Carbon Dioxide Expanded Ethanol Extraction: Solubility and

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Carbon Dioxide Expanded Ethanol Extraction: Solubility and Extraction Kinetics of α‑Pinene and cis-Verbenol Said Al-Hamimi, Alícia Abellan Mayoral, Larissa P. Cunico, and Charlotta Turner* Lund University, Department of Chemistry, Centre for Analysis and Synthesis, P.O. Box 124, SE-22100 Lund, Sweden S Supporting Information *

ABSTRACT: In general, diffusion rates in extractions are enhanced by increasing the temperature. In this study, we instead add compressed liquid carbon dioxide to the extraction phase to accomplish faster mass transfer. The feasibility of using carbon dioxide expanded ethanol (CXE) as the extraction phase was explored, targeting two medium-polar analytes, α-pinene and cis-verbenol in Boswellia sacra tree resin. Hansen solubility parameters (HSP) were first calculated for the analytes and the extraction phases investigated, ethanol, CXE, and supercritical carbon dioxide (scCO2) containing ethanol as a cosolvent. Second, an extraction method with CXE as the extraction phase was optimized using a Box Behnken design, giving optimal conditions of 40 °C, 9.3 MPa, and 0.31 molar fraction of CO2 in ethanol. Third, the developed method was compared with a supercritical fluid extraction (SFE) method and a conventional solid liquid extraction (SLE) method, showing that CXE enables faster and more efficient extraction than both SFE and SLE. In fact, calculations based on Peleg’s equation showed that the initial extraction rate of the new method is up to 10 times faster than SFE when using the highest flow rate tested, 3 mL/min. It was also discovered that it is crucial to cool the makeup solvent in the collection system for efficient analyte collection, at least in modern SFE equipment where pressure is regulated by a backpressure regulator. The use of CXE and pertinently also other CO2-expanded liquids in sample preparation shows a great potential in terms of increasing the extraction rate without elevating the temperature.

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cause opening of the cell matrix as well as breaking of intermolecular interactions between the analyte and the sample matrix, i.e., improved desorption, leading to higher availability of analytes for extraction.5 However, a higher extraction temperature may cause decomposition of thermally unstable compounds6,7 as well as loss of volatile compounds. An alternative to using higher temperature to obtain faster mass transfer is to use a supercritical fluid. Supercritical fluid extraction (SFE) using supercritical carbon dioxide (scCO2) as a solvent has many advantageous properties, including gas-like viscosity, liquid-like density, around hundred times faster diffusivity than in organic solvents at ambient conditions, as well as operation at relatively low temperature.8 There are however also disadvantages of using SFE, including difficult flow rate control and challenging collection of analytes, especially of volatile compounds, due to the immense expansion of CO2 from pressures of typically around 30−60 MPa to ambient conditions.9 In terms of solubility, scCO2 can replace organic solvents such as hexane or heptane, and it typically dissolves nonpolar compounds like oils, fats, and waxes. ScCO2 is however not an appropriate solvent for the

ample preparation is of crucial importance for performing high-quality chemical analysis. A sample preparation procedure can vary in the degree of recovery, selectivity, and speed, depending on the approach and conditions used. In extractions, a high degree of selectivity and recovery can be achieved if there is a compatibility in physicochemical properties between the analytes in the sample matrix, the sample matrix itself, and the extraction phase.1 The performance of an extraction method is also described by the environmental sustainability of the solvent and auxiliary chemicals used. Solid liquid extraction (SLE) is one of the most widely used sample preparation techniques for solid samples. SLE can be described as a an extraction process consisting of desorption from the solid sample surface, diffusion within the sample matrix and through stagnant (mass boundary) layers, and partitioning/solvation to the extraction phase.2 Based on this, the extraction rate is controlled by desorption/diffusion (mass transfer) or partitioning/solubilization (thermodynamics). A major disadvantage of SLE however, is that the extraction rate is often slow, and the process takes long time for completion. One solution is to operate the extraction at higher temperature, in order to increase the diffusivity and decrease the solvent surface tension, thereby giving faster mass transfer.3 A higher temperature also generally improves the solubility in the extraction phase.4 Furthermore, a higher temperature may © 2016 American Chemical Society

Received: November 30, 2015 Accepted: March 22, 2016 Published: March 22, 2016 4336

DOI: 10.1021/acs.analchem.5b04534 Anal. Chem. 2016, 88, 4336−4345

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Analytical Chemistry

Figure 1. Schematic diagram of the apparatus used for CXLE and SFE experiments.

extraction (PLE),24 pressurized hot (subcritical) water extraction (SWE),25 SFE,26 and microwave assisted extraction27 have been used. For the first time, ethanol with added liquefied CO2 (CXE) is explored as the extraction solvent in an analytical chemistry context, considering solubility/partitioning and desorption/ diffusion rate-limiting steps as well as important equipment aspects. The optimized CO2 expanded liquid extraction (CXLE) method is compared to SFE (scCO2/ethanol) and SLE (ethanol). We demonstrate that CXLE offers fast mass transfer without using elevated temperature as well as high solubility of the target compounds (α-pinene and cis-verbenol) based on extraction recovery data and theoretical calculations of the Hansen solubility parameters (HSP) using the group contribution method.

