Comprehensive two-dimensional GC for the analysis of citrus essential oils

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1 136 FLAVOUR L. MONDELLO AND FRAGRANCE ET AL. JOURNAL Flavour Fragr. J. 2005; 20: Published online in Wiley InterScience ( DOI: /ffj.1506 Comprehensive two-dimensional GC for the analysis of citrus essential oils L. Mondello, 1 * A. Casilli, 1 P. Q. Tranchida, 1 P. Dugo 2 and G. Dugo 1 1 Dipartimento Farmaco-chimico, Facoltà di Farmacia Università di Messina, Viale Annunziata, Messina, Italy 2 Dipartimento di Chimica Organica e Biologica, Facoltà di Scienze MM.FF.NN., Università di Messina, Salita Sperone, Italy Received 14 January 2004; Revised 23 March 2004; Accepted 3 April 2004 ABSTRACT: The lemon essential oil volatile fraction is considered one of the most complex amongst all citrus oils being traditionally determined by monodimensional GC. This conventional method, although providing important qualitative/ quantitative information, lacks the necessary resolving power for the separation of several critical compounds. In the past, this has been often accomplished through the employment of conventional multidimensional chromatographic methods, which can only achieve the characterization of single parts of a sample and whose analysis times are generally high. Comprehensive bidimensional gas chromatography (GC GC) is gaining an excellent reputation for the separation of complex samples. This powerful analytical technique is characterized by both an enhanced resolving power and a great potential for the classification of unknowns through chromatographic retention patterns. In this research, these features are exploited in the analysis of a lemon essential oil. Copyright 2005 John Wiley & Sons, Ltd. KEY WORDs: comprehensive GC; lemon essential oil Introduction Citrus essential oils are complex mixtures obtained through distillation, solvent extraction or mechanical pressure. They are used mainly in the food and perfume industries while several isolated components are also employed for their pharmacological properties. These samples contain several components (200 or more) with a wide range of physicochemical properties and are grouped essentially in a volatile (85 99%) and a nonvolatile (1 15%) fraction. The volatile components are mainly a mixture of monoterpene and sesquiterpene hydrocarbons and their oxygenated derivatives; aliphatic hydrocarbons, aldehydes, ketones, acids and esters are also present. 1,2 The volatile constituents are routinely analysed by conventional monodimensional GC methods, using both polar and non-polar stationary phases. Component identification is generally carried out using MS information, linear retention indices (LRI) and co-injection with commercial standards. These well-established methods provide a great deal of information that is often fundamental for the assessment of quality and authenticity. However, it is well known that many 1-D citrus oil peaks are the result of two or more co-eluting compounds and cannot be resolved by any single stationary phase. Whenever further analytical information is requested, this can be * Correspondence to: L. Mondello, Dipartimento Farmaco-chimico, Università di Messina, Messina, Italy. lmondello@pharma.unime.it obtained by performing two separate chromatographic separations. The literature reports several studies concerning the separation of critical compounds in citrus essential oils using on-line column-coupling techniques (LC GC/ MDGC) Some relatively recent papers have reported the on-line HPLC preseparation/concentration and HRGC analysis of various volatile fractions in a fully automated mode. 3 6 These effective separations have concerned monoterpene and sesquiterpene hydrocarbons and their oxygenated derivatives, as well as aliphatic compounds. LC GC has also been employed for the determination of enantiomeric ratios. 7 This type of hyphenation has proved to be very useful as it eliminates the risk of contamination, artifact formation or loss of sample, which were characteristic of the off-line methods. It is characterized, however, by a series of disadvantages, such as the necessity of solvent elimination and the high technical skill required for such an elaborate method. Conventional MDGC is a typical heart-cutting method, which involves the transfer of unresolved compounds from a primary column onto a secondary column. The columns, connected by a switching valve interface, have different separating mechanisms. Several successful applications, especially chiral ones, have been reported in literature Although MDGC is a valuable analytical tool, it is also characterized by a few drawbacks, such as high time costs, complex instrumentation and the fact that the 2-D advantage cannot be exploited by all 1-D peaks. This major handicap has been entirely resolved with the recent introduction of a modern multidimensional

