forensics of explosives - eth z

Forensics of ExplosivesLohmann VictoriaFäh AshlingChanton Michael

What are explosives?

Basics of Explosives

• Chemical reaction that produces heat, light and gas• Solids or liquids in metastable state

3J.T. Thurman, Practical Bomb Scene Investigation, CRC Press, Boca Raton FL, 2006

Various explosives

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Nitroglycerin

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• Explosion heat: 6’276 kJ/kg• Time: 1/100 – 1/1’000’000 s

Liu, J.; Liquid Explosives; Springer-Verlag Berlin Heidelberg, 2015


• Internal source of oxygen• Low explosives: deflagration• High explosives: detonation• Initiation by mechanical means, heat or shock from

detonation



7J.T. Thurman, Practical Bomb Scene Investigation, CRC Press, Boca Raton FL, 2006 p.12


• Internal source of oxygen• Low explosives: deflagration• High explosives: detonation• Initiation by mechanical means, heat or shock from

detonation


Setting the scene

ATM Robbery

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Challenges

• Volatile components• Location of objects with explosive residue• Destruction of evidence by emergency personnel

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Gathering evidence

Locate epicentre

• Analyse damage pattern• Estimate amount of explosive

13Forensic Investigation of Explosions (Second Edition), A. Beveridge, Ed., Taylor & Francis Ltd, CRC Press, Boca Raton FL, 2012

Locate explosive residue

• Fragments of explosive device and container• Surfaces in close proximity (non-porous better than porous)• Gas Wash• Rolled edges• Micro-cratering

14Aspects of Explosives Detection, M. Marshall and J. C. Oxley, Ed., Elsevier, Oxford UK, 2009, Chapter 11: Post-Blast Detection Issues

Locate explosive residue

15Forensic Investigation of Explosions (Second Edition), A. Beveridge, Ed., Taylor & Francis Ltd, CRC Press, Boca Raton FL, 2012J.T. Thurman, Practical Bomb Scene Investigation, CRC Press, Boca Raton FL, 2006

Bag and Tag evidence

• Remove portable items• Swab non-portable• Collect dust, small particles and ground samples from

epicentre• Seal into non-contaminated bags

16Aspects of Explosives Detection, M. Marshall and J. C. Oxley, Ed., Elsevier, Oxford UK, 2009, Chapter 11: Post-Blast Detection Issues

How to build a homemade bomb?And why this is important to know for an analyst.

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Why important to know this?

• Easier for analyst if he knows what he is looking for• Traceability from where the substrates came from• Prevention of future robberies

• Restrict availability of chemicals• Make improvements of ATM design

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First steps

• Information (too) easily accessible

19Screenshot from: https://de.scribd.com/s/Homemade%20explosives (25.Oct.2020)

Example of such dubious PDF's

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Examples for improvised explosives precursors

• Acetone (Internet or hardware store)• Hydrogenperoxide (Pool cleaner, Hairdressers store)• Ammonium nitrate (from fertilizer extracted)• Harder in future Swiss Parlament restricted access to

precursors for HMB. Just lower concentrations and with approval available (EU already in 2014)

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https://www.srf.ch/news/schweiz/spektakulaere-ueberfaelle-die-bancomat-sprenger-ziehen-durchs-land (27.Oct.2020)https://www.luzernerzeitung.ch/schweiz/bundesrat-will-terroristen-das-handwerk-legen-ld.1187210 (28.Oct.2020)

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Source: Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals (2018)Chapter: 2 Precursor Chemicals Used to Make Homemade Explosives

Pipe bombs as an example

• Most prepared HMB in the US

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J.T. Thurman, Practical Bomb Scene Investigation, CRC Press, Boca Raton FL, 2006, p.16, p.91 and p.123

Origins of explosives and improvised explosives

• In case of pipe bomb: Stamp• Fingerprints on bomb possible to find.• Military, commercial < improvised: difficulty in traceabillity• Military and commercial explosives strict regulations

