Application of Photoassisted Electrochemical Detection to Explosive-Containing Environmental Samples

High-performance liquid chromatography (HPLC) with ultraviolet (UV) absorbance and photoassisted electro- chemical detection (PAED) is applied to the determination of explosives in groundwater and soil samples. On-line, solid-phase extraction minimizes sample pretreatment, enabling direct analysis of groundwater samples and soil extracts. Soils are extracted using pressurized fluid ex- traction, which is compared to the Environmental Protec- tion Agency (EPA) sonication method. Limits of detection for explosives in the matrixes of interest are equivalent or superior (i.e., <10 parts-per-trillion for HMX) to those achieved using the EPA method 8330. HPLC-UV-PAED is also shown here to be more broadly applicable, as it is capable of determining nitro compounds of interest (e.g., nitroglycerin) that have poor UV chromophores. Ad- ditional selectivity of amine-substituted nitroaromatic explosives is achieved by using a photochemical reactor with a 366-nm wavelength lamp. By coupling reversed- phase columns of different selectivities together, baseline resolution of all 14 standard explosives is demonstrated.

Because nitro explosives present health hazards to living things due to their carcinogenic and toxic nature,1 their presence in the environment constitutes a potentially serious contamination prob- lem. A contaminated site is typically characterized by collecting and storing soil and groundwater samples until they can be shipped to specialty laboratories for analysis, a process that can take up to a month or more.2 This is detrimental to obtaining accurate results, however, because explosives are known to 8330) despite its inherent lack of selectivity and sensitivity (e.g., the limit of detection for RDX is equal to the risk-based target level as defined by the EPA). Due to this lack of sensitivity, an entire liter of groundwater must be drawn, salted out, and evaporated down to 5 mL for analysis. To achieve adequate selectivity, method 8330 requires that samples be run on a C18 bonded-phase column and, subsequently, on a cyano bonded- phase column for confirmation of analyte identification. A more sensitive, selective method would aid in the analysis of explosive- containing samples.

For about two decades, on-line, postcolumn photolysis follow- ing reversed-phase high-performance liquid chromatography (RP- HPLC) has been used for the determination of a variety of organic nitro explosives.3-10 These studies revealed that nitro explosives are amenable to photoassisted electrochemical detection (PAED) with resulting detection limits in the range of 0.5-4 ng/20 µL injection, or ∼100 µg/L or less in terms of concentration. These studies also described the successful application of HPLC-PAED to the determination of a real-world sample resulting from a pipe bomb explosion under a car, wherein the postblast debris was extracted with acetonitrile and water. However, there are no applications of HPLC-PAED, with or without on-line solid-phase extraction (SPE), for the determination of explosives in ground- water and soil to date.
We have previously described an HPLC-UV-PAED system with on-line SPE that has been validated according to the Resource Conservation and Recovery Act (RCRA).11 RCRA, enacted by Congress in 1976, requires an evaluation of the method with respect to accuracy (80-120%), precision (<20%), repeatability degrade over time. Tetryl exemplifies this problem in that it was reported to decay rapidly in water, even faster than the DNT isomers, which are stable in water for only 23 days at pH <8.2 It is therefore desirable to analyze explosive-containing samples on- site to ensure rapid, accurate results.

HPLC-UV at 254 nm is the accepted method used by the United States Environmental Protection Agency (EPA) for the determination of explosives in groundwater and soil (EPA method the required sample amount from 1 L (EPA method 8330) to 2 mL, and allowing limits of detection ranging from 0.0007 to 0.4 µg/L, well below those achieved with ultraviolet absorption detection for several important explosives.11 Enhanced selectivity enables analyte identification in a single chromatographic run. Having two detectors in-line (HPLC-UV-PAED) offers the advantage of dual detector confirmation, allowing the comparison of retention times on two detectors and response ratios (electro- chemical signal divided by UV signal) of standards versus analytes. Also, having two detectors permits chemometric resolution of overlapping peaks.11

This paper describes the application of that validated system to the determination of explosives in groundwater and soil extracts. Additionally, soils are extracted using pressurized fluid extraction (PFE), and this method is compared to the sonication method outlined in EPA method 8330. The application of PAED to other explosives of interest that are not addressed in method8330 is also investigated. A comparison is made between photolysis at 254 and 366 nm for several nitro explosives; this approach allows one to distinguish between two explosives that coelute in the chromatographic run in that only one responds following photolysis at 366 nm. An alternative approach is via improved chromatographic resolution by combining two different C18 phases of different selectivity.

CONCLUSIONS

HPLC-UV-PAED with on-line SPE is a proven method for the determination of explosives in environmental samples. En- hanced sensitivity and selectivity means that less sample and sample preparation are required. Also, this approach is compatible with on-site analysis of groundwater and soil analyses, allowing fast assessment and profiling of contaminated sites for a wider range of explosives. Furthermore, the unique selectivity of PAED allows one to distinguish between nitro explosives containing amine functionality and those that do not. This attribute enables accurate characterization of explosive-containing samples.

This paper has also explored improvements in the analytical method as to the resolution of explosives that have been problematic since the adoption of EPA method 8330. The use of alternate wavelengths for photolysis provides added selectivity in that only a few of the explosives respond under these unique conditions. However, the assay would require two chromato- graphic runs at different wavelengths to accurately assess a contaminated site. Complete resolution of all 14 explosives of interest as demonstrated in this paper enables a complete, accurate site assessment superior in sensitivity and selectivity to all A-366 other existing methods.