Bull. Chem. Soc. Ethiop. 2015, 29(2), 311-318. ISSN 1011-3924 Printed in Ethiopia  2015 Chemical Society of Ethiopia DOI: http://dx.doi.org/10.4314/bcse.v29i2.13 __________ *Corresponding author. E-mail: mianchem@uom.edu.pk SHORT COMMUNICATION DEVELOPMENT OF AN INDIRECT SPECTROPHOTOMETRIC METHOD FOR DETERMINATION OF METHAMIDOPHOS INSECTICIDE IN SOIL, WATER AND VEGETABLE SAMPLES Jasmin Shah 1 , M. Rasul Jan 1 , Mian Muhammad 1,2,* , Behisht Ara 1,2 and Ibadat Ur Rehman 1 1 Institute of Chemical Sciences, University of Peshawar, Khyber Pakhtunkhwa, Pakistan 2 Department of Chemistry, University of Malakand, Khyber Pakhtunkhwa, Pakistan (Received February 25, 2014; revised February 23, 2015) ABSTRACT. A simple and rapid indirect spectrophotometric method for determination of methamidophos in water, soil and vegetable samples has been described. Methamidophos reacts with acid produced from p- dimethylaminobenzaldehyde (p-DMAB) as a result of Cannizaro’s reaction. The resultant adduct undergoes condensation reaction in acidic medium forming a yellow colored product. Absorbance of the colored product was measured at 405 nm and pH 3 against a reagent blank. The Beer’s law range is obeyed in the range 1-30 µg mL -1 with molar absorptivity of 2.8 ×10 3 L mol -1 cm -1 . The limit of detection and quantification were found to be 0.20±0.03 and 0.60±0.04 µg mL -1 respectively. The proposed method was effectively applied for determination of methamidophos in various samples with percent recoveries in the range of 96±0.08 to 102±0.06 %. KEY WORDS: Spectrophotometric, Methamidophos, p-Dimethylaminobenzaldehyde (p-DMAB), Cannizaro’s reaction INTRODUCTION Methamidophos (O,S-dimethylphosphor-amidothiolate) is a broad spectrum, water soluble, systemic organophosphorus insecticide and acaricide [1]. It is used to control pests of cotton, rice, tobacco, brassica, head lettuce, sugar beet, corn and potatoes (e.g., leaf hoppers, spider mites, trips, aphids, etc.) [2, 3]. It has substantial effects on non-target organisms, humans, animals, plants, and insects. Methamidophos has been reported to exhibit acute and chronic toxicity and has an acute oral and dermal LD 50 of 10-50 and 50-110 mg (a.i.) kg -1 (b.w.) respectively in different test species [3]. Maximum residue level (MRL) of methamidophos in vegetables is 1.0 µg g -1 for cauliflower and 2.0 µg g -1 for tomato [4]. Methamidophos having high solubility in water (200 g L -1 at 20 o C) may contaminate ground and surface water and pose potential threats to human health via drinking water and foods [2, 3]. Due to its potential risk to human it is necessary to establish a selective and sensitive approach for determination of methamidophos in environmental samples. A number of analytical methods have been reported for determination of methamidophos. These include gas chromatography-mass spectrometry [5], high performance liquid chromatography [6], adsorptive stripping square wave voltammetry [7], differential pulse stripping voltammetry [8] and acetylcholine enzyme sensor [9]. Some of these methods like GC and HPLC being highly sensitive and specific, need complicated pre-treatment and clean-up steps [10] and are, therefore, unsuited for rapid assay of samples. In some cases the instrumentations employed is very expensive and can hardly be equipped due to their high setup costs and skills [5]. The present method is based on reaction of methamidophos with p- dimethylaminobenzaldehyde (p-DMAB) in alkaline media followed by acidification and subsequent spectrophotometric detection of the yellow colored condensation adduct formed, against a reagent blank. The proposed method was successfully applied for determination of methamidophos in vegetable and environmental samples. The novelty of the present method lies