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Absorption Spectrophotometry /26 : 5 



little significance. The curves in Figure 1 1 show the type of results to 

 be expected from a stopped-flow and an accelerated-flow reaction, both 

 of which are exponential in their time course. More rapid reaction 



Light Beam 



Photo Multiplier or 

 Other Sensing Device 



Figure 10. Rapid flow apparatus. C 1 and C 2 are three-way 

 stopcocks allowing one to fill the syringes from the storage 

 tanks and then discharge the syringes into the mixing chamber 

 M. The optical density changes are observed at d cm down 

 the flow tube. 



rates can be detected by flow-type measurements than by measurements 

 in cuvettes. However, there are larger experimental errors associated 

 with the flow-type measurements. 



5. Split-Beam and Dual-Beam 

 Spectrophotometers 



In absorption spectrophotometry, the light intensities transmitted by a 

 standard and a test sample are compared. Errors will be introduced 

 into the optical densities so measured if the output of the light source 

 changes. Similarly, if one measures the transmitted intensity at several 

 wavelengths first for the standard and then for the test sample, it is 

 important to compare readings at exactly the same wavelength. These 

 operations can be simplified and the errors reduced by using slightly 

 more complex equipment, namely a split-beam spectrophotometer. In this, 

 the beam from the monochromator is split so that it alternates rapidly, 

 passing first through the standard and then the test sample. There- 

 after, the split beam is recombined to fall on a common detector. The 



