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Noninvasive Detection of Concealed Explosives: Depth Profiling through Opaque Plastics by Time-Resolved Raman Spectroscopy Ingeborg E. Iping Petterson,† María Lopez-Lopez,‡,§ Carmen García-Ruiz,‡,§ Cees Gooijer,† Joost B. Buijs,† and Freek Ariese*,† †
Biomolecular Analysis and Spectroscopy, LaserLaB, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands ‡ University Institute of Research in Police Sciences, Planta Piloto de Química Fina, University of Alcala, Ctra. Madrid-Barcelona Km. 33.600, 28871 Alcala de Henares, Madrid, Spain § Department of Analytical Chemistry, Faculty of Chemistry, University of Alcala, Ctra. Madrid-Barcelona Km. 33.600, 28871 Alcala de Henares, Madrid, Spain ABSTRACT: The detection of explosives concealed behind opaque, diffusely scattering materials is a challenge that requires noninvasive analytical techniques for identification without having to manipulate the package. In this context, this study focuses on the application of time-resolved Raman spectroscopy (TRRS) with a picosecond pulsed laser and an intensified charge-coupled device (ICCD) detector for the noninvasive identification of explosive materials through several millimeters of opaque polymers or plastic packaging materials. By means of a short (250 ps) gate which can be delayed several hundred picoseconds after the laser pulse, the ICCD detector allows for the temporal discrimination between photons from the surface of a sample and those from deeper layers. TRRS was applied for the detection of the two main isomers of dinitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene as well as for various other components of explosive mixtures, including akardite II, diphenylamine, and ethyl centralite. Spectra were obtained through different diffuse scattering white polymer materials: polytetrafluoroethylene (PTFE), polyoxymethylene (POM), and polyethylene (PE). Common packaging materials of various thicknesses were also selected, including polystyrene (PS) and polyvinyl chloride (PVC). With the demonstration of the ability to detect concealed, explosives-related compounds through an opaque first layer, this study may have important applications in the security and forensic fields.
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he detection and identification of explosives and their associated compounds in different environments is a problem of critical interest for security and forensic diagnostics. Many techniques have been investigated for this purpose, but the majority are not ideal for explosives detection in that they are invasive or require sample preparation.1 Raman spectroscopy (RS) is ideal for the rapid detection of potentially hazardous substances because it is noninvasive, and vibrational spectra provide a “molecular fingerprint” that facilitates chemical identification.2 5 Portable surface enhanced Raman spectroscopy (SERS) probes2,3,6,7 and other fieldable Raman spectrometers4,5,8 10 have been used previously for forensic and safety applications. However, conventional RS has its own limitations as it is not suitable for all sample types and scenarios. The Raman signal is generally weak, and enhancement by SERS or resonance is not always achievable. Also, the spectra are often overwhelmed by fluorescence background, and it is not possible to focus within opaque, diffusely scattering sample materials.11,12 When the target substances are obstructed behind such a diffuse scattering material, the spectra will be heavily dominated by Raman photons from the surface layer and provide little information on the compounds of interest. Advanced analytical Raman techniques have been developed in recent years to overcome many of these shortcomings. Depth r 2011 American Chemical Society
resolved Raman spectroscopy techniques such as spatially offset Raman spectroscopy (SORS)13 and time resolved Raman spectroscopy (TRRS) via a Kerr gate14,15 or intensified chargecoupled device (ICCD) camera16,17 hold particular promise in this respect. These techniques allow for depth analysis through diffuse scattering, opaque materials by providing an increased relative selectivity for photons from deeper layers within a sample. This has many practical applications in a variety of fields, including biomedical, pharmaceutical, archeological, art restoration, as well as security and forensic science. In this last context, SORS has recently been implemented for the noninvasive detection of sucrose and hydrogen peroxide concealed in plastic bottles.18 20 Other techniques have also been used to detect bulk substances directly through packaging materials, particularly for pharmaceutical applications. A wide-area illumination Raman scheme was used to average out surface layer irregularities,21 but the spectra were still heavily dominated by the polymer bottle. NIR absorption measurements have also been reported,22,23 but that approach is limited to clear liquids in small bottles Received: July 14, 2011 Accepted: October 3, 2011 Published: October 03, 2011 8517
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Figure 1. Illustration of the concept of TRRS for a two-layer sample in backscattering geometry using a gate of 250 ps. The delay can be adjusted to detect photons predominantly from the second layer.
