Ionic liquids-based crosslinked copolymer sorbents for headspace solid-phase microextraction of polar alcohols

Ionic liquids-based crosslinked copolymer sorbents for headspace solid-phase microextraction of polar alcohols

Journal of Chromatography A, 1245 (2012) 32–38 Contents lists available at SciVerse ScienceDirect Journal of Chromatography A journal homepage: www...

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Journal of Chromatography A, 1245 (2012) 32–38

Contents lists available at SciVerse ScienceDirect

Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma

Ionic liquids-based crosslinked copolymer sorbents for headspace solid-phase microextraction of polar alcohols Juanjuan Feng a,b , Min Sun c , Xusheng Wang a , Xia Liu a , Shengxiang Jiang a,∗ a Key Laboratory of Chemistry of Northwestern Plant Resources, CAS/Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b Graduate University of Chinese Academy of Sciences, Beijing 100039, China c School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China

a r t i c l e

i n f o

Article history: Received 15 March 2012 Received in revised form 6 May 2012 Accepted 8 May 2012 Available online 12 May 2012 Keywords: Crosslinking copolymerization Polymeric ionic liquids Solid-phase microextraction Alcohols Gas chromatography

a b s t r a c t Halogen-based polymeric ionic liquid (PIL) fibers, which have proved efficient for solid-phase microextraction (SPME) of polar compounds, were with very limited fiber lifetimes. In this work, a novel crosslinked PIL sorbent with satisfactory stability and durability was prepared in situ via crosslinking polymerization processes on microstructured-silver coated stainless steel wire. 1,1 -(1,6Hexanediyl)bis(1-vinylimidazolium) bibromide ionic liquid was synthesized and used as the crosslinking agent, with 1-vinyl-3-octylimidazolium bromide as monomer. Extraction properties of the fiber for polar alcohols in polar aqueous matrix were examined using headspace SPME (HS-SPME) coupled to gas chromatography-flame ionization detection (GC-FID). Under the optimized extraction and desorption conditions, the established method exhibited high extraction capacity. Wide linear ranges were obtained with correlation coefficients (R) ranging from 0.9947 to 0.9999. Limits of detection were in the range of 0.5–20 ␮g L−1 . Compared with the non-crosslinked PIL fiber, the proposed crosslinked PIL fiber was with higher thermal stability and durability and longer lifetime. Four different liquor beverages were analyzed as real samples and good results were obtained. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Since the first introduction in 1990s [1], solid-phase microextraction (SPME) has become a very popular sampling method for the extraction of variety of compounds due to its simple, solvent-less, reliable and flexible properties. Various commercially available and in-lab produced fiber coatings have been developed with respect to current requirements in highly specific SPME applications. These coating materials cover a broad polarity scale so that some of them can be selected for sensitive analysis of analytes with different properties. Extraction of non-polar or medium-polar analytes mainly depends on their negligible solubility in water and their hydrophobic interactions with coatings. Because of the easiness of their extraction, these compounds were determined as model analytes in most reports. Efficient extraction of hydrophilic and polar compounds from samples with a polar matrix is comparatively more difficult, owing to their high affinities with sample matrix. Coating materials for polar analytes should have appropriate polar functionally groups. But simply increasing the polarity of the

∗ Corresponding author. Tel.: +86 931 4968266; fax: +86 931 8277088. E-mail address: [email protected] (S. Jiang). 0021-9673/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2012.05.026

sorbent to enhance its affinity for polar analytes is not efficacious because its affinity for the matrix is also enhanced, which may lead to competitive adsorption and finally to the elimination of the analytes [2]. Ionic liquids (ILs) are salts that are liquid at low temperature (<100 ◦ C), and they represent a new class of solvents with nonmolecular and ionic character [3]. The cationic parts of most ILs are organic-based moieties such as imidazolium, N-alkylpyridinium, tetraalkylammonium, and tetraalkylphosphonium ions. The anionic parts can be organic or inorganic and include such entities as some halides, nitrate, acetate, hexafluorophosphate (PF6 − ), tetrafluoroborate (BF4 − ), trifluoromethylsulfonate (OTf− ), and bis(trifluoromethanesulfonyl)imide (NTf2 − ). They deserve special considerations in scientific community for their fascinating properties including wide liquid ranges, low volatilities, good thermal stabilities, electrolytic conductivity, wide range of viscosities, adjustable miscibility, reusability, nonflammability, and so on. ILs may be the most complex of all solvents because of their capacity of most types of interactions with solutes (e.g., dispersive, ␲–␲, n–␲, hydrogen bonding, dipolar, ionic/charge–charge) [4]. The remarkable “dual nature” solvation characteristic makes them dissolve both no-polar and polar compounds. ILs and polymeric ionic liquids (PILs) have been widely used as sorbent materials in SPME concerning the extraction of no-polar or semi-polar analytes [5–17], and a

