Elsevier

Journal of Chromatography A

Volume 1218, Issue 31, 5 August 2011, Pages 5352-5361
Journal of Chromatography A

Oxidized multi-walled carbon nanotubes for the dispersive solid-phase extraction of quinolone antibiotics from water samples using capillary electrophoresis and large volume sample stacking with polarity switching

https://doi.org/10.1016/j.chroma.2011.06.031Get rights and content

Abstract

In this work, a new method for the determination of eleven quinolone antibiotics (moxifloxacin, lomefloxacin, danofloxacin, ciprofloxacin, levofloxacin, marbofloxacin, enrofloxacin, difloxacin, pefloxacin, oxolinic acid and flumequine) in different water samples using dispersive solid-phase extraction (dSPE) and capillary zone electrophoresis with diode-array detection was developed. Oxidized multi-walled carbon nanotubes (o-MWCNTs) were used for the first time as stationary phases for the off-line preconcentration by dSPE of the antibiotics. A 65 mM phosphate buffer at pH 8.5 was found adequate for analyte separation while large volume sample stacking with polarity switching of the analytes dissolved in water containing 10% (v/v) of acetonitrile was carried out in order to improve the sensitivity. dSPE parameters, such as sample volume and pH, o-MWCNT amount, volume and type of eluent in dSPE were optimized. Application of the developed method to the analysis of spiked Milli-Q, mineral, tap, and wastewater samples resulted in good recoveries values ranging from 62.3 to 116% with relative standard deviation values lower than 7.7% in all cases. Limits of detection were in the range of 28–94 ng/L. The proposed method is very fast, simple, repeatable, accurate and highly selective.

Introduction

In the last decade, special concern has arisen regarding the occurrence and fate of emerging contaminants like surfactants, personal care products, industrial additives, pharmaceuticals, and a good number of chemicals purported to be endocrine disrupters in the environment [1]. Their monitoring in water resources should be an important health and safety matter, especially when the existing conventional water treatment plants were not initially designed for their elimination and no specific regulation already exists in this sense.

Quinolones constitute an important class of pharmaceutical compounds with a broad spectrum activity that act by the inhibition of the ADN-gyrase. Their consumption for both human and veterinary purposes is currently increasing and as a result, they have also been found in environmental waters, especially ofloxacin (OFLO), lomefloxacin (LOME), norfloxacin (NORFLO) and ciprofloxacin (CIPRO) [2], [3], [4], [5], [6], [7], [8]. Their analysis is frequently developed by HPLC [9], [10], [11], [12], [13], [14], [15], [16] although CE applications have also been proposed [2], [4], [17], [18], [19]; however, the literature dealing with the analysis of quinolones in environmental waters is still sparse. In this sense, the number of studies using CE is even more limited [2] probably because of the low sensitivity of the technique, which is associated to the injection of low volumes of sample and to the short optical path-length if optical detection is used. To overcome this limitation, on-line preconcentration strategies, also known as stacking or sweeping techniques [20], [21], [22] have been developed, which take advantage of differences in mobility and conductivity between sample and buffer. They can be easily employed since no special devices are required. However, up to now very few works have applied them with success for the on-line preconcentration of quinolone antibiotics [23], [24], [25], [26]. He et al. [23], for example, used field-amplified sample stacking for the CE-UV analysis of nine fluoroquinolones in chicken, while Altria and Chanter [24] used pH mediated sample stacking for the preconcentration of a single quinolone antibiotic. Hernández et al. [25] also compared the use of field-enhanced sample injection with isotachophoresis for the on-line preconcentration of one quinolone (marbofloxacin, MARBO) from pig plasma extract, while in a second work of the same group, Hernández et al. [26] also used isotachophoresis for the determination of three quinolones (enrofloxacin, ENRO, CIPRO and flumequine, FLUME) in the same samples. Therefore and despite their inherent benefits, there is an important gap in the literature concerning the use of stacking techniques in CE to improve the selectivity and sensitivity for this group of compounds as well as to increase the number of studied quinolones.

Two of the stacking techniques most commonly applied in CE and which have not been used for the preconcentration of quinolones are normal stacking mode (NSM) and large volume sample stacking (LVSS). The first of them [20] constitutes the simplest of the stacking modes. In this case, the sample which is dissolved in a low conductivity matrix is hydrodynamically injected into the capillary. Focusing happens at the interface between the BGE and the sample matrix due to a drastic change in the electrophoretic velocity. Concerning LVSS [21], [27], in which only cations or anions can be concentrated at one time, the sample volume injected is much bigger than in NSM. In this case, the sample matrix is pumped out of the capillary moving towards the inlet maintaining the stacked analytes on the sample/BGE interface by manipulation of the EOF which can be done with or without polarity switching. In the first situation, polarity is switched and the current monitored until it reaches 90–99% of the value obtained when the capillary is only filled with the BGE. At this point, the polarity is switched so that the EOF direction is towards the detector. In the second case, for anions it is enough to suppress the EOF with a suitable BGE additive while for cations it is also necessary to reversed the EOF with an appropriate EOF modifier.