(short optical path length) and a reference bottle of the same material must be available. In TRRS, a pulsed laser and a delayed, relatively short detector gate width are used to discriminate just the photons of interest, allowing for greater depth selectivity. The schematic concept of TRRS is shown in Figure 1. For a diffuse scattering two-layer sample in a backscattering geometry, the time profiles show that surface Raman photons arrive at the detector earlier and with a relatively narrow distribution, whereas the Raman signal emigrating from deeper within the sample comes later, has a broader distribution in time, and is also generally less intense than that from the surface.15 Such profiles were first determined by means of a Kerr-gated setup with a 4 ps time resolution and could be reproduced quite well with Monte Carlo simulations.24 Alternatively, an ICCD detector can be used for TRRS measurements, which has the advantage of being independent of laser pulse width, irradiance, or wavelength and can be operated at a 76 MHz repetition rate.25 The temporal resolution is lower (typically 250 ps), but this is not necessarily a disadvantage since it ensures a larger overlap with broadened time profiles. The Raman signal from deeper within a sample is the result of multiple scattering events due to the random walk of photons in a scattering medium. Therefore, the optical pathways of these photons are many factors longer than and can be discriminated from photons from the surface by means of a precisely timed measuring gate. The opening and closing of the detector can be delayed several hundred picoseconds after the laser pulse to correspond with the arrival at the detector of the retarded photons of interest. Although it differs per material, rough estimates can be made of approximate measurement depth based on delay time depth associations modeled or measured with similar scattering materials. For example, when measuring through white polymer materials, a Raman signal is typically delayed on the order of ∼100 ps per millimeter of surface layer thickness.17 The relatively short gate width of time-resolved measurements also considerably reduces interference from fluorescence background; with our time gate, an optimal fluorescence rejection is obtained for interferences with a fluorescence lifetime of 3 10 ns.25 This makes TRRS a powerful tool for the detection of substances obstructed by a broad variety of opaque first layers, including fluorescent materials.
In this work, spectra from the two main isomers of dinitrotoluene, 2,4-dinitrotoluene (2,4-DNT), and 2,6-dinitrotoluene (2,6-DNT) were measured by TRRS through first layers of different, diffusely scattering white plastic materials of various thicknesses. Technical grade DNT is usually manufactured by nitration of toluene with nitric acid in the presence of concentrated sulfuric acid, resulting in a mixture comprised of 76% 2,4-DNT, 18% 2,6DNT, and a small percentage of the other four isomers (2,3dinitrotoluene, 2,5-dinitrotoluene, 3,4-dinitrotoluene, and 3,5dinitrotoluene).26 28 Dinitrotoluenes are commonly used in the production of dyes, of polyurethane foams in the bedding and furniture industries, and in automobile air bags.29 Of particular relevance for security is the association of DNTs with some hazardous and explosive materials. For example, they are used in the munition industry as a modifier for smokeless powders, as a plasticizer in propellants, and are products of degradation and impurities in the synthesis of the explosive 2,4,6-trinitrotoluene (TNT).29 33 Because of structural similarities with other nitroaromatic explosives, the presence of DNTs is an excellent indicator of potential hazards and has been previously used for the localization of landmines.6,34 In order to test the versatility of the detection technique, other explosives-associated compounds such as ethyl centralite (1,2diethyl-1,3-diphenylurea), diphenylamine, and akardite II (1methyl-3,3-diphenylurea) were also detected through a 5.0 mm thick layer of polyethylene. These compounds are used as stabilizers in explosive materials making them an appropriate complement to study in addition to the DNT compounds.
