Free Energy Contribution to Gas Chromatographic Separation of Petroselinate and Oleate Esters
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SAGE-Hindawi Access to Research Chromatography Research International Volume 2011, Article ID 252543, 9 pages doi:10.4061/2011/252543 Research Article Free Energy Contribution to Gas Chromatographic Separation of Petroselinate and Oleate Esters Chanida Sansa-ard,1 Kornkanok Aryusuk,1 Supathra Lilitchan,2 and Kanit Krisnangkura1 1 Biochemical Technology Division, School of Bioresources and Technology, King Mongkut’s University of Technology Thonburi, 49 Tientalay 25 Road. Takham, Bangkhuntien, Bangkok 10150, Thailand 2 Department of Nutrition, Faculty of Public Health, Mahidol University, Rachathewi, Bangkok 10400, Thailand Correspondence should be addressed to Kanit Krisnangkura, [email protected] Received 16 July 2010; Revised 23 September 2010; Accepted 26 October 2010 Academic Editor: Alberto Chisvert Copyright © 2011 Chanida Sansa-ard et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The ease of separation by gas chromatography between petroselinic and oleic acids depends on the alcohol moieties of their esters. The esters of higher molecular weight alcohols tend to be better separated on a 90%-biscyanopropyl-10%-cyanopropylphenyl polysiloxane capillary column (30 m × 0.25 mm i.d.). By analysis of free energies contribution from different parts of the molecules, it is tentatively concluded that the interaction between the double bond and the column stationary phase is interfered by the bulky alkyl group, and it is the major driving force for the separation of the two fatty acids. 1. Introduction addition, Thies [12] reported the use of butyl esters for the separation of these two positional isomers at 220◦Cwith Petroselinic acid (18:1Δ6cis) is a major fatty acid in the a total run time of 13 min. Thies saw good resolution of seed oils of most of the members of the Apiaceae (Umbel- the butyl esters under these conditions, but they were not liflerae), Araliaceae, Garryaceae species, Acer (Aceraceae), baseline separated. On the other hand, with a highly polar and Geraniaceae [1–4]. Petroselinic acid is an interesting column (100%-cyanopropyl polysiloxane) of 50 meters, the oleochemical for food, cosmetics, and pharmaceutical indus- two isomers could almost be baseline separated as their tries [5]. Coriander seed oil, which contained about 70% isopropyl esters [13, 14]. These two isomers in coriander seed petroselinic acid, showed a significant hypolipidomic effect oil could also be separated and quantitated on a 30 m of on animals [6]. High petroselinic acid diets found to lower 100%-cyanopropyl polysiloxane with two steps temperature the n-6 long-chain fatty acids of the phospholipids [7]. program [15], but how well they were separated was not Hence, petroselinic has become more and more important reported. Near-baseline separations of methyl petroselinate in industry and human health. In 1997, Santinelli and and oleate on 100%-cyanopropyl polysiloxane columns from Damiani [8] pointed out that petroselinic acid was always two different manufacturers at 180◦C were reported [16]. accompanied by some oleic acid (18:1Δ9cis), and there A recent report [17] showed that methyl petroselinate and was no single chromatographic technique suitable for their oleate could be separated on ionic liquid stationary phase separation and quantitation. However, methods for its [1,9-di(3-vinyl-imidazolium) nonane bis-(trifluoromethyl) separation from oleic acid have gradually been improved. Lin sulfonyl imidate] columns, with the lengths of 12 (the et al. [9] and Liu and Hammon [10] reported the separation microbore capillary), 30, and 100 meters. of these two positional isomers by reversed phase liquid A detailed study on the effects of alcohol chain lengths chromatography. Kim et al. [11] were able to separate and and branched chain on the separations of petroselinic and quantitate petroselinic and oleic acids in coriander tissues oleic acid esters was reported by Isbell [18]. Analyses were as their methyl esters by gas chromatography (GC) on conducted on a 30 m of 100%-cyanopropyl polysiloxane a 5%-phenyl-95%-methylpolysiloxane capillary column. In column. The authors found that 2-ethyl-1-hexyl esters were 2 Chromatography Research International Δg Δg promising derivatives for quantitative analysis of the oils s lnH f and s lnS f are the changes in standard enthalpy and containing the mixture of petroselinic and oleic acids. entropy of transfer from solution to gas of an hypothetical = g g With the 2-ethyl-1-hexyl esters, the interaction of the polar molecule of z 0, respectively. δs lnH and δs lnS are the carboxyl group with the stationary phase was sterically increments