Lim Chin Chin*, Chia Poh Ling, Su W.J, Michael Tay Ming Kiong
Joint 9th ISADE/4th Finex International Symposium on the Analysis and Detection of Explosives,
Paris, France, July 2007 (oral presentation)
Objective: To present results of a study on the trace level detection of common ingredients of low explosives and their combustion products using Raman microspectrophotometry.
(A) General comparison of oxidisers and combustion products (solid phase)
The Raman spectra of 12 common oxidisers and 19 of their combustion products were compared. Inorganic salts comprising cations such as NH4+, Na+, K+, Ba2+, and anions such as NO3-, ClO3–, ClO4–, NO2–, Cl–, CO32- and SO42-, were examined. The spectra were unique for different anions having the same counter-cation. In solid phase, the counter-cations affect the Raman shifts of the anions, allowing each ion pair to be uniquely identified.
(B) Analysis of aqueous solutions
As water is used to extract post-blast debris for analysis, aqueous solutions of single salts and mixture of salts were analysed to investigate effects of hydration of ions on Raman shifts. Shifts were observed in the peaks between solid state and dissolved salts. The spectra of the latter showed higher base-line, peak broadening, loss of small peaks and lower resolution. Nonetheless, aqueous solutions of salts with a common cation and different counter-anions could still be differentiated. However, different cations having the same anion were not distinguishable ; Raman shifts were of the generic hydrated anions.
(C) Analysis of mixed salts recrystallised from aqueous solutions
Ion exchange upon crystallisation of 13 mixed salts from the evaporation of aqueous solutions was studied. Ion exchange, strongly influenced by ionic radii of the competing ions in solution, was observed in some cases but not in others.
Conclusion: Raman microscopy is a powerful non-destructive technique that can easily and quickly identify oxidisers and their combustion products at low detection limits. It can also conclusively identify the anions in aqueous solutions with minimal sample preparation.