Half-life time

Table C2.6.5 Rapid coagulation half-life time for particles in water at 7 =300 K (equation (C2.6.16)).

Atmospheric photodegradation of DEHP and DBP has been shown to be rapid (51,53) with half-life times of less than 2 days, hence a large proportion of phthalate emissions to the air are broken down by photodegradation. 

Intestinal absorption of digoxin is less complete compared to digitoxin. In order to improve absorption, acetylated- and methylated-digoxin derivates were developed. Digitoxin is metabolised in hepatic microsomal enzymes and can be cleared independently from renal function. The therapeutical serum level of digoxin is 0.5-2.0 ng/ml and 10-35 ng/ml of digitoxin. Steady state plateau of therapeutic plasma concentrations is reached after 4-5 half-life-times using standard daily doses [5]. 

The regulation of receptor synthesis is a second component of receptor downregulation. It involves processes that reduce gene transcription, mRNA stability, and receptor half-life time. It should be noted that mechanisms in addition to the regulation of the receptor number may account for tolerance development. Second messenger levels and enzyme activities that participate in the signaling of a given receptor are... 

The kinetic stability of 17 increases on deprotonation. The half-life times of 17 and its anion N 19 have been estimated [104] from the observed [105, 106] and computed free energy to be only 10 min and 2.2 days, respectively. The high kinetic stability of the anion 19 can be understood in terms of enhanced pentgon stability and aromaticity. The deprotonation raises the energy of lone pair orbitals and promotes cyclic delocalization of o- and rr-electrons. 

When the half-life time of reaction and the half-life time of micromixing in the absence of chemical reaction are of the same order or the former is less than the latter, the role of micromixing may become crucial. For instance, nitration or bromination of resorcinol, even when the ratio of moles of resorcinol to moles of bromine is high, can lead to predominantly disubstituted product contrary to the general belief. In such cases, in many respects, the theory of coupling between reaction and micromixing has parallels with the formalism of theory of mass transfer with chemical reaction (Bourne, 1983). 

The half-life time t of a drug is an important pharmacokinetic parameter. In this simple model it can be obtained immediately from the solution in eq. (39.6) ... 

Fig. 39.5. (a) Plot of plasma concentration Cp (pg 1 ) versus time t. Cp(0) is the extrapolated initial concentration at time zero. At the half-life time tm the plasma concentration is half that of Cp(0). (b) Semilogarithmic plot of plasma concentration Cp (pg 1 ) versus time t. The intercept B of the fitted line is the plasma concentration Cp at time 0. The slope jp is proportional to the transfer constant of... 

Initial plasma concentration Rate constant of elimination Half-life time ... 

In practice, the half-life time of the dmg in the plasma compartment is derived from the P-phase, and is therefore denoted as rf/2 ... 

Likewise the half-life time of the drug in the extravascular compartment ty2... 

Transfer constant of absorption Plasma volume of distribution Half-life time, a-phase ... 

The transfer constant of elimination kp has already been shown to be related to the half-life time of the drug in the plasma compartment (eq. (39.9)) ... 

This should be compared with the initial plasma concentration of 50 pg F which is obtained when the same dose D is injected as a single bolus. The final concentration Cp(x) is also still far from the steady-state concentration (72.2 pg 1" ), since the duration of infusion x is only equal to once the half-life time of the dmg t,/2 (60 min). 

In this special case when the time between dosings is equal to the half-life time of the drug, we can deduce that the minimum (steady-state) plasma concentration with repeated dosing is equal to the peak concentration, obtained from a single dose. Under this condition, the corresponding maximum (steady-state) concentration is twice as much as the minimum one. 

From the curve peeling operation we thus obtained the following intercepts, hybrid transfer constants and half-life times of the a-and (3-phases ... 

This relationship shows the analogy with the previously derived half-life time r,/2 (in Section 39.1.1), which is the time required for a single intravenous dose to be halved in a one-compartment open system. Since we already derived in eq. (39.9) that ... 

For nuclear y-resonance absorption to occur, the y-radiation must be emitted by source nuclei of the same isotope as those to be explored in the absorber. This is usually a stable isotope. To obtain such nuclei in the desired excited meta-stable state for y-emission in the source, a long-living radioactive parent isotope is used, the decay of which passes through the Mossbauer level. Figure 3.6a shows such a transition cascade for Co, the y-source for Fe spectroscopy. The isotope has a half-life time //2 of 270 days and decays by K-capmre, yielding Fe in the 136 keV excited state ( Co nuclei capmre an electron from the K-shell which reduces the... 

Table 1—1. Rate constants k [105 sec-1] and half-life times t1/2 [min] for neutral hydrolysis of iV-acylazoles [ conductivity water, pH 7.0, 25 °C] together with IR frequencies v(C=o) in CC14 and enthalpies.

Table 1—3. Rate constants and half-life times t1/2 [min] for hydrolysis of NJJ -carbonylbisazoles and their benzo and thio derivatives in THF/water (40 1 27°C) and IR frequencies V(C=o> in CHC13.

The rate of hydrolysis of various organic chemicals, under environmental conditions can range over 14 orders of magnitude, with associated half-lifes (time for one-half of the material to disappear) as low as a few seconds to as high as 10 years and is pH dependent. It should be emphasized that if laboratory rate constant data are used in soil models and not corrected for environmental conditions — as is often the only choice — then model results should be evaluated with skepticism. 

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