Explosives detection in soil using a field-portable continuous flow immunosensor

Abstract

A field method for quantitative analysis of explosives in contaminated soil samples is described. The method is based on a displacement immunoassay performed in a commercial instrument, the FAST 2000, engineered by Research International Inc. The method can be used on-site to measure 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) within 5 min. For this study, replicate analyses were performed on soil extracts prepared from each field sample as well as appropriate controls, blanks, and laboratory standards. Statistical analyses were done to assess accuracy, bias, and predictability of the method. The results demonstrated that the immunosensor could be used effectively to screen environmental samples for the presence or absence of explosives. In most samples, the method also provided quantitative values that were in good agreement with standard laboratory analyses using HPLC. A limited number of sample matrices interfered with the immunoassay and produced results that varied significantly from the laboratory data. In each case, the compounds causing the problem have been identified and efforts are being made to minimize these matrix interferences in future field evaluations.

Introduction

Field screening methods to detect environmental contaminants continue to proliferate as microelectronics improve and analytical instruments become miniaturized. One application of on-site testing can be seen in the identification, characterization, and remediation of sites contaminated with 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). These energetic compounds pose a serious site characterization, cleanup, and monitoring problem for the Department of Defense (DoD), Environmental Protection Agency (EPA), and other government institutions. The compounds are most commonly encountered in contaminated soils in close proximity to facilities used to manufacture, assemble, test, store, and demilitarize munitions. Once deposited, they do not rapidly decompose and will leach through soils into the groundwater, creating a serious environmental hazard (Spalding et al. [1]).

The current method of choice for analysis of nitroaromatics and nitroamines is the US EPA Method 8330 utilizing HPLC (Crockett et al. [2], SW-846 [3], Jenkins et al. [4]). Though laboratory analyses provide a more complete, exact profile of contaminants present in a given sample than most field methods, remediation goals require a large number of samples to be run at infrequent intervals. It is difficult, costly and time-consuming to send samples off-site for testing. With rigorous sampling plans and careful attention to QA/QC, accuracy, and precision, on-site methods provide real-time information to remediation managers that can improve decision-making, allow better risk assessments, and significantly decrease analytical costs.

There are a number of analytical laboratory methods available for detection of TNT and RDX including thin-layer chromatography (Douse [5], Haas et al. [6]), capillary electrophoresis (Northrop et al. [7], Oehrle [8], [9]), liquid chromatography (Nair and Huber [10], Hirata and Okamoto [11], Bauer et al. [12], Kleibohmer et al. [13]), ion exchange resin based tube sensor (Heller et al. [14]), and PVC membrane-based sensor (Zhang and Seitz [15]), GC/MS, HPLC (Kolla [16], Yinon and Zitrin [17], Jenkins et al. [18], Caton and Griest [19]). These laboratory methods are often cost prohibitive, time consuming, and labor intensive for use in a field screening application. In the early 1990s, Jenkins and Walsh [20] pioneered a field method for the detection of explosives using a colorimetric technique employing the Janovsky reaction. Since then, several qualitative and quantitative methods have been validated and commercialized for on site testing, including the D-Tech field test kit (Strategic Diagnositics [21], [23]), Ensys immunoassay kit, (Ensys Environmental Products [22]) and other immunochemical based methods (Shriver-Lake et al. [24], [25], Narang et al. [26], [27]). In general, field measurements are considered to be useful for screening purposes, providing reliable data and timely information on the extent of contamination or remediation progress. Assays utilizing immunological reactions to detect a variety of analytes have been well studied (Keuchel et al. [28], [29], [30], Whelan et al. [31], Narang et al. [26], [27]). This report demonstrates the effectiveness of the FAST 2000, a commercially available field version of the continuous flow immunosensor (CFI), for detection of TNT and RDX in soil samples obtained from environmental cleanup sites.