PROPERTIES OF VITAMINS A AND RELATED COMPOUNDS 



720 



250 300 350 400 



WAVE LENGTH IN MJLL 



Fig. 7. The absorption spectra in ethanol: (— ) vitamin Ai; (- -) 

 neovitamin A.'"'''^^^ 



Although solutions of purified vitamin A preparations may be very 

 exactly determined spectrophotometrically at their extinction maxima, 

 there are a nimiber of interfering substances in extracts of natural products 

 which likcAvise absorb light in the 324-332 van area. The presence of 

 appreciable amounts of such contaminants may therefore completely 

 vitiate the determination if corrections are not made for such irrelevant 

 absorption. The chief substances causing the irrelevant absorption are 

 the carotenoids. Although the correction factor for these compounds may 

 be a minor one if the extracts have not been heated, it is markedly increased 

 by saponification or by any procedure which causes the compound to 

 undergo stereoisomerism. Such interfering substances can ordinarily not 

 be removed by saponification. 



There are several methods which can be used to confirm the fact that the 

 material absorbing at 328 m/x is vitamin A. According to Loofbourow,224 ^j^g 

 "normality" of the vitamin A can be established: (i) by the demonstra- 

 tion of the absence of a fine structure in the absorption spectrum indicative 

 of cyclized vitamin A, which possesses no biological activity; and {2) by 

 proof that a well-defined maximum results at 617 m^ when the antimony 

 trichloride test is applied. Moreover, the E (1%, 1 cm., 617 m/x) should 

 be approximately twice that obtained at 580 m/x. 



A second procedure for the confirmation of vitamin A as the material 

 causing the absorption at 328 m/x is the demonstration that the absorption 

 curves are typical for vitamin A. This test was employed by the War 

 Food Administration for purchases of vitamin A during World War II. 

 \Vhen the ratios of E (1%, 1 cm., 300 m/x) and E (1%, 1 cm. 328 m/x) were 

 not more than 0.73, and the ratio of E (1%, 1 cm., 350 m/x) to E (1%, 1 cm. 

 328 m/x) did not exceed 0.65, the samples were considered to be satisfactory, 



"* J. R. Loofbourow, Vitamins and Hormones, 1, 109-155 (1943). 



