- freely available
Chromatography 2015, 2(3), 515-528; doi:10.3390/chromatography2030515
Abstract: In the present work, the performance of carbon nanotubes (c-CNTs) functionalized polydimethylsiloxane (PDMS) based coatings as extractive phases for in-tube solid phase microextraction (IT-SPME) coupled to Capillary LC (CapLC) has been evaluated. Carboxylic-single walled carbon nanotubes (c-SWNTs) and carboxylic-multi walled carbon nanotubes (c-MWNTs) have been immobilized on the activated surface of PDMS capillary columns. The effect of different percentages of diphenyl groups in the PDMS extractive phase has also been evaluated. The extraction capability of the capillary columns has been tested for different organic pollutants, nitrogen heterocyclic compounds and polycyclic aromatic compounds (PAHs). The results indicated that the use of the c-CNTs-PDMS capillary columns improve pyriproxyfen and mainly PAH extraction. Triazines were better extracted by unmodified TRB-35 and modified c-CNTs-PDMSTRB-5. The results showed that the extraction capability of the c-CNT capillary columns depends not only on the polarity of the analytes (as it occurs with PDMS columns) but also on the interactions that the analytes can establish with the immobilized c-CNTs on the PDMS columns. The extraction efficiency has been evaluated on the basis of the preconcentration rate that can be achieved, and, in this sense, the best c-CNTs-PDMS capillary column for each group of compounds can be proposed.
Solid phase microextraction (SPME) is a reliable technique for sample pretreatment owing to its simplicity, rapidity and wide application. SPME integrates sampling, concentration and sample injection in one single step . Moreover, it is a solvent free extraction . These characteristics are especially useful to develop green analytical methodologies . Among the different modalities of SPME, in-tube solid phase microextraction (IT-SPME) provides additional advantages, simplifying the automatization and facilitating on-line coupling to the separation/detection system [4,5]. IT-SPME is based on the extraction of analytes on the coating of a capillary column that is on-line coupled with a liquid chromatographic system [6,7]. Preconcentration can be carried out until equilibrium or until sufficient extraction level is reached.
The nature of the capillary coating for the IT-SPME is a key point to reach satisfactory extraction efficiencies, since the sorption/desorption processes strongly depend on the affinity of the analytes for the coating. Commercially available capillaries have been proposed to determine several analytes for environmental, biological or food applications. However, the utility of conventional coatings is limited mainly by the low extraction efficiencies. Thus, recent research has afforded a variety of alternative capillary coatings to improve extraction efficiency, selectivity, extraction time and coating stability [8,9,10,11].
In this scenario, carbon nanotubes (CNTs)  opened a new investigation area in the development of alternative coating materials for IT-SPME, since their special properties make them very attractive to improve the performance of this technique. Besides the extraordinary electrical, thermal and mechanical properties, CNTs possess a large surface area. Their strong adsorption affinity to a wide variety of organic compounds, together with their large adsorption surface make them excellent material for (micro)extraction [13,14].
An interesting feature of CNTs is that their surfaces can be chemically modified, resulting in functionalized adsorbents that allow the introduction of new interactions with organic molecules . Modifying the structure of the CNTs, either by covalent or non-covalent functionalization, changes the CNTs’ properties, such as CNT hydrophobicity, or introduces specific substituents which modify the affinity of these adsorbents for the analytes .
There are a vast number of publications describing the advantages of using CNTs as adsorbent materials for SPME [16,17,18,19]. Moreover, several review articles can be found dealing with this subject [20,21,22]. However, the use of CNTs or carbon based nanoadsorbents for IT-SPME is still a challenge. Only a few examples of CNTs functionalized capillary columns for IT-SPME can be found [23,24].
The objective of the present work was to evaluate the performance of carboxylic SWNTs-PDMS (c-SWNTs-PDMS) and carboxylic MWNTs-PDMS (c-MWNTs-PDMS) coatings as extractive phases for IT-SPME coupled to Cap-LC. The extraction efficiency, precision and sensitivity for nitrogen heterocyclic compounds and polycyclic aromatic hydrocarbons PAH  compounds compared with the extraction efficiency of commercial PDMS capillary columns has been studied in order to elucidate the potential advantage of using c-CNTs-PDMS as the extractive phase for IT-SPME.