2 COMPREHENSIVE 2-D GC FOR CITRUS ESSENTIAL OILS ANALYSIS 137 technique: comprehensive two-dimensional (2-D) gas chromatography (GC GC). In GC GC the whole sample is subjected to the 2-D interaction and separated on two distinct GC columns connected in series with a modulator device. The function of the modulator is to trap, refocus and then release continuous fractions of the primary column effluent onto a shorter fast column. In order to achieve comprehensive GC analysis and to preserve the 1-D separation, the narrow bands injected onto the secondary column must undergo elution before the modulator performs the following cycle (typically 5 6 s). When analysing extremely complex samples, it is common to observe a first dimension peak untangled into eight to ten components after the brief interaction with the shorter column. It is generally recognized that every 1-D peak must be sampled three or four times in order to obtain satisfactory 2-D information. This means that a 1-D peak with a 20 s base width requires a modulation period of no more than 6 s There have been various approaches regarding the type of modulator employed. The first commercial instrument employed, the thermal sweeper type modulator, was based on a thick stationary phase trap and the subsequential pulsed reinjection of the retained solutes by a rotating heater. 15 A later approach was the longitudinally modulated cryogenic system (LMCS), which achieved low temperature trapping and refocusing by exploiting the freezing effect of a CO 2 stream passing through a moving modulator. 16 All components eluting from the secondary column are represented as oval-shaped peaks scattered onto a 2-D space plane, each pinpointed by two retention time coordinates. Furthermore, compounds belonging to the same chemical group tend to fall within specific zones in the bidimensional chromatogram, forming characteristic patterns. This aspect has been reported recently in literature. 17,18 Comprehensive 2-D GC has been applied successfully in many fields, such as environmental, 19 petrochemical, 20 flavours and fragrances, 21,22 pesticides 23 and foods. 13,24 The unprecedented resolving power of this method, as well as a greater detection sensitivity (due to zone compression), are ideal characteristics for natural volatile product analysis from complex matrices. The present research was designed to evaluate the utility of GC GC in citrus essential oil analysis. The results are discussed by contrast with widely employed conventional GC techniques. Experimental Standard Compounds Hydrocarbon stock solution: 100 ppm of each hydrocarbon (C14:0 C34:0) in n-hexane (Supelco, Milan, Italy). Essential oil components: α-terpineol, nerol, neral, geranial, neryl acetate, geranyl acetate (Sigma- Aldrich, Milan, Italy), α-humulene and (E)-caryophyllene (Extrasynthese, Genay, France). Samples A commercial lemon citrus essential oil, representative of its geographical origin and technology extraction, was diluted 1:10 (v/v) in n-hexane to be employed as the analytical sample. GC GC Analysis GC GC analyses were performed using a Shimadzu Model 2010 gas chromatograph (Shimadzu, Milan, Italy) equipped with a FID system (operated at 50 Hz data acquisition frequency), a AOC-20s auto sampler and a AOC-20i auto-injector (Shimadzu, Milan, Italy), and GC Solution software (Shimadzu, Milan, Italy) for data acquisition. The GC was equipped with a LMCS Everest longitudinally modulated cryogenic system (Chromatography Concepts, Doncaster, Australia), with a mechanical stepper motor drive for movement of the cryotrap. A modulation frequency of Hz (6 s cycle) was applied in all analyses initiated by the GC solution programmed external events that, via the electronic controller, also started the motor operation. CO 2 was supplied