in standard enthalpy and entropy of transfer masked. Therefore, the main interaction would be that from solution to gas per carbon atom; tR and tM are retention between the small polarity differences of the olefins and times of the solute and unretained gas, respectively the polar stationary phase of the column, which caused the At constant z,equation(4) is reduced to (6)orVant’ separation. Hoff’s equation. However, methyl esters of these two acids could also t − t c be separated on a 100 m of 100%-cyanopropyl polysiloxane ln R M = a + ,(6) capillary column [19]. tM T In this study, oils containing petroselinic and oleic acids where are transesterified with alcohols of different chain lengths. The gas chromatographic separations of these two positional a = a + bz, isomers are discussed in term of their standard free energy (7) Δg o c = c + dz. of transfer from solution to gas ( s lnG ). Thus, a general knowledge of GC related to Δg o is briefly reviewed. s lnG It was reported that all the four numeric values of (4)(a, In gas-liquid chromatography, the solute (at infinite b, c,andd) remained unchanged when the column was dilution) partitioning between the stationary and the mobile broken or cut into different lengths [22]. They were still valid phases is assumed to be at equilibrium or very close to Δg o for predicting tR of fatty acid methyl esters (FAMEs) with equilibrium state. Thus, the change in s lnG directly relates good accuracy. On the other hand, for columns of the same Δg o to its equilibrium constant (K). The s lnG , on the other stationary phase but differing in the inside diameters, only Δg g hand, can be divided into s lnG f and δs lnG [20] as described thenumericvalueofa was changed while the other three in the following: remained unchanged [23]. The β value is approximately g g g equal to column inside diameter/(4 × film thickness), while Δ Go = Δ G + zδ G,(1) s ln s ln f s ln the other coefficients are thermodynamic parameters and = Δg = depend on type of solute and solvent (stationary phase) pair. where z is the number of carbon atoms. If z 0, s lnG f Δg o Δg For linear aliphatic solutes (RH and RX), having the same s lnG ,thatis s lnG f is the free energy contributed from the number of carbon atoms but differing in the X functional hypothetical molecule of zero carbon atom or may be simply groups, the difference in the retention indices of R Xand understood as the functional group. Differentiating (1)with z g = Δg o g Rz H is called homomorphic factor [24]. When the solutes respect to z,oneobtainsδs lnG δ s lnG /δz.Thus,δs lnG is are fatty acid alkyl esters (FAAEs) of different alcohols, the the change in Δg Go per carbon atom. In addition, equation s ln general structure would be R1COORy,whereRz = R1COO- (1) can be expanded as described below [21]. and X = Ry. In this study, the homomorphic factors of From basic thermodynamics, fatty esters of different alcohols are evaluated in term of Δ 0 = Δ 0 − Δ 0 thermodynamic parameters. Furthermore, the free energy of G H T S , Δg transfer from solution gas of the double bond ( s lnGu)is 0 (2) Δg ΔG postulated,andtherelativechangein Gu is responsible ln k =− − ln β, s ln Å ff RT for the separations of 18:1 Å6 and 18:1 9estersofdi erent alcohols. where ΔH0 and ΔS0 are the changes in standard enthalpy and entropy, respectively. T is the absolute temperature, k is the retention factor, β is column phase ratio, and R 2. Experimental is the universal gas constant. Equation (3)isobtainedby 2.1. Materials. Coriander seeds were obtained from a plant substitution of (2)into(1), grower (Bangkok, Thailand). Olive oil was from local g g g g supermarket in Bangkok (Thailand). Fatty acid methyl esters Δ H f Δ S f zδ H zδ S ln k =− s ln + s ln − s ln + s ln − ln β (3) (C16–C20) and oleic acid methyl ester were obtained from RT R RT R Sigma-Aldrich Chem. Co. (St. Loius, MO). C1–C3 alcohols or and solvents were reagent grade obtained from Lab Scan Co. (Bangkok, Thailand). Higher alcohols (C4–C8) were from tR − tM c d ln = a + bz + + z ,(4)Sigma-Aldrich Chem. Co. (St. Loius, MO). tM T T where 2.2. Extraction of Coriander Seed Oil. One gram of coriander g g seed kernel was ground in a mortar. The paste was trans- Δ S f δ S a =−ln β + s ln , b = s ln , ferred to a 4 mL vial, and 1 mL of toluene was added. The R R (5) mixture was intermittently vortexed for 1 min. The toluene g g Δ H f δ H layer was decanted into a new vial and transesterified as c =− s ln , d =− s ln . R R described in the next section. Chromatography Research International 3 2.3. Transesterification. Transesterification of FAMEs with Thus, plotting a against 1/T would result in a straight line other alcohols (C2–C8) was carried out via a microreactor with a slope of c and an intercept of a.