Rapid Post-Blast Inorganic Explosive Analysis by Suppressed Ion Chromatography

Transcription

1 Rapid Post-Blast Inorganic Explosive Analysis by Suppressed Ion Chromatography L. Canella, C. Austin, A. Beavis, P. Maynard, M. Dawson, C Roux, and P. Doble Centre for Forensic Science, University of Technology, Sydney; Department of Chemistry, Material and Forensic Science, Broadway, NSW, Australia, 2007 Abstract The typical conditions employed for the analysis of inorganic explosive residues by suppressed ion chromatography result in runs time exceeding 30 minutes. Recent efforts to speed up the analysis time have involved coating monolithic or small particle size reversed phase columns with cationic surfactants. However, eluent ph constraints have made such systems incompatible with suppressed detection, rendering the analysis insensitive. This study employed a Waters hybrid particle 2.5µm silica C18 column coated with di-dodecyldimethylammonium bromide (DDAB) to form an anion exchange column. This column was stable over a ph range of 1-12 which allowed employment of bicarbonate-carbonate eluents and suppressed conductivity detection. At an optimum flow rate of 1.0mL/min seven anions associated with explosive residues were separated in under 20 minutes. The optimised conditions were evaluated for linearity, specificity, repeatability and limit of detection. The limits of detection were below 50ppb for chloride, nitrate and chlorate; 100ppb for nitrite and sulphate; and approximately 500ppb for thiocyanate and perchlorate. The method was used to analyse swabs of surfaces obtained from field trials of five separate explosive devices.

2 Introduction Common oxidizers associated with improvised explosive devices include salts of nitrates, chlorates and perchlorates, while common fuels include sources of carbon (coal, hydrocarbons and sugar), sulfur and aluminium [1]. Other ions of interest include chloride, nitrite, sulfate and thiocyanate. Explosives containing sulfur, such as a mixture of potassium nitrate/sugar/sulfur or black powder may generate thiocyanate in a post blast residue [2]. Nitrite may also be present in post blast residues from the reduction of the nitrate salt oxidizing agent. Chlorate and perchlorate are major components in flash powders and may be combined with sulfur and aluminium. Post blast residues of flash powders may therefore contain not only unreacted chlorate and perchlorate, but also sulfate and chloride as reaction products. Other oxidisers sometimes used include salts of permanganate, persulfate and dichromate. Ion chromatography (IC) was first introduced in 1975 and has since found wide applicability in the separation and determination of inorganic cations and anions, water soluble organic acids/bases and organometallic compounds in complex mixtures [3]. An IC system is composed of an eluent, pump, injection valve, column, suppressor, and detector. The eluent employed is usually a dilute aqueous solution of bases, acids, or salts of weak acids which is pumped through a column with packing which contains either negatively charged groups such as sulfonate or carboxylate for the analysis of cations or positive charged groups such as quaternary amines for the analysis of anions [4]. Limitations of IC for explosive analysis have included the difficulties of eluting certain strongly retained ions such as thiocyanate and perchlorate which have a high polarisability. The elution of these ions significantly increase run times.

3 Gradient flow rates or complex mobile eluents have been used to reduce run times but at the cost of resolution and increased complexity [2, 5]. Recent advances in column packing chemistry have provided the opportunity to revisit some areas of ion chromatography. The XBridge column, from Waters, has a reverse-phase silica packing which is stable over a wider range of ph values than previous models. Carbonate buffers can now be used in separations, allowing the use of suppressed conductivity detection. Using small particle sizes for efficiency, a method was developed which would separate anions of interest with greater speed, selectivity and resolution than previous columns, while the suppressed conductivity detection increased the sensitivity of the method. Materials and Method An XBridge C18 column was coated with DDAB a hydrophobic, ionic surfactant. The coating was found to be stable for extended use, provided that the buffer used in separations did not contain high concentrations of organic solvent. A buffer was developed based on carbonate/bicarbonate. It was found that a high ph, high concentration buffer with 1% acetonitrile flowing at 1.0ml/min gave the best compromise between peak shape, resolution & run time. A suppressor column was placed downstream of the separation column (Figure 1). The suppressor column removes the buffer ions by adding protons to the eluent, greatly reducing the background signal for conductivity detection.

4 Figure 1: XBridge Column (left) and suppressor column (right) for ion chromatography Five explosive devices containing a variety of inorganic components were detonated to provide test samples. Steel plates were placed upright 1 metre from each 250g device and the plates were swabbed for residues after the detonation (Figure 2). Figure 2: Capture plate from field trials 250g device

5 Results and Discussion The optimized separation of the anions is shown in Figure 3; all anion standards were at 5ppm concentration. The run time was 19 minutes. Validation data for the method are shown in Table chloride millivolts nitrite - nitrate - sulfate - chlorate thiocyanate - perchlorate Figure 3: Optimised separation of anion standards. Table 1: Validation of the method Resolution % RSD LOD (ppm) LOQ (ppm) Range (ppm) Chloride Nitrite Nitrate Sulfate Chlorate Thiocyanate Perchlorate

6 The LOD for chloride and for nitrite was affected by the solvent peak at 1.5 minutes. This varied in intensity over the course of the suppressor column lifetime. Swabs obtained from the field trials of explosive devices were successfully analysed for anions of interest (Figure 4) nitrate millivolts - chloride chlorate sulfate -1.0 Figure 4: Analysis of swab from device no. 1 (Potassium Chlorate, sulfur and aluminium). Conclusion A method was successfully developed for the analysis of anions from inorganic explosive residues. The method was sensitive, selective and faster than previous isocratic methods. The method was able to detect residues from swabs taken from field samples. However, the method still required 19 minutes/sample and for crime scene use a faster run time would be desirable. Another limitation is the lifetime of the suppressor column.

7 References 1. Kurowski, P.W.C.a.S.R., Introduction to the Technology of Explosives. 1996: Wiley-Vch. 2. Miller, M.L., Doyle, J.M, Lee, R.A and Gillette, R., Analysis of Anions by Capillary Electrophoresis and Ion Chromatography for Forensic Applications. Forensic Science Communications, (2). 3. Haddad, P.R. and P.E. Jackson, Ion Chromatography: Principles and Applications. 1990: Elsevier Science Publishers. 4. Klassen, S.E., et al., Ion chromatography of energetic materials at Sandia National Laboratories. Thermochimica Acta, (1-2): p Doyle, J.M., et al., A Multicomponent Mobile Phase for Ion Chromatography Applied to the Separation of Anions from the Residue of Low Explosive. Analytical Chemistry, : p