DOI: 10.1002/elan.201400218 Simultaneous Voltammetric Determination of Theophylline and Guaifenesin Using a Multiwalled Carbon Nanotube- Ionic Liquid Modified Glassy Carbon Electrode Mohammad Bagher Gholivand* [a] and Mehdi Khodadadian [a] 1 Introduction Theophylline (TP) (Scheme 1a) is a xanthine derivative that relaxes smooth muscles, relieves bronchospasm and has a stimulant effect on respiration. Guaifenesin (GF) (Scheme 1b) is an expectorant which is used to break up the congestion and mucus and to help breathe more easily [1]. TP and GF in combination induce bronchodila- tion and assist the patient in coughing up viscid mucus, and have been used in the symptomatic treatment of bronchial asthma and other bronchospastic conditions [1,2]. Several methods have been reported for the simultane- ous determination of TP and GF such as high perfor- mance liquid-chromatography (HPLC) [3–5], third deriv- ative UV-spectrophotometry and [6] UV-spectrophotom- etry with partial least squares (PLS) regression [7]. How- ever, these methods are either expensive or not straight- forward to be accepted for routine analysis in quality control experiments. Almost all reported spectrophoto- metric techniques need preprocessing procedures to re- solve overlapped signals and HPLC methods involve in- tensive solvent-usage and are time consuming. On the other hand, electrochemical methods have shown remark- able advantages in the analysis of drugs in pharmaceutical preparations and human body fluids. These advantages are mainly due to the simplicity, sensitivity, low cost and relatively short analysis time, as compared with other techniques. Various electrochemical methods have been reported for individual determination of TP and GF [8– 10]. However, to our knowledge, no electrochemical method has been described for the simultaneous determi- nation of TP and GF in pharmaceutical combinations and biological samples. In recent years, chemically modified electrodes have at- tracted big interest due to their potential applications in electroanalytical chemistry [11]. Modification of electro- des surface with various modifiers such as metal nano- particles [12], carbon nanotubes [13], graphene [11], tran- sition metal complexes [14], ionic liquids [15], enzymes [16], polymers [17] and various nano-composites [18] have been reported. It has been observed that the modifi- cation of electrode substrates with multiwalled carbon nanotubes (MWCNTs) would result in low detection limits, reduced overpotentials and resistance to surface fouling and therefore MWCNTs have been claimed as electrocatalysts [19]. Abstract : A glassy carbon electrode (GCE) modified with multiwalled carbon nanotubes (MWCNTs) and a hy- drophobic ionic liquid (IL), was used for the simultaneous voltammetric determination of theophylline (TP) and guaifenesin (GF). The results showed that the oxidations of TP and GF were facilitated at modified electrode and peak-to-peak separation at MWCNT À IL/GCE (252 mV) was larger than that observed at unmodified GCE (165 mV). Voltammetric signals for TP and GF exhibited linear ranges of 0.5 to 98.0 mM(R 2 > 0.99) and 1.5 to 480.0 mM(R 2 > 0.99), respectively. The method was used to estimate TP and GF contents in some real samples. Keywords: Carbon nanotube · Ionic liquid · Theophylline · Guaifenesin · Modified electrodes Scheme 1. [a] M. B. Gholivand, M. Khodadadian Department of Analytical Chemistry, Faculty of Chemistry, Razi University Kermanshah, Iran tel: + 98 831 4274557; fax: + 98 831 4274559 *e-mail: mbgholivand@yahoo.com www.electroanalysis.wiley-vch.de  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2014, 26, 1 – 10 &1& These are not the final page numbers! ÞÞ Full Paper