Cytochromes Cooled in Liquid Nitrogen 



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after appropriate amplification, is then recorded automatically as a function 

 of wavelength. The sample holder is arranged so that the samples may be 

 raised out of the Dewar, put into liquid nitrogen, or aligned in the light path 

 from the monochromator. 



A classic demonstration of the performance of a spectrophotometer is the 

 spectrum which one can obtain with a didymium nitrate solution. In Fig. 3 



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400 



450 



500 



550 



600 



Wavelength Cm/j) 



Fig. 3. Spectra of didymium nitrate. A 10% solution of didymium nitrate was 

 mixed with an equal volume of glycerol and the spectrum was recorded at room 

 temperature (A-A) using cuvettes with a 10 mm light path. A sample of the 

 didymium nitrate-glycerol mixture was placed in a cuvette of 1 mm light path 

 and cooled in liquid nitrogen and the spectrum recorded (B-B). 



are plotted two spectra as obtained using the instrument developed by 

 Chance. The spectrum marked AA is that of didymium nitrate at room 

 temperature — a rather complex spectrum. When the sample is cooled in 

 liquid nitrogen and then warmed to devitrify the glycerol mixture and then 

 recooled in liquid nitrogen, one obtains the spectrum BB (Note 2). There is 

 a tenfold difference in the optical depth of the cuvettes employed in the two 

 experiments: that is, the upper curve, BB, should be ten times larger than 

 the curve A A. Of interest here, other than seeing the appearance of additional 

 absorption bands at liquid nitrogen temperature, is the resolution of absorp- 

 tion bands which one can obtain with the instrument we are using. For 



