Remote measurements of the atomic and molecular composition of solid samples have a wide range of applications, among them are the analysis of pigments in cultural heritage, detection of explosives and their precursors, analysis of geological samples, measurements for forensics, space exploration, and industrial and environmental monitoring. The most-used techniques for remote molecular analysis of solids are Raman [1
] and Laser-Induced Fluorescence (LIF) [6
] while Laser-Induced Breakdown Spectroscopy (LIBS) supplies information about the elemental composition of the environment [10
] or of the top target layers [11
The three spectroscopic methods (Raman, LIF, and LIBS) provide complementary information and they might have some common components like a laser source, beam focusing, a signal collection system, as well as spectrometers. These facts have prompted an interest and several efforts to develop a sensing device that integrates LIBS and Raman [17
] or all three techniques, as discussed in the following. Osticcioli et. al. [21
] built a combined instrument for proxy (working distance of 2.5 cm) characterization of objects related to cultural heritage. They switched manually a single laser source between 532 nm for Raman and LIBS and 266 nm for LIF, then inserted or removed various filter and changed by PC commands the monochromator gratings to get High Resolution (HR) Raman spectra or lower resolution LIBS and LIF spectra in a larger spectral interval.
The first laboratory instrument for remote sensing by the three techniques was described in [22
], aimed also at planetary exploration. This combined instrument contains a single laser source emitting at 532 nm and a single detector covering the spectral range of 535–680 nm. The Raman signal can be partially separated from LIF emission by restricting the acquisition gate in the proximity of the laser pulse arrival. The LIBS measurements with the described system suffer from a limited detected spectral range where the only lines observed from a rock material belong to Fe, Na, Ca, and Li.
The most advanced Raman-LIF-LIBS device reported until now regards the SuperCam instrument, intended to be mounted on a rover and used by NASA for the Mars 2020 mission [23
]. The SuperCam integrates many remotely-controlled instruments, among them there is LIBS excited at 1064 nm plus Raman/LIF generated at 532 nm, with the measuring range up to 7 m and 12 m, respectively. The instrument is equipped with different spectrometer channels covering certain ranges in UV, VIS, and NIR, thus providing a large number of elements detectable by LIBS.
In the present paper we describe a patent-pending ENEA system ILS for remote material characterization by Raman, LIF, LIBS, and LS techniques. The instrument is equipped with two vision systems, it is fully automatized and has also a scanning capability. Although the ILS might have many applications, some of them were already tested like forensics, characterization of rocks or objects related to cultural heritage, etc., here we describe the system itself and show the preliminary results on explosives in traces, the detection of which still remains a challenging issue [2
], particularly on remote targets [26
]. The contaminants were left on substrates as fingerprints, which are considered as the most important mechanisms for the transfer of trace amounts of explosives during bomb handling and preparation [27
The ILS instrument was initially developed as a laboratory prototype with separate controls for each device and with manual movement of the optical components mounted on micrometric slits. After optimization of the optical lay-out, the components were integrated into a fully-automatized Prototype 2 (Figure 1
), controlled remotely by home-developed software. This system had been successfully tested at the end of 2015 during one demo campaign. Next, we built Prototype 3, where the mechanical stability was improved by integrating the laser head into the rotating platform (Figure 2
). In the following, we describe and discuss the last instrument version (Prototype 3).
The ILS consists of the instrument box and the platform rotating both horizontally and azimuthally. The rotating platform carries the optical head of a high-power laser, the diode laser for measuring the target distance, two video cameras, optical components for launching the laser beams, then the telescope for the signal collection, and behind it, the multiplexer box for signal delivery. The instrument box encloses two spectrometers, various power supplies, control electronics, the motors of the scanning system, an air-cooling system, a PC server, and a WiFi transmitter/receiver. The two units are connected electrically via an umbilical cable while the two optical fibers deliver the signal to the respective spectrometers. The overall system specifications are given in Table 1
The LS measurements here are intrinsically eye-safe while LIF and Raman spectroscopy could be performed in eye-safe regime by limiting the laser energy and number of pulses. For example, according to the ANSI Standard, in eye-safe measurements with only one delivered ns laser pulse at 355 mm, the maximum permissible exposure is 5.6 mJ/cm2.
