Abstract
A headspace solid-phase microextraction (HS-SPME) method was developed for isolation of monocyclic aromatic amines from water samples followed by gas chromatography–flame ionization detector (GC–FID). In this work, the effect of the presence of ionic liquid (namely, 1-hexyl-3-methyl-imidazolium hexafluorophosphate [C6MIM][PF6]) was investigated in the sol–gel coating solutions on the morphology and extraction behavior of the resulting hybrid organic–inorganic sol–gel sorbents utilized in SPME. Hydroxy-terminated poly(dimethylsiloxane) (PDMS) was used as the sol–gel active organic component for sol–gel hybrid coatings. Two different coated fibers that were prepared are PDMS and PDMS-IL ([C6MIM][PF6]) fibers. Under the optimal conditions, the method detection limits (S/N = 3) with PDMS-IL were in the range of 0.001–0.1 ng/mL and the limits of quantification (S/N = 10) between 0.005 and 0.5 ng/mL. The relative standard deviations for one fiber (n = 5) were obtained from 3.1 up to 8.5% and between fibers or batch to batch (n = 3) in the range of 5.3–10.1%. The developed method was successfully applied to real water and juice fruits samples while the relative recovery percentages obtained for the spiked water samples at 0.1 ng/mL were from 83.3 to 95.0%.
Introduction
Monocyclic aromatic amines, or aniline derivatives, are intermediates in the production of rubber, plastics, polyurethane foams, dyes, pesticides and pharmaceuticals. During production, use and disposal of these compounds, emissions of aromatic amines may occur. Aromatic amines, such as aniline and its substitute derivatives, are generally dangerous because of their toxicity and carcinogenicity (1, 2) or else they can be converted into toxic N-nitroso compounds through reactions with nitrosylating agents in the environment (3). Therefore, the monitoring of levels of aromatic amines in environmental matrix is very important.
Owing to the complexity of environmental samples, sample pretreatments, such as extraction, preconcentration and clean-up steps, are often required to improve the sensitivity. Solid-phase microextraction (SPME) is a fairly new sample preparation technique (4) that uses a solid or liquid sorbent coating to extract and concentrate target analytes from a sample. This technique has important advantages over conventional extraction techniques due to its ease of use, being rather rapid, portable and solvent-free. In addition, this extraction technique can be easily coupled to gas chromatography (GC) and high-performance liquid chromatography (HPLC).
In spite of its advantages, it has some important drawbacks, including low thermal stability of fibers coating, stripping of them, instability and swelling of coatings in organic solvents, limitation of selection of the sorbents and short life time of fibers. These disadvantages appear due to the lack of suitable chemical bonding of the stationary phase coating with the fiber surface and relatively high thickness of the conventional fibers (5).
Sol–gel coating technology, established by Malik and co-workers (6), has solved most of these problems. The distinguished character of the sol–gel technique is that it can provide efficient incorporation of organic components into inorganic polymeric structures in the solution under extraordinarily mild thermal conditions. Strong adhesion of the coating onto the support due to chemical bonding is a very important characteristic, which increases the coating stability toward organic solvents and high desorption temperatures (7–10).
Ionic liquids (ILs) are a new class of non-molecular solvents consisting entirely of ionic components, an organic cation and either an organic or an inorganic anion (11). They have many fascinating properties including: wide liquid ranges, negligible vapor pressure, high thermal and chemical stability, good film-forming ability, strong polarity and good solvating properties, wide range of viscosities, high electrolytic conductivity, reusability and nonflammability (12). Owing to these unique characteristics, ILs have received intensive attention in analytical chemistry, especially in separation science over the past few years (13). Recently, ILs have been used as excellent solvents and pore templates to prepare various hybrid organic–inorganic coatings for SPME and capillary microextraction by sol–gel technology (14–16). After the fibers and capillaries were coated with the sol–gel polymers, they were then thermally conditioned above the decomposition temperatures of ILs used and then rinsed with organic solvents. Therefore, ILs did not participate in the extraction process since they had been removed from the sol–gel coatings prior to use. The higher extraction performance of these IL-mediated sol–gel coatings was mainly attributed to their more porous morphology in comparison with the analogous sol–gel coatings prepared without ILs.
