Carbon nanotubes as SPE sorbents for the extraction of salicylic acid from river waterby Encarnación Caballero-Díaz, Miguel Valcárcel

J. Sep. Science


Filtration and Separation / Analytical Chemistry


434 J. Sep. Sci. 2014, 37, 434–439

Encarnacio´n Caballero-Dı´az

Miguel Valca´rcel

Department of Analytical

Chemistry, University of

Co´rdoba, Co´rdoba, Spain

Received November 7, 2013

Revised December 2, 2013

Accepted December 3, 2013

Research Article

Carbon nanotubes as SPE sorbents for the extraction of salicylic acid from river water

This paper deals with the ability of different types of carbon nanotubes to adsorb salicylic acid in river water samples. The use of these nanoparticles as a sorbent in a SPE procedure prior to CE analysis is essential for improving the enrichment factor and the recovery values. Several experimental variables were optimized in order to maximize the extraction efficiency.

The proposed analytical method is simple, fast, and entails low solvent consumption. Furthermore, salicylic acid could be extracted from river water providing good recovery values in the range from 76.2 to 102.0% (RSD<8.2%). The combination of the specific chemical properties of analyte and the unique physicochemical features of carbon nanotubes sheds new light on the use of these nanoparticles as excellent sorbent materials of pharmaceutical compounds in environmental matrices.

Keywords: Capillary electrophoresis / Carbon nanotubes / River water / Salicylic acid / Solid-phase extraction

DOI 10.1002/jssc.201301204 1 Introduction

Pharmaceutical compounds are a class of emerging pollutants widely used throughout the world and as a consequence, are being continuously released into the environment through excreta or the disposal of unused or expired products [1]. Most of them are not entirely eliminated from wastewater treatment plants due to their polarity and high stability [2]. This involves that both parent compounds and their metabolites may finally come into contact with surface waters and, therefore, their environmental monitoring has aroused much attention. For this purpose, chromatographic separation followed by mass spectrometric detection seems to be the most popular choice [3–7]. Nevertheless, this instrumentation is very expensive, requires trained personnel, and is not available in all laboratories. Besides, while GC has drawbacks related to the analyte loss during the derivatization process and the background noise,

LC presents problems owing to the occurrence of matrix effects [6]. On the other hand, there are many compounds that can potentially be assayed by CE, as this is a less complex and expensive alternative to chromatographic techniques, but also appropriate for the analysis of pharmaceuticals in water samples when no severe sensitivity restrictions are required [8–11].

Correspondence: Prof. Miguel Valca´rcel, Department of Analytical

Chemistry, Marie Curie Building (Annex), Campus de Rabanales,

University of Co´rdoba, E-14071 Co´rdoba, Spain


Fax: +34-957-218616

Abbreviations: CNT, carbon nanotube; MWCNT, multiwalled carbon nanotube; SA, salicylic acid; SWCNT, singlewalled carbon nanotube

Nonsteroidal anti-inflammatory drugs are some of the most commonly detected pharmaceuticals [9, 12]. Salicylic acid (SA) belongs to this family of compounds and is produced by the degradation of acetylsalicylic acid [11]. Several analytical methodologies based on GC or LC coupled to MS have been reported for determining SA in different samples [3, 5, 13, 14], however, CE has been also used for this same purpose taking advantage of the ionic nature of this compound [8–10,15, 16].

Pharmaceutical residues are usually present at low concentrations in the environment and consequently, their determination requires an enrichment step prior to detection. To this end, different commercial SPE sorbents have been employed including Strata-X, C8, C18, Oasis HLB, or Lichrolut

EN [1,3,5,17]. The outstanding physicochemical properties of carbon nanotubes (CNTs)make them promisingmaterials in many analytical applications [13]. In this regard, CNTs have been extensively utilized as SPE sorbents thanks to their high specific surface area and excellent ability for sorption of organic compounds [13,18–21]. However, despite their extraordinary features, nanoparticles have been scarcely exploited for extracting pharmaceutical compounds from environmental samples [22, 23].

In this work, a simple and low-cost analytical method for determining SA in river water by using CNTs as a SPE sorbent is proposed. Three types of CNTs were compared in terms of their ability for sorption of SA in samples. SA was selected as target analyte since it is considered as an emerging pollutant and one of the most studied pharmaceutical compounds in the aquatic environment according to related literature [3,5,6,17]. The proposed analytical method involved an initial preconcentration step using CNTs as sorbent material prior to CE–UV analysis. Several experimental variables were optimized and the developed analytical procedure was ultimately applied to the determination of SA in river water.

C© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

J. Sep. Sci. 2014, 37, 434–439 Sample Preparation 435 2 Materials and methods 2.1 Chemicals and reagents

All chemicals were of analytical reagent grade. Sodium tetraborate decahydrate (99.5–105.0%) was used to prepare the electrophoretic buffer and was purchased from SigmaAldrich (Madrid, Spain). SA (2-hydroxybenzoic acid) was selected as target analyte and was also obtained from SigmaAldrich. Sodium hydroxide pellets (98.0%; PA-ACS-ISO) and hydrochloric acid (37.0%; HPLC grade) were supplied by

Panreac (Barcelona, Spain) and were employed for adjusting the pH of electrophoretic buffer and samples, respectively. Methanol (≥99.9%; HPLC-PLUS gradient) and acetone (99.9%; PAI-ACS) solvents were provided by Carlo Erba reagents (Barcelona, Spain) and Panreac, respectively.

Different types of commercially available CNTs were evaluated as sorbents in this work. Multi-walled CNTs (MWCNTs) of 9.5 nm diameter and 1.5 m length were obtained from Nanocyl (Belgium) (MWCNTs-nanocyl).