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Forensic Investigation of Explosions (Second Edition), A. Beveridge, Ed., Taylor & Francis Ltd, CRC Press, Boca Raton FL, 2012 p.432https://www.nature.com/articles/s41598-020-69385-1

Origins of explosives and improvised explosives

• Commercial: Marking by companies• Organic explosives: Certified reference materials or from

manufacturer -> (HPLC, IR, NMR)• Generation of databases • Stable isotope ratios can be characteristic (Isotope Ratio MS)

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J.T. Thurman, Practical Bomb Scene Investigation, CRC Press, Boca Raton FL, 2006, p.16, p.91 and p.37-117Forensic Investigation of Explosions (Second Edition), A. Beveridge, Ed., Taylor & Francis Ltd, CRC Press, Boca Raton FL, 2012 p.547S.Benson et al. Journal of Forensic Sciences, Volume 55, Issue 1. 2010, p.205-212https://en.wikipedia.org/wiki/Isotope-ratio_mass_spectrometry (01.Nov.2020)

Sample Preparation and AnalysisPresentation and Comparisons of various methods

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Sample Preparation: Flowchart

28Forensic Investigation of Explosions (Second Edition), A. Beveridge, Ed., Taylor & Francis Ltd, CRC Press, Boca Raton FL, 2012 p.150

Analysing the Inorganic Fraction

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- What are we looking for?

Specific post-blast ions of common inorganic explosive mixtures• Ammonium nitrate / fuel oil mixtures• Black powder (sulfur / charcoal / potassium nitrate)• Chlorate / perchlorate / sugar mixtures

Anions

Cations

Anal. Chem. 2007, 79, 7005-7013


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- Methods & Instrumentation: Capillary Electrophoresis with Indirect UV-Detection

High voltage ensures swift separation of ions

Capillary has high surface-to-volume ratio

→ efficiently dissipates energyand small cross section

→very low current at high voltage

Handbook of Capillary Electrophoresis Application, 1997, Chapman & Hall, London


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- Principle: Capillary Electrophoresis with Indirect UV-Detection

Separation is based on the electroosmotic flow and the electrophoretic mobility of the ions

The background electrolyte contains internal standards, a buffer system, and a low concentration of UV-active species for the indirect detection, such as chromates for anions and chrysoidine for cations

Column is lined with fused-silica, which is negatively charged above pH=3 and forms a double layer with counter-ions

EOF is reversed by addition of reversal reagents like hexadimethrine bromide to facilitate easier separation of anionsComplexing reagents such as crown ethers are added to ensure proper separation of cations



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- Spectrum: Capillary Electrophoresis with Indirect UV-Detection

Is it possible to quantify ions in CE?• Concentration is proportional to normalised peak area• Peak area depends on migration velocity through detection

window→corrected by dividing peak area by migration time

• Quantification can be achieved through internal standards, well resolved and of the same properties, or an external calibration



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- Why is Quantification Interesting and Important?

Reconstruction of Bomb Composition

• Concentration ratio of present ions to each other allows conclusion about reaction equation of combustion and thereby also original composition of the bomb

• Each bomb has a specific composition that is often unique to the bomb maker → ion concentration as evidence

Trace of Explosion?

• Biggest problem of inorganic explosives analysis: many of the target ions are naturally present in the environment

• Forensic scientist has to prove that significant concentration has been found

J. Chromatogr. A, 1992, 602, 241-247Aspects of Explosive Detection-Chapter 11, 2009, Elsevier, Oxford


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- Alternative Methods & Comparison

Capillary Electrophoresis Ion Chromatography

fast analysis method (~8 min) slower than CE (~20 min)

very small injection volume (in the nL range) larger injection volume (~100 µL)

very sensitive greater separation capacity

not very robust, small changes in background electrolyte and injection change the results

robust and reliable method, small changes in solution and injection are not problematic

→ methods compliment each other well and resolve different things well→ quantitative analysis possible for both methods→ methods are based on different separation principles

Analytical Letters, 2006, 39, 639–657

Analysing the Organic Fraction

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- What Are We Looking For?

