Spectroscopy - June 2009 - (Page 36) 36 Spectroscopy 24(6) June 2009 w w w. s p e c t r o s c o p y o n l i n e . c o m and entrained in the plasma; thus, the carbon and hydrogen contained in the organic substrate will interfere with the carbon and hydrogen atomic emission that originates from the explosive residue. We applied a small amount of RDX, road dust, and oil residue to a red silicone substrate to determine how well we could discriminate explosives from nonexplosives on an organic substrate. We collected 50 single-shot doublepulse LIBS spectra (at 25-m distance) of each residue on the substrate as well as the blank substrate itself. In Figure 2, a representative single-shot double-pulse spectrum of each residue on the organic substrate and the blank substrate are displayed. We used 40 of the 50 spectra for each residue and the blank to generate a model in PLS-DA. From each singleshot spectrum, nine intensities and 20 ratios of these intensities were calculated, as described in reference 44. These 29 variables were the inputs for the PLSDA model. We calculated the model and then used the 10 remaining single-shot spectra from each residue type to test how the model performs. In Figure 3, which shows the predicted scores for the RDX class in the PLS-DA model for both the model and test spectra, all of the RDX residue on red silicone substrate test samples are above the classification threshold determined by the model and are thus classified as explosives. None of the nonexplosive residue test samples classify as explosive. These preliminary results indicate that LIBS can differentiate between explosive and nonexplosive residues on an organic substrate. Current work involves expanding the types of substrate materials and the number of explosive and interferent test samples to further test the PLS-DA models. As described in the introduction, we have been investigating the use of LIBS with biological warfare agent surrogates for several years now. In fact, our first work with coupling chemometrics with LIBS data involved biomaterials (17). We also have performed an extensive study of biomaterials at a 20-m standoff range. These studies include an array of biological warfare agent surrogates, other biomaterials, and interferents. We demonstrated the successful discrimination of the anthrax surrogate BG (2% false negatives, 0% false positives) and ricin surrogate ovalbumin (0% false negatives, 1% false positives) (37). However, biological warfare agents can be grown in a variety of media. The elemental uptake of the bacterial spore will be different depending upon the growth media. Because LIBS tracks all of the elements in order to classify the material as a biological warfare agent or as an interferent, the growth media type could influence how the bioagent is classified. We obtained two spore types, BG and Bacillus thuringiensis (Bt), in two growth media, G-Media (G) and Sporulation Broth (SpBr) from ChemImage (Pittsburgh, Pennsylvania). We performed a small feasibility test on a limited sample set to observe the effect of growth media on the ability to discriminate between the two spores. We collected 10 singleshot LIBS spectra at 40 m for each spore type from each growth media and the blank aluminum substrate. In Figure 4, LIBS spectra of the biomaterials in the two growth media are displayed. In Figure 4b, there are several atomic emission lines that are only observed in the G growth media. In Figure 4c, there are observable differences in the relative intensities of the atomic emission lines for each spore type. The goal was to be able to discriminate between Bt and BG irrespective of growth media. A number of atomic line intensities and ratios of the atomic emission intensities were selected as inputs into the PLS-DA model based upon our earlier work (37). Three classes were defined in the PLS-DA model — a BG class, a Bt class, and a substrate class (aluminum). Six BG in G samples and six BG in SpBr samples were used for the BG class; six Bt in G samples and six Bt in SpBr samples were used for the Bt class; and six aluminum substrates were used for the aluminum class in the model. The remaining 20 spectra were used as test samples for the PLSDA model. In Figure 5, each combination of spore type and growth media test sample matches the correct spore class regardless of the growth media. Conclusions Despite substantial success in the laboratory and in the handful of field trials, standoff LIBS still needs further refinement before it can be considered a fully viable instrument for field use by military or civilian personnel. However, LIBS is one of only a handful of techniques that have been shown to detect explosive signatures at a standoff distance. Other techniques that are being investigated for standoff explosives detection include terahertz (THz) imaging, photofragmentation followed by laser-induced f luorescence (PFLIF), photoacoustic spectroscopy, and Raman spectroscopy (46). THz time domain spectroscopy has been used to collect reflected absorption spectra of RDX at 30 m (47). Even at 30 m, however, atmospheric water vapor obstructs the RDX signal, and the interference will increase as standoff distance is increased (48,49). PF-LIF can be used as a standoff technique and has been demonstrated under laboratory conditions to detect TNT molecules at a distance of up to 2.5 m (50,51). A variation of photoacoustic spectrosocpy using quantum cascade lasers as the optical source for illuminating surface adsorbed explosives and quartz crystal tuning forks as the detectors has been demonstrated recently at a distance of 20 m (52). All of these techniques are still confined to the laboratory and are still