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Stand-off Raman spectroscopy
Trends in Analytical Chemistry, Vol. 28, No. 11, 2009
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Stand-off Raman spectroscopy Alison J. Hobro, Bernhard Lendl Stand-off Raman spectroscopy, where the instrumentation is physically separated from the sample under investigation, can be extremely advantageous for analysis of dangerous, fragile or inaccessible samples. We first review common aspects of stand-off Raman instruments, including laser, optical lay-out, wavelength dispersion and detection, and how their characteristics can affect overall performance. We then provide an overview of applications of stand-off Raman spectroscopy in different fields, including geology, explosives detection, art and archaeology. ª 2009 Elsevier Ltd. All rights reserved. Keywords: Archaeology; Art; Detection; Explosive; Geology; Instrumentation; Laser; Raman spectroscopy; Stand-off; Wavelength dispersion
1. Introduction Alison J. Hobro, Bernhard Lendl* Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164-AC, A-1060 Vienna, Austria
*
Corresponding author. Tel.: +43 (0) 15880115140; Fax: +43 (0) 15880115199; E-mail: [email protected]
Raman spectroscopy measures vibrational transitions in a molecule or a sample and, as this information is specific to the particular chemical structure of a molecule, Raman spectra can be thought of as a ÔfingerprintÕ of a sample. Raman spectroscopy is non-invasive and non-destructive and can be used to analyze a wide range of substances, with no requirements for sample composition or labeling prior to analysis. Stand-off Raman spectroscopy is usually defined as Raman spectroscopy performed where the spectrometer (and therefore the operator) are some distance from the sample under interrogation [1]. These stand-off systems are of great benefit where samples are dangerous (e.g., chemical spills or suspected explosive packages), as the operator is distanced from potential danger. Art and archaeological artifacts can be fragile to analyze, and stand-off Raman analysis not only removes the need for contact but also allows samples to be measured in situ, something that provides distinct advantages for wall paintings and murals, or where artifacts are housed in museum displays. Stand-off measurements can also be advantageous for geological studies where it can be difficult to gain direct access to samples, one such example is that Raman spectroscopy is being included in
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the European Space Agency ExoMars mission [2]. Stand-off Raman systems may also be applicable for the study of extremophiles {i.e. organisms that live in extreme environmental conditions [e.g., high-temperature or high-sulfur environments (e.g., those found in volcanoes), which are difficult to access directly]}. The principle of stand-off detection using Raman spectroscopy was first proposed in the mid-1960s [3]. Although theoretically possible, the field of stand-off Raman spectroscopy suffered from controversy regarding the adequacy of sensitivity arising from the inherently low signal levels, long-distance ranges and low sample concentrations [4]. Early experiments in the 1960s and 1970s showed a number of different design features were critical in obtaining better signal-to-noise ratios. These early design improvements centered on optimizing the laser and the optical pathway, as well as careful selection of detection equipment and gating apparatus [4]. These are all features of the spectrometer that have been continually developed since these early experiments so that systems currently in use can operate over considerable distances, in daylight and under various other environmental conditions. Prior to 1990, a number of remote Raman-based Light Detection and Ranging (LIDAR) and differential absorption LIDAR (DIAL) systems had been developed for atmospheric measurements of up to a few km in distance (e.g., [5–8]), but the instrumentation requirements for light collection – high-powered lasers and large telescopes – often limited the portability of such systems. In 1992, Angel et al. [9] published details of their small, portable, Raman system that could measure a range of solid and liquid samples, analogous to substances found in waste tanks, at distances of 6.3 m and 16.7 m and collection times of 1–2 min with both 488-nm and 1235
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809-nm-laser excitation [9]. Since then, and with improvements in component size and performance, the details of a number of more portable stand-off Raman systems have been published (e.g., [10–13]).