J. Feng et al. / J. Chromatogr. A 1245 (2012) 32–38

small number of reports also investigated their extraction performances for polar analytes [18–20] based on their multiple solvation interactions. Anderson and colleagues [19,20] exploited the hydrogen bond accepting property of PILs with chloride as counter anion for selective SPME of polar analytes. Higher extraction efficiency and selectivity with poly(ViHIm+ Cl− ) were obtained for the extraction of analytes with high hydrogen bond donating compared to the PIL containing the same cation but with the NTf2 − as anion. This was assigned to the high hydrogen bond basicity of chloride-based ILs, which offered strong interactions with polar and hydrogen bond donating analytes. But the authors found that because of the inherently lower thermal stability and higher volatility of chloride-based ILs, lifetime of the Cl− based PIL fiber was only 30–40 times under the conditions of their work, and that was much shorter than that of the NTf2 − based PIL fibers. In polymer chemistry, crosslinking is a common strategy to enhance thermal properties and alter properties of the resulting polymer systems. Commercially available polydimethylsiloxane/divinylbenzene (PDMS/DVB) and carbowax/divinylbenzene (CW/DVB) fibers are both adopted this strategy, with divinylbenzene as crosslinking agent, to diversify the properties of the SPME fibers. Under the consideration of that, bis(1-vinylimidazolium)based dibromide IL was synthesized and used as crosslinking agent to prepare a novel crosslinked copolymeric ILs sorbent in this work, with the aim to improve the durability and fiber lifetime of PIL fibers. Based on data reported by Lungwitz et al. [21–25], chloride and bromide based ILs were with similar hydrogen bond accepting ability ([Bmin+ Cl− ]/[Bmim+ Br− ], 1.00/0.93), dipolarity and polarizability ([Bmin+ Cl− ]/[Bmim+ Br− ], 1.13/1.14), while thermal stability of bromide-based IL was much higher than that with chloride as anion (Tstart ([Bmin+ Cl− ]/[Bmim+ Br− ]), 150/215 ◦ C; Tonset ([Bmin+ Cl− ]/[Bmim+ Br− ]), 264/273 ◦ C). So bromide-based ILs were used in this work instead of those chloride ILs. Coupled to gas chromatography-flame ionization detection (GC-FID), the proposed fiber was used to extract aliphatic alcohols in aqueous samples with headspace (HS) techniques. The proposed HS-SPME–GC-FID method was applied to determine the alcoholic flavoring components in liquor beverages. 2. Experimental 2.1. Materials and reagents The stainless steel wire was purchased from the Yixing Shenglong Metal Wire Net. Co. (Jiangsu, China). Methanol, ethanol, n-propanol, iso-butanol, n-pentanol, n-hexanol and silver nitrate were purchased from the Tianjin Chemical Reagent Plant (Tianjing, China). sec-Butanol was purchased from the Beijing Fuxing Chemical Reagent Plant (Beijing, China). isoPentanol was obtained from the Shantou Chemical corporation (Shantou, China). Aqueous ammonia (NH3 ·H2 O, 28 wt.%) was purchased from the Baiyin Liangyou Chemical Reagent Factory (Baiyin, China); 1,6-dibromohexane, 1-vinylimidazole, 3-mercaptopropyltrimethoxysilane (MPS), vinyltrimethoxysilane and glucose were purchased from the Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). 1,8-Octanedithiol was obtained from the Alfa Aesar (Ward Hill, MA, USA). Azodiisobutyronitrile (AIBN) was obtained from Shanpu Chemical Co. (Shanghai, China) and purified through recrystallization before use. 1-Vinyl-3-octylimidazolium bromide (VOIm+ Br− ) was purchased from Shanghai Chengjie Chemical Co. (Shanghai, China). All chemicals are analytical reagents. Real samples of liquor beverages were obtained from local market. Stock solutions of alcohols were prepared in acetone with concentration at 10 mg mL−1 , and stored at 4 ◦ C for use. Working

