Percentage absorption

FIGURE 1 The optical absorption spectrum of chlorophyll as a plot of percentage absorption against wavelength. Chlorophyll a is shown in red, chlorophyll b in blue. 

Since the amount of exposure via this route is perceived to be small under normal circumstances it is probably not unrealistic to simply presume 100% absorption. This is not always the case and frequently there is information from metabolism studies which would indicate the actual percentage absorption. 

In atomic absorption analysis, the sample cell is an atomizer which in most cases has a very reproducible path length, 6, and Beer s Law relationships are followed in most cases. The percentage absorption reading is converted to absorbance, log10(/0// ) and then related to the sample concentration. For example, a sample absorption of 12.9% = 0.06 absorbance, and 1% absorption = 0.0044 absorbance. The most important point to note about the Beer—Lambert Law is that the concentration of the sample is directly proportional to absorbance A and not to percentage absorption. 

The molar ratio of phytate/zlnc of some mid-eastern diets was estimated to be 20 ( ), but only small amounts of meat were generally available. Sandstrom et al. obtained results that indicate absorption of zinc from a meal may be appreciably Influenced by the type of protein in the meal (, ). Our menus provided protein from meat in most of the meals which may have affected bioutilization of the dietary zinc. In one comparison Sandstrom et al. found that whole wheat bread decreased percentage absorption, but the absolute amount of zinc absorbed was greater because total zinc in the whole wheat bread meal was greater. 

Currently, for regulatory purposes, if the area dose levels applied in the in vivo study are at or below the area dose levels anticipated to occur in the scenarios under investigation, no underestimation of dermal absorption is expected. However, if the area doses are higher than the anticipated exposure levels, the percentage absorbed, as derived from the study, should not be used as it may underestimate dermal absorption, as in general, the percentage absorption decreases with increasing dose. If the values are used, the uncertainty associated with this decision should be documented in the risk assessment. 

Fig. 3.4-3 shows the infrared emission spectrum (the spectral radiance) of a sample of butadiene gas alp = 107 mbar in a cuvette v, th a thickness of 10 cm at 800 K, compared to the emi.ssion spectrum of a black body at the same temperature. It is interesting to note that - as theory predicts - the percentage emission of butadiene compared to that of a black body almost equals the percentage absorption of the same gas at room temperature as recorded with an ordinary spectrometer. 

Rgure 9.15 Relationship between the log P of solute and the percentage absorption through the buccal mucosa of human subjects for bases (O) and acids ( )... 

Thiamine absorption occurs primarily in the proximal small intestine by both a saturable (thiamine transporter) process at low concentration (Ipmol/L, or lower) and by simple passive diffusion beyond that, though percentage absorption diminishes with increased dose. The absorbed thiamine undergoes intracellular phosphorylation, mainly to the pyrophosphate, but at the serosal side 90% of the transferred thiamine is in the firee form. Thiamine uptake is enhanced by thiamine deficiency and reduced by thyroid hormone, diabetes, and ethanol ingestion. The gene for the specific thiamine transporter has been identified, and the transporter cloned. Thiamine is carried by the portal blood to the liver. The firee vitamin occurs in the plasma, but the coenzyme, TPP, is the primary cellular component. Approximately 30 mg is stored in the body with 80% as the pyrophosphate, 10% as triphosphate, and the rest as thiamine and its monophosphate. About half of the body stores are found in skeletal muscles, with much of the remainder in heart, liver, kidneys, and nervous tissues (including the brain, which contains most of the triphosphate). 

Figure 2. Iron absorption by adults from a range of foods. The bars represent the mean absorptions and standard errors, calculated from the logarithms of the percentage absorptions. (Reproduced, with permission, from Ref. 15. Copyright 1971, Grune and Stratton, Inc.)...

Figure 3. Photoacoustic signal of PMMA as a function of percentage absorption.

The challenge in determining the dermal absorption values most relevant to factor into a risk assessment process for environmental contaminants is to define the most relevant parameter (i.e., flux, permeability coefficient, percentage absorption, or total systemic load). For example, when using in vitro techniques to determine potential dermal absorption of a contaminant in water (in which it will probably be present at low concentrations), the most common model would use an infinite dose aqueous application. This will allow the determination of flux and the calculation of a permeability coefficient. However, from such an experiment, the percentage absorption value will be practically meaningless. The total systemic load will be dependent on many other factors, such as concentration and solubility of contaminant within the medium, pH of the medium (and thus the degree of ionization of the contaminant). 

Table 10.2, Equation 10.7 was shown to correlate the cumulative evaporation data from a controlled human forearm evaporation study involving 11- and 12-component fragrance mixtures (Vuilleumier et al., 1995) with values of 0.80 and 0.73, respectively (Saiyasombati and Kasting, 2003b). The compounds tested and their physical properties are shown in Table 10.1, and the correlations are shown in Figure 10.2. The corresponding estimated percentage absorption, calculated as % abs(< ) = 100 - % evap(oo), is shown in Figure 10.3.

FIGURE 10-37. Calibration curves of atomic absorption signals of zinc using absorbance and percentage absorption versus zinc concentration in /ig/ml. [From W. G. Schrenk, Evaluation of Data, in Flame Emission and Atomic Absorption Spectroscopy, Vol. 2, Edited by J. A. Dean and T. C. Rains, Marcel Dekker, New York (1971), Chap. 12. Used by permission of Marcel Dekker, Inc.]... 

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