2.2. Optical System
The optical system of the ILS is schematically drawn in Figure 3
. The high-power laser (Q-smart 850, Quantel, Newbury, UK) emitted 6 ns-long pulses at a repetition rate up to 10 Hz and with a maximum energy of 850 mJ and 230 mJ at wavelengths of 1064 nm and 355 nm, respectively. Before and after the frequency conversion unit, there is a motorized slit carrying two laser mirrors each, used to bypass the nonlinear crystals and so delivering the beam at fundamental harmonics for LIBS measurements (not shown). Alternatively, the mirrors do not intercept the laser beam exiting the resonator, so the fundamental beam is converted to 355 nm and used for Raman and LIF spectroscopy. Both the UV and the IR beam are centered at the exit aperture of the laser head unit and aligned angularly to follow the same optical path.
In order to reduce the length of the rotating unit, the high-power laser beam exiting the laser head is bent by mirrors (HR 1064/355) and brought to the back side of the platform. The laser radiation then passes through the beam expander/focuser, the last lens of which has a diameter of 100 mm. The first lens of this optical unit is mounted on a motorized slit in order to adjust the focus according to the target distance and the laser wavelength. The expanded high-power laser beam is injected into the telescope’s axis by means of two dielectrically-coated mirrors that are transparent to radiation of the laser diode (650 nm) emitted by the distance-meter (DIMETIX, FLS-C10 nm). The red beam is brought along the telescope’s axis by two metallic mirrors, and the second one is placed behind the last mirror for the high-power laser beam.
The optical signal from the target is collected and focused by a Cassagrain telescope, the primary mirror of which has a diameter of 400 mm. Both telescope mirrors are coated with AlMgF in order to maximize the reflectivity in the UV spectral region. The primary mirror, together with the multiplexer box, is mounted on a motorized stage that keeps the target in focus when the working distance changed. The multiplexer box distributes the optical signal to one of the two optical fibers or to the internal camera, and it also inserts/removes the razor edge filter that cuts the UV laser radiation (not shown).
LIBS and LIF spectra are detected by the Echelle spectrometer (Mechelle 5000, Andor, Belfast, UK) equipped with an ICCD (iStar DH334T, Andor), covering the range of 200–900 nm. The quartz fiber connecting this unit with the multiplexer has an 800-µm core diameter. For collecting the intrinsically-weak Raman signal, we use a bundle containing 100-µm diameter fibers arranged at the exit into an array to match the slit of the Czerny–Turner spectrometer (iHR-320, HORIBA, Kyoto, Japan) equipped with a CCD detector (iKon-M934, Andor).
Due to the large achievable laser beam expansion (up to 10×) the minimum theoretical spot size on a target, calculated for the beam quality M2
= 2, is relatively small (Table 2
). However, for LIBS, we consider the optimal spot size as a value corresponding to the maximum plasma emission that occurrs when focusing the beam beyond the target surface. In order to collect as much Raman signal as possible, for this technique, the high-power laser beam is defocused to match the diameter of the target area imaged by the telescope on the fiber bundle. For simplicity, the same laser spot size is used for LIF, although here and in the LS measurements, the signal is collected from a smaller area, corresponding to the image of the single, large core fiber. The lateral resolution of the measurements is given in Table 2
, and it could be improved in Raman and LIF measurements by a tighter focusing of the high-power laser beam.
Software for control of the ILS instrument was developed in a Net.4 Windows environment. The program core activates and controls all the devices, which correct functioning allows launching the interactive interface to perform the spectroscopic measurements. The user’s interface on the desktop, photographed after target scanning by Raman technique, is shown in Figure 4
. The central image is continuously taken by the external camera.
The signal processing routines, like background subtraction and de-noising of the Raman spectra, as well as peak identification to classify nitro-compounds starting from the built-in database, were written in LabView and recalled by the main program. The analogue routines were written for recognition of a generic nitro-compound by LIBS. For both techniques, the areas where a nitro-compound was detected appear in real-time as red cells on the target image. In the case of Raman measurements, the name of the identified nitro-compound and the corresponding measured spectrum appear on the desktop if clicking on the specific target’s point.
The internal camera is used only in specific cases, and it is presently controlled out of the main instrument’s program.
2.4. Materials and Methods
For the first outdoor campaign, the task was to identify fingerprints of nitro-compounds left on a vehicle. In preliminary laboratory measurements, we placed, with a silicon finger, residues up to the 10th generation on black polypropylene from the internal car and on white varnished metal from the external car. The analyzed residues included different military-grade explosives: trinitrotoluene (TNT), cyclotrimethylene-trinitramine (RDX), cyclotetramethylene-tetranitramine (HMX), and pentaerythritol-tetranitrate (PETN), plus their possible precursors as analytical-grade ammonium-nitrate and urea-nitrate, plus some common interferents like dust, wax, hand-cream, diesel, lubricating oil, and grease. Except for the LIBS, performed at 1–4 points per fingerprint, the measurements regarded the central part of fingerprint left on the substrate. Furthermore, inside the second outdoor campaign, we tested a limited number of fingerprints per substance on other substrates like textile, jeans, card, and some other plastic materials.