2. Instrumentation 2.1. Laser The choice of laser wavelength affects the performance of a Raman spectrometer. At near-ultraviolet (UV) and visible wavelengths, the Raman spectrum can be covered by fluorescence, originating directly from the sample or from impurities within the sample. This can be avoided using laser wavelengths in the UV (where fluorescence lies outside the Raman spectral range) or nearinfrared (NIR) regions (where the lower laser energies required to generate a Raman spectrum do not give rise to fluorescence) of the spectrum [14]. However, Raman spectral intensity is inversely related to the fourth power of the laser wavelength. This means that the intensity of
a Raman spectrum is much greater when exciting in the UV region than in the NIR region [12]. In practical terms, this results in higher sensitivity and shorter dataacquisition times when exciting in the UV as compared to the NIR [14]. Using a UV laser, therefore, has the advantages of producing both high spectral intensity and being unaffected by fluorescence. Using an excitation laser in the UV region below 300 nm has an additional advantage for stand-off Raman, as the Hartley absorption band of ozone will block solar radiation, meaning that the collection of Raman spectra will not be greatly affected by ambient light. This factor allows UV-based Raman systems to function efficiently in daylight conditions without complex instrumentation required for effective light discrimination [12]. An additional consideration when using laser excitation in the UV region is the increased risk of photodegradation or photo-induced dissociation. Using an excitation laser in the UV region can also cause some difficulties when it comes to resolving the Raman spectra obtained. The dispersion of a Raman spectrum, in nm,
1400
Dispersion 1200
355 nm excitation
1000
Dispersion (cm-1 /mm)
Raman spectrum covers 55 nm 800
600
532 nm excitation Raman spectrum 400
covers 140 nm 266 nm excitation
200
Raman spectrum covers 30 nm
0 200
300
400
500
600
700
800
900
1000
1100
1200
Wavelength (nm) Figure 1. Relationship between excitation wavelength and dispersion of the Raman spectrum for the second, third and fourth harmonics of a Nd:YAG laser. Here, a full Raman spectrum is taken to be 3800 cm 1.
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Trends in Analytical Chemistry, Vol. 28, No. 11, 2009
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depends on the excitation wavelength. The closer to the IR region the laser-excitation wavelength is, the greater the dispersion of the Raman spectrum, as shown in Fig. 1. In practical terms, this means that achieving high-resolution measurements is much more difficult in the UV region than it is in the NIR region. Previous stand-off Raman instruments have utilized several different laser wavelengths, with some authors indicating that their choice of laser wavelength was primarily due to available equipment rather than on a scientific basis. The most popular choice of laser wavelength has been 532 nm (usually a frequency-doubled Nd:YAG laser) (e.g., [15,16]) but studies using 355 nm and 266 nm, and 488 nm and 809 nm, have also been reported [9,17]. 2.2. Optical geometry The geometry of the laser and the telescope relative to the sample can be configured in two ways – coaxial and oblique (sometimes referred to as bi-axial). Fig. 2 shows these lay-outs. Co-axial geometry allows measurements from samples at different distances without the need for realignment. Oblique geometry ensures that all the laser power reaches the sample, by careful aligning laser and telescope. Of the two lay-outs, the oblique design shows a higher performance than the co-axial lay-out, in terms of signal intensities, through avoiding loss of power through reflection and scattering from prisms in the near field [18]. An assessment of the sampling depth (i.e. the range of sample positions at which Raman spectra can be observed) using benzene as a target substance showed that, for the oblique system, while high signal intensity was
recorded at 20–50 m (±15 m at 35 m), the sample was still detectable at 10 m and 65 m. For the co-axial set-up, the detectable range was much wider with spectra recordable at 10 m and 120 m [18]. 2.3. Coupling regime To date, most stand-off Raman systems have been designed with optical-fiber couplings to collect light from the telescope, via a holographic filter, and direct the light into the spectrograph. However, the fiber-optic coupling can decrease the efficiency of the Raman system, if the optical fibers are small. Due to the recent decrease in the size of spectrographs, direct coupling between telescope and spectrometer is now possible [18]. Comparisons of fiber-optic and directly-coupled systems, using benzene as a target, have shown that the directly-coupled systems show a 10-fold increase in performance over the fiber-optic systems [18]. However, directly-coupled systems can be considerably bulkier than fiber-optic systems, as all components need to be in a sequential line. Fiber-optic couplings give more freedom and flexibility in placing individual components within the instrument and allow a smaller final product. 2.4. Wavelength dispersion To date, there have been two methods of wavelength dispersion utilized in stand-off Raman spectroscopy. A small number of researchers have used an acousto-optical tunable filter (AOTF) [19,20], while the majority have used spectrographs. An AOTF uses an acoustic wave (generated by radio frequency) in an anisotropic crystal in order to diffract a selected wavelength. By altering the radio frequency, the AOTF can quickly switch to diffract at a different wavelength, a distinct advantage when the
Co-axial geometry Laser Sample
To spectrograph Telescope
Oblique geometry Laser Sample
To spectrograph Telescope
Figure 2. Co-axial vs. oblique geometries for laser and telescope alignment (adapted from [18]). Raman scattering from the sample occurs in all directions; the diagram here only represents those Raman-scattered photons returning to the collection optics.
