NO. 19 CHLOROPHYLL DETERMINATION — JOHNSTON-WEINTRAUB 3 



30-fold. It is a rugged type of high sensitivity and has a uniform 

 response over the wide range from 2500 A. to 6.5 /x. This transmission 

 range has been obtained by the use of a so-called "bubble window" 

 which is a thin (.025-.050 mm. thick) disk of glass that is fused to the 

 cell body and then sucked in. The couple, whose zero stability is 

 excellent, has a resistance of about 15 ohms and gives 3.3 microvolts 

 per microwatt per square millimeter. Its time response is less than 

 that of commercial galvanometers. 



The couple is connected directly to a Moll galvanometer without 

 intermediate means of amplification. This galvanometer has a period 

 of 1.3 seconds with an internal resistance of 50 ohms and the external 

 resistance for critical damping may be varied from 120 to o ohms. 

 At a scale distance of 1 m. a deflection of 1 mm. corresponds to a 

 current of 6 x io~ u amperes. 



The absorption cell ( C) is 5 cm. long and has a volume of 10 ml. 

 It is constructed of Pyrex glass with fused-on ends. 



PREPARATION OF EXTRACT 



The plant material is thoroughly ground by hand in a mortar with 

 sand and acetone, and the solution decanted and filtered under reduced 

 pressure. The residue, which is retained in the mortar, is ground and 

 the solution filtered twice more, all the filtrates being combined. This 

 requires from 10 to 15 minutes. Carotenoids are not separated from 

 the chlorophyll. It is essential that all suspended material be removed 

 from the solution since the slightest turbidity reduces the transmitted 

 energy and introduces an error in the chlorophyll determinations. 

 After trying several kinds of filters the one illustrated in plate 2 has 

 been found very satisfactory. Several layers of close-grained filter 

 paper are placed in position between the ground faces of the upper and 

 lower portions of the filtering tube. A brass collar fits against a 

 shoulder on each part of the filter tube and by means of screws the 

 ground faces are held tightly against the filter paper. 



DETERMINATIONS 



Before making determinations of unknown chlorophyll solutions it 

 was necessary to construct a calibration curve (fig. 2) from known 

 concentrations of purified chlorophyll solutions. 3 It has been deter- 



J The purified chlorophyll solutions and the unpurified leaf extracts gave 

 identical absorption curves in the region from 6000 A. to 7400 A. We are 

 indebted to Dr. K. S. Gibson and H. J. Keegan of the Colorimetry Section, 

 National Bureau of Standards, for the determination of these curves with the 

 General Electric recording spectrophotometer. 



