RECENT DEVELOPMENTS IN ESSENTIAL OILS :291 particularly the introduction of improved instruments that give readily reproducible results, and of modern automatic recording devices that eliminate tedious plotting by hand. (A) Use of ultraviolet spectra has been given great impetus by the publication of Sale, et al. (143), of his work on lemon oils. (B) Use of infrared spectra has been greatly extended. A number of laboratories, under the direction of experienced researchers in the United States and abroad, are engaged in the isolation and purification of essential oil constituents, and in the measurement of their infrared spectra. Some of this valuable work has been published in the literature, some is available through commercial laboratories specializing in this field, but much is still in the development stage, hence unpublished. Libraries of reference spectra are being compiled, with modern machine-indexing of the spectra. Once a great many of the spectra have been assembled and indexed, it will be relatively easy to identify a compound isolated from an essential oil. One of the advantages of infrared spectroscopy is that minute quanti- ties of a substance, liquid or solid, are sufficient for examination. The uses of infrared spectra include: (a) Identification of components of essential oils. (b) Elucidation of structural formulas of newly detected components. (c) Selection of raw material for the manufacture of isolates. (d) Detection of adulteration. (e) Evaluation of pure materials by determination of the percentage of important constituents (56) e.g., true pipefine content of oleoresin pepper. (f) Control of manufacturing processes. The importance of infrared spectra to our industry is demonstrated by the fact that the Scientific Section of the E.O.A. has established a per- manent subcommittee to study infrared techniques and their application. (C) New distillation equipment. Modern laboratory fractionation columns are so efficient that they permit sharp separation of the various fractions, the separating power sometimes being equal to that of 150 single operations in the older, conventional stills. III. Liquid-Liquid Extraction This technique has been found very helpful in research work on fruit flavors. It involves distribution of a substance between two immiscible solvents. Multi-stage apparatus with counter-current distribution halve been developed (44). IF. Chromatography Chromatographic techniques have become standard methods in every modern laboratory investigating essential oils and their components. Much of the rapid success recently achieved in the isolation of essential

292 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS oil constituents, their separation from isomeric or azeotropic mixtures, and preparation in very pure form has been accomplished by the skillful use of chromatography. Taking advantage of different versions and techniques it is now possible to isolate trace compounds which, although present in an essential oil only in minute quantities, nevertheless exert great influence upon its odor and flavor (60, 133, 183). (Kirchner and Miller (84) have shown that chromatography can be applied also for the preparation of terpeneless oils.) The classical method of chromatography is that of selective adsorption on a column of active solid, development of the column, and elution with a series of solvents. Automatic fraction collectors add to the efficiency of the method. Another technique is partition chromatography (either paper or column partition), which also allows the examination of minute quanti- ties of material (173). Variations are the chromatostrip (100) and the chromatoplate (137) methods which permit the use of very small quanti- ties for simple and rapid characterization of an essential oil. New possibilities are being opened by the recent introduction of gas chromatography which gives very sharp separation (82). Once the nec- essary spectra have been accumulated and properly interpreted, gas chromatography will be a relatively simple and highly efficient tool in the hands of the essential oil and terpene chemist. Striking results have been obtained by a combination of chromato- graphic and spectroscopic methods: isolation of very small quantities of a substance in pure form, and rapid identification by means of its infrared spectrum. Entirely new vistas appear on the horizon with the emergence of Raman, mass, microwave, nuclear and magnetic resonance spectra (173), but in this far-reaching new field much more experience must be gathered, and better apparatus deyeloped before the techniques can be applied effec- tively in the essential oil laboratory. ELUCIDATION OF STRUCTURAL FORMULAS OF ESSENTIAL OIL CONSTITUENTS Reviewing the work of the early researchers (Wallach, Semmler, etc.) in the field of terpene chemistry toward the close of the last century one cannot help being amazed at how much was accomplished with the rela- tively primitive tools and techniques available at a time which has justly been called the "Elizabethan Age of the essential oil industry." Within less than thirty years the structures of most of the sesquiterpenes and their oxygen derivatives occurring in essential oils were established only the configurations of the more complex sesquiterpenes and azulenes defied at- tempts at elucidation. Representing formidable problems, these com- pounds could be attacked only after fundamentally new techniques of