Methods and apparatus for remote Raman and laser-induced breakdown spectrometry

An spectrometer including Raman and LIBS spectroscopy capabilities is disclosed. The spectrometer includes a laser source configurable to produce a lased light directable towards a target substance, the laser source having a single wavelength and having sufficient power to cause a portion of the target to emit Raman scattering and sufficient to ablate a portion of the target substance to produce a plasma plume. A separate remote light collector is optically configurable to collect light emitted from the portion of the target emitting Raman scattering and from the portion of the target producing the plasma plume. A filter is optically coupled to the remote light collector to remove reflected light and Rayleigh-scattered light, and a spectroscope is optically coupled to the filter and configured to separate the collected and filtered light into a frequency spectrum comprising a Raman spectrum and a laser-induced breakdown spectrum. Finally, an electronic light sensor is used to record the frequency spectrum.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to spectrometry and more particularly to methods and apparatus for remote Raman and Laser-Induced Breakdown Spectrometry.

BACKGROUND OF RELATED ART

Spectrometry is a technique used to assess the concentration or amount of a given material. The apparatus that performs such measurements is a spectrometer or spectrograph. Spectrometry is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them. Spectrometry is also often used in remote sensing applications, such as explosive detection.

One example method of spectrometry is Raman spectroscopy. Raman spectroscopy uses the inelastic scattering of light to analyze vibrational and rotational modes of molecules. Because vibrational information is specific to the chemical bonds, atomic mass of the atoms in the bond and symmetry of molecules, Raman spectroscopy provides a “fingerprint” by which the molecule can be identified.

Another example method of spectrometry is Laser-induced breakdown spectrometry (LIBS). LIBS uses a high-power laser focused onto the surface of a sample to produce plasma. Light from the plasma is captured by spectrometers and the characteristic spectra of each element can be identified, allowing concentrations of elements in the sample to be measured.

DETAILED DESCRIPTION

The following description of the disclosed examples is not intended to limit the scope of the invention to the precise form or forms detailed herein. Instead the following description is intended to be illustrative of the principles of the invention so that others may follow its teachings.

Raman spectroscopy typically uses either a continuous wave (CW) or a pulse visible laser of modest average power (e.g., approximately 100-700 mW/cm2) to identify the molecular fingerprint of the sample from its Raman spectrum. For a continuous wave laser based remote Raman system, there are generally two significant issues: interference of the high-ambient light background during the day; and long-lived fluorescence with the Raman spectra of the sample. However, as described herein, by utilizing a pulsed laser system and gated receiver, these limitations may be overcome. Pulsed Raman spectroscopy offers two important benefits: the ability to discriminate against unwanted light; and the ability to discriminate against long-lived fluorescence emissions from the sample.

In general, LIBS uses a pulsed laser having a typical wavelength of 1064 nm, and a high peak power (e.g., >1 GW/cm.sup.2) for ablating material from the surface of a sample and to probe elemental composition. During LIBS, a small amount of the target is ablated and atomized, and the resulting atoms are excited to emit light. The emitting elements are identified by their unique spectral peaks, and the process yields semi-quantitative abundances of major, minor, and trace elements simultaneously. Advantageously, laser ablation profiles through dust and weathering layers, meaning that the LIBS technique is effective in cleaning sample surfaces. By repetitive sampling with a high intensity laser at the same spot, the LIBS technique permits ablation through weathered surfaces to reach the underlying bulk rock.

Both remote Raman and remote LIBS spectroscopic techniques interrogate samples using pulsed laser beams and both employ dispersive spectrographs over over-lapping spectral ranges with approximately the same spectral resolution. Therefore, as described herein, it is possible to combine the two techniques into one system. In particular, at low laser power density at the sample, one obtains the Raman spectra and at high laser power, with high optical power density, one can also generate the LIBS plasma spectra and record the LIBS spectra from the same material. Both the Raman and LIBS spectra can be recorded simultaneously with a focused laser where the area falling in the high energy density region produces the LIBS spectra and molecules exited by the edges of the focused beam corresponding to a low energy density region generates the Raman spectra.

