Optical density-time relationship

In systems such as the 2- and 6-hydroxypteridines, sudden addition of an alkaline solution to a neutral buffer, or of a neutral solution to an alkaline buffer, displaces the equilibrium between hydrated and anhydrous species (because the anions are less hydrated than the neutral molecules). By measuring the time-dependent change of optical density at a selected wavelength, a first-order rate constant, obs5 can be obtained. This constant is a composite one, and to see its relationship to other quantities some discussion is necessary. 

Figure 7 6 Relationship between the optical density (as in Figure 75) and the water content of cod white muscle caught at different times of the year. (After Love, 1962.)...

Some 30 years later, Jean-Henri Lambert (1728-1777) proposed the first mathematical relationship the logarithm of the decrease in light intensity (today we would say the inverse of the transmittance) is equal to the product of the opacity of the medium times its thickness. Finally in 1850, Auguste Beer established a relation between concentration and optical density (now called absorbance), which led to the current form of the Beer-Lambert law (also called Lambert-Beer law or even Lambert-Beer-Bouguer law). 

There is evidence that not only are the inherent assumptions of many of the semiquantitative approaches not wholly valid (see above), but that the measure itself (e.g., the ratio of optical density in the region relative to the optical density of white matter) may be altered by factors unrelated to glucose utilization. For example, the relationship of tissue isotope concentration to optical density is not linear, and the relationship varies with the exposure time in the preparation of the autoradiograms. As a... 

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