extraction of oxygenated components like phenolic compounds and terpenoids. In order to overcome this drawback, a polar cosolvent, a so-called modifier, is usually added to increase the solubility of the analytes in scCO2. For instance, essential oil has been extracted from plants and herbs with scCO2 containing 5 and 10% ethanol.10 An alternative to supercritical fluids is to add compressed (liquefied) CO2 gas to a conventional organic solvent, giving a so-called CO2-expanded liquid (CXL).11 Depending on temperature, pressure, and chemical composition of the system, at equilibrium there will be either a two-phase system with a volumetrically expanded liquid phase containing CO2 and a gas phase containing mainly CO2 or a one-phase liquid mixture.12 In terms of properties, dissolving compressed CO2 in an organic solvent decreases its dielectric permittivity and subsequently its polarizability as well as its solubility parameters. This feature of tunability has been confirmed by measuring the polarizability and illustrated with Kamlet−Taft plots of π (polarizability) and β (basicity or hydrogen-bond accepting ability).13,14 Furthermore, dissolving compressed CO2 in an organic solvent will decrease its surface tension and viscosity, and thereby improve its mass transfer properties.15 So far, CXLs have been used in chemical engineering studies for particle formation, polymer processing, separation and crystallization processes, and in homogeneous and heterogeneous catalysis.16 However, there are limited studies in utilizing CXLs in analytical chemistry. Susan Olesik has developed the concept of enhanced fluidity liquid chromatography in which compressed CO2 is added to an organic solvent, commonly methanol, to obtain gradient separation with different retention mechanisms.17,18 CXLs have also been used as extraction solvent by Ibañ e z’s group for γ-linolenic acid 20 and astaxanthin21 (J. Supercrit. Fluids) and by Jessop’s group for lipids19 (Bioresource Technol.), although none of these studies presented any information about extraction kinetics. As an appropriate example, in this study we have investigated two compounds found in relatively high abundance in most aromatic plants and natural resin complexes, α-pinene and cisverbenol. These are organic compounds of terpenes classified as monoterpenes. An extraction phase typically used for these compounds is ethanol, and conventional SLE is often used as an extraction technique,22 but also Soxhlet,23 pressurized liquid

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EXPERIMENTAL SECTION Chemicals and Reagents. Ethanol (99.7%, Solveco, Rosenberg, Sweden) was used as a solvent. α-Pinene 98% was purchased from Alfa Aeser (Karlsruhe, Germany) and cisverbenol from Sigma-Aldrich (Steinheim, Germany). The internal standard compound n-decane was purchased from BDH (VWR, Pennsylvania). Ultrapure CO2 in cylinders with a dip tube was provided by Air Products (Amsterdam, The Netherlands). Plant Material. Authentic sample of the oleo-gum resins Hoojri was collected from the Dhofar region of Oman and was botanically identified by Dr. Sulaiman Al-Khanjari, a biodiversity researcher at Horizon for Medical and Scientific, Oman. The resin was crushed with a mortar and pestle to a powder and kept in a sealed bottle at room temperature. The initial moisture of the resins was measured by incubating in an oven at 80 °C overnight and was found to be 2% (w/w). Solubility Parameters. In this study, the software Hansen Solubility Parameters in Practice (HSPiP)28 was used to predict the Hansen solubility parameter (HSP) values for cis-verbenol because the original values of HSP from the Hansen work29 (obtained from liquid solubility data sets) were not available. HSP values were available in the literature for α-pinene and ethanol and were accordingly used in this study.29 The temperature dependence of the HSP for all compounds was calculated by the Jayasri and Yaseen30 method. However, the HSP values for the analytes varied only marginally with 4337


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Analytical Chemistry temperature, hence only the values for 40 °C are given. Temperature and pressure effects were considered in the prediction of HSP for CO2, as proposed by the Williams et al. model.31 The values for density used in the molar volume calculation necessary in Williams et al.31 method were obtained from NIST web book.32 When experimental data that was not available at the considered pressures in this work, a linear correlation was performed to extrapolate the experimental data for density of CO2 at 60, 70, and 80 °C. For a mixture of solvents, each of the HSPs were considered linear with the composition of the solvents in the mixture.33 Carbon Dioxide Expanded Liquid Extraction (CXLE). An MV-10 ASFE (Waters Technologies, Manchester, U.K.) SFE system was used for CXLE. The system operation and parameters setting were controlled by ChromScope Software. The system as illustrated in Figure 1 is equipped with dual piston pumps for CO2 and cosolvent connected with a Tjunction, an extraction oven that can hold up to 10 extraction vessels of 5.0 mL each, a back pressure regulator (BPR), a transfer line heated with a heat exchanger, a makeup solvent pump, and a 12-bottles collection tray. Collection of analytes downstream was optimized for this system using a number of different methods. The extractions conducted for collection optimization were performed by spiking glass beads with αpinene and extracted at 40 °C, 7.5 MPa, 2.0 mL/min flow rate, and 1:1 (molar fraction of CO2−ethanol) for 10 min and collection in 25-mL flasks. For real sample extraction, 1 g of Boswellia sacra (Frankincense) resin was weighed and loaded into a 5 mL stainless-steel extraction vessel and filled with glass beads. CO2 and ethanol were pumped at a constant flow rate and mixed at the T-junction and then passed through a 200 cm coil for preheating inside the oven. The pressure was controlled by the BPR. The eluting extract was mixed with makeup solvent (ethanol placed in an ice bath) before the outlet, downstream of the BPR and the transfer line. The outlet tubing was inserted into 5 mL of liquid solvent (ethanol) in a 25-ml volumetric flask placed in an ice bath. The extract was collected as fractions every 5 min up to 20 min then every 10 min up to 60 min, and finally every 20 min until 100 min. The system was flushed with the CO2/ethanol fluid mixture after each run for cleaning of the system. The extracts were stored at −20 °C until further analysis. The molar fraction of CO2 in ethanol was calculated after recording the temperature of CO2 at the pump at each investigated flow rate and pressure. The density of the CO2 was calculated by an online program Peace Software,34 the values of temperature and pressure were the input. Calculated density values were used to calculate the number of moles of CO2 delivered to the T-junction per minute. The number of moles of ethanol per minute was calculated based on the fact that the density of liquid is not affected significantly by pressure and flow rate. Subsequently, the molar fractions of CO2 in ethanol were calculated using the calculated values of number of moles of both CO2 and ethanol. Information about the calculation of molar fractions of CO2 in ethanol along with the experimental points marked in a vapor/liquid phase diagram is shown in Table S1 and Figure S2 in the Supporting Information. Experimental Design. Response surface methodology (RSM) was used to explore the functional relationship between extracted amount of α-pinene and cis-verbenol as responses and the independent variables extraction pressure, extraction temperature, and molar fraction of CO2. The experimental