By combining remote Raman spectroscopy and remote LIBS in a single apparatus a more complete analysis of a sample in terms of mineralogy and elemental composition is possible. For example, Raman spectroscopy can determine whether the sample is calcite or aragonite, both of which have the same chemical composition but different mineral structures that often indicate different geological formation processes. LIBS analysis is sensitive to typical cationic elements and is also capable of remotely detecting hydrogen, carbon, oxygen, and sulfur. Raman spectroscopy mostly distinguishes mineral structure and anionic species from Raman active lattice modes of the mineral, and can provide only indirect information about cations from internal vibrational mode of polyatomic cations that are sensitive to cationic composition. Usually Raman active lattice modes are much more sensitive to composition and polymorphism than the internal modes of vibrations.

Additionally, for the detection of organic compounds, Raman spectroscopy is a more desirable technique than LIBS as it can distinguish between various hydrocarbons such as benzene, naphthalene, methane, and their various chemical isomers (e.g., o-, m-, p-xylene, etc.); and different kind of organic molecules, such as protein, lipids, amino acids, complex molecules such as pigments, mycosporines, etc.

Moreover, LIBS can detect small amounts of impurities in a bulk sample, but the trace analysis is difficult with Raman spectroscopy because the Raman signal is proportional to the number of molecules excited by the laser in the sample. A few molecules of impurity produce a very small Raman signal compared to the signal produced by the host bulk sample.

Accordingly, there is a desire to combine the two techniques and to develop various methods and apparatus that include a single instrument capable of analyzing a sample from a remote distance and capable of obtaining both Raman and LIBS spectra. The combined Raman and LIBS methods and apparatus described herein may be used for a variety of applications, including, for example, in the remote identification of explosives, explosive residue, chemical, minerals, gases, fume clouds, and/or other chemicals of interest. Additionally, by providing for a remote sensing capability, the described apparatus and/or operator may be located a distance away from the sample, providing for a greater range of detection and/or safety.

Referring now toFIG. 1, a diagram illustrating an example combined Raman spectroscopy and LIBS apparatus10is shown. The example apparatus10generally includes a laser source12, an emitted light collector14, and a spectrograph16. Specifically, in this example, the laser source12emits a lased light such as, for example, a laser beam18. The example laser beams18may travel through a beam expander20to focus and shape the laser beam as desired. The resultant laser beam18is then directed toward the intended remote target22. In this example, a plurality of prisms24are used to direct the laser beam18, however, any suitable object, device, and/or means of directing the beam18may be utilized including, for example, a splitter, mirror(s), etc. In each example, the laser beam18is of sufficient power to cause the generation of an emission signal26at the target22having at least one of Raman excitation and LIBS plasma formation as described herein.

As noted, Raman excitation generally relies upon inelastic scattering, or Raman scattering, of the laser beam18, such as, for instance, the vibrational, rotational, and/or other low-frequency excitation modes in the target22. In particular, the laser light18interacts with molecular vibrations or other excitations in the target22, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives detectable information (e.g., the signal26) about the Raman active vibrational and rotational modes in the target22.

Similarly, as described, LIBS operates by focusing the laser beam18onto a small area at the surface of the target22. During LIBS, the laser beam18is of sufficient power to ablate at least a portion of the target22to generate a detectable plasma plume. In particular, as the plasma plume expands and cools, the plume emits atomic emission lines (e.g., the signals26) characteristic of the elements comprising the target22.