Full, intact molecule is targeted for organic explosives as the decomposition products are mostly gases

• Nitrogen-based explosives → PETN, RDX, TNT, EDGN, NG• Peroxide-based explosives → HMTD, TATP

An idea of the present substances is necessary to choose the correct analysis method• Thermal instability• Involatile substances• Difficult fragmentation patterns in MS• Solvent effects

Trends Anal. Chem. 2020, 131, 241-247


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- Methods & Instrumentation: Thin Layer Chromatography

TLC as screening technique

Requires no further purification or sample preparation

Fast, cheap and easy

Well researched, Rf-values for most organic explosives can be found in literature for a multitude of solvent systems

J Forensic Sci, 1975, 20(2), 254-256

J. Mater. Chem. 2012, 22, 2908-2914

Only small amount of sample necessaryAdvancement in plate coatings and work-up methods have made even better separations possibleRequires at least one more analysis to verify the results, as it is not a selective analysis method


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Detection method based on chemiluminescence

Only analysis of nitrate esters, nitroso-group containing, and nitro-group containing compounds→ insensitive to solvents and many matrix elements

- Methods & Instrumentation: Gas Chromatography – Thermal Energy Analyser

TEA is very sensitive, but not selective→ selectivity is provided by separation method

Compounds are identified by comparison to chromatogram of reference solution containing different explosives before and after sample elucidation, retention reference markers are added for reproducibility

J. Hazard. Mater. 2004, 106A, 1-8


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- Quantification: Gas Chromatography – Thermal Energy Analyser

J. Forensic Sci. 1995, 40(3), 406-411

Relative analysis of peak areas, one substance is normalised to 1Composition of commercially available explosives (e.g. dynamite) is unique to the manufacturer

→ ratio of DNT isomers importantInternal standard has to be added for absolute quantification


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- Problems: Gas Chromatography – Thermal Energy Analyser

J. Forensic Sci. 1996, 41(6), 975-979

DNPMT is a common polymer blowing agent that can lead to false positive results

→ verification analysis necessary

On-column injection is not possible→ unvapourisable matrix elements contaminate column

Some compound degrade at lower temperature→ GC conditions must be optimised for them to be elucidated first

Nitro-groups are very polar→ column material must have low polarity, otherwise irreversible adsorption

Only nitrogen containing explosives can be analysedJ. Chromatogr. A, 1994, 674, 309-319


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- Methods & Instrumentation: HPLC-APCI-MS/MS

Peroxide-based explosives are very labile and undergo thermal decomposition

→ GC is not suitable as separation method

Tandem MS increased reliability→ (pseudo)-molecular ions are selected by first MS detector→ are then fragmented with collision gas to obtain full mass spectrum

For trace analysis, the Selected Reaction Monitoring is preferred, which lowers the LOD

→ a couple selected m/z ions are monitored throughout the analysis

J. Forensic Sci. 2004, 49(6), JFS2003440-7


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- Alternative Methods & Comparison

GC-TEA HPLC-MS GC-MS TLC

Substances Analysed Nitrogen-containing organic explosives

Organic explosives Organic explosives Organic explosives except NC

Thermal Decomposition Yes No Yes No

Matrix Effects DNPMT contamination Very stable to contamination, dirty samples can be analysed

Problems with involatile compounds

LOD 1-10 ng per 100 µL Single MS/full spectrum2.6-33 ng per 100 µLTandem MS/SRM0.8-8 ng per 100 µL

Similar to HPLC MS very dependent on investigated explosive

~200 ng per application

Sensitivity High Medium Order of magnitude poorer than GC-TEA

Low

Selectivity Very selective method MS can be ambiguous for nitrate esters and RDX in complex matrices

MS can be ambiguous for nitrate esters and RDX in complex matrices

Unselective

forensics of explosives - eth z

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