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solutions were prepared daily by diluting the stock solutions with distilled water. 2.2. Apparatus Analysis of the model compounds was performed with an Agilent 7890 GC system (Agilent Technologies, USA) equipped with a flame ionization detector (FID) and a split/splitless injector. The column for the separation of alcohols was AT-FFAP capillary column (Analytical Technology, Lanzhou) (30 m × 0.32 mm id. × 0.33 ␮m film thickness). Ultrapure nitrogen (>99.999%) was used as the carrier and makeup gas at 2.5 mL min−1 and 25 mL min−1 , respectively. The injector was used in splitless mode at 200 ◦ C. The detector temperature was fixed at 300 ◦ C. Separation of alcohols was achieved using temperature programs as follows: initial temperature was held at 45 ◦ C for 6 min and programmed at 2 ◦ C min−1 to 46 ◦ C, finally at 8 ◦ C min−1 to 160 ◦ C. Temperature program for analysis of thermal stability was performed as follows: initial temperature was held at 45 ◦ C, and programmed at 10 ◦ C min−1 to 220 ◦ C. 1 H NMR (UNITY INOVA-400 MHz) was used to confirm the formation of dicationic imidazolium monomer. Surface properties of the proposed fiber were characterized by a scanning electron microscope (SEM, JSM-5600LV, JEOL Ltd., Japan). 2.3. Preparation of PILs-coated SPME fiber 2.3.1. Synthesis of the dicationic imidazolium bromide salt 1,1 -(1,6-Hexanediyl)bis(1-vinylimidazolium) bibromide was synthesized according to the following procedures: 1vinylimidazole (9.42 g, 0.10 mol) was dissolved in 40 mL acetone; 7.78 g of 1,6-dibromohexane (0.05 mol) was dissolved in 10 mL acetone and slowly added into the 1-vinylimidazole solution. The mixture was stirred for 24 h at room temperature. The precipitate was obtained by filtering the liquid phase. Purification was performed by recrystallization in acetone three times. The structure was confirmed by 1 H NMR (see supplementary material). 1 H NMR (DMSO-d , 400 MHz; ı, ppm, relative to TMS): 6 9.429–9.436 (t, 1H), 8.180–8.189 (t, 1H), 7.891–7.899 (t, 1H), 7.236–7.297 (q, 1H), 5.909–5.954 (q, 1H), 5.408–5.435 (q, 1H), 4.151–4.187 (t, 2H), 1.792–1.826 (t, 2H), 1.265–1.300 (t, 2H). 2.3.2. Preparation of the crosslinked PILs-coated fibers The stainless steel wire was firstly coated by a microstructured silver layer via silver mirror reaction which was described in detail in our previous study [26]. After that, the metal wire support was modified according to procedures [27] shown in Fig. 1. The microstructured-silver coated fiber was immersed in an ethanol solution of MPS (20.0 mM) for 12 h to form a self-assembled monolayer of MPS on it. Then, it was dipped into acidic water (pH 1.0) for hydrolysis. Finally, the SAM-modified wire was functionalized with vinyl groups by immersing it in vinyltrimethoxysilane solution in ethanol (20.0 mM) for 12 h. After the modification, the fiber was coated with PILs via crosslinking copolymerization according to the following procedures: 30 mL of VOIm+ Br− (2.6 g, 0.3 M) solution was put into a flask with dimethyl sulfoxide as solvent. 2.4 g of crosslinker and 0.05 g of AIBN was added into the solution. The pre-polymerization solution was stirred at room temperature for 10 min. 2 mL of the solution and the modified fibers were transferred into a 15-mL glass tube and purged with nitrogen gas for 10 min. Then the tube was sealed immediately to perform the crosslinked copolymerization at 70 ◦ C for 6 h. After the reaction, the solution turned to semi-solid state. The fiber was pulled out from the tube carefully. A thin layer of PIL coating was observed on the fiber surface. After the vapourization of the solvent, the fiber was immersed again into another