It is well known that the number of particles and their distribution vary significantly from one fingerprint to successive prints [28
], where the transfer efficiency also depends on the adhesive properties of the substrate [30
]. For example, the first-generation fingerprint containing RDX particles might be heavily loaded, with a mass in the order of 1 mg, while already the fifth fingerprint contained a particle mass 1000-times lower [15
]. The first prints rapidly loose the large-diameter heavy particles and then the transfer efficiency to the next prints, containing mainly small and light particles, is high. For example, in [29
], it was found that the RDX mass measured for the fifth fingerprint was of 1.7 µg, while the 50th fingerprint had a mass of 0.23 µg. Furthermore, the prints did not exhibit a gradual progression from the heaviest loading to the lightest, and this might explain why we missed detecting RDX by Raman technique on some fingerprints, but recovered the signal at successive print generations.
To estimate roughly the detection sensitivity of the ILS techniques, we started from the typical RDX particle mass per unit area in fingerprints, based on the data reported in [29
]. We encircled the area of the shown fingerprint on the dimensionally-calibrated photo of 29 × 18 mm and found that the print occupied about 4.1 cm2
. The extrapolated particle mass of RDX inside the 10th fingerprint was about 1 µg, corresponding to the mean mass density of MD10
= 0.24 µg/cm2
The LS-LIF signals at a distance of 10 m were collected from a spot diameter of 3.3 mm (area 0.085 cm2
). On a non-fluorescing or weakly-fluorescing substrate, all the examined nitro-compounds excluding PETN showed their own LIF spectrum, detectable up to the 10th fingerprint, which corresponded to the mean probed particle mass of 20 ng. Furthermore, the LIF signal intensity might be improved by increasing the laser fluence inside the target area captured by the optical collection system. This could be achieved both by focusing the high-power laser beam more tightly down to a diameter of 3.3. mm or by increasing the laser energy from 16 mJ, used here, to, for example, 200 mJ. Raising the laser energy to 200 mJ in the same focusing conditions would lead to an energy density of 0.30 J/cm2
, too low to induce ablation on most of the materials. In the case of RDX, the lower damage threshold under 355-nm irradiation by ns pulses is about 1.1 J/cm2
]. However, when working on the black polypropilene substrate we noticed some sound released during the beam-target interaction for laser energies above 60 mJ, caused by rapid surface heating although the substrate remained unaltered under visual inspection. Thus, in order to avoid thermal effects on any target, in the systematic measurements we limited the UV laser energy. From the previous considerations, we might conclude that for the fluorescing nitro-compounds on a non-fluorescing substrate placed at a 10-m distance, the particles might be detected by the LIF technique (of the ILS system) down to ~10 ng without altering the target. This sensitivity is sufficient to observe high-generation fingerprints although the LIF technique, itself, cannot be considered discriminatory for nitro-compounds. On the other hand, the examined liquid organic residues were highly fluorescing, but with the emission peak close to the laser excitation wavelength, and they might be detected in much smaller quantities than the nitro-compounds.
The detection sensitivity analogue to the LIF might be deduced also for the LS measurements on dark substrates. Here, the back-scattered signal from the continuous-wave red laser could be increased by applying longer acquisition times. While surface particles increased the LS signal compared to the dark substrate, the presence of greasy residues heavily lowered the backscattered signal, if present on the bare substrate. In this way, the combination of LS-LIF measurements was extremely useful for tagging the target areas containing some surface contaminants, supplying also information about the residue state: particles or liquid/gel.
In the case of a highly fluorescing and scattering substrates like the examined white varnished metal, the presence of residues might be deduced from changes in the LS-LIF spectral intensity and distribution (for LIF). In such a case, the sensitivity, both of the LIF and the LS measurements, is more difficult to estimate because the detection of residues is based on differences in the response between the bare and the contaminated area, and not on the absolute signal intensities.