In this work, according to our previous work (16) we prepared an IL-mediated sol–gel coatings for SPME containing of PDMS as sorbent and (1-hexyl-3-methyl-imidazolium hexafluorophosphate [C6MIM][PF6] as porogen and co-solvent. The developed sol–gel coating was then utilized for the determination of monocyclic aromatic amines in environmental water samples.
Experimental
Chemical and regents
Monocyclic aromatic amines standards include aniline (99%) that was obtained from Fluka, N,N-dimethlaniline (99%) was purchased from Merck and 2-chloroaniline (99%) and 3-chloroaniline were purchased from Riedel-de Haen. Hydroxy-terminated poly(dimethylsiloxane) (PDMS) was bought from Fluka (Buchs, Switzerland), and methyltrimethoxysilane (MTMOS), trifluoroacetic acid (TFA) and poly(methylhydrosiloxane) (PMHS) were bought from Merck (Darmstadt, Germany). 1-Hexyl-3-methyl-imidazolium hexafluorophosphate [C6MIM][PF6] was obtained from Ionic Liquids Technologies (Denzlingen, Germany). The stock solution (500 mg/L) containing the monocyclic aromatic amines, aniline, N,N-dimethylaniline, 2-chloroaniline and 3-chloroaniline was prepared in methanol and stored in the dark at 4°C. Working solutions were made daily just before use to the required concentration with deionized water. Water samples (river water and wastewater) were collected from Mashhad, Razavi Khorasan Province, Iran, and stored in amber-glass bottles without headspace and maintained in the dark at 4°C until their analysis.
Instrumentation
Analysis of gas chromatographic was carried out using a Chrompack CP9001 (Middleburg, the Netherlands) equipped with the flame ionization detector (FID) and a split/splitless injector. Helium (99.999%; Sabalan Co., Tehran, Iran) was used as the carrier gas and was set at 1 mL/min. GC separation was achieved with a CP-Sil 24CB (50% phenyl, 50% dimethylsiloxane) capillary column, WCOT Fused silica, 30 m × 0.25 mm ID with 0.25 µm stationary film thickness (Chrompack, Middelburg, The Netherlands). Temperature of injection and detector was set at 280 and 300°C and fiber desorption was carried out in the splitless mode for 3 min, after this time the injection port was set to split mode (split ratio = 100). The oven temperature was programed: 80°C was used for initial oven temperature and holding for 5 min, then temperature was increased to 180°C by a linear thermal gradient of 5°C and held for 5 min. A Branson ultrasonic cleaner model, 1510 (Danbury, USA) was employed for mixing of various solution ingredients. Also the surface characteristics of the coating fibers were studied by scanning electron microscopy (SEM) (LEO, model 1450VP, Germany).
Preparation of SPME fiber
In this work, two different coated fibers which were prepared are PDMS and PDMS-IL ([C6MIM][PF6]) fibers.
Pretreatment of fused silica
The outer surface of the fused silica fiber (the diameter of bare fiber was 250 μm) was pretreated prior to the formation of the sol–gel coating on it. First, the polyimide layer was eliminated from a 1 cm segment of the fiber at one of a 3-cm-long fused fiber end. This was accomplished by burning it with flame. The burnt section of the fiber was cleaned with methanol, methylene chloride, acetone and deionized water and dried. Then, the burnt section of the fiber was soaked in 1 mol/L sodium hydroxide solution for 1 h so that all surface groups were in the form of Si–O–H on silica surface of the fibers, and then into 0.1 mol/L HCl for 30 min to neutralize the excess sodium hydroxide. Finally, the fibers were cleaned again with deionized water and dried in the oven.