’ EXPERIMENTAL SECTION First Layers (Polymer Materials). First layer materials were opaque, white polymer materials of different thicknesses, including polytetrafluoroethylene (PTFE), 3.0 mm; polyethylene (PE), 3.0 and 5.0 mm; polyoxymethylene (POM), 3.0 mm; polystyrene (PS), 0.5 mm; polyvinyl chloride (PVC), 1.0 mm. PTFE, PE, and POM were cut to desired thicknesses from stock sheets. PS and PVC were cut from white plastic containers and packaging materials commonly found in the supermarket. Their thicknesses were measured with a hand-held caliper. 8518
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Analytical Chemistry Second Layers (Explosives Materials). 2,4-DNT (97% m/m) and 2,6-DNT (98% m/m) were purchased from Sigma-Aldrich (Zwijndrecht, The Netherlands). Ethyl centralite, diphenylamine, and akardite II samples were kindly provided by Jeroen Carol-Visser of TNO Defense, Safety and Security (Rijswijk, The Netherlands). Sample Preparation. The basic sample design consisted of a two-layer system: a first layer of a block of polymer of a known thickness clamped in front of a second layer of a powdered compound in a 1 cm quartz cuvette. The time associated with photon migration through the clear quartz walls is negligible relative to the temporal width of the detector.
Figure 2. Instrumental setup for TRRS with an ICCD camera.
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Instrumentation. The experimental setup, shown in Figure 2, includes a frequency-doubled Nd:YVO4 laser (532 nm; Coherent type Verdi-V18, Santa Clara, CA) operated at 11 W. It pumps a tunable Ti-sapphire laser with a wavelength range of 690 980 nm that produces 3 ps pulses at 76 MHz (Coherent Mira 900P). Through the use of a frequency doubler/tripler (TP2000B from U-Oplaz, Chatsworth, CA), additional excitation wavelength ranges are available between 350 and 480 nm and between 240 and 320 nm. For this work, excitation was applied at 460 nm with a typical power of ∼30 mW. The excitation beam was focused onto the sample by a small prism (5 mm) that has a minor shadowing effect on the much larger collection optics (diameter lens 2 = 38 mm). The diameter of the laser beam at the sample was determined to be ∼120 μm, by means of a WinCamD beam profiler (Dataray Inc., Bella Vista, CA). A dielectric stack filter (Omega Optical 470 AELP, Brattleboro, VT) was used to block the laser line and Rayleigh scattering. Lens 3 focused the backscattered Raman emission on the 70 μm entrance slit of a 50 cm single-stage spectrograph (SpectraPro, Acton, MA). Time-gated detection of the Raman spectra was done using an ICCD camera (LaVision Picostar HR Picostar, G€ottingen, Germany) with a multichannel plate (MCP) operated at 750 850 V, T = 11 °C and triggered by a photodiode receiving a fraction of the laser output. For comparative measurements in the SORS mode, the small prism was moved laterally to create an offset of several millimeters relative to the focal point of the collection lens 2. Continuous wave detection was carried out using a nongated CCD camera (model DV420-OE) from Andor Technology (Belfast, U.K.), side-mounted to the same spectrograph and operated at 50 °C. Measurements. Steps of 50 100 ps were used to increment the delay and thus obtain depth profiles of the two-layer samples. For temporal calibration a small volume of cyclohexane was used. The onset of the nonretarded Raman signal of cyclohexane corresponds to t0, and in this work the delay settings
Figure 3. TRRS spectra of 2,4-DNT (left) and 2,6-DNT (right) in quartz cuvettes behind various plastics of known thicknesses at 2 s 100 accumulations. Each pair of spectra is taken at a short delay (∼0 100 ps) and a longer delay (∼300 500 ps), as indicated in the figure. First layer materials are (from top to bottom) 1.0 mm PVC, 3.0 mm POM, 0.5 mm PS, 5.0 mm PE, and 3.0 mm PTFE. The DNT reference spectra were recorded at 250 ms 40 accumulations. 8519
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Figure 4. (A) TRRS of 2,6-DNT behind 3.0 mm thick PE illustrating spectral subtraction. From top to bottom at 2 s 100 accumulations: long delay time (300 ps); short delay time (0 ps); subtraction difference of long short delay time spectra (short delay spectrum scaled by a factor of 1.4 prior to subtraction). The reference of 2,6-DNT at the bottom at 250 ms 40 accumulations. The same sample was measured (B) in TRRS mode with 2 s 1 accumulation and (C) using CW SORS, with 2 mm and 0 mm offset (2 s acquisition time).
are expressed relative to that point. Spectral calibration was done using powdered paracetamol and a second-order polynomial fit.35 Reference spectra of all compounds (as a powder in a quartz cuvette) and solid polymer materials were measured with TRRS under similar conditions as the depth measurements. The spectral acquisition times for specific spectra are indicated in the figure captions.
