Small transit time

Chronopotentiograms were traditionally evaluated graphically, which with small transition times is difficult and often inaccurate. It is more convenient to measure the time values—which are proportional to concentration—electronically via the first or second derivative. In this way empirical time values are obtained which always differ to some degree from the theoretical transition times [34]. 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

In practice is a small number and the sing-around frequencies are scaled up for display. In one example, for a pipe 1 m in diameter and water flowing at 2 m/s, the frequency difference is 1.4 Hz (10). Frequency difference transit time meters provide greater resolution than normal transit time ultrasonic meters. The greatest appHcation is in sizes from 100 mm to 1 m diameter. 

This equation holds only at short times when thickness 5. is small compared to the steady-state dilfnsion-layer thickness 5j.(, which will be attained under given experimental conditions, particularly when the solntion is stirred. As soon as attains the value of 5,.(, the transitory processes end and a steady state is attained there is no fnr-ther change in concentration distribution with time (Fig. 11.3a). It follows from Eq. (11.7) that the transition time of the transient process... 

Producing electron-hole pairs by light excitation in the small particles (d < dg ) electrons and holes can easily be transferred to an electron acceptor and donor, respectively, provided that the energetic requirements are fulfilled. The quantum efficiency of the reaction depends on the transfer rate at the interface, on the recombination rate within the particle and on the transit time, the latter being given by ... 

SITT small intestinal transit time (4.5h = 270min)... 

E Parry, R Shields, AC Turnbull. Transit time in the small intestine in pregnancy. J Obstet Gynaecol 77 900-901, 1970. 

In order to determine the optimal number of compartments, literature information on small intestinal transit times was utilized. A total of over 400 human small intestinal transit time data were collected and compiled from various publications, since the small intestinal transit time is independent of dosage form, gender, age, body weight, and the presence of food [70]. Descriptive statistics showed that the mean small intestinal transit time was 199 min with a standard deviation of 78 min and a 95% confidence interval of 7 min. The data set was then analyzed by arranging the data into 14 classes, each with a width of 40 min. Figure 9 shows the distribution of this data set. 

Figure 9 Distribution of small intestinal transit time in humans. The transit time was measured by y-scintigraphy based on the difference in time between 50% of the drug arriving at the colon and 50% of the drug leaving the stomach. The distribution was constructed from over 400 literature data points. (From Ref. 64 with kind permission from Elsevier Science-NL, Amsterdam.)...

The cumulative curve obtained from the transit time distribution in Figure 9 was fitted by Eq. (48) to determine the number of compartments. An additional compartment was added until the reduction in residual (error) sum of squares (SSE) with an additional compartment becomes small. An F test was not used, because the compartmental model with a fixed number of compartments contains no parameters. SSE then became the only criterion to select the best compartmental model. The number of compartments generating the smallest SSE was seven. The seven-compartment model was thereafter referred to as the compartmental transit model. 

F(t) Cumulative distribution of the small intestinal transit time... 

LX Yu, GL Amidon. Characterization of small intestinal transit time distribution in humans. Int J Pharm 171 157-163, 1998. 

The risk inherent in long transition times, characteristic of free convection with small driving force, is twofold ... 

The transit time from the hole to the reflector and back again corresponds to the period of oscillation (v). Thus the microwave frequency can be tuned (over a small range) by adjusting the physical distance between the anode and... 

SIWV = small intestinal water volume (mL), assumed to be c. 250 mL SITT = small intestinal transit time (min), assumed to be 4.5 h = 270 min. 

Fig. 8.3. Average transit time in the small intestine measured using y-scintigraphy following administration of various pharmaceutical dosage forms to humans. Data from reference [611-... 

The ACAT model is loosely based on the work of Amidon and Yu who found that seven equal transit time compartments are required to represent the observed cumulative frequency distribution for small intestine transit times [4], Their original compartmental absorption and transit (CAT) model was able to explain the oral plasma concentration profiles of atenolol [21]. 

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