Improvements in LA-ICP-MS for Trace Forensic Analysis of Common Household Products Using a Short Pulse Ultraviolet Laser

Transcription

1 Improvements in LA-ICP-MS for Trace Forensic Analysis of Common Household Products Using a Short Pulse Ultraviolet Laser Megan Mekoli Jonna Berry, Maria Bock, Jeneé Jacobs, Stan Bajic and R. Sam Houk Iowa State University

2 Overview Develop a method for use in forensic evidence analysis using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) Compare nanosecond-la-icp-ms to femtosecond-la- ICP-MS Duct and electrical tape Used in bombs and explosive devices Binding during a crime Copper speaker wire Binding during a crime Used in bombs Galvanized steel pipe Used to make pipe bombs and other explosive devices

3 LA-ICP-MS Photo courtesy of Dr. R.S. Houk, 3

4 LA-ICP-MS Photo courtesy of Dr. R.S. Houk, 4

5 LA-ICP-MS and Forensic Science Low detection limits Isotope abundances measureable Sample destruction limited Range of sample sizes Homogeneous 1 Quantification without matrix matched standards is difficult. 2 1 Anal. Bioanal. Chem. 2003, 376, J. Anal. At. Spectrom., 2003, 18, 872.

6 Why Femtosecond Ablation? fs shorter pulse duration (~100 fs vs. 10 ns) Ablated particles from fs smaller, more consistent ns ablated matter can contain large particles Large spikes in signal, worse stability fs less heat transfer to sample Less melting Good for tapes especially Less fractionation problems

7 Intensity (cps) Intensity (cps) Can You Tell the Difference Between These Mass Spectra? Ti 48 Fe 56 Ni 58 Duct- Nashua, Backing Side Ni 60 Ti 46 Ti 47 Ti 49 Ti 50 Mn 55 Ti 48 Mass (u) Fe 56 Ni 58 Ni 61 Ni 62 Zn 64 Ga 69 Cu 63 Cu 65 Zn 68 Duct- 3M, Backing Side Ti 46 Ti 47 Ti 49 Ti 50 Mn 55 Ni 60 Ga 69 Ni 61 Ni 62 Zn Cu 63Zn64 Cu Zn 68 Mass (u) While you may be able to spot some differences by eye, what really matters are the analytical differences in the data. To quantify these differences, I m going to use Principle Component Analysis (PCA). 7

8 PCA Multi-variable statistical technique First consider variables (intensities) with the most variance, then second, third, etc. Score Plot scores Eigenvector Solo 4.1 software (Eigenvector Technologies) Approach meant for pairwise head-to-head comparison analysis Not intended to be for attribution studies

9 Tapes Duct and Electrical Tapes Often used for binding during a crime Can be used in bomb making 3 mm 3 mm Duct- 3M, Backing Side Duct- 3M, Adhesive

10 Method 1 revolution-waste Sample on large steel washer cut off excess Ablate backing side Flip, ablate adhesive side. 3 different samples per roll (0 ft., 3 ft., 9 ft.) 1.75 in. 6 ft. 3 ft. 9 ft. 3 ft. 0 ft. 5

11 Instrumental Conditions Laser: CETAC LSX 500 nanosecond Nd:YAG (~10 ns) 9.1 mj/pulse (100% power) Frequency: 20 Hz Wavelength: 266 nm Spot diameter: 100 μm 350 μm/s rastering This is fast to avoid ablating through the tape. Thermo Element 1 ICP-MS: Cones: Ni Medium resolution (m/δm=4000) Dual (counting and analog) mode Analysis: Principal Components Analysis (PCA) Eigenvector Solo 4.1 Software (Eigenvector Technologies) 1.75 in.

12 Instrumental Conditions Laser: COHERENT LIBRA femtosecond Ti:Sapphire (~100 fs) 140 μw (100% power) Frequency: 1000 Hz Wavelength: 266 nm Spot diameter: 50 μm 300 μm/s rastering This is fast to avoid ablating through the tape. Thermo Element 1 ICP-MS: Cones: Ni Medium resolution (m/δm=4000) Dual (counting and analog) mode Analysis: Principal Components Analysis (PCA) Eigenvector Solo 6.2 Software (Eigenvector Technologies) 1.75 in.

13 Scores on PC 1 (98.08%) Femtosecond Ablation Homogeneous? PCA Plot- Duct Tape, Backing Side, 3M Brand x ft. 3 ft. 9 ft x x Scores on PC 2 (1.78%) x 10 5

14 Scores on PC 1 (99.9%) Femtosecond Ablation PCA Plot- Duct Tape, Adhesive Side, Duck Brand x ft. 3 ft. 9 ft x 10 6 Scores on PC 2 (0.00%)

15 Autoscaled Scores on PC 2 (12.53%) Nanosecond Ablation PCA Plot-Duct Tape, Backing Side 10 3M Duck Nashua Staples Autoscaled Scores on PC 1 (14.51%)

16 Average Q-Residual Model Q-Residual Plot- Duct Tape, Backing Side 3M Duck Nashua Staples 95% C.I m_3m m_duck m_nashua m_staples

17 Electrical tape 1.75 in.

18 Scores on PC 3 (0.12%) x PCA Plot-Electrical Tape, Adhesive Side, UL Brand Femtosecond Ablation ft. 3 ft. 9 ft x Scores on PC 1 (96.92%)

19 Scores on PC 1 (95.35%) Nanosecond Ablation PCA Plot- Electrical Tape, Adhesive Side x Old Super 33 6 Plymouth Super 33 UL x Scores on PC 2 (2.77%)

20 Average Q-Residual 1.00E+13 Q-Residual Plot Adhesive Side 1.00E+12 Old Super 33 Plymouth 1.00E+11 Super 33 UL 1.00E+10 95% C.I. 1.00E E+08 m_old Super 33 m_plymouth m_super 33 m_ul Model

21 Copper Speaker Wire Can be used for binding in a crime Can be used to make bombs and explosive devices Method Remove plastic coating Separate individual strands within the bundle Mount on device Single line ablation ns ablation 2 cm

22 Average Q Residual Q-Residual Plot for 10 Wires Nanosecond Ablation Wire 1 Wire 2 Wire 3 Wire 4 Wire 5 Wire 6 Wire 7 Wire 8 Wire 9 Wire 10 95% C.I Model for Wire #

23 Galvanized Steel Pipe Commonly used for making pipe bombs and other explosive devices Representative for bulk metal analysis Steel is mostly Fe, coating is Zn Coating varies by manufacturer conditions Number of times dipped Cooling time, temperature Method Cut into pieces Line depth profile, ns laser 3.81 cm

24 Intensity (cps) Depth Profile: Outside of Galvanized Pipe X Fe(56) Zn(64) Zn(66) a b c d e f Line

25 Conclusions Electrical and Duct tape homogeneous within roll Tape brands may be distinguished from one another Bulk copper speaker wire bundles are homogeneous within a roll Individual copper speaker wire strands can be distinguished from one another Could allow us to do attribution studies Galvanized pipe coating depth may be sampled Layer by layer analysis

26 Acknowledgments NIJ for the funding of this grant Dr. R. Sam Houk Dr. Stan Bajic Jonna Berry (copper wire) Maria Bock (tape) Jeneé Jacobs (galvanized pipe) Dan Zamzow