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Raman intensity needs to be recorded at only a small number of wavelengths. However, in order to collect a complete Raman spectrum, the AOTF needs to be tuned to each wavelength sequentially as, unlike spectrographs, the Raman spectrum cannot be collected in one shot [20]. Although AOTF systems should provide higher throughput [20], Carter et al. found that their performance was limited by the optical fiber used to couple the AOTF or spectrograph to the telescope, suggesting the expected higher throughput of the AOTF can be exploited only in directly-coupled systems [20]. 2.5. Detector Most stand-off Raman systems utilize charge-coupled device (CCD) cameras as detectors. Previous studies have used gated intensified CCD (iCCD) detectors, and Carter et al. [20] showed that the increase in gain provided by the iCCD detector also gives rise to a higher noise level, generating signal-to-noise ratios similar to those obtained using conventional CCD cameras. However, when coupled to a pulsed laser system, the ability to gate the detector can have two advantages: the gating of the detector can be co-ordinated with the laser pulse to restrict data collection to the time period where Raman-scattered photons are expected to reach the detector, while excluding the ambient light photons outside of this time period; and, accurate gating around the laser pulse also allows effective discrimination of the Raman signal from possibly longer-lived fluorescent or luminescent components in the target sample (as these signals will reach the detector after the Raman-scattered photons) [20,21].
3. Performance Raman spectra have been collected at different stand-off distances in the range 3–533 m (e.g., [9,12,18,22]). The limits of detection (LODs) for a particular sample will vary for each stand-off Raman instrument but will be especially affected by the excitation-laser wavelength and the stand-off distance. Carter et al. [15] used a stand-off Raman system operating at 27 m and utilizing a 532-nm excitation laser with a maximum power of 140 mJ per pulse and pulse width of 5 ns. They estimated that their LOD was of the order of 250 parts-permillion (ppm) for RDX [15]. Others state much higher LODs (e.g., the remote Raman system developed at Brookhaven National Laboratory in the USA, whose overall estimation is that their system is capable of detection, at levels sufficient for identification, from samples with a minimum of 850 g/m2 at a distance of 533 m and with 1 min of signal integration [12]). However, LODs for stand-off Raman analyses of the majority of samples are not currently known. 1238
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The sensitivity to environmental conditions (e.g., temperature, rain, snow or air pollution) has not been extensively studied. However, Pettersson et al. [16] have shown that it is possible to record stand-off Raman spectra in heavy rain and through snow with little appreciable difference to the spectra obtained [16]. Wu et al. found that measurements through light marine fog could reduce the Raman signal by two orders of magnitude [12]. This difference between rain/snow and fog stems from the fact that, during the timescale of a Raman experiment, snow and raindrops can be viewed as being large and stationary and the chances of them being in the path of the laser beam are small, so they do not greatly affect the passage of the laser light. When we consider fog, however, the particles are much smaller and more dispersed through the path of the laser beam. This results in greater diffraction and reflection of the laser beam by the fog particles, reducing the strength of the Raman signal reaching the collection optics. A field-portable system designed by Sharma et al. [13] has successfully measured benzene, in both clear-glass and brown-glass bottles, at a distance of 10 m. While a Raman spectrum of the sample in a clear-glass bottle only required a collection time of 1 s and the sample in a brown glass bottle required 5 s [13], this shows the potential of stand-off Raman spectroscopy for identifying substances in a range of different containers.