design was of Box−Behnken type (MODDE 10.1, Umetrics, Umeå, Sweden), i.e., second-order designs based on three level incomplete factorial designs. A total of 15 experiments with three replicates in the central point were performed. The extraction pressure ranged from 6 to 10 MPa, temperature ranged from 40 to 80 °C, and CO2 molar fraction of CO2 in ethanol ranged from 0.1 to 0.5. For optimization, the flow rate was set to 2.0 mL/min while the extraction time was set to 5 min. The makeup solvent flow rate was set to 0.3 mL/min. All responses were centered and scaled to unit variance. Multiple linear regression was used to calculate the fitting model and response surface. The optimum processing conditions were obtained by using graphical and numerical analysis based on the criteria of the desirability function and the response surface plots. Supercritical Fluid Extraction (SFE). In order to compare the performance of the proposed method of CXLE, extraction of the resin with SFE was run according to a described method in the literature35 with little modifications using same system of CXLE with the same setup except that the make up solvent was connected after the BPR but before the transfer line. Briefly, 1 g of resin powder was placed in a 5-mL extraction vessel and mixed with glass beads. The extraction was conducted under the following experimental conditions: temperature 55 °C, at 30 MPa, 5% (v/v) ethanol as modifier, and a total extraction time of 80 min. The extracts were collected in the same manner as described in the Carbon Dioxide Expanded Liquid Extraction (CXLE) section. The extracts were stored at −20 °C until further analysis. Conventional Solid Liquid Extraction (SLE). Conventional SLE was conducted in batch-mode based on a typical method described in the literature.22 A total of 3 g of the powder resin was extracted with 15 mL of ethanol in the dark at ambient conditions. The extraction was carried out with stirring for 2.5 h at 550 rpm. The extract was filtered using filter paper and the residue was subjected to the extraction with another 15 mL of ethanol. This procedure was repeated twice. The extracts were pooled and stored at −20 °C until further analysis. Continuous-flow SLE with ethanol as a solvent was performed using a home-built system consisting of an highpressure liquid chromatography (HPLC) pump (model 9012, Varian) connected to a preheating coil and an extraction vessel placed inside a heated gas chromatography (GC) oven. In total, 1 g of resin powder was placed in a 5-mL extraction vessel and mixed with glass beads. The extraction temperature was set to 40 °C, and the pressure was atmospheric (between 0.2 and 0.5 MPa built up due to tubing). The extracts were collected in fractions as described in the Carbon Dioxide Expanded Liquid Extraction (CXLE) section without using makeup solvent during an 80 min extraction period. The extracts were stored at −20 °C until further analysis. Extraction Rate. A study of the extraction rate (extraction kinetics) was performed at the optimal extraction condition of CXLE and selected SFE and continuous-flow SLE methods. The flow rate was varied between 1, 2, and 3 mL/min and the fractions were collected at defined time periods during a certain extraction time. The extraction curves obtained using the three different extraction methods were fitted to a model derived by Peleg.36 In Peleg’s model,36 the absorption of solutes (analytes) into a solvent is considered, and is described by the following equation (eq 1): 4338


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Table 1. Hansen Solubility Parameters Consisting of Dispersive Interactions (δD), Polar Interactions (δP), and Hydrogen Bonds (δH) for α-Pinene, cis-Verbenol, Neat Ethanol, CXLE, and SFE Conditions cmpd/solvent

δD (MPa1/2)

δP (MPa1/2)

δH (MPa1/2)

δTotal (MPa1/2)

condition

α-pinene cis-verbenol ethanol CXLE: ethanol with 0.31 molar fraction of CO2 (69:31, mol/mol)

16.6 16.8 15.4 8.5

1.8 3.1 8.7 5.0

3.0 8.4 18.8 11.1

17.0 19.0 25.8 14.9

SFE: scCO2 with 5 volume fractions of ethanol (95:5, vol/vol)