2. Experimental Section
SWNTs, MWNTs, 3-aminopropyl trirthoxysilane (APTS), 1,3-dicyclohexylcarbodiimide, glutaraldehyde, and the triazines (atrazine, propazine, terbuthylazine-desethyl (DES-TBA) and terbuthylazine-2-hydroxy (2-OH-TBA)) were purchased from Sigma-Aldrich (Germany). Pyriproxyfen were obtained from Sigma (St, Louis, MO, USA). Terbuthylazine (TBA) and PAH-Mix 18 (acenaphthene acenaphthylene, anthracene, benzo[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[g,h,i]perylene, benzo[a]pyrene, chrysene, dibenzo[a,h]anthracene, fluoranthene, fluorine, indeno[1,2,3,c,d]pyrene, naphthalene, phenanthrene, perylene and pyrene) were supplied by Dr Ehrenstorfer (Germany). Acetonitrile was of HPLC grade (Scharlau, Barcelona Spain). The ultrapure water was obtained through a system Nanopure II (Sybron, Barnstead, United Kingdom).
The capillary chromatographic system consisted of a capillary LC binary gradient pump (Agilent 1200 Series, Waldbronn, Germany), a six-port injection valve (Rheodyne 7725) and a UV-Vis diode array detector (HP 1200 series) equipped with an 80 nL flow cell. The analytical signal was recorded between 210 and 360 nm. A Zorbax SB C18 (150 mm × 0.2 mm i. d., 0.5 µm) particulate column (Agilent) was employed for analyte separation.
The c-CNTs were characterized using FTIR-ATR Agilent Technologies Cary 630 FTIR (Agilent, USA). The micrographs of scanning electronic microscope (SEM) were obtained with a HITACHI-S4800 operating at 20 kV.
2.3. Functionalization of the Capillary Columns
SWNTs and MWNTs were carboxylated prior to their immobilization on the capillary column inner surface. Figure 1 shows the structure of the CNTs used in this work. Carboxylation of CNTs was carried out following the procedures described in [26,27] for SWNTs and MWNTs, respectively. Briefly, to prepare c-SWNTs, 5 mL of H2SO4:HNO3 (3:1) were added to 0.025 g of SWNTs and ultrasonicated for 90 min (50 W, 60 Hz). Then, this mixture was diluted with water (500 mL) and filtered through a 0.45 μm cellulose acetate filter. Finally, c-SWNTs were washed with 1 M HCl and left to dry at room temperature. The c-MWNTs were prepared by adding 80 mL of H2SO4:HNO3 (3:1) to 0.020 g of MWNTs, and ultrasonicated for 5 h. Next, the suspension was diluted with water (1.5 L), filtered and washed with water .
The following step was the c-CNT immobilization on the inner surface of the PDMS capillary columns (see Figure 1). Firstly, activation of the PDMS capillary column was carried out. For this purpose, the column was rinsed with a NaOH solution (2 M) and left to act for 24 h . Next, ultrapure water was passed through the capillary column (5 min) followed by 2% (v/v) APTS prepared in anhydrous acetone (15 min), water (5 min) and methanol (5 min) to eliminated the excess of APTS. In the next step, 10% (v/v) glutaraldehyde dissolved in 50 mM borate buffer at pH 9.0 was passed through the capillary for 10 min. Then, c-CNTs (5 mg/mL) dispersed in 4.5 mL of dimethylformamide containing 0.5 mg of 1,3-dicyclohexylcarbodiimide were passed through the capillary for 30 min. Finally the capillary was rinsed with water to remove the unreacted c-CNTs.