Regarding the Raman measurements, the considerations were analogous to the LIF technique, but in this case the signal was collected from a larger area, corresponding to 0.66 cm2. Assuming the presence of 1 µg of a nitro-compound in the 10th-generation fingerprints, the Raman signals of which were weak but generally observable (excluding TNT), the instrument’s detection sensitivity at a 10-m distance was of about 150 ng. This detection limit could be further improved by increasing the energy of the high-power laser a few times in the absence of an intense substrate fluorescence that could mask the weak Raman signal.
In the LIBS measurements at a distance of 10 m, the energy density on the target was 24.9 J/cm2
. This laser fluence was sufficient to create plasma on all the tested samples and it was well above the 100% ignition threshold for RDX [31
] and PETN [32
] under the analogue laser excitation. On residues of these secondary explosives the measured sum S achieved values > 11, more than six-times higher than those measured on the bare substrate. A few random missing detection points (S < 1.6) for RDX on the black polypropylene and for PETN on the white varnish could be attributed to a locally low particle presence inside the small laser spot. TNT was tested only on the white varnish and it was always detected by LIBS, although with S values < 5. Given that this residue was not observed by Raman, we believe that its presence on the target was lower than expected because of its relatively easy evaporation.
Ammonium-nitrate and urea-nitrate, which are not explosives but oxidizers, are less sensitive to shock than the explosives. For these nitrates the determined S values were always <7. In particular, very low S values in some prints of ammonium-nitrate compared to the bare black substrate might be attribute to the hygroscopic properties of this substance, where the presence of the absorbed water highly reduced the plasma emission intensity. Beside the very low plasma emission intensity when the substrates were covered by liquid/gel residues, it is interesting to note that in the presence of dust contamination, the S values were equal to or smaller than on the clean substrate. This indicates that the increased LIBS intensities of the H I and N I (the elements also present in air) lines on areas containing explosives or oxidizers were not caused by an improved laser–particle interaction compared to the laser-substrate, but to the plasma temperature increase due to the laser-induced micro-explosions and endothermic reactions [31
Considering that the laser spot in the LIBS measurements was of only 1.3 mm in diameter and assuming an uniform particle distribution, the 10th-generation fingerprint would contain about 3 ng of particles inside the probed spot. This mass could be approximately considered as the detection limit reached on the tested samples. In the future, the effect of higher laser energy density on the detection sensitivity by LIBS could be examined, but taking into account that a higher incident energy might increase the contribution of the target to the plasma produced on a surface residue.
The herein applied approach for the detection of nitro-compounds by LIBS, based on the spectral differences after the first and the second laser pulse at the same position, was much simpler and straightforward than the methods proposed in [15
]. Our approach was not affected by missing plasma stoichiometry [36
] and it reduced the influence of substrate material on the classification results. Furthermore, this method exploited only the emission lines in the red-NIR spectral region, which are less sensitive to the atmospheric attenuation than UV spectral features, and this is a promising factor for the stand-off LIBS detection of trace explosives at longer distances than reported in this work.
The herein described ILS system allowed scanning of remote targets, producing 2D thematic maps. By simultaneous LS and LIF and measurements it was possible to observe changes of the material from one point to another, to identify areas with the presence of surface residues and to have indications if the surface contaminants were liquid or in the form of particles. Visualization of the surface details was also possible by the internal camera, which works with a high magnification optical system. Raman probing allowed recognizing the molecular composition of residues (if present in a sufficient quantity) and tagging the areas where nitro-compounds were identified. Similarly, target scanning by LIBS tagged areas containing a generic nitro-compound. The estimated detection sensitivities for nitro-compound particles for a target distance of 10 m were in the order of 10 ng, 100 ng, and 1 ng for LS-LIF, Raman, and LIBS, respectively.
The ILS instrument is fully automatized, equipped with a user-friendly software interface controlled via wireless communication. These characteristics permits operating also in hazardous conditions, either from a fixed position or mounting the system on a robotic vehicle. Except for LIBS, the measurements could be performed also in eye-safe regime, but with reduced detection sensitivity. The tests were performed up to a distance of 30 m and using less than 10% or 40% of the available laser energy at 355 nm and 1064 nm, respectively. Consequently, it could be expected that the instrument’s maximum working distance might largely exceed 50 m.
In this work we focused on the instrument performances and its ability to detect specifically the nitro-compounds. Nevertheless, by recalling different data processing algorithms, the ILS could produce many other thematic maps, both molecular and elemental, among them for example: recognizing the polymer type or producing a distribution of heavy elements inside the scanned area.
The ILS is a highly-performing scientific instrument and it is intended as a starting point for the development of other simplified instruments for some specific applications, such as characterization of cultural heritage objects, forensic, environmental monitoring and many others.