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Fig. 1. Schematic representation of functionalization of stainless steel wire and preparation of copolymeric ionic liquid-coated fiber.

fresh pre-polymerization solution, and the procedure was repeated for several times to thicken the coating for sufficient extraction capacity. 2.3.3. Preparation of non-crosslinked PIL-coated fiber For comparison of thermal stability, non-crosslinked PIL-coated fiber was prepared according to the procedures in previous report [28]. Briefly, the microstructured silver coated fiber was immersed in the solution of 1,8-octanedithiol in toluene (20 mM) for 24 h to form a self-assembled monolayer of dithiol, which acted as chain transfer agent in the following polymerization. Afterwards, the fiber was immersed into the solution of VOIm+ Br− (0.5 M) containing appropriate amount of AIBN (5% of the monomer, w/w) for polymerization at 70 ◦ C for 12 h under nitrogen atmosphere. The procedure was also repeated for several times. After the reaction, the fiber was taken out and washed with ethanol. 2.4. Headspace solid-phase microextraction procedures The fibers were installed into a homemade SPME device which was a modification of 5 ␮L syringe [5]. Before the HS-SPME

experiments, the proposed fibers were activated at 200 ◦ C for 10 min in GC injector. For all the extraction analysis, 10 mL of water sample was placed into a 15 mL glass vial. A magnetic stirring bar was put in the vial to accelerate the extraction, and it was fixed at 1000 rpm. Ionic strength of the sample solution was adjusted to optimum value with NaCl (30 wt.%). The extraction equilibrium was conducted by immersing the fiber in the headspace of the sample at 80 ◦ C for 40 min. Once the extraction was over, the fiber was withdrawn into the needle, removed from the vial and then immediately put into the GC injector at 200 ◦ C for desorption. Possible carryover effect was avoided by a second time injection. 3. Results and discussion 3.1. Influence of crosslinking on fiber performances The crosslinked PIL-based fibers were prepared using 1,1 -(1,6hexanediyl)bis(1-vinylimidazolium) bibromide as crosslinking agent and 1-vinyl-3-octylimidazolium bromide as monomer. Extraction performances of crosslinked and non-crosslinked fibers

Table 1 Influencea of crosslinking on fiber performances. Content of crosslinking agent (g per 30 mL)

Content of IL monomer (g per 30 mL)

State of the post-copolymerization system

0 2.0 2.2 2.4 2.6 2.7

4.3 2.6 2.6 2.6 2.6 2.6

Liquid Liquid Soft semi-solid Soft semi-solid Soft semi-solid Hard semi-solid

a

Fiber performanceb

Fiber lifetimes

Reproducibility (n = 5, %)

30c 42 50 61 64 68

3.7–12.0d 4.3–10.3 3.0–13.4 4.6–14.2 11.7–23.1 15.7–24.4

Available to the reaction system mentioned in preparation procedures. Conditions: Extraction temperature, 80 ◦ C; extraction time, 20 min; content of NaCl, 30 wt.%; concentration of analytes, 4 ␮g mL−1 ; desorption temperature, 200 ◦ C; desorption time, 3 min. c Extraction times of fibers with RSDs of peak areas less than 25%. d RSDs of peak areas obtained by five different fibers for the extraction of eight alcohol analytes. b

J. Feng et al. / J. Chromatogr. A 1245 (2012) 32–38

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Fig. 2. Influence of desorption temperature on baseline of crosslinked (A) and non-crosslinked PIL (B) fibers (pA = 10−15 ampere (A)).