to the trap, so its expansion cooled the trap, which was thermostically regulated at about 0 C. A small internal flow of nitrogen gas (about 10 ml/min) prevented ice formation inside the trap. Data were collected using GC Solution software and its export function; ASCII data were obtained through a list of detector responses every 0.02 s. The ASCII data were converted into a matrix with rows corresponding to a 6s duration and data columns covering all successive second-dimension 6 s chromatograms, using the 2-D GC Converter 2.0 (Chromatography Concepts, Doncaster, Australia). Contour representation of the 2-D chromatograms was created through Transform version 3.3 software (Fortner Software, VA, USA). The column set for GC GC analysis consisted of two columns, which were serially connected by a mini-union (SGE, Rome, Italy). In this study we used the following set of columns: the main conventional column was a Supelcowax-10 (30 m 0.25 mm i.d, 0.25 µm film thickness), while the secondary fast column was a SPB-5 (1 m 0.10 mm i.d., 0.10 µm film thickness) (Supelco Italy, Milan, Italy). The operational conditions were as follows: temperature programmed from 50 C to 280 C at 2.5 C/min. The GC was equipped with a split/splitless injector (270 C); an injection volume of 1.0 µl was employed and a split ratio of 30:1 was used. The carrier gas was hydrogen, with an average linear velocity of 70 cm/s, and the column head pressure was kpa. FID, at constant linear

3 138 L. MONDELLO ET AL. velocity. FID, 280 C; H 2, 50 ml/min; air, 400 ml/min; make-up, 50 ml/min (N 2 ). GC MS Analysis GC MS analyses were carried out using a Shimadzu QP5050 (Shimadzu, Milan, Italy) system equipped with commercial libraries (Wiley and NIST) and a laboratory constructed library. The operational conditions were as follows: Supelcowax-10 column (30 m 0.25 mm i.d, 0.25 µm film thickness) (Supelco Italy, Milan, Italy). Temperature program: C at 3 C/min. The carrier gas was He, delivered at a constant pressure of 26.7 kpa; injection volume, 1.0 µl; split ratio, 1:50 (250 C); interface temperature, 250 C; ionization energy, 1.50 kv; acquisition mass range, amu; solvent cut, 3 min. Pressurized CO 2 cylinder An increase in the CO 2 tank pressure was achieved through compression of the CO 2 with nitrogen. The introduction into the cylinder of this lighter gas enabled an increase in the CO 2 pressure of about 100 atm. A tube insert placed inside the gas cylinder allowed the exit flow of the sole CO 2. The two-stage regulator connected to the cylinder was modified in order to deliver a 150 atm CO 2 flow to the LMCS system. Results and Discussion Many citrus essential oil analytes reported in the literature are characterized by high volatility. In preliminary GC GC applications on the lemon oil and other matrices, the minimum modulation temperatures supplied by the pressurized CO 2 in the LMCS system proved to be insufficient for the entrapment of these components, which ended either unseparated or grossely broadened by the lack of zone compression. It is well known that a modulation temperature of a 100 C below the analyte elution temperature is required for effective entrapment. In this research, several components eluted within the 70 C range, and so trapping temperatures of about 30 C, were required for the satisfactory analysis of these analytes. Commercial CO 2 cylinders have a maximum pressure of 50 atm and are capable of providing modulation temperatures of just under 0 C, setting quite a large restriction on the number of volatiles that can be entrapped. In the present research, this problem was overcome with a substantial decrease of the minimum modulator temperature, obtained through an increase in the CO 2 cylinder pressure, as explained in Experimental. A monodimensional GC application was initially performed on the lemon essential oil sample. This analysis was carried out on a polar column (see Experimental), which achieved separation on the basis of solute polarity differences. The major components of this matrix were all identified using combined GC MS and LRI information. Substantial interferences occurred between the oxygenated monoterpene and sesquiterpene hydrocarbon fractions on this type of stationary phase. As a consequence, several peaks underwent partial or complete co-elution. It must be noted