Of the five senses that of odor perception is perhaps the most subtle and certainly the least understood.
The advantages afforded by the corroborative use of objective analytical methods such as gas-liquid and thin-layer chromatography, infrared and ultraviolet spectrography, and the classical analytical techniques, together with the subjective evaluation of odor and flavor are illustrated by descriptions of its application to the evaluation of cinnamic aldehyde, the establishment of analytical limits, the differentiation among various synthetic and natural types of geraniol, the influence of optical rotation on the odor of carvone, the determination of the geographical source of essential oils especially American peppermint oils, the detection of residual solvents in floral concretes, the sclareol content of concrete of clary sage, the differentiation of lemon oils of various origins, the spoilage of orange and lemon oils, the control and evaluation of terpeneless, sesquiterpeneless, and concentrated oils, and the odor evaluation of rectified peppermint oil.
Though the technique is very useful, it is subject to many limitations. Instances are cited in which the human nose is a far more sensitive detector than any instrument. New types of essential oils or aromatic chemicals require readjustment of objective criteria.
As regards perfume compounds, objective-subjective methods are useful primarily for duplication. The esthetic value of perfumes can only be judged by subjective means such as panel evaluations. Because of the many limitations, objective-subjective correlations should be attempted only after exhaustive study of the many factors involved in each individual problem.
At the present state of knowledge, the correlation of odor with molecular structure is of very limited value.