were investigated in detail in this work. We found that fibers produced with different amount of crosslinking agent showed comparable extraction efficiency, but lifetimes and reproducibilities (fiber-to-fiber repeatability) of them were greatly influenced. As shown in Table 1, acceptable fiber lifetime (≥60) is obtained with amount of crosslinking agent no less than 2.4 g, but reproducibility becomes bad (≥20%) with amount of crosslinking agent more than 2.6 g. Higher content of crosslinking agent was favorable for the increase of fiber durability because that will increase the degree of crosslinking of the copolymer. But if the content was too high, fibers will be easily damaged by the hard copolymer during the pulling out process, and that will lower the reproducibility. So the amount of crosslinking agent in pre-polymerization solution should be in the range between 2.4 and 2.6 g to get satisfactory fiber performance. In this work, fiber produced with 2.4 g of crosslinking agent was selected as candidate and used in all following characterization and extractions. The baseline of non-crosslinked and the proposed crosslinked fiber are recorded at different desorption temperatures (180, 200, 220, 250 and 280 ◦ C), and the chromatograms are shown in Fig. 2 (all fibers were activated at 200 ◦ C for 10 min before the desorptions). The bleeding signal of the crosslinked PIL fiber (Fig. 2A) is not obviously observed until the temperature is up to 220 ◦ C. When the temperature is up to 250 ◦ C, decomposes becomes strongly. Baselines of the non-crosslinked PILs-coated fiber (Fig. 2B) are more or less the same with that of the crosslinked one at 180 and 200 ◦ C. But when the temperature is above 220 ◦ C, bleeding of the non-crosslinked fiber becomes much more serious. The crosslinked fiber can be used approximately 60 times without significant loss of extraction efficiency (RSDs for peak areas less than 25%), but the lifetime of the non-crosslinked one was only about 30 times. So the crosslinking polymerization was favorable for increasing thermal stability and life span of the PIL coating. But durability of the bromide-based PIL coated fibers was still inferior compared with PILs with other anions, such as PF6 − and NTf2 − . Besides the inherently thermal instability, Anderson and colleagues attributed this to their swelling through absorption of water, especially when working at elevated temperatures [20].

3.3. Optimization of HS-SPME conditions Extraction and desorption conditions, such as extraction time, extraction temperature, ionic strength, and desorption time were optimized to obtain the highest extraction efficiency. All the experiments were carried out in triplicate and the averages were taken in the discussion. 3.3.1. Extraction temperature Temperature has a twofold impact on extractions. Higher temperature accelerates the mass transference, whereas partition coefficients of the analytes between the fiber coating and the

3.2. Surface properties of the crosslinked PIL fiber Fig. 3 shows the scanning electron microscope (SEM) images of the proposed crosslinked PIL fiber. The coating has smooth surfaces and porous structures. Partially rough surface may be produced during the pulling out process from the semi-solid copolymer solutions. Average thickness of the coating was about 35 ␮m.

Fig. 3. SEM images of crosslinked copolymeric IL fibers at 400-fold (A) and 2000-fold (B) magnifications.

30 20 – 7 – – – – a

b

0.5–100 0.5–100 0.1–100 0.1–100 0.05–100 0.05–100 0.01–100 0.01–100

Calibration levels, n = 9. Standard addition level, 4 ␮g mL−1 . c Polyacrylate. d Polydimethylsiloxane/divinylbenzene. Conditions: Extraction temperature, 80 ◦ C; extraction time, 40 min; content of NaCl, 30 wt.%; concentration of analytes, 4 ␮g mL−1 ; desorption temperature, 200 ◦ C; desorption time, 3 min.

30 15 – 5 – – – – – – – – – – 4.8 2.1 30 20 – 10 – – – – 14.5 8.3 7.3 6.6 9.6 10.5 14.4 4.5 6.0 8.3 2.9 4.2 7.1 5.1 8.0 4.8 0.4 0.4 2.7 0.4 6 4 4 8 ± ± ± ± ± ± ± ± 11.9 29.8 97.7 68.9 141 135 143 309 20 15 7 5 2 2 1 0.5

PAc (85 ␮m) Poly(ViHIm+ Cl− ) (8 ␮m) [20]

Methanol Ethanol sec-Butanol n-Propanol iso-Butanol iso-Pentanol n-Pentanol n-Hexanol

0.9970 0.9996 0.9997 0.9999 0.9947 0.9982 0.9980 0.9985

Single fiber repeatabilityb (n = 5, %) Slope ± SDa LODs (␮g L−1 ) Correlation coefficient (R) Linear ranges (␮g mL−1 )

Fig. 5. Effect of extraction time on peak area. Conditions: Extraction temperature, 80 ◦ C; content of NaCl, 30 wt.%; desorption time, 3 min; concentration of analytes, 4 ␮g mL−1 .