that, while the components of these chemical classes are characteristic for each citrus essential oil, the monoterpene hydrocarbon group presents a similar profile in citrus oils. A 26 min expansion of the 2-D space plane relative to the same matrix is illustrated in Figure 1. This GC GC separation was achieved on a 1-D polar and 2-D apolar column set, with volatiles separated by the former on a polarity basis and by the latter on a volatility basis. The 2-D peaks relative to the main oxygenated monoterpenes and sesquiterpene hydrocarbons were determined and are reported in Table 1 (these components are present within a specific range in lemon oil). Contour plot peak identification was achieved through information derived from monodimensional GC separations and internal standards. The superior resolving power of GC GC is clearly demonstrated by the fact that β-bisabolene (peak 15), neryl acetate (peak 16) and bicyclogermacrene (peak 14), which form a triple component peak in the monodimensional application, elute separately after the secondary column interaction. It is quite astonishing that while these volatiles eluted together after ca. 35 min on the polar 1- D column, their separation was achieved in under 5 s on the apolar 2-D column. (Z)-β-Farnesene (peak 10) and neral (peak 9), also unseparated after monodimensional analysis, are completely resolved by the multidimensional application. Another 1-D triple coelution concerning peaks 6, 7 and 8 is totally unravelled on the apolar 2-D column. As can be seen from the circled areas in Figure 1, monoterpene oxygenated compounds and sesquiterpene hydrocarbons all fall within specific zones in the bidimensional chromatogram. However, the distribution of the lemon essential oil volatiles, does not follow a regular pattern, as can be observed, for example, when analysing homologous groups of compounds. The reason for this is that, although essential oil analyte groups derive from the same number of terpene molecules, they have different hydrocarbon structures and/or functional groups. While the distribution of the sesquiterpene molecules inside their 2-D region seems to be completely random, the contour plot positions relative to oxygenated monoterpenes with the same functional group (esters, alcohols and aldehydes) are more related. Although these components have different hydrocarbon structures, peaks 1, 12, 22 and 24 (alcohols), 9 and 17 (aldehydes), 5, 7,

4 COMPREHENSIVE 2-D GC FOR CITRUS ESSENTIAL OILS ANALYSIS 139 Figure 1. A 26 min expansion of the 2-D chromatogram relative to a lemon essential oil Table 1. Separation of selected sesquiterpene and oxygenated monoterpene components from lemon essential oil Peak Compound 1 Linalool 2 cis-α-bergamotene 3 (E)-Caryophyllene 4 trans-α-bergamotene 5 Linalyl acetate 6 (Z)-β-Santalene 7 Citronellyl acetate 8 α-humulene 9 Neral 10 (Z)-β-Farnesene 11 Germacrene D 12 α-terpineol 13 Valencene 14 Bicyclogermacrene 15 β-bisabolene 16 Neryl acetate 17 Geranial 18 Geranyl acetate 19 cis-α-bisabolene 20 trans-α-bisabolene 21 Citronellyl formiate 22 Nerol 23 p-cymen-8-ol 24 Geraniol 21 and 16, 18 (esters) tend to align themselves in distinct bands. The present GC GC application can be considered a preliminary investigation on this food matrix. In fact, besides the aforementioned major lemon oil components, a considerable number of minor unidentified compounds are present in the whole 2-D space plane. Their total number is certainly higher than what can be obtained from a single, or even two distinct, monodimensional GC runs. Several of these are the result of multi-component 1-D peaks, present especially in the central zones of the chromatogram, which are illustrated in the expansion. For example, the sesquiterpene hydrocarbon region, wellresolved in the 2-D space plane, is of particular interest, both for its complexity and because it can vary greatly in citrus oils. The accurate determination of this fraction is very useful to reveal adulterations or for taxonomy classification, but it also can be quite problematic. In fact, five sesquiterpene hydrocarbon peaks (6, 8, 10, 14 and 15) of the 12 separated and identified through comprehensive GC were not resolved by the single polar column application. The analysis of these minor constituents can been achieved, as indicated, through on-line HPLC HRGC.