11.5

4.8

10.6

16.3

T = 40 °C T = 40 °C T = 40 °C T = 40 °C P = 9.3 MPa T = 55 °C P = 30.0 MPa

C(t ) = Co +

t K1 + K 2t

Principles of Green Chemistry37 concerns the use of safer solvents and auxiliaries. Water, scCO2, and ethanol among a few others are considered as green solvents.38 As discussed in the introduction, SWE with pressurized hot water as extraction phase brings disadvantages in terms of thermal degradation of the analytes, and the drawbacks of SFE with scCO2 as extraction phase include limited solubility of medium-polar and polar compounds, the necessity of using high pressure (typically 30−60 MPa), and a rather difficult flow rate control. Hence, the aim of this study was to explore the use of ethanol as a green extraction solvent, with the addition of compressed CO2 to enhance the mass transfer rates without using excessive temperatures as in PLE or SWE. The addition of compressed CO2 to the ethanol will also tune the dielectric and overall solvating properties of ethanol.39,40 Rationale Behind Solvent Selection−solubility Parameters. The solubility of the target analytes in the extraction phase is crucial since a high solubility could potentially lead to high recovery in short time with minimum usage of solvent. Solubility should be taken into account when selecting an appropriate extraction solvent. Instead of using time-consuming experimental procedures, a theoretical approach is advantageous as a first approximation. Hansen solubility parameter (HSP) provides a quick numerical method of rapidly predicting the extent of interaction between materials.29 Furthermore, the HSP approach is well acknowledged for its predictive power of the solubility of many compounds in liquid solvents. By using the molecular structure of the analytes, group contribution methods allow the calculation of HSPs.28 In this approach, molecular interactions are categorized into three groups: dispersive interactions (δd), polar interactions (δp), and hydrogen bonding (δh). This approach has been applied to predict the three interaction values of the analytes and solvents investigated in this study (Table 1). As Williams et al.31 report, both temperature and pressure affects the HSP for compressed CO2. At constant temperature, an increase of the pressure will increase the total solubility parameters due to the density affect. At constant pressure, an increase of temperature will decrease the total solubility parameters due to vapor pressure effects. Different ranges of HSP values for CO2 are observed at the liquid, gas, or supercritical fluid phase region.31 Thus, Williams et al.31 consider the density dependence for the HSP calculation. Therefore, the calculated HSP values for CO2 obtained in this work using Williams et al. method31 differ numerically from the original HSP given by Hansen at normal conditions of temperature and pressure. As shown in Table 1, ethanol at 40 °C has larger polar and hydrogen bonding interactions compared to α-pinene and cisverbenol, particularly for α-pinene. Adding CO2 to ethanol results in reducing these two interactions. Table 1 reveals that

(1)

where C(t) is the concentration of the analyte at time t (min), C0 is the initial concentration of extracted analyte at time t = 0 (mg/g of sample), K1 is Peleg’s rate constant (g of sample × min extraction time/mg analyte), and K2 is Peleg’s capacity constant (g of the sample matrix/mg of analyte). The Peleg rate constant K1 relates to the extraction rate (B0, in mg/min × g) at the beginning of the extraction (eq 2). At the same time, the capacity constant K2 is related to the equilibrium concentration (Ce, in mg/g), where the concentration of the extracted compound has its maximum (eq 3). In this study, K1, K2, B0, and Ce were calculated for the extraction curves obtained for the different extraction methods. B0 =

1 K1

(2)

Ce =

1 K2

(3)

Quantification of Essential Oil by GC/MS. Gas chromatography/mass spectrometry (GC/MS) analysis of the extracted essential oil was performed using an Agilent 6890 series GC, coupled with an Agilent 5973 mass selective detector (Agilent Technologies) and a HP-5 MS fused-silica capillary column (5% phenyl/95% dimethylpolysiloxane; 30 m × 0.25 mm i.d., film thickness 0.25 μm). The system was operated at 70 eV ionization energy, 0.5 s/scan, and the mass range m/z 40 to 350. The ion source and quadrupole temperatures were maintained at 250 and 150 °C, respectively. The oven temperature was held at 60 °C for 3 min then increased to 270 °C at a rate of 3 °C/min, both injector and detector temperatures were held at 250 °C. Helium was used as the carrier gas with a flow rate of 1.0 mL/min and a split ratio of 1:20. Calibration curves were generated by measuring the standards ranging in concentration between 1 and 250 μg/mL for both compounds. The first two fractions of the extracts were diluted two times to fit the calibration curve. Internal standard n-octane was added (10 μg/mL) to each extract fraction and calibration standards. The concentrations of the individual compounds were expressed relative to the internal standard. The final results were expressed as micrograms of each compound per gram of dry sample. The software used to handle mass spectra and chromatograms was ChemStation (Agilent Technologies).

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RESULTS AND DISCUSSION There is a growing consciousness in the world about the environment and the importance of replacing hazardous and polluting substances with safer ones. One of the Twelve 4339


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temperature of the extract, which leads to degradation alternatively sweeping out of volatile compounds from the collection flask. In SFE on the other hand, additional solvent is needed to purge the extracted compounds since the CO2 is rapidly expanding to gas, and in addition, heating is not an issue due to the cooling from the rapidly expanding fluid. Figure 2

