Trace Analysis of Explosives in Soil Samples Using the Agilent 1290 Infinity LC System Equipped with an Agilent Max-Light 60-mm Cartridge Cell - Gerd Vanhoenacker, Frank David, Pat Sandra & Edgar Naegele

This Application Note describes how the Agilent 1290 Infinity LC system equipped with the Agilent Max-Light 60 mm Diode Array Detector cartridge cell was used for residue analyses of nitroaromatic explosives in soil. Excellent sensitivity can be achieved with detection limits below 1 μg/L for standard solutions. Chromatographic selectivity for the structurally related compounds was obtained using an Agilent Poroshell 120 EC-C18 column operated at 44°C. The developed method is validated and applied to a set of soil samples.

Introduction
Explosive residues are found in groundwater, sediment, and soil that have been contaminated by military or terrorist activities and civil activities such as mining and construction.Trinitrotoluene (TNT) and its metabolites and hexogen (RDX) are the most commonly used explosives1,2. Bacteria in the soil transform TNT to the toxic and mutagenic metabolites 2-amino- 4,6-dinitrotoluene (2A-DNT) and 4- amino-2,6-dinitrotoluene (4A-DNT).

The United States Environmental Protection Agency has published EPA Method 8330 for the analysis of nitroaromatics and nitramines3,4. The structures of the 14 compounds studied in this work are shown in Table 1. The determination of explosives in environmental samples is a challenging task. The inherent limited thermal stability of some of the explosives makes them unsuitable for GC analysis. Consequently, LC is the method of choice for these compounds. LC/MS using atmospheric pressure chemical ionization (APCI) is applicable for explosive residue analysis2 because the sensitivity for certain explosives is as low as 1 to 10 ng/kg or ng/L. However, for some compounds the sensitivity is not within standards and the instrumental cost is high.

Explosive residues can be detected at relatively low levels with UV or DAD detectors. One of the disadvantages of using UV-based detection compared to mass selective detection is the lack of selectivity. This can be problematic because some of the compounds are structurally very similar, which makes it difficult to separate them chromatographically. The EPA Method 8330, therefore, recommends the use of two columns. A C18 phase is used as the primary column while an additional analysis on a CN phase is required for confirmation purposes. Coupling columns in series produces a combination of both stationary phases in the same analyses5. However, sample matrixes can vary significantly and produce interferences during sampleanalyses. Nevertheless, LC with UV detection (EPA 8330) is still the method of choice for this analysis.

This Application Note shows the advantages of using state-of-the-art equipment combined with a sensitive detector for the analysis of all explosives, especially those in soil samples.The analyses were performed using the Agilent Poroshell 120 column.