5 140 L. MONDELLO ET AL. The GC GC method presented in this work is certainly capable of supplying the same information without the drawbacks connected to the LC GC approach. Conclusions The scope of this research was to evaluate the effectiveness of 2-D comprehensive GC in citrus essential oil analysis. The results obtained confirmed the enormous potential of GC GC and also the superiority of this approach in comparison to traditional mono- and multidimensional techniques (LC GC/GC GC). Hopefully, the method presented here will prove to be advantageous for the routine characterization of citrus oils. The qualitative/quantitative analysis of the compounds reported, at a reasonable cost in time, would certainly be more than sufficient for the assessment of quality and the detection of even the most subtle adulterations, especially when suitable software for 2-D peak quantitation becomes available. We intend, in future investigations, to advance in the determination of all the unidentified compounds present in the bidimensional chromatogram and to carry out further applications on the more economically important citrus essential oils. This will be achieved with the support of time-of-flight (TOF) mass spectrometry, which is undoubtedly the instrument of choice for rapid-eluting 2-D peaks. 24,25 References 1. Dugo G, Cotroneo A, Verzera A, Bonaccorsi I. In Citrus, Dugo G, Di Giacomo A (eds). Taylor & Francis: London and New York, 2002; Dugo G. Perfum. Flavor 1994; 19: Mondello L, Bartle KD, Dugo G, Dugo P. J. High Resol. Chromatogr. 1994; 17: Mondello L, Bartle KD, Dugo P, Gans P, Dugo G. J. Microcol. Sep. 1994; 6: Mondello L, Dugo P, Bartle KD, Frere B, Dugo G. Chromatographia 1994; 39: Mondello L, Dugo P, Dugo G, Bartle KD. J. Chromatogr. Sci. 1996; 34: Dugo G, Verzera A, Cotroneo A et al. Flavour Fragr. J. 1994; 9: Mondello L, Catalfamo M, Proteggente AR, Bonaccorsi I, Dugo G. J. Agric. Food Chem. 1998; 46: Mondello L, Catalfamo M, Cotroneo A, Dugo G. J. High Resol. Chromatogr. 1999; 22: Mondello L, Catalfamo M, Dugo P, Dugo G. J. Microcol. Sep. 1998; 10: Phillips JB, Beens J. J. Chromatogr. A 1999; 856: Dallüge J, Beens J, Brinkman UAT. J. Chromatogr. A 2003; 1000: Shao Y, Marriott PJ, Shellie R, Hügel H. Flavour Fragr. J. 2003; 18: Shellie R, Mondello L, Marriott PJ, Dugo G. J. Chromatogr. A 2002; 970: Marriott PJ, Kinghorn RM, Ong R et al. J. High Resol. Chromatogr. 2000; 23: Marriott J, Kinghorn RM. Anal. Chem. 1997; 69: de Geus H, Aidos I, de Boer J, Luten JB, Brinkman UAT. J. Chromatogr. A 2001; 910: Mondello L, Casilli A, Tranchida PQ, Dugo P, Dugo G. J. Chromatogr. A 2003; 1019: Seeley JV, Kramp FJ, Scarpe KS, Seeley SK. J. Sep. Sci. 2002; 25: Blomberg J, Schoenmakers PJ, Brinkman UAT. J. Chromatogr. A 2002; 972: Shellie R, Marriott PJ, Cornwell C. J. High Resol. Chromatogr. 2000; 23: Marriott PJ, Shellie R, Fergeus J, Ong R, Morrison P. Flavour Fragr. J. 2000; 15: Dallüge J, van Rijn M, Beens J, Vreuls RJJ, Brinkman UAT. J. Chromatogr. A 2002; 965: Adahchour M, van Stee LLP, Beens J, Vreuls RJJ, Batenburg MA, Brinkman UAT. J. Chromatogr. A 2003; 1019: Debonneville C, Chaintreau A. J. Chromatogr. A 2004; 1027: