424 HAROLD F. BLUM 



where C is the concentration, and e the molecular extinction coefficient. 

 This is the common form of expression of Beer's law, which is the 

 general basis for absorption spectrophotometry of solutions. An- 

 other term in common use is the density, D, which is defined as: 



log {h/I) = D (9) 



The length of the absorbing path is not specified. In equations (3-8) 

 the numerical value of k, k' , or e depends upon the unit in which the 

 path length, I, is measured; this is usually the centimeter. The 

 molecular extinction coefficient is generally defined as the value of D a 

 solution must have if the concentration is 1 mole per liter, when the 

 absorbing path is 1 centimeter. 



The nomenclature for expression of the absorption laws is by no 

 means uniform, nor are the symbols or the units. It is therefore 

 necessary in absorption spectroscopy to understand the basic rela- 

 tionships clearly, and to ascertain the units used by a particular 

 author; it is equally important to be sure that your own results are 

 clearly and completely expressed. While the Bougeur-Lambert 

 law holds for any homogeneously absorbing medium, Beer's law some- 

 times fails when applied over wide ranges of concentration. Thus, it 

 is well to state the approximate concentrations at which measure- 

 ments are made. 



The numerical values of k, k', or e apply to monochromatic radia- 

 tion of a specific wavelength for which they have been determined. 

 Hence an absorption spectrum may be expressed by plotting k, k', or 

 e against the wavelength measured in mju (Figs. 1 and 2) or some other 

 unit. Sometimes, however, they are plotted against the frequency v 

 in sec.~^ or against the wave number (= 1/X, when X is measured in 

 centimeters) . For purposes of comparison it is sometimes convenient 

 to plot the logarithm of the absorption coefficient as in Figure 2; 

 this is particularly useful when dealing with solutions of unknown 

 concentration. 



In biological experiments, the concentration of a substance is often 

 unknown, and hence data cannot be expressed in terms of molecular 

 extinction coefficients or Beer's law; but absorption coefficients, ex- 

 tinction coefficients, or, in some cases, density may be used. In much 

 of the discussion of action spectra that follows, it makes no difference 

 which of these units is used, because only relative values are important . 



The intensity, /, was defined above as the number of quanta per 

 unit area per unit time. Measurements of radiant energy are usually 



-^. 