Preparation of IL-mediated sol–gel coatings
PDMS-IL was synthesized according to our previous work (16). For the preparation of PDMS-IL, 100 mg PDMS, 100 μL methylene chloride and 50 μL of IL ([C6MIM][PF6]) were added to an eppendorf tube and dissolved thoroughly by ultrasonic agitation for 5 min. Then, 100 μL of MTMOS and 30 μL PMHS were added and mixed by ultrasonication for another 5 min. Then, ∼50 μL TFA containing 5% water was sequentially added to the resulting solution with ultrasonic agitation for 5 min. The sol solution was then centrifuged at 15,000 rpm for 5 min to remove the precipitate. The top clear sol solution was transferred to a clean plastic tube and was further used in the coating process. Then, the treated fiber was inserted vertically into the sol solution for 30 min. For each fiber, this coating process was repeated three times using a freshly prepared sol solution until the desired thickness of the coating was obtained. The coated fiber was kept in a desiccator at room temperature for 24 h and then conditioned in a GC injection port under a helium flow rate of 1 mL/min, at 100, 200, 270 and 320°C for 1 h, respectively. After conditioning, the fiber was ready for SPME experiments.
For comparison purposes, a PDMS coating fiber that did not contain any ILs was prepared in a similar manner except that 150 μL of methylene chloride was used as solvent instead of a mixture of methylene chloride (100 μL) and the IL (50 μL). The sol–gel coated fibers prepared without IL were thermally conditioned analogous to their IL-mediated counterparts for comparative purposes.
Headspace SPME procedure
In all experiments, 15 mL of the aqueous sample was spiked with monocyclic aromatic amines standards in a 25-mL glass vial containing 5.25 g of sodium chloride and a magnetic stir bar (PTFE coated). Then, vial was sealed with a PTFE-faced septum and parafilm to prevent possible losses of target analytes from the headspace during extraction. Extractions were carried out in a water bath equipped with a temperature control system. The extraction was performed by exposing the fiber coating to the headspace over the sample for 40 min at 30°C. The samples were continuously stirred at a constant speed with a magnetic stirrer. After extraction, the fiber was withdrawn into the needle, and then immediately inserted into the GC injector port for thermal desorption at 280°C for 3 min.
Results
Optimization of SPME operating conditions
The HS-SPME method is based on the multiphase (coating/headspace/aqueous) equilibration principle. To achieve the best extraction efficiency of the new coating for monocyclic aromatic amines, several parameters including extraction temperature, extraction time, ionic strength, stirring speed and desorption parameters such as desorption temperature and time were investigated and optimized. The optimization was carried out on an aqueous solution containing 100 ng/mL of each analyte.
To ensure complete desorption of analytes from the fiber and avoid memory effect or carryover, suitable desorption temperature and desorption time are critical. Regarding the volatility of the compounds under study, only temperatures between 250 and 300°C were tested. On the other hand, to avoid carryover effect, desorption time should be sufficient for the quantitative desorption of the extracted analytes from the surface of the coating SPME fiber. For this reason, experiments were performed with various desorption times to test the effect of desorption time (20 s up to 5 min) on the chromatographic peak areas of the analytes. The results showed that all of the components are completely desorbed at 280°C, so it was selected as the optimum desorption temperature and 3 min was selected as the optimal desorption time.
The extraction temperature has contrast effects on the extraction efficiency. An increase in extraction temperature causes an increase in the diffusion rate of the analytes and a simultaneous decrease in the analyte distribution constant between the headspace and fiber. A temperature range of 10–50°C was used to study the effect of extraction temperature on the extraction efficiency of analytes. The extracted quantity of analytes increased up to 30°C and decreased at higher temperature. According to these results, an extraction temperature of 30°C was considered to be the optimum condition for subsequent experiments.
SPME is an equilibrium-based technique, and there is a direct relationship between the extracted amount of analyte and the extraction time. The range of extraction times was investigated between 10 and 50 min for determination of optimum extraction time. The extraction efficiency for all amines under investigation increased up to 40 min. After this time, the extraction efficiency was not increased significantly for most of the analytes. Extraction time of 40 min was finally chosen as the optimal time for the subsequent evaluation.
Mass transfer from a liquid sample to the headspace can also be accelerated by stirring the sample, and sample stirring may therefore improve the efficiency of the extraction process. In this work, various stirring speeds from 200 to 1,000 rpm were investigated. On the basis of observations, the stirring speed of 1,000 rpm was selected as the optimum stirring speed.
Adjustment of the pH can enhance extraction, as dissociation equilibrium is affected together with the solubility of the acidic/basic target analytes. Aromatic amines are weak organic bases and exist in two neutral and ionized forms in aqueous solutions. For partitioning them into the headspace, they must be in the neutral form. The pH of sample solutions was adjusted by the addition of strong bases. The amount extracted onto the fiber increased with increasing NaOH concentration. A final concentration of 1.0 M NaOH was employed for further extractions.
