True absorption

Fig. 8. Calculated solid echo 2H NMR powder spectra for jumps between two sites related by the tetrahedral angle for ij =0, i.e. true absorption spectrum and Tj = 200 ps. xc is the correlation time of motion. R is the reduction factor, giving the total normalized intensity of the spectra for x, = 200 ps. (For x, = 0 all the spectra have total intensity 1)...

As shown in Figure B2.1, double-beam spectrophotometers automatically record the true absorbance by measuring log(IR/Is), thanks to a double compartment containing two cuvettes, one filled with the solution and one filled with the solvent. Because the two cuvettes are never perfectly identical, the baseline of the instrument is first recorded (with both cuvettes filled with the solvent) and stored. Then, the solvent of the sample cuvette is replaced by the solution, and the true absorption spectrum is recorded. 

Figure 4-30. Calculated and true absorption spectra of species A, B and C (from left to right).

While there thus appears to be evidence for apparent excess absorption of solar radiation by clouds, there is substantial controversy over whether this is indeed true absorption or whether there is some other explanation for the discrepancies (e.g., see Stephens and... 

We define measured and true absorptance by the pair of relations... 

Examining Eq. (47), we see two possible ways to go beyond a modified (i.e., fully deconvolved) y4M(x), that is, beyond our best estimate AT(x) of the true absorptance AT ... 

Fig. 17 Base-line variation effects. Trace (a) is the original spectrum o(x), trace (b) the convoluted spectrum i(x). Traces (c)-(g) are the deconvolution results after offsets of —5, — 2, 0,2, and 5%, respectively, have been applied to the convolved data i.e., the base line used for trace (c) was placed at the true absorptance of —0.05 (on a scale of 0.0 to 1.0).

Many of the absorption lines in SN 1987A are quite deep, and are therefore produced largely by scattering rather than true absorption. This is consistent with the large deviations from LTE that must exist in the rapidly expanding, tenuous envelope. In this situation the line formation can be approximately represented by the Schuster-Schwarzschild model of a reversing layer superposed on top of a continuum, in which oase the residual intensity of a line r is related to the optical depth at line center t by the relation (Mihalas 1969)... 

Figure V-IOtt shows the apparent absorption cocIFicicnts of NO in the region 1500 to 2300 A and Fig. V-106 shows those in the region 1100 to 1500 A. Because of the lack of resolution of the monochromator, the absorption coefficient given in Fig. V-IOn is a function of the slit width and the pressure of the gas (sec Section 1-8.1 for details). The true absorption coefficient may be obtained by computation from the oscillar strength and the assumed widths of the rotational lines. According to the computation by C ieslik and Nicolct (214) the true absorption coefficient of individual...

In Chandrasekhar s theory, the true absorption coefficient Oty = Kvp(l-uJ0) and the true scattering coefficient av = Kvpw0. Klier has found the relations between the Chandrasekhar and SIM coefficients... 

Klier s contribution consists of demonstrating that the ratio of the true absorption to scattering coefficients (a /o. ) can be determined as... 

The drawbacks of SC and IM injections include potentially decreased bioavailability that is secondary to variables such as local blood flow, injection trauma, protein degradation at the site of injection, and limitations of uptake into the systemic circulation related to effective capillary pore size and diffusion. The bioavailability of numerous peptides and proteins is, for example, markedly reduced after SC or IM administration compared to their IV administration. The pharmacokine-tically derived apparent absorption rate constant is thus the combination of absorption into the systemic circulation and presystemic degradation at the absorption site. The true absorption rate constant ka can then be calculated as ... 

The practical absorptivities indicate the overall effect of the transition element when it is incorporated into fused silica by the flame hydrolysis method. The absorptivities do not reveal the absorbing power of the individual valence state (i.e. true absorptivity). 

The last condition provides a mechanism for measuring the absorption spectra under steady state conditions. In the absence of this condition, it is necessary to make transient measurements and extrapolate back to the true absorption coefficient prior to excitation. 

In diffuse reflectance, the reflected light is analyzed, and attempts have been made to relate the reflectance to the true absorption coefficient. 

Klier (1972) deduced that for 0.6 < p < 1, which corresponds to 0.13 > F(P) > 0, the Kubelka-Munk absorption coefficient should be nearly proportional to the true absorption coefficient. Deviations from the proportionality up to a factor of two occurred for lower reflectance values. In the range p > 0.6, the Kubelka-Munk function should be nearly proportional to the absorber concentration. Through comparison with the radiative-transfer equation formulated by Chandrashekhar (1960), Klier related the phenomenological coefficients to the true absorption and scattering coefficients a and [Pg.142]

A recent study in weanling rats by Weigand et al. (25) showed that at this age, tissue Mn homeostasis can effectively be achieved over a wide range of dietary Mn levels. A major level of control appears to be by lowering the true absorption as expressed as percent of Mn uptake thus, with increasing dietary Mn, absorption varied from 29% down to 2%. However, additional control was exerted by increased endogenous loss of Mn (presumably via bile). Therefore, if Mn absorption in infancy already is high and the excretion pathway not yet developed, homeostasis may not occur. 

TRUE ABSORPTION accounts for all absorbed activity Including that which Is secreted back Into the gut. True absorption meets the definition given previously and accounts for all material which crosses the Intestinal epithelium Into blood or lymph. True absorption, however, cannot be easily determined except In those rare cases In which endogenous secretion Into the gut Is minimal or can be determined separately from fecal excretion of unabsorbed substance. 

Apparently the zinc of these water insoluble complexes dissociates in the digestive tract or the complex is not strong enough to prevent the intestine from extracting the zinc. However, when phytate is present in the diet in 12 to 15-fold molar excess over zinc rats exhibit signs of decreased bioavailability of dietary zinc ( , 0). The metabolic balance does not measure true absorption, but a measure can be obtained over time of the amount of bioavailable nutrient under a given dietary regimen. Decreased bioavailability would be indicated by a decrease in apparent... 

---