4. Applications Today, stand-off Raman detection is used for a wide range of studies where access to the sample can be difficult, dangerous or destructive. Such applications include: atmospheric and geological measurements (e.g., [21]), where direct access to the sample is often limited or prohibitively difficult; identification or monitoring of explosives or chemical spills (e.g., [9,15]), where direct access to the sample could be dangerous or toxic; and, studies of ancient artifacts or artworks (e.g., [23]), where more direct measurements may damage a priceless object. Table 1 summarizes the stand-off distances and instrumentation parameters for these different applications. 4.1. Geology and geophysics One of the most active groups in the field of stand-off Raman is that of Sharma in Hawaii, whose research focuses on geological and environmental measurements using both field-portable and laboratory-based stand-off Raman instruments. These instruments operate at a laser wavelength of 532 nm (a frequency-doubled Nd:YAG laser), with a power of 35 mJ per pulse, a width
Application Geology Examples include calcite, marble, feldspar, Chemical Detection CCl4, sodiumferrocyanide Acetonitrile and cyclohexane on topsoil Air pollutants SO2 and kerosene Explosives Examples include TNT, PETN, TATP, RDX, HMX, Semtex
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Art and Archaeology Objects of art in museums, paintings on chapel wall Other Fundamental studies of ATOF vs. spectrograph and CCD vs. iCCD
Stand-off distance (m)
Laser wavelength (nm)
10–65 10–50 10–120
532 (pulsed) 532 (pulsed) 532 (pulsed)
16.7 533 3–30
Repetition rate / Laser power at source
Optical geometry Coupling regime
Wavelength dispersion
Detector
Ref.
20 Hz, 35 mJ/pulse 20 Hz, 35 mJ/pulse 20 Hz / -
Co-axial Co-axial & oblique Co-axial
Fiber optic Direct Direct
Spectrograph Spectrograph Spectrograph
iCCD iCCD iCCD
[12,17,20] [12,17,20,23] [26]
488 & 809 (CW) 266 (pulsed) 266 (pulsed)
- / 100 mW 30 Hz / 20 Hz, 7 mJ/pulse
Coaxial Coaxial
Transfer optics Transfer optics Transfer optics
Spectrograph Spectrograph Spectrograph
CCD iCCD iCCD
[8] [11] [21]
200
2 J, 2 pps ruby
-
-
-
-
Photo-multiplier
[3]
10 27 & 50 30 20 & 55
532 (pulsed) 532 (pulsed) 532 532 (pulsed)
Coaxial Coaxial
Fiber optic Fiber optic
Coaxial
Fiber optic
Spectrograph Spectrograph Spectrograph Spectrograph
iCCD iCCD iCCD iCCD
[12] [14] [16] [15]
-
514.5 (CW)
20 Hz, 35 mJ/pulse 10 Hz, 140 mJ/pulse 13 Hz, 20 mJ/pulse 10 Hz, 280 mJ/pulse (20 m) 10 Hz, 450 mJ/pulse (55 m) - / 500 mW
Fiber-bundle
Fiber-bundle
ATOF
Photo-multiplier
[18]
<1
785 (CW)
- / 300 mW
Probe head
Fiber optic
Spectrograph
CCD
[10,23,28,29]
15
532 (pulsed)
10 Hz, 47 mJ/pulse
Co-axial
Fiber optic
Spectrograph & ATOF
CCD and iCCD
[19]
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Table 1. Summary of instrumentation parameters for stand-off Raman systems discussed in the text
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of 8 ns and with a beam divergence less than 0.8 mrad [18,21]. The laboratory-based system can achieve clear, well-resolved Raman spectra of a range of minerals at a distance of 10 m with single laser-pulse excitation [24], as well as recording clear spectra of various minerals and chemicals at 50–100 m with a fast integration time of 1 s [25]. Both laboratory-based and field-portable stand-off Raman spectrometers have been used to identify a wide range of different mineral substances, including quartz, mica, calcite and marble [18,21,26]. The studies showed that these Raman systems can distinguish between closely-related minerals (e.g., dolomite, gypsum and fluorapatite), which are all Ca2+ based but contain differing anions [24]. They also showed that, when studying glasses of plagioclase composition, it was possible to distinguish between glasses formed during volcanic processes and those formed as a result of impact [18]. 4.2. Explosives detection Several studies have assessed the applicability of standoff Raman systems for remote identification of high explosives and associated compounds. They utilized both visible and UV excitation to characterize a range of explosives, including RDX, TATP, PETN, TNT and urea nitrate [15]. Additionally, explosive chemicals (e.g., ammonium nitrate and potassium perchlorate) have been detected at distances of 100 m with a 1-s integration time [25]. These explosives and related substances cover both nitrogen-based and peroxide-based explosives, showing the versatility of Raman spectroscopy for explosives detection. Carter et al. [15] developed a stand-off Raman system capable of detecting explosive compounds at 27 m and 50 m, with the 27 m set-up providing the best signalto-noise ratio, due to the overlap of laser-spot size and collection optics giving very efficient light collection [15]. This system utilizes a frequency-doubled Nd:YAG laser, producing 532-nm excitation, and a 10-ms gate width to allow good spectra