Determination of Phenylurea Herbicides in Tap Water and Soft Drink Samples by HPLC–UV and Solid-Phase Extraction

Mar 01, 2012

A simple and sensitive high performance liquid chromatography (HPLC) method with ultra-violet (UV) detection has been developed for the analysis of phenylurea herbicides — namely, monuron, diuron, linuron, metazachlor and metoxuron — that involves a preconcentration step using solid-phase extraction (SPE). The mobile phase used was acetonitrile–water at a flow-rate of 1 mL/min with direct UV absorbance detection at 210 nm. Separation of analytes was studied on a C18 column. The method was applied successfully to the analysis of the herbicides in three soft drink brands and tap water. Good linearity and repeatability were observed for all the pesticides studied.

Phenylurea herbicides are used widely in a broad range of herbicide formulations as well as for nonagricultural use. Consequently, their residues are frequently detected as major water contaminants in areas where these are used extensively (1). Diuron and linuron are substituted urea compounds that are soluble in water and can migrate in soil and enter the food chain (2). These herbicides are of significant toxicological risk to humans and wildlife. Diuron, which is used in cotton-growing areas and with fruit crops, is rated as the third most hazardous pesticide for groundwater resources. These herbicides are also applied on railway tracks to maintain quality and provide a safer working environment (3) but this may lead to groundwater contamination as their leaching potential is significant. Phenylureas enter the environment through pathways such as spray drift, runoff from treated fields and leaching into groundwater. Most of the excess material penetrates into the soil where it is subjected to the action of microorganisms (4) and degradation as studied by Canonica and colleagues (5). Phenylureas are unstable photochemically, as discussed by Khodja and colleagues (6) but these can persist in water for several days or weeks depending on the temperature and pH. Cases of incidental pesticide pollution of water reservoirs (2–4,7–13) have become more numerous in recent years.

Phenylurea residues can be found in water sources, processed products and on the crops where these are applied. In India, most of the soft drink bottling plants use surface water from canals and rivers, which have a high risk of pesticide contamination. The water treatment measures used are insufficient for complete removal of these pesticide residues, which have been found to be above permissible limits. The evidence for the above stated facts was provided in a 2003 Centre for Science and Environment (CSE, New Delhi, India) report that found several pesticide residues in many soft drink samples of leading international brands procured from all over India. The CSE findings were confirmed further by a Joint Parliamentary Committee (JPC) created to verify the facts. In 2006, CSE conducted another round of tests and found pesticides yet again in soft drink samples. Keeping this in mind, the present work has great importance, as it involves the determination of phenyl urea herbicides in soft drink samples and tap water.

Therefore, it is imperative that sensitive, selective and efficient methods for herbicide analysis be designed. The common analytical methods used are high performance liquid chromatography (HPLC)–UV (2–4,7–9), solidphase microextraction (SPME)–HPLC (10), diode array (11), immunosorbent trace enrichment and HPLC (12,14), LC–mass spectrometry (MS) (15,16), gas chromatography (GC)–MS (13), capillary electrophoresis (17,18, 19), photochemically induced fluorescence (20,21) and derivative spectrophotometry (22). A useful review is presented by Sherma (23) on the use of thin-layer chromatography (TLC) and its modified versions for the analysis of these herbicides. Solid-phase extraction (SPE) of phenylurea herbicides has been reported in literature by several workers (24–29). The SPE of soft drinks has been reported extensively (30–36). As the use of polar and degradable pesticides becomes widespread, it is urgent that more sensitive analytical methods be developed for their residual analysis in various matrices. HPLC has several advantages over GC because it can be used for simultaneous analysis of thermally unstable, nonvolatile, polar and neutral species without a derivative step. Because of the thermally unstable nature of phenylurea herbicides, the direct application of GC to these compounds is not possible and derivatization prior to the detection is needed. For this reason, HPLC with UV absorption or fluorescence detection (7–10) is preferred over GC. As a result, HPLC is gaining popularity and preference as a pesticide analysing technique.

The present work describes a simple and sensitive HPLC–UV method for the analysis of phenyl urea herbicides (namely, monuron, diuron, linuron, metazachlor and metoxuron) and it involves a single-step preconcentration by SPE.