Retention Indices for Frequently Reported Compounds of Plant Essential Oils V. I. Babushok,a) P. J. Linstrom, and I. G. Zenkevichb) National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA (Received 1 August 2011; accepted 27 September 2011; published online 29 November 2011) Gas chromatographic retention indices were evaluated for 505 frequently reported plant essential oil components using a large retention index database. Retention data are presented for three types of commonly used stationary phases: dimethyl silicone (nonpolar), dimethyl sili- cone with 5% phenyl groups (slightly polar), and polyethylene glycol (polar) stationary phases. The evaluations are based on the treatment of multiple measurements with the number of data records ranging from about 5 to 800 per compound. Data analysis was limited to temperature programmed conditions. The data reported include the average and median values of retention index with standard deviations and confidence intervals. VC 2011 by the U.S. Secretary of Commerce on behalf of the United States. All rights reserved. [doi:10.1063/1.3653552] Key words: essential oils; gas chromatography; Kova´ts indices; linear indices; retention indices; identification; flavor; olfaction. CONTENTS 1. Introduction The practical applications of plant essential oils are very 1. Introduction................................ 1 diverse. They are used for the production of food, drugs, per- fumes, aromatherapy, and many other applications.1–4 The 2. Retention Indices ........................... 2 need for identification of essential oil components ranges 3. Retention Data Presentation and Discussion . 2 from product quality control to basic research. The identifi- 4. Summary.................................. 45 cation of unknown compounds remains a complex problem, in spite of great progress made in analytical techniques over 5. Acknowledgments .......................... 45 the last several decades.3,5,6 Gas chromatography (GC) and 6. References................................. 45 gas chromatography-mass spectrometry (GC/MS) techniques are commonly employed for the identification of essential oil components.1,3,7,8 Measurement of retention indices (RI) of List of Tables chemical compounds and comparison with available reten- tion data collections is the usual approach in the confirmation 1. Cross-reference list of essential oil components of compound identification. Retention data are more useful (common names, other chemical identifiers and in combination with mass spectrometry, because the combi- average retention index values). .............. 4 nation can provide a nearly unambiguous identification of 2. Related enantiomers and racemic mixtures.. 16 isomers, which is difficult using mass spectra alone. Thus 3. Retention indices of essential oil components differences in the structures of branched alkyl substituents for dimethylsilicone stationary phase . ........ 17 and cis/trans isomers, which normally do not result in signif- 4. Retention indices of essential oil components icant mass spectral differences, can be identified with the use for dimethylsilicone stationary phase with 5% of retention data. phenyl groups. ............................. 26 Gas chromatographic retention data are very attractive for 5. Retention indices of essential oil components applications due to the simplicity of the measurements, for polar phase ............................. 35 application, and interpretation. To some extent retention data 6. Averaged standard deviation and confidence are available at no additional cost or effort as a by-product of range size for commonly reported constituents the use of chromatographic separation for mass spectrometry of essential oils............................. 45 measurements. It is known that a large number of compo- 7. The structural assignment of four isomers of nents of plant essential oils are common to many species. farnesol using retention data . ............... 45 Adams7 indicated that the ability to identify 500 compounds would enable one to identify more than 90% of the constitu- a)Electronic mail: [email protected]. ents of an essential oil of most species. Thus a relatively b) On leave from the Chemical Research Institute, St. Petersburg State Uni- small and reliable RI data collection can substantially versity, St. Petersburg, Russia. VC 2011 by the U.S. Secretary of Commerce on behalf of the United States. increase the identification effectiveness in the analysis of All rights reserved. essential oil constituents. 0047-2689/2011/40(4)/043101/47/$47.00043101-1 J. Phys. Chem. Ref. Data, Vol. 40, No. 4, 2011 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.6.105.191 On: Tue, 30 Dec 2014 17:37:42 043101-2 BABUSHOK, LINSTROM, AND ZENKEVICH 0 0 0 0 The purpose of this communication is to evaluate retention Ix ¼ 100n þ 100½logðtxÞlogðtnÞ=½logðtnþ1ÞlogðtnÞ; (1) indices for the most frequently reported components of essential 9–13 0 0 oils using the retention data from the NIST data-collection. where tn and tnþ1 are adjusted retention times of the refer- A large body of gas chromatographic retention data exists for ence n-alkane hydrocarbons eluting immediately before and the compounds of essential oils, thus reliable values of RI data after compound “X,” and tx is the adjusted retention time of can be provided. The goal was to evaluate retention data for compound “X.” Linear indices (nonisothermal indices in about 500 commonly identified components of essential oils. accord with the definition of Van den Dool and Kratz32 from We undertake the analysis of data distributions of multiple (rep- temperature-programming measurements) are defined by the licate) measurements to provide statistically justified RI values. following: Retention indices also find applications in the characteriza- tion of selectivity of stationary phases, in structural analysis, Ix ¼ 100n þ 100ðtx À tnÞ=ðtnþ1 À tnÞ; (2) and in studies of physico-chemical properties of analytes and 5,14 stationary phases. Relationships between Kova´ts indices where tn,tnþ1, and tx are net retention times. and thermodynamic properties are used for determination of A large volume of retention data is available for essential vapor pressures, enthalpies, and entropies of adsorption and oil compounds. Thus 1967 data records for limonene can be vaporization of different analytes.15–28 Zabiegala et al.29 found in the last release of the NIST database. The observed employed retention indices for the estimation of the calibra- RI data distributions are the result of several factors includ- tion constants of permeation passive samplers with polydime- ing small variations in stationary phase polarity, temperature thylsiloxane membranes. RI is also used as an aid in the conditions, and ratios between amounts of characterized ana- development of new medical and perfume formulations.8,30 lytes and reference compounds.34 The observed data spread The current status of retention data collections and of the is also the result of errors in measurements and compound development of computerized databases has been summar- misidentification.35 Discussion of retention data distributions ized by Babushok et al.9 The collection and processing of can be found in Refs. 9, 10, 35, and 36. gas chromatographic properties of chemical compounds began at NIST in the late 1990s with the aim of developing a 3. Retention Data Presentation and comprehensive and evaluated database of retention indi- Discussion ces.9,13 Currently the collection contains 346 757 data records for 70 839 compounds measured on nonpolar and po- A list of the most frequently reported compounds was gen- lar stationary phases (a data record corresponds to a single erated based on the number of data records available for dif- RI measurement for chemical compound). Data were ferent compounds in the NIST database.9,11,12 At first, the extracted from original papers, technical reports, conference list of compounds was determined based on the number of proceedings, and Internet sources (1958–2011). The collec- all available data records in the database. This list was tion contains over 10 000 sources of gas chromatographic adjusted in accord with the frequency of data reporting in the properties. The third release of the RI database is available journals publishing the results of research related to the as a part of NIST/EPA/NIH Mass Spectral database (June, essential oils, such as “Journal of Essential Oil Research,” 2011),11 as well as on the Internet (NIST Chemistry Web- “Flavor and Fragrance Journal,” “Phytochemistry,” and Book).12 The database allows one to study and evaluate “Biochemical Systematics and Ecology.” Additionally, we retention index variability for different chemical compounds. verified the presence of these compounds in the available This variability results from the differences in the column collections of data for essential oil compounds.6,7,37–39 As properties (brand, column size, etc.), differences in the con- the result, the list of frequently reported components of ditions of measurements, and experimental errors. essential oils was determined (Table 1). We limited our anal- ysis to the 505 commonly identified compounds. Only com- 2. Retention Indices pounds with at least five data-records were included in the final list. The retention index system suggested first by Kova´ts,31 The following describes our procedure for the treatment of and