Detection of Bulk and Explosive Traces on Metallic Surfaces Using Laser Induced Breakdown Spectroscopy Technique Abdul Kalam Shaik 1 , and Venugopal Rao Soma 1,* 1 Advanced Center of Research in High Energy Materials (ACRHEM), University of Hyderabad, Telangana, 500046, India e-mail: soma_venu@yahoo.com, soma_venu@uohyd.ac.in; phone +91 40 2313 8811 Discrimination and labeling of high energy materials (HEMs)/explosives is an immediate concern to avoid their illegitimate transport, thaw the terrorist activities and safe guard the common citizens [1]. Examining the traces poses a true challenge due to the low sampling amount and low vapour pressures associated with the HEMs. In alternative to lab based detection techniques several laser-based spectroscopy techniques including laser induced breakdown spectroscopy (LIBS) are developed as they can provide in-situ real-time monitoring/detecting explosives in bulk/trace forms especially in standoff mode [2][3]. Though laser-induced breakdown spectroscopy (LIBS) can potentially be implemented to investigate the explosive in bulk as well as residues several limitations restrict its widespread use. Further, nanosecond pulses are widely used for investigating the distant targets (~100 m), these suffer with beam intensity distortion while propagating in air, thereby resulting in a decreased intensity at the focal volume [4]. In this study, we have utilized femtosecond (fs) filaments that deliver substantial intensity at remote locations (with less beam damage) were utilized to investigate explosives in bulk (150 mg pellets) as well as traces/ a layer of residue. Control of the onset, length and in turn the energy in filament reservoir is an added advantage of fs filaments [5]. The fs filaments were generated through focusing the fs pulses (~50 fs pulse duration and ~2 mJ energy) delivered by a Ti:sapphire amplifier laser system (M/s Coherent, Libra, ∼4 mJ, 1 kHz) using a combination of two lenses. L1 is a plano-concave lens (PCV) of −50 cm focal length, and L2 is a plano-convex lens (PCX) of focal length 100 cm. Figure 1 depicts the schematic of fs ST-FIBS setup where femtosecond pulses were focused to form a filament of ∼30 cm length at 6.5 m away (as measured from L2). The emissions emanated during the laser matter interaction are collected through a Schmidt Cassegrain Telescope (SCT) recorded with ANDOR Mechelle spectrograph 5000 (resolution 0.05 nm @500 nm) at 6.5 m in standoff mode. In the same setup we have earlier examined metals (Al, Cu) alloys (brass, steel) bimetallic targets (Ag@Au, Ag@Cu with varying weight percentages) and a set of six nitro- imidazoles (HEMs in bulk 150 mg pellets) [6,7]. All these were distinguished by analyzing FIBS spectra of with principal component analysis (PCA). While investigating the residues residing on various surfaces, the emission spectra of these residues can be potentially influenced/altered by the nature, composition of the targets owing to the partial ablation of the target itself [8]. As an example, when a 1 mg of CL-20 residue was examined on brass target, the obtained FIBS spectra had the spectra signature of both CL-20 and brass. Thus, the obtained LIBS spectra contain spurious spectral signals which have contribution from the residue as well as the entrained substrate [2]. In this context, to mitigate the substrate contribution femtosecond (fs) pulses can be potentially utilized in the form of filaments [9]. Thus, we have investigated the residues of common military explosives namely RDX, HMX and TNT. The residues of these explosives were prepared at different known concentrations