collection in ambient-light conditions. They also studied the effects of laser power on their explosive samples and found that, for most of those studied, there appeared to be no significant laserinduced degradation of the sample. However, for TNT, spectroscopic and visual analysis indicated that laser powers greater than 3.4 · 105 W/cm2 can induce photo-degradation or thermal degradation of the sample, highlighting the importance of experimental parameters in measuring unknown explosive samples [15]. Gaft and Nagli [17] studied the pros and cons of using both green and UV excitation wavelengths for detecting explosives. They found that, for 532-nm excitation, Raman detection of explosives could be hampered by the presence of other materials in the background (e.g., metal surfaces, plastics or cloth that 1240
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the explosives may be contained in or deposited on) with such materials giving rise to luminescence [17]. When moving to 355-nm and 266-nm excitation using conventional laboratory-based systems – third and fourth harmonics of a Nd:YAG laser – the problem of luminescence from other materials could be compensated for by the increasing Raman cross-section of the explosive molecules at UV wavelengths [17]. For a number of explosive materials, with fundamental absorption bands in the UV, the move to UV-excitation wavelengths increases the likelihood of generating resonance or pre-resonance Raman signals that will provide an additional increase in signal intensity [27]. Gaft and Nagli found that, for the majority of explosives, the signal-to-noise ratio was much better when using 266 nm than when using 355 nm. However, RDX and its derivatives were the exception to this as a much clearer spectrum was recorded using 355 nm [17,27]. A further study [27] investigated the Raman crosssections of some explosives at different wavelengths in more detail. Finally, Sharma et al. also used a field-portable system to identify two explosives: b-HMX and TATB (distance 10 m, 10 s integration) with both low wavenumber (150–1800 cm 1) and high wavenumber (2990– 3225 cm 1) regions, giving rise to characteristic spectra that were well-defined fingerprints for the two explosives [13]. 4.3. Chemical detection A number of chemicals (e.g., cyclohexane and acetonitrile) have been used as substances to test the performance of newly-developed stand-off Raman systems. However, several authors have extended the application of such systems to more complex chemical samples. Wu et al. [12] studied carbon tetrachloride in acetonitrile and carbon disulfide in cyclohexane as models of chemical spills, while Ray et al. [22] showed that it is also possible to detect cyclohexane when it is placed on topsoil. Angel et al. [9] designed their system to measure solids, semi-solids and gases in storage tanks. They stated that a typical mixed waste tank will contain both liquid and solid wastes and studied a mixture of 10% sodium ferrocyanide and 10% sodium nitrate in a sodium-chloride matrix as a model system for such mixed wastes. Their stand-off Raman system could detect both substances in the waste mixture simultaneously, as the Raman bands of NO3 and ferrocyanide do not overlap [9]. 4.4. Art and archaeology While the stand-off distances required for many of the other applications described above are generally on the scale of meters, for reasons of accessibility and safety, the stand-off distances required for measuring objects of art are often much smaller, but there still needs to be sep-
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aration between the spectrometer and the object to prevent damage or contamination of the artifact. Also, the use of pulsed lasers is not advisable, due to the potential damage to the delicate sample surface. Vandenabeele et al. [23] have studied the performance of a number of commercially-available and bespoke Raman instruments designed with measurements at short standoff distances in mind. They used these spectrometers to study titanium oxide and strontium yellow (SrCrO4) as models for paints on objects of art. These studies were also extended to paintings on canvas and the identification of organic elements (e.g., beeswax) in pottery vessels [23]. The portable bespoke Raman instrument described in ref [23], described in full in [10], has been used to study a wide range of different art and archeological objects. These objects ranged from Egyptian sarcophagi to paintings on paper, textile and wood. In all cases, even with problems arising from ambient light and fluorescence from varnish layers, Vandenabeele et al. could identify the composition of most pigments present on these artifacts [28]. Such mobile Raman instruments can also detect the degradation products of pigments, identifying areas where paint colour has changed as a result of aging (e.g., medieval wall paintings at the chapel of Ponthoz in Belgium [29]).