2.4. IT-SPME Procedure and Chromatographic Conditions
The injection loop of an injection valve was replaced by a 40-cm segment of a GC capillary column of PDMS functionalized with c-SWNTs and c-MWNTs. The PDMS capillary columns used in this work were TRB-35 (35% diphenyl and 65% dimethyl polysiloxane) and TRB-5 (5% diphenyl and 95% dimethyl polysiloxane) (0.32 mm i. d. y 3.0 μm thickness). Capillary connections to the valve were facilitated by the use of 2.5 cm sleeve of 1/16 in polyether ether ketone (PEEK) tubing; 1/16 in PEEK nuts and ferrules were used to complete the connections. The schematic diagram of the IT-SPME system is shown in Figure 2. Aliquots of 4 mL of standard solutions of triazines (50 µg/L), and pyriproxyfen (50 µg/L) and 2 mL of PAHs (up to 100 µg/L) were manually loaded into the system by means of 1.0 mL precision syringe. A clean-up step was also carried out by injecting 50 µL of ultrapure water after the sample loading. The injection of the retained analytes was performed by rotating the valve to the inject position.
The analysis of triazines was carried out in isocratic elution mode using acetonitrile: water (70:30). Gradient elution mode was used for the chromatographic separation of PAHs: the percentage of acetonitrile was increased from 70% at zero min to 90% at 10 min, then to 100% at 24 min; and decreased to 70% at 27 min. Pyriproxyfen were also analyzed in gradient elution mode with acetonitrile/water, in this case, the initial percentage of acetonitrile was 70%; this percentage was increased to 100% at 8 min, remained constant until 18 min, and then decreased to 70% at 20 min. In all the cases the flow rate was 10 μL/min. Acetonitrile and water were filtered through 0.45 μm nylon membrane before use.
3. Results and Discussion
3.1. Characterization of c-CNTs and Capillary Columns
Characterization of c-CNTs after the carboxylation process was carried out by ATR-FTIR. Figure 3 shows the IR spectra obtained for c-SWNTs and c-MWNTs. Besides to the CH2 band at 1360 cm−1, absorption bands at 1120 and 1710 cm−1 were in correspondence with C–O and C=O stretching, respectively, corroborating the existence of carboxyl groups in the functionalized SWNTs and MWNTs [26,27].
Figure 3c,d depict the SEM micrographs of the inner surface of c-SWNTs-PDMSTRB-5 and c-MWNTs-PDMSTRB-5 capillary columns, showing the resulting coating on the surface of the capillary column. It should be remarked, that the coatings were homogeneous all along the capillary columns. A similar coating was observed in the case of c-CNTs-PDMSTRB-35 capillary columns (micrographs not shown).
3.2. Study of the Extraction Capability of the c-CNTs-PDMS Coated Capillary Columns for Nitrogen Heterocyclic Compounds
The target analytes were 2-OH-TBA, DES-TBA, propazine, atrazine, TBA and pyriproxyfen. 4 mL of a mixture of triazines (50 µg/L) and pyriproxyfen (10 µg/L) were processed in the IT-SPME-Cap-LC system. Figure 4 shows the analytical responses obtained for triazines with the different capillary columns at the above mentioned concentration level. The structures of the analytes are given in Table 1.
Figure 4 show that the use of commercial PDMSTRB-35 capillary columns for triazines extraction was advantageous compared with the use of PDMSTRB-5. PDMSTRB-5 coating contains a 5% phenyl groups that provides a characteristic affinity towards aromatic rings while the percentage of phenyl groups in the PDMSTRB-35 coating is higher, which increases the polarity of the capillary column. Therefore, the extraction of triazines was favored when the PDMSTRB-35 capillary column was used. In the case of pyriproxyfen, the extraction efficiency was slightly better with the PDMSTRB-5 capillary column. The extraction efficiency determines the analyte responses. However, the limits of detection (LODs) are determined by other factors, mainly the features of the baseline in the region where the analytes eluted. Therefore, the LODs were experimentally established as concentrations that produced a peak with a signal to noise ratio of 3. Table 1 summarizes the LODs under the experimental conditions described in Section 2. C-SWNTs-PDMSTRB-5 capillary column provided better LODs values for atrazine and terbuthylazine, similar to those achieved with the PDMSTRB-35 capillary column.
