The variation of the extinction coefficient of vitamin A with solvent

Abstract

THE ultra-violet absorption spectrum of vitamin A, with its characteristic maximum near 328m,u, is now used extensively as a means both for identification and determination of the vitamin in the various natural sources in which it occurs. When, however, intensity of absorption of light is used for the quantitative determination of any absorbing entity in solution it has to be borne in mind that the intensity of absorption is not an exclusive property of the solute but also depends somewhat upon the nature of the solvent. In spectrographic determinations of vitamin A this source of discrepancies has usually been avoided in the past, and comparative results have been obtained, by using the same solvent whenever possible (most commonly alcohol). It is, however, sometimes convenient to use media other than alcohol for these determinations as, for instance, when Carr-Price tests are being carried out on the same sample in chloroform solution and only limited quantities of material are available.It was therefore decided to determine the ratios of the intensities of absorption of vitamin A in several typical solvents, i.e. alcohol, chloroform, benzene and light petroleum. In view of the fact, however, that the vitamin is usually present in natural products in only minute amounts, e.g. order 0 03 % in cod liver oil and 0-001 % in butter fat, it was necessary to determine whether the chemical substances constituting the natural environment of the vitamin cause any variation in the light-absorbing properties of the solute-solvent complex.For these reasons we have separately considered vitamin A in four different environments:(a) Fat and sterol-free fish liver oil concentrates prepared by distillation or chromatography and containing some 50-70 % of the vitamin.(b) The gross fat-free unsaponifiable matter of fish liver oils containing sterols as the major impurities (vitamin A content order 10-25 %).(c) Fish liver oils consisting mainly of glycerides with only relatively small amounts of vitamin A (0-5-3 %, or more rarely 10%, in halibut liver oils; 0 1-0-5 % in most other fish liver oils).(d) Butter fat and butter unsaponifiable matter. The vitamin content of the former is so low (order 0-001 %) that it is only accurately measurable after concentration in the unsaponifiable matter (order now 0 1-1 0 %).