Compound

3.3.3. Ionic strength Ionic strength strongly influences the solubility of organic compounds in water solutions [30]. The effect of ionic strength was tested by varying the NaCl concentration from 0, 10, 20 and 30 wt.% (30 wt.% is close to the saturation degree of NaCl in water). The extraction efficiency increases with the ionic strength increasing (see supplementary material). So 30 wt.% was used during the extractions.

Table 2 Analytical parameters for alcohols based on the proposed crosslinked-copolymeric ionic liquid fiber coupled to GC-FID.

3.3.2. Extraction time Extraction efficiency usually varies with extraction time and reaches the highest when the extraction equilibrium is reached [29]. The extraction time was investigated from 10 to 80 min in this work. Effect of extraction time on extraction efficiency is shown in Fig. 5. For most of the analytes, the approximate extraction equilibrium is reached at 40 min. So the extraction time was fixed at 40 min for all the extraction procedures.

Fiber-to-fiber repeatabilityb (n = 5, %)

solution decrease with the increase of temperature. The extraction temperature was investigated from 15 to 90 ◦ C, and the influence of it on the peak area was shown in Fig. 4. The extraction efficiency increases with the solution temperature rising up to 80 ◦ C, then decreases from 80 to 90 ◦ C. So the optimum extraction temperature was 80 ◦ C. Because of the high affinity of the polar analytes with the water matrix, the extraction temperature selected was much higher compared with other HS-SPME procedures with non-polar or semi-polar compounds as analytes.

Poly[(StyrIm+ )2 C6 2NTf2 − ] (50 ␮m) [18]

LODs with other PILs-based fibers (␮g L−1 )

Fig. 4. Effect of extraction temperature on peak area. Conditions: Extraction time, 40 min; content of NaCl, 30 wt.%; desorption time, 3 min; concentration of analytes, 4 ␮g mL−1 .

PDMS/DVBd (65 ␮m)

J. Feng et al. / J. Chromatogr. A 1245 (2012) 32–38 LODs with commercial fibers (␮g L−1 ) [18]

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Fig. 6. GC-FID chromatograms of four different liquor beverage real samples with tenfold dilution prepared by proposed HS-SPME method. Peaks: methanol (1); ethanol (2); sec-butanol (3); n-propanol (4); iso-butanol (5); iso-pentanol (6); n-pentanol (7); n-hexanol (8).

3.3.4. Desorption time Under the consideration of the PIL thermal stability and fiber lifetime, desorption temperature was fixed at 200 ◦ C. Desorption time was carefully tested from 1 to 5 min. Analytes with lower boiling point get completely desorption earlier than the others (see supplementary material). Finally, desorption time was fixed at 3 min. 3.4. Method validation Under the optimized conditions, the proposed HS-SPME–GC method was evaluated by testing the analytical parameters of alcohols in water samples. The results are shown in Table 2. The method shows wide linear ranges with correlation coefficients (R) ranging from 0.9947 to 0.9999. Limits of detection (LODs) were calculated as three times the signal to noise ratio, and investigated by extraction of water samples spiked at different concentration levels to meet such signal levels. Values of LODs are in the range of 0.5–20 ␮g L−1 . LODs with other PILs-based and commercial fibers for some

analytes were also listed in Table 2. Performance of the proposed crosslinked PIL fiber is much comparable with the commercial polyacrylate (PA) fiber, which is with high polarity. Compared with the other PILs-based fibers, LODs in this work are lower but in comparable order of magnitudes. It is necessary to highlight the differences in coating thickness among these fibers. Because higher coating thicknesses in SPME are usually accompanied by higher extraction efficiencies. Single-fiber repeatability was investigated with five replicate runs. RSDs for different analytes are from 2.9 to 8.3%. Fiber-to-fiber repeatability was tested by controlling the film preparation conditions and performing the HS-SPME procedures under the same conditions. RSDs obtained from five different fibers are from 4.5 to 14.5%. Slopes of calibration lines were used to show the sensitivity of the method for different analytes. The fiber showed better sensitivity for the more hydrophobic analytes than the less hydrophobic ones. From the analytical performances of the method we can see that, hydrophobic interactions still existed besides the polar interactions during extractions. Analytes with higher hydrophobicity

Table 3 Determinations of alcohols with proposed HS-SPME–GC method in four different liquors. Compound