The results obtained with the c-SWNTs or c-MWNTs-PDMS coatings must be addressed considering the affinity of the PDMS capillary towards triazines but also the interactions that the c-CNTs-PDMS based capillary coatings can establish with the triazines. CNTs can interact with triazines through π-π interactions, however in this work c-CNTs have been used and then, ion exchange interactions and hydrogen bonds could also be involved . Experimentally, it was observed that the presence of c-CNTs in the PDMSTRB-35 capillary column resulted in a decrease of the analytical response for triazines, and so an increase of their LODs (see Figure 4 and Table 1). However, the pattern for both PDMSTRB-5 and c-CNTs-PDMSTRB-5 was different. Similar or better results were obtained for atrazine, propazine and terbutylazine with the c-SWNTs-PDMSTRB-5 capillary column than those achieved with PDMSTRB-5 capillary column. This slight improvement on the extraction could be explained considering π-π interactions, since hydrogen bonds and ion exchange would provide a more significant increase on the extraction efficiency. As mentioned above, similar results were obtained with PDMSTRB-35 and c-SWNTs-PDMSTRB-5 for trade triazines. In the case of pyriproxyfen, Table 1 shows that the c-MWNTs-PDMSTRB-35 capillary coating was the best option since the LOD was lower compared with the other studied capillary coatings. Probably, this enhancement was also due to the π-π interactions between the aromatic systems of the analyte and the c-CNTs immobilized on the surface of the capillary column .
3.3. Study of the Extraction Capability of the c-CNTs-PDMS Capillary Columns for PAHs
PAHs include a number of hydrocarbons compounds with different polarity . Naphthalene, benzo[b]fluoranthene and dibenzo[a,h]anthracene were chosen as target analytes (100 µg/L). Noted that the retention times at the experimental chromatographic conditions were tr = 5.2 min, tr = 13.5 min and tr = 23 min for naphthalene, benzo[b]fluoranthene and dibenzo[a,h]anthracene, respectively. As expected, the use of the commercial PDMSTRB-5 capillary column provided better results than the PDMSTRB-35 column for the more apolar PAHs, while the PDMSTRB-35 capillary was more adequate for the more polar PAHs (Figure 5 and Table 1). These results can be explained by the lower polarity of the PDMSTRB-5 compared with the PDMSTRB-35 capillary.
The presence of c-CNTs on the surface of the PDMS capillary columns gave rise to differences on the interactions of the capillary coating towards the analytes that resulted in differences to the analytical response, and so on with the LODs. As can be seen in Figure 5, the extraction efficiency of the PDMS columns for PAH compounds was strongly enhanced by the presence of c-CNTs in the case of TRB-5, particularly when c-SWNTs were immobilized on these capillaries. This improvement can be explained by the π-π interactions between the aromatic rings of the PAHs and the c-CNTs.
Precision was evaluated for all the analytes at concentration level of 50 µg/L for triazines and pyriproxifen and 100 µg/L for PAHs. Relative standard deviations (%RSD) obtained with the different capillary coatings are shown in Table 2. As can be seen, run-to-run %RSD values were lower than 9%. These results indicated that satisfactory %RSD values were achieved with all the capillary columns studied. Capillary-to-capillary precision was also evaluated. For this aim, we compared the extraction efficiency obtained for a mixture of the triazines (50 µg/L) or PAHs (100 µg/L) with two c-CNTs-PDMS capillaries columns prepared under identical conditions. Capillary-to-capillary %RSD values were lower than 19% for all the analytes. The low RSD values bear evidence to the fact that the use of c-CNTs-PDMS capillary columns was a reliable and reproducible approach to be employed for IT-SPME extraction.
|PDMSTRB 5||c-SWNTs-||c-MWNTs-||PDMSTRB 35||c-SWNTs-||c-MWNTs-|
|PDMSTRB 5||PDMSTRB 5||PDMSTRB 35||PDMSTRB 35|
3.5. Evaluation of the Preconcentration Ration Achieved with the c-CNTs-PDMS Capillary Coatings
A ratio PR was defined in order to compare the results obtained from all extractive phases:
The PR values for PAHs were higher than those ratios obtained for the other families of compounds tested, in particular when the c-SWNTs-PDMSTRB-5 was used. Such behavior is attributed to the strong π-π interactions between the aromatic rings of PAHs and the c-CNTs based coating. It should be noted that extraction of PAHs depends on the size of the π-electron conjugated system of organic molecules that are adsorbed on the surface of CNTs . The c-SWNTs-PDMSTRB-5 capillary column provided the highest analytical response when it was used as the extractive phase for IT-SPME-Cap-LC for PAHs and similar responses for trade triazines than TRB-35 coating. However, pyriproxyfen is better extracted by c-MWNTs-PDMSTRB-35 and c-MWNTs-PDMSTRB-5 capillary columns, respectively.