5. Stand-off Raman combined with other laserbased techniques Sharma and co-workers have proposed that additional developments could centre around combining stand-off Raman detection with other techniques [e.g., laser-induced breakdown spectroscopy (LIBS) and laser-induced native fluorescence (LINF)] [18]. SharmaÕs conventional stand-off Raman system can be modified using a flash lamp and a delay generator to illuminate the sample at the correct time, allowing Raman and LINF spectra with 10-ns resolution at a 9-m stand-off distance. The LINF spectra give complementary information to the Raman spectra, allowing measurements of rare-earth and transition-metal ions at ppb levels as well as sensitive detection of organic materials [18]. A prototype instrument has been developed, combining Raman with LIBS and using the frequency-doubled Nd:YAG wavelength of 532 nm [18,30]. Due to the differences in power-density requirements for LIBS and Raman spectroscopies, the two are collected sequentially, with the laser power for Raman-spectra collection reduced slightly, through reducing the laser power or defocusing the laser, in order to minimize the possibility of sample ablation [30]. Further studies have shown that, with a pulsed laser operating at dual wavelengths (532 nm and 1064 nm), it is possible to measure LIBS and Raman spectra of minerals (e.g., calcite and gypsum) [31].
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Although the two techniques cover different spectral ranges, with LIBS a total of 650 nm and Raman (at 532 nm) 144 nm, they have compatible resolution requirements, with LIBS-emission lines resolved at 2500 (k/Dk) corresponding to a resolution of 7.5 cm 1 for Raman spectroscopy [30]. Finally, while the detection parameters are less significant for LIBS, due to the higher signal intensity than that of Raman spectroscopy, both techniques can benefit from the advantages of a gated detector reducing the effects of ambient-light contributions to the background signal [30]. 6. Future directions Stand-off Raman spectroscopy is still an emerging field and, for the most part, its future directions are likely to be application-driven. Advances that make the instrumentation smaller, and therefore more portable, as well as improvements in performance providing better signalto-noise ratios and LODs will not only improve the applicability of stand-off Raman to the topics described in this review but also broaden the range of fields of application. The rejection of ambient light, through choice of excitation wavelength or gating the detector, has been applied where the stand-off distances are on the scale of meters (e.g., explosives detection and geological measurements). However, discrimination of ambient light for measuring art and archaeology has not been developed extensively. Most stand-off Raman instruments that we described contain a single laser wavelength, which, whilst often making the overall design more portable, can lead to problems analyzing certain substances at particular excitation wavelengths {e.g., identifying azurite (Cu3(CO3)2(OH)2), a blue pigment used in Renaissance paintings, that does not scatter strongly when a 785-nm laser is used [23]}. In such cases, where samples may be colored, the ability to switch between different excitation wavelengths would be advantageous. Acknowledgements The research leading to these results received funding from the European Union-funded Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 218037. References [1] Committee on the Review of Existing and Potential STandoff Explosived Detection Techniques, Existing and potential standoff explosives detection techniques, 0-309-09130-6, National Research Council, Washington, DC, USA, 2004. [2] European Space Agency, 2009 (http://www.esa.int/SPECIALS/ ExoMars/SEMSZIAMS7F_0.html). [3] J. Cooney, Proc. Symp. Electromagnetic sensing of the Earth from Satellites, Brooklyn Polytechnic, Brooklyn, New York, USA, 1965.