in the early stages of development. While these other techniques have been applied to the detection of explosive materials at a standoff distance, to our knowledge, none have demonstrated discrimination between explosives and interferent materials at a standoff distance. Raman spectroscopy is another optical technique that has demonstrated success for detecting explosive signatures (53). Raman can be used to obtain molecular information about the sample and does not leave an imprint, although photodegradation of the sample can occur. However, the signal intensity is weak and thus, acquisition time can be much longer relative to LIBS. Using LIBS and Raman together to provide orthogonal information is currently under investigation by several groups. The instruments conceivably can share the same laser and spectrometers while obtaining elemental and molecular information. The data can be fused from http://www.spectroscopyonline.com Table of Contents for the Digital Edition of Spectroscopy - June 2009 Spectroscopy - June 2009 Contents News Spectrum Auger Spectroscopy Statistics and Chemometrics for Clinical Data Reporting, Part 1 Understanding and Interpreting the New GAMP 5 Software Categories Current Status of Standoff LIBS Security Applications at the United States Army Research Laboratory Spectral Studies on the Interaction of [Ru(bpy)2(BTIP)]2+ with DNA and Determination of Nucleic Acids at Nanogram Levels Product Resources Calendar Ad Index Spectroscopy - June 2009 Spectroscopy - June 2009 - (Page BB1) Spectroscopy - June 2009 - (Page BB2) Spectroscopy - June 2009 - Spectroscopy - June 2009 (Page Cover1) Spectroscopy - June 2009 - Spectroscopy - June 2009 (Page Cover2) Spectroscopy - June 2009 - Spectroscopy - June 2009 (Page 3) Spectroscopy - June 2009 - Spectroscopy - June 2009 (Page 4) Spectroscopy - June 2009 - Spectroscopy - June 2009 (Page 5) Spectroscopy - June 2009 - Spectroscopy - June 2009 (Page 6) Spectroscopy - June 2009 - Spectroscopy - June 2009 (Page 7) Spectroscopy - June 2009 - Contents (Page 8) Spectroscopy - June 2009 - Contents (Page 9) Spectroscopy - June 2009 - Contents (Page 10) Spectroscopy - June 2009 - Contents (Page 11) Spectroscopy - June 2009 - News Spectrum (Page 12) Spectroscopy - June 2009 - News Spectrum (Page 13) Spectroscopy - June 2009 - Auger Spectroscopy (Page 14) Spectroscopy - June 2009 - Auger Spectroscopy (Page 15) Spectroscopy - June 2009 - Auger Spectroscopy (Page 16) Spectroscopy - June 2009 - Auger Spectroscopy (Page 17) Spectroscopy - June 2009 - Statistics and Chemometrics for Clinical Data Reporting, Part 1 (Page 18) Spectroscopy - June 2009 - Statistics and Chemometrics for Clinical Data Reporting, Part 1 (Page 19) Spectroscopy - June 2009 - Statistics and Chemometrics for Clinical Data Reporting, Part 1 (Page 20) Spectroscopy - June 2009 - Statistics and Chemometrics for Clinical Data Reporting, Part 1 (Page 21) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 22) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 23) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 24) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 25) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 26) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 27) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 28) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 29) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 30) Spectroscopy - June 2009 - Understanding and Interpreting the New GAMP 5 Software Categories (Page 31) Spectroscopy - June 2009 - Current Status of Standoff LIBS Security Applications at the United States Army Research Laboratory (Page 32) Spectroscopy - June 2009 - Current Status of Standoff LIBS Security Applications at the United States Army Research Laboratory (Page 33) Spectroscopy - June 2009 - Current Status of Standoff LIBS Security Applications at the United States Army Research Laboratory (Page 34) Spectroscopy - June 2009 - Current Status of Standoff LIBS Security Applications at the United States Army Research Laboratory (Page 35) Spectroscopy - June 2009 - Current Status of Standoff LIBS Security Applications at the United States Army Research Laboratory (Page 36) Spectroscopy - June 2009 - Current Status of Standoff LIBS Security Applications at the United States Army Research Laboratory (Page 37) Spectroscopy - June 2009 - Current Status of Standoff LIBS Security Applications at the United States Army Research Laboratory (Page 38) Spectroscopy - June 2009 - Spectral Studies on the Interaction of [Ru(bpy)2(BTIP)]2+ with DNA and Determination of Nucleic Acids at Nanogram Levels (Page 39) Spectroscopy - June 2009 - Spectral Studies on the Interaction of [Ru(bpy)2(BTIP)]2+ with DNA and Determination of Nucleic Acids at Nanogram Levels (Page 40) Spectroscopy - June 2009 - Spectral Studies on the Interaction of [Ru(bpy)2(BTIP)]2+ with DNA and Determination of Nucleic Acids at Nanogram Levels (Page 41) Spectroscopy - June 2009 - Spectral Studies on the Interaction of [Ru(bpy)2(BTIP)]2+ with DNA and Determination of Nucleic Acids at Nanogram Levels (Page 42) Spectroscopy - June 2009 - Spectral Studies on the Interaction of [Ru(bpy)2(BTIP)]2+ with DNA and Determination of Nucleic Acids at Nanogram Levels (Page 43) Spectroscopy - June 2009 - Spectral Studies on the Interaction of [Ru(bpy)2(BTIP)]2+ with DNA and Determination of Nucleic Acids at Nanogram Levels (Page 44) Spectroscopy - June 2009 - Product Resources (Page 45) Spectroscopy - June 2009 - Product Resources (Page 46) Spectroscopy - June 2009 - Product Resources (Page 47) Spectroscopy - June 2009 - Calendar (Page 48) Spectroscopy - 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