|Log Kow ||c-SWNTs-PDMSTRB 5||c-MWNTs- PDMSTRB 5||c-SWNTs-PDMSTRB 35||c-MWNTs-PDMSTRB 35|
In the present work, the extraction capability of c-CNTs-PDMS coated capillary columns has been evaluated as an alternative for their use in IT-SPME-Cap-LC. C-SWNTs-PDMS and c-MWNTs-PDMS coated capillary columns have been successfully synthetized and their analytical performance has been shown for different compounds such as nitrogen heterocyclic compounds and PAHs. In addition, the percentage of diphenyl groups (5% and 35%) on the PDMS extractive phase has also been considered.
The enhancement of the extraction efficiency of each coating is a function of the affinity of the PDMS coating towards the analytes and the different interactions that can take place between the analytes and the c-CNTs present in the coating material. A significant enhancement of the extraction efficiency of PAHs occurs with the c-CNTs-PDMS capillary columns, and this improvement can be attributed to π-π interactions. The results indicated that this enhancement depends on the presence of biphenyl groups in the PDMS and on the structure of CNTs, c-SWNTs or c-MWNTs. Under the experimental conditions of this work, we have demonstrated that the best extraction for PAHs was achieved with the c-SWNTs-PDMSTRB-5 capillary column. This extractive phase also shows improved results compared with unmodified PDMSTRB-5 for trade triazines. Pyriproxyfen is better extracted by c-MWNTs-PDMSTRB-35 and c-MWNTs-PDMSTRB-5, respectively. Therefore, the structure of the analytes plays a key role for practical applications of c-CNT based PDMS coatings for IT-SPME coupled to a chromatographic system. However, if all tested compounds want to be screened, the best capillary option is the sorbent c-SWNTs-PDMSTRB-5 achieving a good compromise bearing in mind its extraction efficiency.
The authors thank the Ministerio de Economía y Competitividad of Spain (Grant CTQ2014-53916-P) and the Generalitat Valenciana (Grants PROMETEO Program 2012/045) for financial support received.
The manuscript was written through contributions of all authors and all authors have given approval to the final version.
Conflicts of Interest
The authors declare no conflict of interest.
- Risticevic, S.; Niri, V.H.; Vuckovic, D.; Pawliszyn, J. Recent developments in solid phase microextraction. Anal. Bioanal. Chem. 2009, 393, 781–795. [Google Scholar] [CrossRef] [PubMed]
- Wang, T.-T.; Chen, Y.-H.; Ma, J.-F.; Hu, M.-J.; Li, Y.; Fang, J.-H.; Gao, H.-Q. A novel ionic-liquid-modifies organic-polymer as the sorbent for in-tube solid-phase microextraction of acidic food additives. Anal. Bioanal. Chem. 2014, 406, 4955–4963. [Google Scholar] [CrossRef] [PubMed]
- Aufartová, J.; Mahugo-Santana, C.; Sosa-Ferrera, Z.; Santana-Rodríguez, J.J.; Novákova, L.; Solich, P. Determination of steroids hormones in biological and environmental samples using Green microextraction techniques. Anal. Chim. Acta 2011, 704, 33–46. [Google Scholar] [CrossRef] [PubMed]
- Queiroz, M.E.C.; Melo, L.P. Selective capillary coating materials for in-tube solid-phase microextraction coupled to liquid chromatography to determine drugs and biomarkers in biological samples: A review. Anal. Chim. Acta 2014, 826, 1–11. [Google Scholar] [CrossRef] [PubMed]