After the laser beam18strikes the target22, the emitted signals26(e.g., at least one of Raman and/or LIBS) excited by the laser beam18are collected by the collector14. The example collector14may be any suitable collector, including, for instance, a telescope. In this example, the telescope includes a beam finder27which may be used to align the telescope with the beam18. The collected signals26are then optically directed into the spectrograph16for processing. In at least one example, however, the collected signals26are first passed through a filter28, such as, for example, a notch filter to remove the reflected and Rayleigh-scattered light from the signal26. Additionally, in this example, the signal26is focused into the spectrograph16by passing through at least one lens30. The example signal26may then be processed by the spectrograph16. In particular, the spectrograph16is configured to separate the signal26into a detectable frequency spectrum. For example, to detect the generated frequency spectrum, the example apparatus10includes an intensified charge-coupled device (ICCD)32optically coupled to the spectrograph16. Once detected by the ICCD32, the frequency spectrum may be processed by any suitable process, including, for example, by spectrum analysis software.

In one example of the integrated remote Raman and LIBS apparatus10, the laser source12is a frequency doubled mini Nd:YAG laser source, such as a Model ULTRA CFR, Big Sky Laser, 532 nm, 20 Hz. The use of a double pulse separated, by, for example approximately 0.15.mu.s to approximately 1.0.mu.s may enhance the LIBS signal by at least a factor of 10. The example 532 nm pulsed laser beam is focused to a spot (e.g., 300.mu.m diameter) on the target22placed a distance (e.g., nine meters) away with a 10.times. beam expander20. For simple Raman measurements the laser power at 532 nm may be electronically adjusted to approximately 25 mJ/pulse, and for combined LIBS and Raman measurements the laser power may be increased to approximately 35 mJ/pulse.

In still other examples, the laser source12may produce a lased light with any suitable wavelength, such as, for example, 248 nm, 266 nm, 532 nm, 785 nm, and 830 nm for Raman spectra, and 532 nm and 1064 nm for LIBS spectra. Still further, it will be appreciated that a Raman spectra may first be produced at a lower laser power and/or by defocusing the laser beam, thereby giving a lower optical power density which is not enough to decompose the sample. The LIBS spectra may then be obtained after the Raman measurement on the same target with a higher laser power and/or a focused laser beam, thereby giving a high enough optical power density to ablate the sample.

In one example for the combined Raman and LIBS measurements, the signals26excited by the laser source12are collected with a 203-mm diameter reflecting telescope in 180-degree geometry, such as, for example, a Meade LX200R Advanced Ritchey-Chrétien, 203 mm clear aperture, f/10. In other example, the laser source12may be collected with any suitable light collector, including, for example, a suitable camera lens. Additionally, in one example, the collected signal26passes through a 532-nm holographic super-notch filter (NF) to remove the reflected and Rayleigh-scattered laser light, and enters the spectrograph16, which in this example is a Kaiser F/1.8 HoloSpec transmission-grating spectroscope with a 100-μm slit coupled to an intensified CCD detector, such as a Princeton Instruments, PI-MAX. The HoloSpec spectrometer measures the Raman spectra from 70 cm−1to 4500 cm−1when excited with a 532 nm laser. The grating covers the spectral region between 534 nm to 699 nm, which is sufficient for identifying some of the major atomic emission lines of measured targets22from their LIBS spectra. It will be understood, however, that the components and/or operating parameters of the example apparatus10may be suitably changed, combined, and/or configured as desired.

FIG. 2is a graph illustrating one example of both Raman, and Raman/LIBS spectra as measured by the example apparatus10. In particular,FIG. 2shows a Raman spectra200and a combined Raman and LIBS spectra250of calcite (CaCO3)200A,250A and of Gypsum (CaSO4.2H2O)200B,250B located at approximately nine meters in air and in the 534-615 nm wavelength range. In this example, the Raman spectra200of these samples were excited with 25 mJ/pulse of the 532 nm laser and accumulated for one second in the gated mode with a 2 μs gate. As illustrated, the lattice modes202of calcite at 155 cm−1and 282 cm−1are clearly visible, along with internal modes204of carbonate ions at 711 (ν4) cm−1, 1085 (ν1) cm−1, and 1434 (ν3) cm−1. In addition, a combination mode206of carbonate ions at 1748 cm−1and the symmetric stretching modes208of atmospheric O2and N2, respectively at 1556 cm−1and 2331 cm−1are also detected.