the solubility parameters for ethanol containing CO2 in a molar ratio of 31/69, the optimal condition for CXLE as discussed later, and the scCO2/ethanol (95/5, volume/volume) solvent used in the selected SFE reference method, have similar values as those for α-pinene and cis-verbenol, especially considering polar and hydrogen bonding interactions. Although CO2 is nonpolar, under high pressure and with a polar cosolvent, its polarity interaction increases. Our hypothesis is that α-pinene and cis-verbenol are more soluble and have faster extraction rate in CXE compared to in neat ethanol, at the same temperature of 40 °C. Equipment Considerations and Optimization of the Collection Method. Extraction for quantitative analysis has never before been conducted using CXL as a solvent; hence, there is no commercial equipment available for CXLE. Choosing between PLE and SFE equipment, the latter is preferred since it is designed to deliver compressed CO2 as a fluid. The drawback of using an SFE system for CXLE is that high proportions of organic solvents might damage sealing parts. However, ethanol is generally a safe to use solvent with SFE equipment even at large proportions. The first aspect to consider in extractions using a compressed gas as the extraction phase is the collection step. Controlling the expansion of CO2 downstream, the extraction vessel is crucial in order to achieve quantitative recovery particularly for volatile compounds. Several methods have been developed for analyte trapping in SFE, such as liquid solvent and solid phase trapping,41,42 although the references are outdated. In the 1990s and 2000s, in SFE systems based on liquid solvent trapping, the flow rate and pressure were controlled manually by needle and capillary restrictors.9 In such systems, expansion of CO2 takes place inside the collection solvent trap. Modern systems for SFE are equipped with backpressure regulators (BPR) to control the pressure during the extraction. A BPR is based on a spring-loaded opening that changes in size depending on the upstream pressure. A BPR has a larger dead-volume than a needle or capillary restrictor, i.e., expansion of CO2 takes place inside the BPR as well as inside the transfer line right after the BPR (Figure 1). Hence, with modern analytical SFE instrumentation, methods for analyte trapping need to be revised. The use of neat scCO2 or with low percentage of cosolvent might lead to clogging of the BPR or the transfer line due to the rapid cooling of the fluid, which takes place upon expansion of CO2 to atmospheric pressure. In addition, potential sample losses due to aerosol formation during the expansion could occur. To prevent or reduce the impact of these issues, a makeup solvent is introduced downstream of the BPR to be mixed with the extraction phase containing the extracted compounds. Heating the transfer line after the BPR (Figure 1) also reduces the risk of clogging. In this study, for the first time, collection efficiency was studied in an analytical context for a modern SFE system equipped with BPR. The collection method was optimized using glass beads spiked with α-pinene (0.688 mg). It was investigated whether the collection efficiency would be affected by cooling the makeup solvent as well as the trapping solvent. Also, the position where the make up solvent is introduced (before or after the heated transfer line) has been investigated and it was found that adding the makeup solvent after the transfer line is enhancing the recovery in CXLE, and the opposite was found for SFE (data not shown). In CXLE, the explanation could be that the heat exchanger of the transfer line increases the

Figure 2. Extraction efficiency of α-pinene using different collection methods: (A) chilled makeup and trap solvents (EtOH), (B) chilled trap solvent only (EtOH), (C) ambient temperature for both solvents (EtOH), and (D) chilled makeup solvent (EtOH) and chilled trap solvent (EtOH/ISP). Control is a standard solution of α-pinene in ethanol (27.5 μg/mL). Error bars are relative standard deviations (RSD) for n = 3.

shows that the highest recovery, 89% of α-pinene, was found with chilled (3−5 °C) makeup solvent and the use of ethanol as a liquid solvent trap placed in an ice bath. When a mixture of isopropanol and ethanol (1:1) was instead used as a solvent trap, the recovery was found to be 81%. Using the default setting of the instrument at ambient temperature of both the makeup solvent and the trapping solvent gave the lowest recovery, 63%. Even worse recovery was found when the collection flask was initially empty (data not shown). The reduction in recovery can be attributed to a sweep out of the trap by the flow of gaseous CO2 and aerosols. Similar observations have been reported previously with volatile compounds extraction using scCO 2 as the extraction phase.43,44 Our obtained results clearly point out the importance of cooling both the makeup and trap solvents, and this method was used for the extraction study. It should be noted that in the extraction of real samples, the flow of analytes to the trap is slower due to the impact of desorption and diffusion from the sample matrix, as compared to extracting spiked glass beads. Hence, collection is pertinently easier with real samples. All of the remaining results reported below have been obtained using real samples, i.e., Boswellia sacra (Frankincense) resin. Optimization of Solvent and Solubility in CXLE. To understand the impact of temperature, pressure, and molar fraction of CO2 in ethanol on the resulting recovery of αpinene and cis-verbenol in the extraction process, a design of experiment was conducted based on a Box-Behnken design (BBD), see Table 2. The variables for the optimization were selected based on factors controlling solubility in extraction. One of the benefits of CXLE is that a relatively low pressure and moderate temperature is in general needed to achieve desired solubility and mass transfer properties. The range of the variables temperature and pressure is selected based on this fact, i.e., excessive pressure and temperature are not used (up to 10 MPa and 80 °C, respectively). In addition, the mixture of CO2 and ethanol should be used as a one-phase liquid mixture 4340


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Analytical Chemistry Table 2. Levels of Independent Variables Used in the Design of Experiment level independent variable

symbol

low (−1)

temperature (°C) pressure (MPa) molar fraction of CO2 in ethanol

T P M

40 6.0 0.1

middle (0)

high (+1)