- Moliner-Martinez, Y.; Herráez-Hernández, R.; Verdú-Andrés, J.; Molins-Legua, C.; Campins-Falcó, P. Recent advances of in-tube solid-phase microextraction. Trends Anal. Chem. 2015. [CrossRef]
- Eisert, R.; Pawliszyn, J. Automated in-tube solid phase microextraction coupled to high-performance liquid chromatography. Anal. Chem. 1997, 69, 3140–3147. [Google Scholar] [CrossRef]
- Kataoka, H.; Ishizaki, A.; Nonaka, Y.; Saito, K. Developments and applications of capillary microextraction techniques: A review. Anal. Chim. Acta 2009, 655, 8–29. [Google Scholar] [CrossRef] [PubMed]
- Liu, X.Y.; Ji, Y.S.; Zhang, H.X.; Liu, M.C. Highly sensitive analysis of substituted aniline compounds in water samples by using oxidized multiwalled carbon nanotubes as an in-tube solid phase microextraction medium. J. Chromatogr. A 2008, 1212, 10–15. [Google Scholar] [CrossRef] [PubMed]
- Gonzalez-Fuenzalida, R.A.; Moliner-Martinez, Y.; Verdú-Andrés, J.; Molins-Legua, C.; Herráez-Hernández, R.; Campins-Falcó, P. Nanoparticle Microextraction in Sample Extraction Techniques for Biological Samples: Recent Advances and Novel Applications; De Vooght-Johnson, R., Ed.; Future Science Ltd, unitec House, 2 Albert Place: London, UK. (in press)
- Moliner-Martínez, Y.; Prima-Garcia, H.; Ribera, A.; Coronado, E.; Campíns-Falcó, P. Magnetic in-tube solid phase microextraction. Anal. Chem. 2012, 84, 7233–7240. [Google Scholar] [CrossRef] [PubMed]
- Campíns-Falco, P.; Coronado-Miralles, E.; Moliner-Martínez, Y.; Ribera, A.; Prima-García, H. Magnetic in-tube solid phase microextraction. Patent 201100823 Spain, 2014. [Google Scholar]
- Lijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354, 56–58. [Google Scholar] [CrossRef]
- Xu, J.; Zheng, J.; Tian, J.; Zhu, F.; Zeng, F.; Su, C.; Ouyang, G. New materials in solid-phase microextraction. TrAC-Trend. Anal. Chem. 2013, 47, 68–83. [Google Scholar] [CrossRef]
- Valcarcel, M.; Cardenas, S.; Simonet, B.M.; Moliner-Martínez, Y.; Lucena, R. Carbon nanoestructures as sorbent materials in analytical processes. TrAC-Trend. Anal. Chem. 2008, 27, 34–43. [Google Scholar] [CrossRef]
- Pyrzynska, K. Carbon nanotubes as sorbent in the analysis of pesticides. Chemosphere 2011, 83, 1407–1413. [Google Scholar] [CrossRef] [PubMed]
- Kueseng, P.; Pawliszyn, J. Carboxylated multiwalled carbon nanotubes/polydimethylsiloxane, anew coating for 96-blade solid-phase microextraction for determination of phenolic compounds in water. J. Chromatogr. A 2013, 1317, 199–202. [Google Scholar] [CrossRef]
- Song, X.Y.; Ha, W.; Chen, J.; Shi, Y.P. Application of beta-cyclodextrin-modified, carbon nanotube-reinforced hollow fiber to solid-phase microextraction of plant hormones. J. Chromatogr. A 2014, 1374, 23–30. [Google Scholar] [CrossRef] [PubMed]
- Feng, X.; Li, Y.; Jing, R.; Jiang, X.; Tian, M. Detection of Organophosphorous Pesticides in Soil Sample with Multiwalled Carbon Nanotubes Coating SPME Fiber. B. Environ. Contam. Tox. 2014, l93, 769–774. [Google Scholar] [CrossRef] [PubMed]
- Bahzadi, M.; Noroozian, E.; Mirzaei, M. A novel coating based on carbon nanotubes/poly-ortho-phenylenediamine composite for headspace solid-phase microextraction of polycyclic aromatic hydrocarbons. Talanta 2013, 108, 66–73. [Google Scholar] [CrossRef] [PubMed]