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Stand-off Raman spectroscopy Alison J. Hobro, Bernhard Lendl Stand-off Raman spectroscopy, where the instrumentation is physically separated from the sample under investigation, can be extremely advantageous for analysis of dangerous, fragile or inaccessible samples. We first review common aspects of stand-off Raman instruments, including laser, optical lay-out, wavelength dispersion and detection, and how their characteristics can affect overall performance. We then provide an overview of applications of stand-off Raman spectroscopy in different fields, including geology, explosives detection, art and archaeology. ª 2009 Elsevier Ltd. All rights reserved. Keywords: Archaeology; Art; Detection; Explosive; Geology; Instrumentation; Laser; Raman spectroscopy; Stand-off; Wavelength dispersion
1. Introduction Alison J. Hobro, Bernhard Lendl* Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164-AC, A-1060 Vienna, Austria
*
Corresponding author. Tel.: +43 (0) 15880115140; Fax: +43 (0) 15880115199; E-mail: [email protected]
Raman spectroscopy measures vibrational transitions in a molecule or a sample and, as this information is specific to the particular chemical structure of a molecule, Raman spectra can be thought of as a ÔfingerprintÕ of a sample. Raman spectroscopy is non-invasive and non-destructive and can be used to analyze a wide range of substances, with no requirements for sample composition or labeling prior to analysis. Stand-off Raman spectroscopy is usually defined as Raman spectroscopy performed where the spectrometer (and therefore the operator) are some distance from the sample under interrogation [1]. These stand-off systems are of great benefit where samples are dangerous (e.g., chemical spills or suspected explosive packages), as the operator is distanced from potential danger. Art and archaeological artifacts can be fragile to analyze, and stand-off Raman analysis not only removes the need for contact but also allows samples to be measured in situ, something that provides distinct advantages for wall paintings and murals, or where artifacts are housed in museum displays. Stand-off measurements can also be advantageous for geological studies where it can be difficult to gain direct access to samples, one such example is that Raman spectroscopy is being included in
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the European Space Agency ExoMars mission [2]. Stand-off Raman systems may also be applicable for the study of extremophiles {i.e. organisms that live in extreme environmental conditions [e.g., high-temperature or high-sulfur environments (e.g., those found in volcanoes), which are difficult to access directly]}. The principle of stand-off detection using Raman spectroscopy was first proposed in the mid-1960s [3]. Although theoretically possible, the field of stand-off Raman spectroscopy suffered from controversy regarding the adequacy of sensitivity arising from the inherently low signal levels, long-distance ranges and low sample concentrations [4]. Early experiments in the 1960s and 1970s showed a number of different design features were critical in obtaining better signal-to-noise ratios. These early design improvements centered on optimizing the laser and the optical pathway, as well as careful selection of detection equipment and gating apparatus [4]. These are all features of the spectrometer that have been continually developed since these early experiments so that systems currently in use can operate over considerable distances, in daylight and under various other environmental conditions. Prior to 1990, a number of remote Raman-based Light Detection and Ranging (LIDAR) and differential absorption LIDAR (DIAL) systems had been developed for atmospheric measurements of up to a few km in distance (e.g., [5–8]), but the instrumentation requirements for light collection – high-powered lasers and large telescopes – often limited the portability of such systems. In 1992, Angel et al. [9] published details of their small, portable, Raman system that could measure a range of solid and liquid samples, analogous to substances found in waste tanks, at distances of 6.3 m and 16.7 m and collection times of 1–2 min with both 488-nm and 1235
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809-nm-laser excitation [9]. Since then, and with improvements in component size and performance, the details of a number of more portable stand-off Raman systems have been published (e.g., [10–13]).
2. Instrumentation 2.1. Laser The choice of laser wavelength affects the performance of a Raman spectrometer. At near-ultraviolet (UV) and visible wavelengths, the Raman spectrum can be covered by fluorescence, originating directly from the sample or from impurities within the sample. This can be avoided using laser wavelengths in the UV (where fluorescence lies outside the Raman spectral range) or nearinfrared (NIR) regions (where the lower laser energies required to generate a Raman spectrum do not give rise to fluorescence) of the spectrum [14]. However, Raman spectral intensity is inversely related to the fourth power of the laser wavelength. This means that the intensity of
a Raman spectrum is much greater when exciting in the UV region than in the NIR region [12]. In practical terms, this results in higher sensitivity and shorter dataacquisition times when exciting in the UV as compared to the NIR [14]. Using a UV laser, therefore, has the advantages of producing both high spectral intensity and being unaffected by fluorescence. Using an excitation laser in the UV region below 300 nm has an additional advantage for stand-off Raman, as the Hartley absorption band of ozone will block solar radiation, meaning that the collection of Raman spectra will not be greatly affected by ambient light. This factor allows UV-based Raman systems to function efficiently in daylight conditions without complex instrumentation required for effective light discrimination [12]. An additional consideration when using laser excitation in the UV region is the increased risk of photodegradation or photo-induced dissociation. Using an excitation laser in the UV region can also cause some difficulties when it comes to resolving the Raman spectra obtained. The dispersion of a Raman spectrum, in nm,
1400
Dispersion 1200
355 nm excitation
1000
Dispersion (cm-1 /mm)
Raman spectrum covers 55 nm 800
600
532 nm excitation Raman spectrum 400
covers 140 nm 266 nm excitation
200
Raman spectrum covers 30 nm
0 200
300
400
500
600
700
800
900
1000
1100
1200
Wavelength (nm) Figure 1. Relationship between excitation wavelength and dispersion of the Raman spectrum for the second, third and fourth harmonics of a Nd:YAG laser. Here, a full Raman spectrum is taken to be 3800 cm 1.