60 8.0 0.3

80 10.0 0.5

and not a two-phase gas−liquid system, which depends on the temperature, pressure, and the proportion of CO2 in ethanol. Several studies have been conducted to evaluate vapor−liquid equilibrium of a binary systems composed of CO2/ethanol.45,46 The experimental data in such studies have been modeled by plotting pressure versus molar fraction of CO2 in ethanol at fixed temperature to illustrate the isothermal plot.45,46 Such data was also taken into consideration when selecting the range of the variables studied, see Table 2. Finally, the SFE equipment limitations were also considered when selecting the factor space, for instance, a pressure of less than 6 MPa was not obtainable due to the high tank pressure of the CO2. An empirical relationship expressed by a second-order polynomial equation with interaction terms was fitted between the experimental results obtained from experimental design and the input variables by applying multiple linear regression. The fitted model showed a total explained variance of 94% (R2 = 0.94) and a cross-validated predictability of 78% (Q2 = 0.78), where R2 shows the model fit and Q2 shows an estimation of the future prediction and precision, see Table S3 in the Supporting Information. The comparison of experimental data and predictive modeling by RSM was performed by the coefficient of determination (R2) and adjusted coefficient of determination (adj. R2), which were calculated to be 96.8% and 91.3% for α-pinene and 91.6% and 89.4% for cis-verbenol, respectively. These values indicate that the developed model by RSM is satisfactorily compatible with the experimental results. The results also showed that the model predicted the extracted amount adequately, as indicated by error analysis that showed nonsignificant lack-of-fit (0.26 and 0.50). Low residual values indicated a good agreement of the experimental data with the mathematical model. Contour plots (Figure 3) were obtained to illustrate main and interactive effects of independent variables on response variables. Results for analysis of variance (ANOVA) and regression coefficients of the fitted quadratic equation are found in Tables S3 and S4 in the Supporting Information. The contour plots in Figure 3 show for both compounds that the extracted amount was in large positively correlated with pressure and negatively with temperature. The coefficient plot showed that pressure, temperature, and the interaction of M × M were the most influential factors in the extraction of both compounds as well as the interaction of T × T for cis-verbenol (Figure S5 in the Supporting Information). Other parameters have small coefficients signifying that those factors can be ignored. In CXLE, an increase in pressure will lead to an increment in density of the extraction phase,47 since the compressibility of a GXL is larger than it is for a neat organic solvent. Such increase in density will most likely increase the solubility of the analytes in the extraction phase. Further, temperature was found to have a negative impact on the extracted amount for both compounds, where its influence was statistically significant for α-pinene and insignificant for cis-

Figure 3. Contour plots showing the interaction effect of pressure (MPa), temperature (°C) and molar fraction of CO2 in ethanol on the extracted amount of α-pinene and cis-verbenol (mg/g). The responses are illustrated as functions of two factors while the third factor is kept constant at the middle value.

verbenol. The negative impact of temperature on the extracted amount can possibly be explained by the fact that a temperature elevation decreases the density of the ethanol/CO2 mixture,48 which may lead to a reduction in the solubility of the analyte. The other explanation could be that the analytes are volatile compounds and increasing the temperature may increase the risk to sweep out the analytes from the collection flask. Molar fraction of CO2 in ethanol had a significant influence on the obtained concentrations of both compounds. Increasing the CO2 molar fraction from 0.1 to 0.3 increased the recovery, most likely due to an increasing solubility of the analytes in the extraction phase since the addition of compressed CO2 to the ethanol decreases its static relative permittivity.13 Table 1 further shows that adding compressed CO2 to ethanol improves the similarities in intermolecular interactions with the analytes in terms of polar interactions and hydrogen bond interactions but not in terms of dispersion interactions. The overall total solubility parameters calculated in Table 1 indicate that the CXL in this case is a more suitable solvent for the analytes than neat ethanol. Another effect of increasing the CO2 molar fraction is a decrease in surface tension and viscosity, which improves the mass transfer rate in the extraction phase. However, a relatively short extraction time (5 min) was used in 4341


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Figure 4. Extracted amount (mg/g) of α-pinene (A) and cis-verbenol (B) versus the extraction time (min) and the extracted amount (mg/g) of αpinene (C) and cis-verbenol (D) versus the solvent volume used (mL) at three different flow rates at the optimal condition of CXLE (40 °C, 9.3 MPa, 0.31 molar fraction of CO2 in ethanol). Error bars represent standard deviation for n = 3.

diffusion-controlled extraction. A low flow rate on the other hand will lead to a so-called solubility-controlled extraction, since the extraction approaches equilibrium.50,51 Plots of the CXLE extracted amount of α-pinene and cisverbenol vs time and solvent volume are shown in Figure 4 for three different flow rates, 1, 2, and 3 mL/min, using the optimal extraction conditions found (40 °C, 9.3 MPa, 0.31 CO2 molar fraction). Plots of extracted amounts for both compounds show that during the first 15 to 20 min, the extraction rates increase proportional to the flow rate of the extraction phase (Figure 4A,B), which is even more obvious in Figure 4C,D where the extracted amount is plotted versus volume of solvent used. This observation demonstrates that the extraction is limited primarily by the solubility rather than by desorption/diffusion. This can be explained by that the analytes are readily available at the interface solid/solvent. Subsequently, increasing the flow rate in this period will increase the concentration gradient and increases the mass transfer to the solvent bulk. For α-pinene, after 15−20 min of the extraction process, the extraction rate slows down at every flow rate investigated, indicating that the desorption/diffusion kinetics begins to dominate the extraction rate. Further, the extraction rate of cis-verbenol was faster than for α-pinene particularly at high flow rate. One reason could be that the solubility of cis-verbenol in the CXL is higher than that for α-pinene, which seems to be true considering polar and hydrogen-bonding interactions calculated as HSPs in Table 1. In addition, the extractable concentration of cis-verbenol was 5 times lower than that of α-pinene. Nevertheless, the extraction kinetics of cis-verbenol is solubility-controlled all through the extraction as clearly seen in Figure 4D. While the initial portion of the extraction is highly dependent on the flow rate, the extraction is sufficiently fast that good recovery for both compounds from the resin is achieved in about 60 min, regardless of the flow rate. After 60 min there was very little increase in the total extracted amount. Hence, it