’ RESULTS AND DISCUSSION Raman Spectra of DNTs through Different First Layers. The spectra and depth profiles of samples composed of different combinations of first and second layer materials were measured using TRRS. Figure 3 depicts the TRR spectra in the range of ∼600 1800 cm 1 of 2,4-DNT and 2,6-DNT as second layers behind various plastics of different thicknesses. A series of measurements over a range of delay times was performed for each sample, and two spectra were selected from each series for this figure. One spectrum is taken at a relatively short delay time, in which the spectrum is primarily composed of the first layer material, and this is paired with
another spectrum from a longer delay time, in which peaks from the second layer have a stronger contribution. Reference spectra of 2,4- and 2,6-DNT can be found in the top of the figure. The spectra of the DNTs showed the most intense peaks around 790 cm 1 (NO2 scissoring), 1200 cm 1 (ring breathing), 1360 cm 1 (NO2 symmetric stretching), 1529 cm 1 (NO2 asymmetric stretching), and 1610 cm 1 (ring stretching).6,26,27,34,36,37 A major difference in the spectra of the two DNT compounds occurs as variations in intensities of the peaks in the ranges 790 850 and 1529 1610 cm 1. These peaks are clearly visible when the DNTs are used as a second layer behind plastics and therefore can be used to distinguish between the two isomers in these measurements. Differences in the series of peaks in the range of 900 1200 cm 1, from different C H vibrations,6,26,27,36,37 can also be noted in the reference spectra; however, these peaks are less visible in second layer measurements primarily due to the overlap with the first layer peaks. The most prominent peak(s) occurring in the spectra of the DNTs are those of the NO2 symmetrical stretches, which with our spectral resolution appear as one intense, broad peak at 1357 cm 1 for 2,4-DNT and 1366 cm 1 for 2,6-DNT. These 8520
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Figure 5. TRRS of ethyl centralite (EC), diphenylamine (DPA) and akardite II (AK) behind a 5.0 mm thick layer of polyethylene (PE). Two s 100 accumulations for depth measurements. Akardite II and ethyl centralite references at 500 ms 100 accumulations, diphenylamine reference 200 ms 100 accumulations. For each compound, three spectra are shown: the two-layer sample measured at a relatively long delay (top), the spectrum obtained (middle) after scaled subtraction of the spectrum of the first layer material (PE), and the spectrum of the pure explosive substance (bottom, graphically scaled for ease of comparison). The first layer (PE) reference is shown at the very bottom.
strong peaks are often the most clearly distinguishable in TRRS measurements with DNTs as a second layer, and can be used as a marker of DNT compounds. The first layer materials provide different photon retardation times based on their different scattering properties, refractive indices and thicknesses. Therefore, different optimum delay settings were found for the second layer Raman signals. Spectral Subtraction for Identification of Second Layer. Figure 4 demonstrates the application of the TRRS ICCD technique simulating its possible use in a real-life security setting. 2,6-DNT was measured at different delay times through a first layer of 3.0 mm thick PE. A spectrum obtained using a delay of 300 ps (top of Figure 4A) shows a contribution from both first and second layers, while a spectrum from 0 ps that has only contribution from the first layer (in this case PE) is depicted below it. The spectrum from 0 ps was subtracted from the spectrum with a longer delay time, and the resulting difference is a spectrum almost purely composed of peaks from the second layer material, which can be compared to the 2,6-DNT reference spectrum. In the example of Figure 4A, before subtraction, the 0 ps spectrum was scaled by a factor of 1.4 in order to have approximately the same intensity of the first layer in both spectra. It should be mentioned that for these measurements relatively long acquisition times (2 s 100) were used, in order to record also the minor Raman bands with a good signal-to-noise. However, in a security setting one would probably focus on only a few major bands, and acquisition times of a few seconds would be sufficient. As an illustration, Figure 4B shows the
