PROPERTIES OF VITAMINS A AND RELATED COMPOUNDS 731 



Another method for estabhshing the amount of absorption at 328 m/t 

 which is caused by vitamin A is by the use of acid clays which specifically 

 adsorb vitamin A and thus separate it from the impurities which interfere 

 with the spectrophotometric determination. Awapara, Mattson, Mehl, 

 and DeueP^^ have demonstrated that the vitamin A in margarines or but- 

 ters can be quantitatively removed from a benzene solution by passing the 

 non-saponifiable extract over a thin layer of floridin.^^'^ When the absorp- 

 tion curve of the eiuate is subtracted from that of the untreated extract, 

 the difference shows a typical vitamin A curve. One complication which 

 arises is the fact that the carotenoids are also adsorbed by fioridin, even 

 ^^'hen an extremely thin layer is employed. In case such products are 

 present in the extract, corrections must be made for their absorption. The 

 vitamin A-floridin reaction involves the development of a deep blue color 

 on the clay, which is extremely sensitive. Apparently, a chemical reaction 

 occurs, since it has been impossible to remove the vitamin A from the 

 floridin by means of other eluents. 



Since vitamin A is particularly sensitive to irradiation with ultraviolet 

 light, ^^ another method for establishing the proportion of the absorption at 

 328 m/x which is referable to vitamin A is from the difference in absorption 

 before and after irradiation of the sample. ^^^-"^ Neal and Luckmann^^^ 

 have successfully applied this method to the determination of vitamin A in 

 margarine. 



Wilkie and DeWitt^^" employ chromatographic adsorption of vitamin A 

 extracts obtained after saponification of margarine, which aids in removing 

 the irrelevant absorption before the spectrophotometric determination is 

 carried out. HoAvever, the eluates of the vitamin-A-containing layer of the 

 chromatographic column still ilo not show characteristic curves for vitamin 

 A. 



Morton and Stubbs^^^ have developed a formula to correct for irrelevant 

 alisorption. By determining the extinction values of a known mixture of 

 anthracene and vitamin A, it is possible to obtain correction for the E (328 

 m/i) value based upon observations at 313 and 338.5 m^u to afford a true 

 vitamin A value. The Morton and Stubbs correction averages 6% for 

 molecular distillates, while correction factors of 11-16% are usually neces- 

 sary for oils.^^^ The need for correction arises from the presence of anhy- 

 di'ovitamin A and sometimes oxidation products. 



=■-« J. Awapjuu, F. II. Miiltsuii, J. \V. Melil, and II. J. Deuel, Jr., Science, 104, 602-604 

 (1946). 



227 A. Emmerie and C. Engel, Rec. trav. chim., 58, 283-289 (1939). 

 22S R. W. Little, Ind. Eng. Chem., Anal. Ed., 16, 288-293 (1944). 



229 R. H. Xoal and F. II. Luokmann, Jnd. Eng. Chem., Anal. Ed., 16, 358-362 (1944). 



230 J. B. Wilkie and J. B. De Witt, J. Assoc. Official Agr. Chem., 28, 174-186 (1945). 



231 R. A. Mort<)n and A. L. Stubbs, Analyst, 71, 348-356 (1946). 



232 R. A. Morton and A. L. Stubbs, Biochcm. J., 40, Iviii (1946); 42, 195-203 (1948). 