Methanol Ethanola Ethanolb sec-Butanol n-Propanol iso-Butanol iso-Pentanol n-Pentanol n-Hexanol a

Concentration (mg per 100 mL)

Sample 1#

Sample 2#

Sample 3#

Sample 4#

21.8 ± 3.2 52%, v/v (55 ± 2)%, v/v 12.1 ± 1.4 35.7 ± 3.9 12.5 ± 1.3 63.7 ± 5.6 0.023 ± 0.006 3.65 ± 0.45

2.1 ± 0.7 44%, v/v (45 ± 2)%, v/v 2.84 ± 0.67 44.6 ± 2.4 13.1 ± 2.1 60.1 ± 9.1 0.049 ± 0.005 7.15 ± 1.77

7.5 ± 1.0 38%, v/v (40 ± 3)%, v/v 3.58 ± 0.24 23.7 ± 4.1 1.17 ± 0.81 25.5 ± 3.5 – 3.70 ± 0.31

23.4 ± 2.2 44%, v/v (46 ± 2)%, v/v d 1.24 ± 0.59 6.3 ± 0.7d 16.1 ± 1.1 40.7 ± 2.9 – 1.25 ± 0.37

Concentration marked in brand label. Concentration obtained in this work. c With sample #1 as matrix and diluted at tenfold. d Unreliable detection value. Conditions: The same as shown in Table 2. b

Recovery for standard additionc (%)

82.2 ± – – 107.0 ± 87.5 ± 89.5 ± 90.3 ± 95.9 ± 97.0 ±

3.1

3.5 5.5 3.6 10.3 10.0 4.0

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were given wider linear ranges, lower LODs and higher sensitivities. Besides their lower solubilities, analytes with higher hydrophobicity kept stronger affinity with the long alkyl chain of monomers. That was also favorable for their extraction.

new proposed fiber has been proved to be a suitable candidate for headspace extraction of polar analytes in polar matrix with sufficient sensitivity, repeatability and wide linear ranges. Acknowledgement

3.5. Application Aliphatic alcohols are flavoring components in liquor beverages in trace level. Absolute and relative contents of them determine the quality and style of a certain Chinese alcoholic beverage. In this work, the as-established HS-SPME–GC method was applied to determine the concentration levels of model analytes in four different liquors. Contents of ethanol in these samples were labeled at 38–52% (v/v). Recoveries were investigated at different diluting level with distilled water in sample #1, with standard addition at 10 mg per 100 mL sample. Results showed that when the sample was diluted sevenfold, the lowest recoveries for some analytes was 73.0%; when the sample was diluted eightfold, the lowest recoveries was up to 78.4%; when the sample was diluted tenfold, recoveries for analytes except ethanol were in the range of 82.2–107%. So the external standard calibration method was feasible for the determination with tenfold dilution. Fig. 6 presents the GC chromatograms of the samples prepared by the proposed HS-SPME process. Because of the huge difference of concentrations between ethanol and other analytes, each sample was diluted fiftyfold for the quantification of ethanol, tenfold for the rest. Result of the determination is shown in Table 3. Concentrations of different analytes are with great diversity. For example, methanol is harmful component in liquors. Limit of its existence is regulated at 40 mg per 100 mL liquor beverages. Sample #4 keeps the highest concentration level at 23.4 mg per 100 mL, while in sample #2 is 2.1 mg per 100 mL. n-Propanol and iso-pentanol are the second major compositions among the proposed analytes in all sample (except the n-propanol in sample #4), which are from 23.7 to 63.7 mg per 100 mL. Content of ethanol are with good correspondence with label indications. 4. Conclusions In this work, a new laboratory-made crosslinked-copolymeric IL-based SPME fiber was prepared in situ on silver-coated stainless steel wire, with 1,1 -(1,6-hexanediyl)bis(1-vinylimidazolium) bibromide as crosslinking agent and VOIm+ Br− as monomer. Coupled to GC, the fiber was successfully used for detection of polar alcohols in aqueous matrix by headspace mode. Improved thermal stability and long life span were the main advantages of the proposed fiber compared with non-crosslinked PIL fibers. The as-established HS-SPME–GC method was applied successfully to determine aliphatic alcoholic components in liquor beverages. The

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