488 Absorption Spectrophotometry /26 : 3 



Provided that Beer's law is valid for all the molecular species involved, 

 it is possible to determine the contribution to /x of any one type of com- 

 pound by measuring /x for solutions with and without that compound. 

 The difference in the two values of /x rather than the two absolute values 

 is important. Most spectrophotometers are constructed so as to read this 

 difference directly. 



In actual practice, instead of using /x which is defined by 



it is more customary to measure the optical density D where 



D = log 10 J j (9) 



The two are simply related; since 



logic (/«>//) =2^7] 



one may write 



" = 2X* < 10 > 



The values of fx and /3 may be specified in a number of ways, depending 

 on the units used for x and c; different symbols are often used for the 

 same form. Table II on page 489 gives some of the more common terms 

 and symbols. All of these and other coefficients are used at one place or 

 another in the literature. Usually, x = 1 cm, and its dimensions are 

 ignored. Probably the most widely used member of this group of 

 coefficients is 



mM' 



A familiarity with the symbols and units indicated allows one to 

 compare and correlate the work of different authors and the contents of 

 different textbooks. In terms of these units, one may use measured 

 optical densities to compute concentrations and to identify biologically 

 significant molecular species. 



3. Spectrophotometers 



The remainder of this chapter is devoted to specific equipment useful in 

 spectrophotometry. The large number of variations and combinations 

 in existence illustrate the widespread use of spectrophotometry, in both 

 research and routine clinical procedures. The purpose of this section 

 is to review the general types of spectrophotometric equipment, their 

 advantages and disadvantages. From the equipment standpoint, every 





