26 : 5/ Absorption Spectrophotometry 



497 



optical density of the test sample relative to the standard is found 

 electronically or by mechanically controlling the intensity of the source 

 so that the light passing through the standard gives rise to a constant 

 voltage. 



Stopped Flow -Pen - writing Recorder 

 Start Stop 



O.D. 



10.001 



Time 



0.1 sec 



Region I . Old reactants removed by flow down tube. 

 Region 2. New reactants do not react during time to 

 flow down tube. 



(a) 



Accelerated Flow — Oscilloscope Tracing 



0.010- 



c 

 .o 



0.005 



0.000 



l/=co 

 O.D. =0.010 



Removal of Used Reactants 



2 4 6 8 10 12 14 16 

 Linear Flow Velocity {Arbitrary Units) 



(b) 



Figure II. Rapid flow records, (a) Stopped flow as made 

 by a pen-writing recorder, (b) Accelerated flow as indicated 

 by an oscilloscope tracing. 



The split-beam method has another advantage, besides reducing the 

 sensitivity to light-source fluctuations and increasing the ease of obtaining 

 spectral measurements. For example, the absorption spectra of a 

 suspension of cells in the presence and absence of oxygen can be com- 

 pared by using one of these suspensions as a standard and the other as 