In the illustrated Raman spectrum of gypsum200B, the strongest Raman peaks210observed at 1006 cm−1originate from the symmetric stretching vibrations of the SO42−ion, ν1(SO4), and its position depends on degree of hydration. The other internal mode of vibration of sulfate ions212appear at 1136 (ν3) cm−1, 415 cm−1and 495 (ν2) cm−1, and 621 cm−1and 671 (ν4) cm−1are clearly visible.

In the combined LIBS and Raman spectra250of calcite250A and of gypsum250B the Raman lines are marked with the letter “R”. In the example spectrum of calcite250A, the 1085 cm.sup.-1 Raman line of carbonate252appears only as a weak line. In the example combined LIBS and Raman spectra of gypsum250B, most of the prominent Raman lines of sulfate254are clearly visible indicating that the LIBS spectrum is produced only by a few hot spots in the focused laser beam. As both calcite and gypsum contain Ca cations, the LIBS lines250inFIG. 2originate from Ca I excited states. In the combined LIBS Raman spectrum of calcite250A the LIBS lines256at 588.99 and 589.59 nm indicate presence of trace amount of Na ion in the calcite sample.

Similarly,FIG. 3is a graph illustrating one example of LIBS spectra as measured by the example apparatus10. Specifically,FIG. 3shows an example of a remote LIBS spectrum300of minerals magnetite (Fe3O4)302, hematite (Fe2O3)304, α-quartz (α-SiO2, Qz)306, and forsterite (Mg2SiO4, Fo)308, respectively. The example graph is limited to the 540-690 nm spectral range, the respective target was located approximately nine meters away, and was excited with a 532 nm laser pulse of 35 mJ/pulse. The LIBS illustrated spectra300of magnetite302and hematite304and are dominated by the Fe I emission lines310. Both of these iron oxides contain a few ppm of Na as indicated by the presence of Na I emission lines312at 589.0 and 589.6 nm.

Additionally, the remote LIBS spectra of Fo308and Qz310show strong emission lines of Si314at 634.7 and 637.1 nm, and weak emission lines of Na316and Li318indicate the presence of these elements in trace amounts. The Fo LIBS spectrum308contains strong emission lines of Mg320at 552.8 nm and a weak line322at 571.1 nm. A number of weak Fe emission lines324are also observed in the LIBS spectrum of Fo indicating that this sample is indeed Mg-rich olivine (Fo92) containing .about.8% of iron.

In another example data set reproduced as a graph400inFIG. 4, another operation of the example apparatus10is illustrated. In this example, the apparatus10was used to detect the Raman shift of calcite at a distance of approximately ten meters, using only a Raman spectrum. In a first operating example410, the apparatus10was operated in a continuous wave mode with room lights on and having an average room light of 13 W/m2. In a second operating example420, the apparatus10was again operated in a continuous wave mode with the room lights turned off. Finally, in a third operating example430, the apparatus10was operated in a gated mode, with the room lights turned on and having an average room light of 13 W/m2.

In another example use of the apparatus10illustrated as a graph500inFIG. 5, the apparatus10was used to detect the Raman shift of a homemade explosive chemical at a distance of approximately fifty meters. The laser12of the example apparatus10was operated at 532 nm, 20 Hz, 30 mJ/pulse, and included a 50 ns gate width. In a first example510, the apparatus10was utilized to detect potassium perchlorate (KClO4). In a second example520, the apparatus10was utilized to detect potassium nitrate (KNO3). Finally, in a third example530, the apparatus10was used to detect ammonium nitrate (NH4NO3). In each example, the apparatus10provided clear, sharp peaks for chemical identification, with a high signal-to-noise ratio.