this experiment; hence, mass transfer effects should be quite small. Adding more than 0.3 molar fraction of CO2 to the ethanol decreased the extracted amount, which is clearly observed in Figure 3. This could be due to the inherent risk that ethanol and CO2 could split into two phases, a liquid phase and a gas phase. An optimizer function based on a simplex algorithm with a nonlinear desirability function was applied to find the optimum condition in terms of T, P and M. The optimizer was set to maximize the extracted amount of both compounds. Both responses were given equal weights. The optimal condition was found to be a temperature of 40 °C, a pressure of 9.3 MPa, and a CO2 molar fraction of 0.31. This point is marked in Figure S2, Supporting Information. Extraction Rate Study Using the Optimized CXLE Conditions. Varying the flow rate in extraction fulfills two purposes; first, to find an appropriate flow rate giving as fast extractions as possible with as little dilution of the sample as possible; and second, to gain a deeper understanding of the extraction kinetics. Many models have been developed to describe the extraction kinetics of essential oils from plant materials.49 The diffusion model is one of these models based on the fact that the difference (or gradient) between equilibrium and fluid phase concentration may be the main driving force of the extraction process. Changing the solvent flow rate affects the concentration of extracted compounds in the solvent inside the extraction vessel and subsequently the magnitude of the diffusion coefficient inside the mass boundary layer. A high solvent flow rate results in a large concentration gradient, which can make diffusion inside the sample matrix and through the stagnant mass boundary layer go faster. In addition to diffusivity, the magnitude of the affinity of the analytes to the adsorption sites on the sample matrix (initial desorption and readsorption/desorption during the extraction) will affect the extraction rate. The situation above is called a desorption/ 4342


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Figure 5. Comparison of extraction rate using three different extraction methods: CXLE, SFE, and SLE at a flow rate of 3 mL/min for α-pinene (A) and cis-verbenol (B). Error bars represent standard deviation for n = 3.

Table 3. Values of Extraction Extent, Peleg’s Constants (K1 and K2), Initial Extraction Rate (B0) for α-Pinene and cis-Verbenol Extraction from the Resin with Correlation Coefficient (R2) and the Root Mean Squared Deviation (RMSD) Using Investigated Extraction Methods at Different Flow Rates type of extraction GXLE

cmpd α-pinene

cis-verbenol

SFE

α-pinene

cis-verbenol

Cont. SLE

α-pinene

cis-verbenol

flow rate (mL/min)

K1 (min g/mg)

K2 (g/mg)

B0 (mg/g min)

Ce (mg/g)

R2

RMSD

1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0

0.272 0.158 0.098 1.191 0.494 0.193 0.408 0.209 0.131 2.871 1.685 1.612 0.442 0.248 0.135 1.612 0.868 0.502

0.040 0.042 0.042 0.190 0.196 0.200 0.039 0.042 0.042 0.203 0.212 0.201 0.040 0.043 0.045 0.201 0.211 0.216

3.67 6.31 10.21 0.84 2.03 5.18 2.48 4.77 7.62 0.35 0.59 0.62 2.26 4.04 7.38 0.62 1.15 1.99

24.72 23.54 23.68 5.24 5.08 4.98 25.54 23.77 23.41 4.91 4.69 4.95 24.84 23.25 22.16 4.95 4.72 4.61

0.915 0.988 0.957 0.978 0.985 0.996 0.686 0.945 0.990 0.987 0.997 0.899 0.694 0.902 0.941 0.938 0.979 0.982

0.365 0.137 0.273 0.085 0.073 0.037 0.683 0.296 0.129 0.057 0.028 0.177 0.659 0.386 0.301 0.137 0.083 0.078

was decided that the optimal flow rate of the CXLE method was to use a flow rate of 3 mL/min for first 10 min then 1 mL/ min until 60 min. Comparison of CXLE with SFE and SLE Using Fractionated Collection. Extractions were performed using a typical SFE method used for essential oils in the literature,35 based on scCO2/ethanol (95:5, v/v) at 55 °C, 30 MPa, 1 mL/ min for 60 min; as compared to a classical SLE method operated in continuous-flow mode using ethanol at 40 °C, a flow rate of 1 mL/min for 60 min extraction. Extraction rate curves for both analytes were plotted for the two reference methods at three different flow rates, see Figures S6 and S7 in the Supporting Information. The curves from both methods (SFE and SLE) showed that the extraction of the analytes behave in a similar way as in CXLE, with the extraction being limited by solubility in the beginning and then by desorption/ diffusion kinetics. However, the extraction rate is slower for SFE compared with CXLE, particularly for cis-verbenol. The extraction rate of α-pinene using continuous-flow SLE has a similar extraction profile as SFE. In contrast, cis-verbenol shows a faster extraction rate by continuous-flow SLE compared to SFE. Figure 5 shows the extraction profiles for both analytes using the three extraction methods at the flow rate giving the fastest

