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In most naturally occurring vegetable fats and oils, unsaturated constituents contain only isolated, i.e. nonconjugated, double bonds in the cis configuration. These cis bonds may be isomerized to the trans configuration during extraction and processing procedures. Oxidation and partial hydrogenation promote isomerization from the naturally occurring cis to the trans isomers. Animal and marine fats may contain measurable amounts of naturally occurring trans isomers. Isolated trans bonds in long-chain fatty acids, esters and triglycerides are measured in an infrared spectrophotometer. An absorption band with maximum at about 10.3 µ, arising from a C-H deformation about a trans double bond, is exhibited in the spectra of all compounds containing an isolated trans group. This band is not observed in the spectra of the corresponding cis and saturated compounds. Measurement of the intensity of this absorption band under analytically controlled conditions is the basis for a quantitative method for the determination of isolated trans content. This method is applicable to the determination of isolated trans bonds in natural or processed long-chain acids, esters, and triglycerides which contain only small amounts (less than 5%) of conjugated materials. The spectra of long-chain fatty acids exhibit a band of medium intensity at about 10.6 µ band, arising from a vibration of the carboxyl group. Correction for the contribution of this band to the 10.3 µ band, along with correction for any "background" absorption at this wavelength, can be made by the baseline technique. However, if the isolated trans content of the long-chain fatty acid is sufficiently small, the correction will become the major factor in the measured absorption at 10.3 µ and quantitative accuracy is not attainable. Therefore, long-chain fatty acids containing less than 15% isolated trans isomers must be converted to their methyl esters before analysis by the infrared spectrophotometric method. The method is not applicable or is applicable only with specific precautions, to fats and oils containing large quantities (over 5%) of conjugated unsaturation, such as tung oil; to materials containing functional groups which modify the intensity of the C-H deformation about the trans double bond, such as castor oil containing ricinoleic acid or its geometrical isomer ricinelaidic acid (12-hydroxy-9-octadecenoic acid); to mixed glycerides having long and short-chain moieties, such as diacetostearin, or in general, to any materials containing constituents which have functional groups which give rise to specific absorption bands at or sufficiently close to interfere with the 10.3 µ band of the C-H deformation of the isolated trans double bond.