In yet another example use of the apparatus10illustrated as a graph600inFIG. 6, the apparatus10was used to detect the Raman shift of various chemicals through a container at a distance of approximately fifty meters. In a first example610, the apparatus10was utilized to detect water (H2O). In a second example620, the apparatus10was utilized to detect isopropyl alcohol (2-propanol). Finally, in a third example630, the apparatus10was used to detect acetone (OC(CH3)2). In each example, the apparatus10was able to detect the various liquids through plastic and/or glass containers.

In still another example use of the apparatus10illustrated as a graph700inFIG. 7, the apparatus10was used to detect and distinguish between the Raman shift associated with similar chemical, during daytime and at a distance of approximately fifty meters. In a first example710, the apparatus10was used to detect nitrobenzene (C6H5NO2). In a second example720, the apparatus10was utilized to detect ethylbenzene (C6H5CH2CH3). Finally, in a third example630, the apparatus10was used to detect benzene (C6H6).

As illustrated in a graph800inFIG. 8, in another example, the apparatus10was used to detect gypsum at approximately fifty meters. In this example, the apparatus10was used to detect the Raman shift of both the target gypsum and/or the ambient atmosphere. In particular, in a first example810, the apparatus10was used to detect both the ambient atmosphere and the gypsum target. In a second example820, the apparatus10was used to detect only the ambient atmosphere. Lastly, in a third example830, the apparatus10was used to detect only the gypsum target.

In another example use of the apparatus10illustrated as a graph900inFIG. 9, the example apparatus10was used both in a single pulse mode910, and in a one-second, twenty pulse mode920. In this example, the apparatus was used to detect the Raman shift of cyclohexane (C6H12) at a distance of approximately fifty meters. As is illustrated, the detected signals910and920are proportional to the number of laser pulses.

In still another example use of the apparatus10illustrated as a graph1000inFIG. 10, the example apparatus10was used to detect the Raman Shift of naphthalene (C10H8). In a first example1010the apparatus10was placed approximately fifty meters from the target naphthalene. In a second example1020the apparatus10was placed approximately one hundred meters from the target naphthalene. As is shown, the detected signals1010and1020are substantially similar in shift detection and proportional in intensity.

In another example data set reproduced as a graph1100inFIG. 11, the example apparatus10was used to detect the Raman shift of a target at a distance of approximately one hundred meters. In a first operating example1110, the apparatus10was used to detect ammonium nitrate (NH4NO3). In a second operating example1120, the example apparatus10was used to detect Potassium perchlorate (KClO4).

In another example data set reproduced as a graph1200inFIG. 12, the example apparatus10was used to detect the Raman shift of an organic target at a distance of approximately one hundred meters. In a first operating example1210, the apparatus10was used to detect nitromethane (CH3NO2). In a second operating example1220, the example apparatus10was used to detect Nitrobenzene (C6H5NO2) and is graphed at ×0.2.

In an example data set reproduced as a graph1300inFIG. 13, the results of the operation of the example apparatus10is illustrated. In this example, the apparatus10was used to detect the LIBS spectrum of Trinitrotoluene (TNT) (C.sub.6H.sub.2(NO.sub.2).sub.3CH.sub.3). In this example, the LIBS spectrum was produced with a single pulse excitation of a 532 nm laser powered to 35 mJ. The target was placed at approximately nine meters from the apparatus10. Additionally, in this example, the gate size of the detector32was 8.mu.s, and the produced laser spot on the target TNT was approximately 629.mu.m. As can be seen from the graph1300, the notch filter28creates an area1310of filter (e.g., zero) data.

In an example data set reproduced as a graph1400inFIG. 14, the result of the operation of the example apparatus10is illustrated. In this example, the apparatus10was used to detect the LIBS spectrum of eight percent Cyclotrimethylenetrinitramine (8% RDX) on silica. In this example, the LIBS spectrum was produced with a single pulse excitation of a 532 nm laser powered to 35 mJ. The target was placed at approximately nine meters from the apparatus10. As can be seen from the graph1400, the notch filter28creates an area1410of filter (e.g., zero) data.