- Xu, J.; Zheng, J.; Tian, J.; Zhu, F.; Zeng, F.; Su, C.; Ouyang, G. New materials in solid-phase microextraction. TrAC-Trend. Anal. Chem. 2013, 47, 68–83. [Google Scholar] [CrossRef]
- Ali, M.; Zanjani, A.; Ovasis, M. Advances for sensitive, rapid and selective extraction in different configurations of solid-phase microextraction. TrAC-Trend. Anal. Chem. 2013, 51, 13–22. [Google Scholar] [CrossRef]
- Ferial, G.; Amirhassan, A.; Robian, Y. Methods for coating solid-phase microextraction fibers with carbon nanotubes. TrAC-Trend. Anal. Chem. 2014, 58, 133–143. [Google Scholar]
- Wang, X.; Li, X.; Li, Z.; Zhang, Y.; Bai, Y.; Liu, H. Online coupling of in-tube solid phase microextraction with direct analysis in real time mass spectrometry for rapid determination of triazine herbicides in water using carbon-nanotubes-incorporated polymer monolith. Anal. Chem. 2014, 86, 4739–4747. [Google Scholar] [CrossRef] [PubMed]
- Moliner-Martinez, Y.; Serra-Mora, P.; Verdú-Andrés, J.; Herráez-Hernández, R.; Campins-Falcó, P. Analysis of polar triazines and degradation products in waters by in-tube solid-phase microextraction and capillary chromatography: an environmentally friendly method. Anal. Bioanal. Chem. 2015, 407, 1485–1497. [Google Scholar] [CrossRef] [PubMed]
- EU Parliament. Directive 2008/105/EC of the European Parliament and Council on environmental quality standards in the field of water policy. Official J. Eur. Union 2008, L348, 84–97. [Google Scholar]
- Suarez, B.; Moliner-Martínez, Y.; Cardenas, S.; Simonet, B.M.; Valcárcel, M. Monitoring of carboxylic carbon nanotubes in surface water by using multiwalled carbon nanotube-modified filter as preconcentration unit. Environ. Sci. Technol. 2008, 48, 6100–6104. [Google Scholar] [CrossRef]
- Sombra, L.; Moliner-Martínez, Y.; Cardenas, S.; Valcárcel, M. Carboxylic multi-walled carbón nanotubes as immobilized stationary phase in capillary electrochromatography. Electrophoresis 2008, 29, 3850–3857. [Google Scholar] [CrossRef] [PubMed]
- Slentz, B.E.; Panner, N.A.; Lugowaska, E.; Regnier, F. Nanoliter capillary electrochromatography columns based on collocated monolithic support structures molded in poly(dimethylsiloxane). Electrophoresis 2001, 22, 3736–3743. [Google Scholar] [CrossRef]
- Wang, X.; Liu, Y.; Tao, S.; Xing, B. Relative importance of multiple mechanisms in sorption of organic compounds by multiwalled carbon nanotubes. Carbon 2010, 48, 3721–3728. [Google Scholar] [CrossRef]
- Moliner-Martínez, Y.; González-Fuenzalida, R.A.; Herráez-Hernández, R.; Campíns-Falcó, P.; Verdú-Andrés, J. Cleaning sorbents used in matrix solid-phase dispersion with sonication: Application to the estimation of polycyclic aromatic hydrocarbons at ng/g levels in marine sediments. J. Chromatogr. A 2012, 1263, 43–50. [Google Scholar] [CrossRef] [PubMed]
- Kragulj, M.; Tričković, J.; Dalmacija, B.; Kukovecz, Á.; Kóya, Z.; Molnar, J.; Rončević, S. Molecular interactions between organic compounds and functionally modified multiwalled carbon nanotubes. Chem. Eng. J. 2013, 225, 144–152. [Google Scholar] [CrossRef]
- Schmitt, P.H.; Garrison, A.W.; Freitage, D.; Kettrup, A. Separation of s-triazine herbicides and their metabolites by capillary zone electrophoresis as a function of pH. J. Chromatogr. A 1996, 723, 169–177. [Google Scholar] [CrossRef]
© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).