TRRS spectra of the same sample recorded with only a single 2 s accumulation. In practice, to identify an unknown sample concealed behind an opaque material, the measurement of a short-delay spectrum, a long-delay spectrum, and subsequently subtracting them will result in an isolated spectrum of the concealed unknown material. The subtracted spectrum could be compared to a reference database of spectra of potentially harmful or dangerous compounds. The presence of a peak in the range of 1366 cm 1, for example, could be a red flag in security screening, as it could indicate the presence of a concealed nitro-aromatic compound. For more complex samples, spectra of individual components can be distinguished using automated PCA techniques that have been used in pharmaceutical screening.23,38 40 For comparison, we also performed continuous wave (CW) SORS measurements using the same excitation source and sample but a nongated CCD camera and an adjustable offset between the excitation beam and the focal point of the collection optics. The spectra recorded at 0 and at 2 mm offset are shown in Figure 4C and illustrate how both techniques are capable of selectively recording Raman signals of a deeper layer. When comparing the relative intensities of the first-layer peaks and the second-layer peaks, changing the delay in TRRS mode has a greater effect than changing the offset in SORS mode. The TRRS spectrum recorded at a short delay results in a cleaner spectrum of the first layer than the zero-offset measurement in SORS mode. On the other hand, comparing parts B and C of Figure 4 illustrates the better signal-to-noise that is typically obtained in 8521
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Analytical Chemistry SORS mode. For instance, the 790 cm 1 peak has a S/N ratio of 25 (SORS, 2 mm offset, where N is rms noise) versus 16 in TRRS mode (500 ps delay). This difference can be understood since in TRSS only a temporal selection of photons is detected. Furthermore, the ICCD camera used in these experiments could only be cooled to 11 °C versus 50 °C in the case of the CW camera. Detection of Other Explosives Components. Other common constituents of explosives were also detected through a relatively thick, opaque, plastic material. Figure 5 shows the TRRS spectra of ethyl centralite, diphenylamine, and akardite II behind a 5.0 mm thick layer of PE. Similar to the procedure in Figure 4A, each TRRS spectrum from a longer delay time (mixed contribution of first and second layers) is grouped with two other spectra, a reference spectrum of the second layer material at the bottom of the group and a spectrum in between in which the contribution from the first layer was removed by scaled subtraction. A reference spectrum of the first layer material (PE) is shown at the bottom of the figure. In all cases studied, the second layer spectrum is clearly visible with TRRS, even through a relatively thick first layer (5.0 mm, which is a factor 10 thicker than many common packaging materials and containers). In general, the thinner the packaging material, the shorter delay time necessary in order to obtain a spectrum from the concealed contents inside, with some variation due to differences in the scattering properties of the materials.
’ CONCLUSIONS The two main isomers of dinitrotoluene, 2,4-dinitritoluene and 2,6-dinitrotoluene, have been noninvasively detected through opaque, white plastics of several millimeters thicknesses by means of time resolved Raman spectroscopy. To show the utility of the method, other explosive-related compounds including ethyl centralite, diphenylamine, and akardite II were also detected through several millimeters thick polyethylene. This technique has potential for the detection of a wide variety of compounds. We are currently testing the same approach at NIR wavelengths. This would provide more versatility in the materials through which we can measure and the samples we can detect. For colored materials, the possibility of interference from absorption is reduced. NIR excitation will also increase the potential penetration depth of the excitation light and could reduce fluorescence background in addition to the reduction by the 250 ps measuring gate. In comparison with SORS, the approach described here requires a picosecond laser system and fast-gated detector and is therefore somewhat more complex and more expensive. Also, CW SORS measurements typically show a somewhat better signal-to-noise ratio than TRRS17 and therefore require shorter acquisition times. On the other hand, we have observed that use of TRRS for this kind of depth analysis has an advantage over SORS in that TRRS provides better selectivity over first layer photons17 and also provides the advantage of fluorescence reduction. It can be expected that with the ongoing developments in laser technology, future systems will be more rugged, userfriendly, and affordable and thus more compatible with everyday screening applications. ’ AUTHOR INFORMATION Corresponding Author
*Address: Freek Ariese, Biomolecular Analysis and Spectroscopy, LaserLaB, Vrije Universiteit, De Boelelaan 1083, 1081 HV
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Amsterdam, The Netherlands. E-mail:
[email protected]. Phone: +31 20 598 7524. Fax: +31 20 598 7543.