extraction, 3 mL/min. It is obvious that CXLE is the most efficient extraction method among investigated methods, especially for cis-verbenol. Although SFE has the potential to enable fast mass transfer due to low viscosity and surface tension, a low solubility of the analytes will negatively affect the extraction rate. It is also interesting to note that none of the methods led to any thermal degradation during the extraction, since this would have been observed as a lower reachable extracted amount when a lower flow rate is used. In order to better compare the extraction rate between the different extraction methods, the Peleg’s equation has been used to quantify the initial extraction rates. Peleg’s equation is a nonexponential empirical model, initially used to fit data of moisture content of a sample vs time of exposure.52 In Peleg’s model,36 the absorption of solutes (analytes) into a solvent is considered. However, since the sorption shapes exhibited by Peleg’s model was found to be similar to extraction kinetics described for natural products, the model has been applied to profiling the extraction behavior of compounds from solid sample matrixes.53 In Table 3, the calculated parameters of Peleg’s model (constants K1, K2, Ce, and B0), correlation coefficient (R2), and the root mean squared deviation (RMSD) are shown. The Ce values represent the total extracted amount obtained at different flow rates throughout the extraction 4343


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Although CXLE does not give significantly higher recovery results compared to SFE and SLE in continuous-flow mode, the extraction rate results emphasized that CXLE gives a faster extraction rate and, again, has the potential to be run at even higher flow rate.

process (Table 3). Clearly, all three methods gave similar accumulative extracted amounts of the analytes. However, the B0 values indicated that the initial extraction rate of CXLE is faster in the extraction of both analytes, compared to both SFE and SLE. Most remarkable results were found at high flow rate (3.0 mL/min), where CXLE has a 10-fold higher extraction rate for cis-verbenol compared to SFE (Table 3). This suggests that CXLE enables fast diffusion rates and is beneficially run at high flow rates, perhaps even higher than was studied in this work. Comparison of the Final Extraction Methods. The efficiency of the developed CXLE method at optimal conditions was compared to the other extraction methods investigated in this study. CXLE was run under the optimal condition at a flow rate of 3 mL/min for the first 10 min and then 1 mL/min until 60 min as concluded from kinetics study, while the other methods were used as described in the Experimental Section, a flow rate of 1 mL/min for 80 min for SFE and SLE. Figure 6 represents the extracted amount of α-

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CONCLUSIONS In this study, we have shown that CO2-expanded ethanol (CXE) is a high-diffusion extraction phase that can be used in fast and efficient extraction of medium polar compounds from solid complex samples, in this case α-pinene and cis-verbenol in Boswellia sacra resin. Hansen solubility parameters were initially calculated for the analytes and the extraction phases (ethanol, CXE, and scCO2/ethanol), demonstrating the feasibility of analytes solubility in the extraction phases. The software Hansen Solubility Parameters in Practice turned out to be a useful tool to predict the HSP for the compounds based on their molecular structure when HSP values were not available from the literature or the HSP database. On the basis of initial calculations of HSPs, the CXLE was experimentally optimized using SFE equipment and a Box Behnken design. Our results point at the importance of cooling the makeup solvent in the collection step in CXLE using modern SFE equipment with a backpressure regulator. The optimal extraction condition was found at 40 °C, 9.3 MPa, and 0.31 molar fraction of CO2 in ethanol. A kinetics study showed that CXLE is initially controlled by solubility and then limited by internal desorption and diffusion of the analytes from matrix to the bulk solvent, i.e., it is proposed to use a high flow rate (3 mL/min) in the beginning of the extraction, followed by a lower one (1 mL/ min). The study also showed that CXLE is faster and more efficient than both SFE and SLE methods, for instance for cisverbenol the extraction rate using CXLE was 10-fold faster than SFE when operated at the highest flow rate studied, 3 mL/min. Here, the Peleg’s model was applied to calculate the extraction rates of the three extraction methods (CXLE, SFE, and SLE) at different flow rates (1, 2, and 3 mL/min). In summary, CXE is a promising extraction phase for use in high-speed “green” sample preparation, with operation at a relatively low temperature. Further research is needed to explore the potential of a variety of different CO2 expanded green solvents for use in sample preparation, especially considering thermally unstable analytes that could benefit from the mild extraction conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b04534. Additional experimental data and data analysis (PDF)

Figure 6. Total extracted amount (mg/g) of α-pinene (A) and cisverbenol (B) using the new CXLE method, SFE, continuous-flow SLE ,and batch-mode SLE. Error bars represent RSD for n = 3.

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pinene (Figure 6A) and cis-verbenol (Figure 6B) obtained by the four extraction methods. The results showed that continuous-flow extraction methods are more efficient compared to classical SLE in batch-mode. This is probably because the continuous addition of fresh solvent maintains a desorption/diffusion-controlled extraction at as high diffusion rates as possible and at the same time preventing the extraction from reaching equilibrium before the extraction is quantitatively completed. CXLE, SFE, and SLE in continuous-flow mode gave fairly similar recovery of both analytes, which was also observed in the experiments with fractionated collections (Figure 5).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +46 46 222 8125, +46 706 222 752. Fax: +46 46 222 8209. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by Antidiabetic Food Centre AFC at Lund University and the Swedish Research Council (VR, Grants 622-2010-333, 621-2013-4356, and 621-2014-4052). 4344


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The authors want to thank Sulaiman Al-Khanjari, Horizon for medical and supplies, and Oman for providing resin sample and botanic taxonomy.

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Carbon Dioxide Expanded Ethanol Extraction: Solubility and

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