310. Spectroscopic Examination of Oils.—The presence of chlorophyll or of its alteration products is a characteristic of crude oils of vegetable origin. In refined oils, even when of a vegetable origin, all traces of the chlorophyll products may disappear. The absorption bands given by oils are not all alike and in doubtful cases a suspected sample should be compared with one of known origin.
In conducting the examination, the oil in a glass vessel with parallel sides, is placed in front of the slit of the spectroscope and any absorption band is located by means of the common divided scale and by the color of the spectrum on which it falls. Olive and linseed oils give three sharply defined absorption bands, a very dark one in the red, a faint one on the orange and a well marked one in the green.
Sesame, arachis, poppyseed and cottonseed oils also show absorption bands. Castor and almond oils do not affect the spectrum.
Fig. 96.
Lard Crystals × 65.
Fig. 97.
Refined Lard (beef fat) Crystals × 65.
A. Hoen & Co., Lithocaustic.