Absorption difference

Circular dichroism employs standard dispersive or interferometric instmmentation, but uses a thermal source that is rapidly modulated between circular polari2ation states using a photoelastic or electro-optic modulator. Using phase-sensitive detection, a difference signal proportional to the absorption difference between left- and right-polari2ed light, AA is recorded as a function of wavenumber. Relative differential absorptions... 

Chemicals have to pass through either the skin or mucous membranes lining the respiratory airways and gastrointestinal tract to enter the circulation and reach their site of action. This process is called absorption. Different mechanisms of entry into the body also greatly affect the absorption of a compound. Passive diffusion is the most important transfer mechanism. According to Pick s law, diffusion velocity v depends on the diffusion constant (D), the surface area of the membrane (A), concentration difference across the membrane (Ac), and thickness of the membrane (L)... 

Figure 17. Optical absorption spectra of the polyimide implanted with 5x 10 Au m in presence of dry air and of methanol vapor (6000 ppm). Inset optical absorption difference calculated taking into account both spectra. (Reprinted from Ref. [68], 2005, with permission from Elsevier.)...

Drug absorption is highly variable in neonates and infants [21,22]. Older children appear to have absorption patterns similar to adults unless chronic illness or surgical procedures alter absorption. Differences in bile excretion, bowel length, and surface area probably contribute to the reduced bioavailability of cyclosporine seen in pediatric liver transplant patients [22a]. Impaired absorption has also been observed in severely malnourished children [22b]. A rapid GI transit time may contribute to the malabsorption of carbamazepine tablets, which has been reported in a child [23]. Selection of a more readily available bioavailable dosage form, such as chewable tablets or liquids, should be promoted for pediatric patients. 

Inhaled tetraethyl and tetramethyl lead vapors behave as gases in the respiratory tract and, as a result, their pattern and extent of deposition and absorption differ from that of inhaled inorganic lead particles (EPA 1994a Overton et al. 1987 Overton and Miller 1988). These differences result in a higher fractional absorption of inhaled tetraethyl and tetramethyl lead (Heard et al. 1979). 

In bulk samples, X-ray yields need to be adjusted by the so-called "ZAF" correction. Z stands for the element number (heavier elements reduce the electron beam intensity more than lighter elements, because they are more efficient back-scatterers), A for absorption (different elements have different cross sections for X-ray absorption), and F for secondary fluorescence (the effect described above). Corrections are much less important when the sample is a film with a thickness of 1 pm or less, because secondary effects are largely reduced. The detection limit is set by the accuracy with which a signal can be distinguished from the bremsstrahlung background. In practice, this corresponds to about 100 ppm for elements heavier than Mg. 

Optical and FTIR instrumentation. Absorption spectra were taken on three instruments. A Hewlett Packard 8450 UV/Vis Spectrophometer was used for the visible region of the spectrum. This instrument was computer control led and used to measure standard absorption spectra as well as absorption difference spectra. A Cary 14 Spectrophotometer was used for the near IR region, and an IBM IR/90 Series... 

Cma, fmax- The latter is a measure of the rate of absorption whereas the former is also dependent on the extent of absorption. Differences in either extent or rate of absorption can markedly alter the plasma concentration profQe and produce different clinical effect (Figure 2.6). 

Figure 7. Trace of an Os uptake measurement. The uptake of Os by the instrument (see Figure 6) and by a solution in the bubble tube is measured by the UV spectral absorption difference. The ppm of (Os) concentration was calculated based on an extinction coefficient of 138 atm (22). Scrub out refers to a valve change resulting in the removal of the indicated scrub from the system. Linolenic acid (100 fxM, final concentration in 0.5 ml acetone) is added where indicated 0.5 ml (-j-). The total solution in the bubble tube is 50 mM Tris-Cl pH 8.2 (50 ml). An injection of acetone alone provided the control trace (—).

Two differential spectrophotometric methods were used by Chatterjee et al. for the simultaneous analysis of diloxanide furoate and metronidazole in pharmaceutical formulations [24]. The first method involved measurement of the absorbance of a methanolic solution of the two drugs at 259 and 311 nm. Since the absorbance of diloxanide furoate at 311 nm is zero, the concentration of metronidazole is directly measured, and a simple equation based on absorbance ratios is used to calculate the concentration of diloxanide furoate. The second method was a differential spectrophotometric determination based on pH-induced spectral changes, on changing from an acidic to an alkaline solution. A marked bathochromic shift was exhibited by metronidazole, while diloxanide furoate showed a slight hypsochromic shift. The wavelength of maximum absorption difference for diloxanide furoate was 267 nm, where metronidazole did not absorb. Similarly, diloxanide furoate did not interfere with metronidazole at when measured at 322 nm. 

Pharmacokinetics Absorption differs between formulations. Protein binding 50%-80%. Bound to serum albumin. Metabolized to salicylate glucuronides and salicyluric acid. Excreted in urine. 

Fig. 17. Comparison of the transition temperatures for RNase-A (circles), RNase-S (triangles), and S-protein (squares) as determined by optical rotation (open symbols) and ultraviolet absorption difference spectroscopy (filled symbols). Reproduced from Sherwood and Potts (387).

Fig. 1. Transient absorption spectra (AOD = absorption difference) 100 ps after flash photolytic excitation of fluorenone (i) in the presence of tetramethyl- 1,3-dioxole (4) in various solvents a) acetonitrile b) benzene c) cyclohexance...

The presence of a 4-(4-dimethylaminophenyl) group enhances the visible absorption differences between the pyrylium and thiopyrylium salt analogues. Thus, on reaction of 625 with hydrosulfide ion a change in hue from magenta (540 nm) to blue (580 nm) is observed this color change serves as the basis for a new chemodosimeter selective for hydrosulfide (Scheme 240) <2003JA9000>. 

To determine the effects of the deprivation of specific micronutrients on the water hyacinth (Eichhornia crassipes), Colley et al. (1979) studied the rate of uptake of iron and manganese in comparison with phosphorus. Results indicated that all three elements were actively absorbed by the root systems, but the rates of absorption differed markedly. The rate of absorption of manganese by roots was 13 and 21 times that for radio-iron and -phosphorus, and iron was taken up by the roots at nearly twice the rate of phosphorus. Manganese translocation appeared to be faster than phosphorus translocation by an order of magnitude and 65 times faster than iron translocation. 

Figure 3 Transient absorption difference spectra obtained 40 ns following 355-nm pulsed excitation (10-ns pulse width). (A) Model complex 11 (B) metal-organic dyad 10a. Reprinted with permission from Ref. 83.

Figure 5 Transient absorption difference spectra of 20b obtained following 295-nm excitation (800-fs pulse width). Delay times are listed above each spectrum. (Reprinted with permission from Ref. 91.)...

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