Determination of sugar cane herbicides in soil and soil treated with sugar cane vinasse by solid-phase extraction and HPLC-UV

This work reports on the development and validation of a small-scale and efficient SPE-HPLC-UV method for the simultaneous determination of the most used herbicides (diuron, hexazinone, and tebuthiuron) applied to soil and soil treated with sugar cane vinasse (soil-vinasse) in areas where sugar cane crops are grown in the state of São Paulo, Brazil. The analytical procedure was optimized for solvent extraction and HPLC-UV conditions. Extraction and clean-up were combined in a single step employing solid-phase extraction, avoiding sophisticated techniques, organic-solvent-water mixtures and consequently a longer concentration step. Recovery studies with soil and soil-vinasse samples spiked at two herbicides levels (around 0.25 and 2.0 mg kg⁻¹) and sample stability (sample frozen for 20 days before analysis) were applied as parameters to control the efficiency of the method. Good accuracy and precision were achieved with average recoveries ranging from 78% to 120% and relative standard deviations less than 10% throughout the whole recovery test. The method's limit of detection ranged between 0.025 and 0.050 mg kg⁻¹ for diuron, hexazinone, and tebuthiuron in soil and soil-vinasse. The feasibility of this method was applied to determine the herbicide half-lives (t_1/2) in soil and soil-vinasse in a laboratory study. Sugar cane vinasse added to soil increased the degradation of diuron and tebuthiuron (p < 0.05), reducing the t_1/2 from 80 to 7 days and 128 to 73 days, respectively. This method is presented as an alternative which could be applied to assess herbicide behavior in soil in order to prevent water contamination and to contribute to establish pesticide limits in soil.