Addition of salt, usually sodium chloride (NaCl), decreases the solubility of the compounds in water and forces more of these analytes into the headspace and extracting phase. The salting-out effect was examined by monitoring the variation of peak areas in the presence of different concentrations of NaCl (from saltless up to 35% w/v). Results showed that the extraction efficiency of the compounds under study increased with the salt content in the range tested. Thus, the optimum salt addition was selected to be 35% (w/v) for subsequent experiment.
Discussion
Comparison of PDMS and PDMS-IL fibers
For comparison, two kinds of fibers (PDMS and PDMS-IL) were used to extract desired compounds at the optimized conditions and their chromatographic peak areas were compared. Sol–gel coatings with greater porous morphology obtained with the help of ILs can be expected to provide better performance in extraction. According to Figure 1, the extraction efficiency of the PDMS-IL fiber for target compounds was better than the PDMS, and higher amounts of analytes were extracted.
Figure 1.
Comparison of the amount of analytes by PDMS and PDMS-IL. This figure is available in black and white in print and in color at JCS online.
The SEM images of both fibers show that the sol–gel PDMS-IL coating possesses a more porous structure than its counterpart prepared without IL (16). The SEM micrograph roughly shows off a film thickness of 5 μm for PDMS, and 17 μm for PDMS-IL coating fibers. Such a porous structure should significantly increase the available surface area on the fiber. A high surface area will be able to provide large stationary phase load and high extraction capacity. The results indicated that the IL acted as a porogen, which alters the morphology of the coating.
Validation of the method
Linearity, limits of quantitation (LOQs) and limits of detection (LODs) have been evaluated to assess the performance of SPME with sol–gel-coated fiber. The results are summarized in Table I. The linear range of the method was tested by extracting different aqueous standards with increasing concentrations. The HS-SPME procedure with PDMS-IL fiber showed a wide liner ranges with correlation coefficients ranging between 0.9902 and 0.9922. The LODs, minimum detectable concentration, were obtained by the concentration of monocyclic aromatic amines that provide chromatographic signals three times the background noise, and limits of quantification (LOQs), the lower limit of the linear range, were obtained by minimum concentration of analytes that produce chromatographic signals ten times the background noise. As could be seen, LODs and LOQs were from 0.001 to 0.1 and 0.005 to 0.5 ng/mL, respectively.
Table I.
Figures of Merit of the HS-SPME with PDMS-IL Coating Fibers for the Determination of Monocyclic Aromatic Amines
| Analyte | LOD (ng/mL) | LOQ (ng/mL) | Linear range (ng/mL) | Correlation coefficient (r) |
|---|---|---|---|---|
| Aniline | 0.01 | 0.05 | 0.05–500 | 0.9902 |
| N,N-Dimethlaniline | 0.001 | 0.005 | 0.005–500 | 0.9922 |
| 2-Chloroaniline | 0.1 | 0.5 | 0.5–500 | 0.9905 |
| 3-Chloroaniline | 0.1 | 0.5 | 0.5–500 | 0.9903 |
Repeatability of the method was assessed through five consecutive extractions of independently prepared solutions at concentrations of 1, 100 and 500 ng/mL of the aromatic amines. The observed repeatability ranged 3.1–8.5% depending on the compound considered. In addition, the reproducibility, relative standard deviation (RSD) between fibers (batch to batch), was investigated at above concentrations by preparation of three similar PDMS-IL fibers, which ranged from 5.3 to 12.4%. (Table II). The lifetime of the coating layer of SPME is very important for practical applications. The change of extraction efficiencies of the PDMS-IL fiber in extracting aromatic amines from the aqueous solution after being used for 30, 60, 90, 120 and 150 times is studied at 280°C. No obvious decline was observed, which indicated that the fiber was still stable and usable at least for 150 times. This long life span is because of the thinness of the coating and the heat-resistant properties due to strong chemical bonding between the sol–gel generated organic–inorganic composite coating and silica fiber surface.
Table II.