’ ACKNOWLEDGMENT I.E.I.P. and M.L.-L. contributed equally to this work. We would like to thank Jeroen Carol-Visser and TNO (Rijswijk, The Netherlands) for providing explosives samples. We also extend many thanks to Cecilia Vidami-Negoescu for Gaussian calculations. M.L.-L. thanks the Ministry of Science and Innovation (Government of Spain Project CTQ2008-00633-E) and the University of Alcala and LaserLaB Europe (EU-Integrated Infrastructures Initiative Program No. 2008-1-228334) for their respective mobility grants. ’ REFERENCES (1) Moore, D. S. Rev. Sci. Instrum. 2004, 75 (8), 2499–2512. (2) Hatab, N. A.; Eres, G.; Hatzinger, P. B.; Gu, B. J. Raman Spectrosc. 2010, 41, 1131–1136. (3) Botti, S.; Cantarini, L.; Palucci, A. J. Raman Spectrosc. 2010, 41, 866–869. (4) Lewis, I. R.; Daniel, N. W., Jr.; Chaffin, N. C.; Griffiths, P. R.; Tungol, M. W. Spectrochim. Acta, Part A 1995, 51, 1985–2000. (5) Lewis, M. L.; Lewis, I. R.; Griffiths, P. R. Appl. Spectrosc. 2004, 58 (4), 420–427. (6) Sylvia, J. M.; Janni, J. A.; Klein, J. D.; Spencer, K. M. Anal. Chem. 2000, 72, 5834–5840. (7) Wackerbarth, H.; Gundrum, L.; Salb, C.; Christou, K.; Vi€o, W. Appl. Opt. 2010, 49 (23), 4362–4366. (8) Moore, D. S.; Scharff, R. J. Anal. Bioanal. Chem. 2009, 393, 1571–1578. (9) Carter, J. C.; Angel, M. S.; Lawrence-Snyder, M.; Scaffidi, J.; Whipple, R. E.; Reynolds, J. G. Appl. Spectrosc. 2005, 59 (6), 769–775. (10) Izake, E. L. Forensic Sci. Int. 2010, 202, 1–8. (11) Everall, N.; Hahn, T.; Matousek, P.; Parker, A. W.; Towrie, M. Appl. Spectrosc. 2001, 55 (12), 1701–1708. (12) Matousek, P. Chem. Soc. Rev. 2007, 36, 1292–1304. (13) Matousek, P.; Clarke, I. P.; Draper, E. R. C.; Morris, M. D.; Goodship, A. E.; Everall, N.; Towrie, M.; Finney, W. F.; Parker, A. W. Appl. Spectrosc. 2005, 59 (4), 393–400. (14) Matousek, P.; Towrie, M.; Stanley, A.; Parker, A. W. Appl. Spectrosc. 1999, 53 (12), 1485–1489. (15) Matousek, P.; Everall, N.; Towrie, M.; Parker, A. W. Appl. Spectrosc. 2005, 59 (2), 200–205. (16) Ariese, F.; Meuzelaar, H.; Kerssens, M. M.; Buijs, J. B.; Gooijer, C. Analyst 2009, 134, 1192–1197. (17) Iping Petterson, I. E.; Dvorak, P.; Buijs, J. B.; Gooijer, C.; Ariese, F. Analyst 2010, 135, 3255–3259. (18) Eliasson, C.; Macleod, N. A.; Matousek, P. Anal. Chem. 2007, 79, 8185–8189. (19) Eliasson, C.; Macleod, N. A.; Matousek, P. Vib. Spectrosc. 2008, 48, 8–11. (20) Bloomfield, M.; Loeffen, P. W.; Matousek, P. Proc. SPIE 2010, 7838, 783808–1 15. (21) Kim, M.; Chung, H.; Woo, Y.; Kemper, M. S. Anal. Chim. Acta 2007, 587, 200–207. (22) Broad, N. W.; Jee, R. D.; Moffat, A. C.; Eaves, M. J.; Mann, W. C.; Dziki, W. Analyst 2000, 125, 2054–2058. (23) Luypaert, J.; Massart, D. L.; Vander Heyden, Y. Talanta 2007, 72, 865–883. (24) Everall, N.; Hahn, T.; Matousek, P.; Parker, A. W.; Towrie, M. Appl. Spectrosc. 2004, 58, 591–597. (25) Efremov, E.; Buijs, J. B.; Gooijer, C.; Ariese, F. Appl. Spectrosc. 2007, 61, 571–578. (26) Liu, G.; Ma, X.; Ma, S.; Zhao, H.; Ma, M.; Ge, M.; Wang, W. Chin. J. Chem. 2008, 26, 1257–1261. 8522
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