkylated lactam, relative to that of flecainide is 50 and 10%, respectively. The plasma concentrations of the two metaboUtes relative to that of flecainide are 3—25%. Elecainide is mainly excreted by the kidneys, 30% unchanged, the rest as metaboUtes or conjugates about 5% is excreted in the feces (1,2). 

The stagnant-film model discussed previously assumes a steady state in which the local flux across each element of area is constant i.e., there is no accumulation of the diffusing species within the film. Higbie [Trans. Am. Jn.st. Chem. Eng., 31,365 (1935)] pointed out that industrial contactors often operate with repeated brief contacts between phases in which the contact times are too short for the steady state to be achieved. For example, Higbie advanced the theory that in a packed tower the liquid flows across each packing piece in laminar flow and is remixed at the points of discontinuity between the packing elements. Thus, a fresh liquid surface is formed at the top of each piece, and as it moves downward, it absorbs gas at a decreasing rate until it is mixed at the next discontinuity. This is the basis of penetration theoiy. 

If the entry of a molecule into the body were simply a temporally restricted absorption process, then a steady-state concentration would be achieved given enough time for complete absorption. However, what in fact is observed in drug pharmacokinetics is a complex curve reflecting absorption of the drug into the body and the diminution of the concentration that is absorbed back down to negligible levels. The reason for this complex pattern of rise and fall in... 

In a steady-state process, a gas is absorbed in a liquid with which it undergoes an irreversible reaction. The mass transfer process is governed by Fick s law, and the liquid is sufficiently deep for it to be regarded as effectively infinite in depth. On increasing the temperature, the concentration of reactant at the liquid surface CAi falls to 0.8 times its original value. The diffusivity is unchanged, but the reaction constant increases by a factor of 1.35. It is found that the mass transfer rate at the liquid surface falls to 0.83 times its original value. What is the order of the chemical reaction ... 

In a continuous steady state reactor, a slightly soluble gas is absorbed into a liquid in which it dissolves and reacts, the reaction being second order with respect to the dissolved gas. Calculate the reaction rate constant on the assumption that the liquid is semi-infinite in extent and that mass transfer resistance in the gas phase is negligible. The diffusivity of the gas in the liquid is 10" 8 m2/s, the gas concentration in the liquid falls to one half of its value in the liquid over a distance of 1 mm, and the rate of absorption at the interface is 4 x 10"6 kmol/m2 s. 

A soluble gas is absorbed into a liquid with which it undergoes a second-order irreversible reaction. The process reaches a steady-state with the surface concentration of reacting material remaining constant at (.2ij and the depth of penetration of the reactant being small compared with the depth of liquid which can be regarded as infinite in extent. Derive the basic differential equation for the process and from this derive an expression for the concentration and mass transfer rate (moles per unit area and unit time) as a function of depth below the surface. Assume that mass transfer is by molecular diffusion. 

The kinetics of the reactions taking place when contacting flue gas t ntaining NO and O2 with aqueous Fe (EDTA) solutior were detennined. With this information, a steady state, rate based BiodeNOx absorber model was developed for a counter current packed colunm djsorber unit. The model has been tqipli to determine the optimum reaction conditions for the absorber. Absorber perfiirmance is improved considerably when operating at the low end of the tempaature range. 

Adsorbed nuclides. Obtaining a solution to Equation (1) requires knowledge of the absorbed abundance of the nuclide of interest (Iads)- Assuming that steady state abundances have been achieved at each location, supply by adsorption is balanced by desorption and decay. No supply is expected to occur by decay of adsorbed parent atoms in the cases where a recoil drives daughters instead into solution or into the underlying mineral (although modification may be made in the case of p decay). Then,... 

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