Carbon nanotubes (CNTs) are relatively new nano-materials that have found an extensive variety of applications in analytical sciences. In particular, the role of CNTs in analytical chemistry is becoming more important due their demonstrated applications [28], which also involve their use as stationary phases in solid-phase extraction (SPE). In this sense, CNTs have revealed to have an adequate sorption capacity for the extraction of both organic and inorganic compounds as shown in a recent review article [29]. From a detail revision of the literature, it is also clear that few works have used CNTs-SPE in combination with CE analysis [30] and regarding the specific use of CNTs for the SPE of pharmaceuticals from water samples, the number of applications developed till now is also relatively low. Up to now, they have only been used for the extraction of tetracyclines from surface waters [31] (using multiwalled CNTs, MWCNTs, of 20–50 nm diameter and 5–20 μm long), sulfamides and cephalosporins from tap and well water [32] (using MWCNTs of 30–60 nm diameter), sulfonamides from eggs and pork after extraction with ACN, ferrite potassium cyanide and zinc sulfate [33] (using MWCNTs of 5–15 μm long and diameter of 60–100 nm) and chloramphenicol from eggs, honey and milk after a suitable dilution with water at 60 °C [34] (using MWCNTs of 5–15 μm long and diameter of 40–60 nm). While concerning quinolone extraction, up to now, only one work has used then for this purpose [35]. However, in this case magnetic nanoparticles (MNPs) were deposited onto MWCNT surface to simplify sample treatment, which were later used for the extraction of ten of these analytes (i.e. enoxacin, NORFLO, OFLO, CIPRO, danofloxacin, DANO, LOME, ENRO, difloxacin, DIFLO, cinoxacin and nalidixic acid) from human plasma. The use of free MWCNTs (not modified with MNPs) of 110–170 nm diameter and 5–9 μm length, revealed that such CNTs (which are one of the longest commercialized nowadays) could be used for the extraction of these analytes; however, a full study regarding the applicability of these or other nanotubes for real sample analysis especially waters, has not yet been developed.

Therefore, the aim of this work is to study the application of MWCNTs of 110–170 nm diameter and 5–9 μm length, specially their oxidized form, for the extraction of eleven quinolone antibiotics (moxifloxacin, MOXI, LOME, DANO, CIPRO, levofloxacin, LEVO, MARBO, ENRO, DIFLO, pefloxacin, PEFLO, OXO and FLUME) from different types of water samples. CE with DAD has been used in combination with LVSS with polarity switching as on-line preconcentration technique in order to improve instrumental sensitivity. For the extraction and preconcentration of the selected quinolones from water samples, a SPE procedure was developed in the dispersive mode. Although, dSPE is normally used for matrix clean-up purposes, which means that the dispersive sorbent is added to the bulk solution or matrix containing the analytes and the possible matrix interferences/components are retained onto it to finally discard the sorbent and analyze the supernatant, it can also be used with the aim of trapping the target analytes which are later eluted or desorbed with an appropriate solvent [36], [37], [38]. In this last case which is not so widely used, the extraction time is considerably reduced and at the same time, the whole procedure is simplified becoming faster and easier than conventional SPE. Concerning the particular use of CNTs for SPE purposes, up to now and, to the best of our knowledge, only one work dealing with the extraction of pesticides and which has also been developed by our group [39], has used CNTs in the dSPE mode, providing good results. Besides, this is the first work in which oxidized-MWCNTs (o-MWCNTs) are used as stationary phases for the extraction and preconcentration of quinolone antibiotics from water samples and also the first work dealing the use of LVSS with polarity switching for the on-line preconcentration of quinolones in CE.

Section snippets

Chemicals and samples

All chemicals were of analytical reagent grade and used as received. HPLC grade methanol, acetonitrile (ACN) and acetone were purchased from Panreac Química S.A. (Barcelona, Spain), while HPLC grade dichloromethane was provided from Scharlau (Barcelona, Spain). Sodium dihydrogen phosphate dihydrate, hydrochloric acid, nitric acid, ammonium hydroxide, formic acid and diethyl ether were obtained from Merck (Darmstad, Germany). Sodium hydroxide was from Fluka (Madrid, Spain). Distilled water was

CE-DAD method

Among the different quinolones selected for this study nine of them (MOXI, LOME, DANO, CIPRO, LEVO, MARBO, ENRO, DIFLO and PEFLO) belong to the fluoroquinolone family, which have a fluor atom in 6th position and a piperazinyl moiety in 7th position. Since most fluoroquionolones have two relevant ionizable functional groups, their acid-base chemistry involves two equilibria: the dissociation of the carboxylic group (pKa1 in the range 5.0–6.5) and the deprotonation of the N4 of the piperazine

Conclusions

In this work, MWCNTs were functionalized (oxidized) and used for the first time as stationary phases for the simultaneous dSPE of eleven quinolones from different water samples (Milli-Q, mineral, tap and wastewater) using CE-DAD as separation technique. LVSS was used as on-line preconcentration method, providing an overall sensitivity of the method of 28–94 ng/L. The proposed procedure is simple, selective, repeatable and effective, as demonstrated by the recovery and precision and accuracy

Acknowledgements

A.V.H.H. thanks the Spanish Ministry of Education for the FPU grant at the University of La Laguna. J.H.B. thanks the Spanish Ministry of Science and Innovation for the Ramón y Cajal contract at the University of La Laguna. Authors would also like to thank the Servicio de Medio Ambiente de la Universidad de La Laguna (SEMALL) for providing the wastewater samples and for the BOD analysis. This work has been supported by the Spanish Ministry of Science and Innovation (projects AGL2008-00990/ALI

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