Selective Determination of TNT in Soil Extracts by Sequential Injection Spectrophotometry

A rapid sequential injection spectrophotometric method for the selective determination of 2,4,6-trinitrotoluene (TNT) in soil samples is presented. The method is based on a derivatization reaction of TNT with sodium sulfite in a basic acetone medium. The reaction conditions, namely the percentage of acetone used in the reaction, the sample and reagent volumes, the mixing coil volume, and the reaction time, were optimized. The reaction was found to be particularly sensitive to the concentration of acetone; an acetone/water medium of 88% (v/v) acetone was found to be optimal. A study of the response of the method to other explosives demonstrated that the method is selective for TNT. In particular, it was shown that the method had little (<2%) or no response to other secondary explosives such as 2,4- or 2,6-dinitrotoluene (DNT). An average precision of 6.1% RSD was established for five soil samples ( n = 4). The limit of detection was 0.5 μg mL - 1 for aqueous standards and 80 μg g - 1 for ∼300 mg soil samples. Analysis time was approximately three minutes per sample. Quantitative results from the determination of TNT in authentic contaminated soil samples compared favorably to those obtained using a standard method. The extension of the method to water samples and the means by which samples can be preconcentrated on-line using solid-phase extraction (SPE) techniques are discussed.