The Repeatability and Reproducibility of the HS-SPME Method for the Analysis of Monocyclic Aromatic Amines
| Concentration (ng/L) | RSD (%) one fiber (repeatability) (n = 5) |
RSD (%) batch to batch (reproducibility) (n = 3) |
||||||
|---|---|---|---|---|---|---|---|---|
| Aniline | N,N-Dimethylaniline | 2-Chloroaniline | 3-Chloroaniline | Aniline | N,N-Dimethylaniline | 2-Chloroaniline | 3-Chloroaniline | |
| 1 | 6.8 | 6.6 | 8.5 | 6.4 | 10.1 | 12.4 | 6.7 | 8.3 |
| 100 | 4.8 | 3.5 | 5.3 | 5.7 | 8.4 | 9.5 | 6.0 | 7.9 |
| 500 | 3.5 | 3.1 | 4.4 | 4.1 | 6.3 | 7.1 | 6.0 | 5.3 |
Real water analysis
Analysis different water samples was accomplished with the IL-mediated sol–gel coating to investigate the practicability of the proposed analytical methodology for the determination of contents of aromatic amines. The contents, precisions and relative recovery of analytes in these water samples are shown in Table III. Some of the aromatic amine compounds were detected in the wastewater sample. The data show that for all analytes, the relative recoveries were higher than 83.3% and RSD were less than 10.6%. These results clearly demonstrate the absence of significant matrix effects on the efficiency of SPME. GC chromatogram of a wastewater sample is shown in Figure 2.
Table III.
Content of Monocyclic Aromatic Amines in Real Water Samples and the Accuracy of the Established Proposed Method
| Analyte | River water | Wastewater |
|---|---|---|
| Aniline | ||
| Mean (ng/mL) | – | 0.08 |
| RSD (%) | 6.2 | 10.6 |
| Relative recovery (%) | 93.2 | 83.3 |
| N,N-Dimethlaniline | ||
| Mean (ng/mL) | – | 0.56 |
| RSD (%) | 7.3 | 8.2 |
| Relative recovery (%) | 92.8 | 87.8 |
| 2-Chloroaniline | ||
| Mean (ng/mL) | – | – |
| RSD (%) | 7.8 | 8.7 |
| Relative recovery (%) | 90.2 | 90.0 |
| 3-Chloroaniline | ||
| Mean (ng/mL) | – | 0.61 |
| RSD (%) | 8.2 | 7.9 |
| Relative recovery (%) | 95.0 | 93.1 |
Figure 2.
Typical chromatogram obtained from wastewater sample with the sol–gel PDMS-IL fiber. Peak numbers correspond to (1) aniline, (2) N,N-dimethylaniline and (3) 3-chloroaniline.
Comparison with other related SPME methods
The main analytical statistics of the proposed method was compared with some previous research studies that were used for aromatic amine analysis. The data are shown in Table IV. The repeatability was acceptable and comparable with other methods reported in the literature. The method exhibited a vast linear response and adequately low detection limits. It was known that the good extraction performance has been improved greatly due to the three-dimensional network structure of sol–gel coating which larger surface area and using IL-mediated sol–gel coating via role of co-solvent and porogen and thus causing access to highly porous structure for adsorption capacity.
Table IV.
Comparison of Analytical Characteristics for Proposed Fiber with Other Fibers in the Determination of Monocyclic Aromatic Amines
Conclusion
In this study, the effect of IL was studied on the morphology and extraction efficiency of IL-mediated PDMS sol–gel coating fibers. Due to enhancement of porosity by ILs, IL-mediated PDMS coatings exhibited more porous morphology in comparison to the no-IL coatings. The extraction efficiency of the PDMS-IL fiber for target compounds was better than the PDMS, and higher amounts of analytes were extracted. The porous structure of sol–gel coating increases the surface area on the fiber, the speed of extraction and desorption steps and sample capacity. Better performance due to the use of IL in the sol–gel process that creates a large adsorption surface for analytes and thus has been improved the LOD of the method. In addition, sol–gel coatings possess porous structures which should significantly increase the surface area availability on the fibers.
Acknowledgments
The authors are thankful to Ferdowsi University of Mashhad, Iran, for financial support of this work.
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