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depends on the excitation wavelength. The closer to the IR region the laser-excitation wavelength is, the greater the dispersion of the Raman spectrum, as shown in Fig. 1. In practical terms, this means that achieving high-resolution measurements is much more difficult in the UV region than it is in the NIR region. Previous stand-off Raman instruments have utilized several different laser wavelengths, with some authors indicating that their choice of laser wavelength was primarily due to available equipment rather than on a scientific basis. The most popular choice of laser wavelength has been 532 nm (usually a frequency-doubled Nd:YAG laser) (e.g., [15,16]) but studies using 355 nm and 266 nm, and 488 nm and 809 nm, have also been reported [9,17]. 2.2. Optical geometry The geometry of the laser and the telescope relative to the sample can be configured in two ways – coaxial and oblique (sometimes referred to as bi-axial). Fig. 2 shows these lay-outs. Co-axial geometry allows measurements from samples at different distances without the need for realignment. Oblique geometry ensures that all the laser power reaches the sample, by careful aligning laser and telescope. Of the two lay-outs, the oblique design shows a higher performance than the co-axial lay-out, in terms of signal intensities, through avoiding loss of power through reflection and scattering from prisms in the near field [18]. An assessment of the sampling depth (i.e. the range of sample positions at which Raman spectra can be observed) using benzene as a target substance showed that, for the oblique system, while high signal intensity was
recorded at 20–50 m (±15 m at 35 m), the sample was still detectable at 10 m and 65 m. For the co-axial set-up, the detectable range was much wider with spectra recordable at 10 m and 120 m [18]. 2.3. Coupling regime To date, most stand-off Raman systems have been designed with optical-fiber couplings to collect light from the telescope, via a holographic filter, and direct the light into the spectrograph. However, the fiber-optic coupling can decrease the efficiency of the Raman system, if the optical fibers are small. Due to the recent decrease in the size of spectrographs, direct coupling between telescope and spectrometer is now possible [18]. Comparisons of fiber-optic and directly-coupled systems, using benzene as a target, have shown that the directly-coupled systems show a 10-fold increase in performance over the fiber-optic systems [18]. However, directly-coupled systems can be considerably bulkier than fiber-optic systems, as all components need to be in a sequential line. Fiber-optic couplings give more freedom and flexibility in placing individual components within the instrument and allow a smaller final product. 2.4. Wavelength dispersion To date, there have been two methods of wavelength dispersion utilized in stand-off Raman spectroscopy. A small number of researchers have used an acousto-optical tunable filter (AOTF) [19,20], while the majority have used spectrographs. An AOTF uses an acoustic wave (generated by radio frequency) in an anisotropic crystal in order to diffract a selected wavelength. By altering the radio frequency, the AOTF can quickly switch to diffract at a different wavelength, a distinct advantage when the
Co-axial geometry Laser Sample
To spectrograph Telescope
Oblique geometry Laser Sample
To spectrograph Telescope
Figure 2. Co-axial vs. oblique geometries for laser and telescope alignment (adapted from [18]). Raman scattering from the sample occurs in all directions; the diagram here only represents those Raman-scattered photons returning to the collection optics.
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Raman intensity needs to be recorded at only a small number of wavelengths. However, in order to collect a complete Raman spectrum, the AOTF needs to be tuned to each wavelength sequentially as, unlike spectrographs, the Raman spectrum cannot be collected in one shot [20]. Although AOTF systems should provide higher throughput [20], Carter et al. found that their performance was limited by the optical fiber used to couple the AOTF or spectrograph to the telescope, suggesting the expected higher throughput of the AOTF can be exploited only in directly-coupled systems [20]. 2.5. Detector Most stand-off Raman systems utilize charge-coupled device (CCD) cameras as detectors. Previous studies have used gated intensified CCD (iCCD) detectors, and Carter et al. [20] showed that the increase in gain provided by the iCCD detector also gives rise to a higher noise level, generating signal-to-noise ratios similar to those obtained using conventional CCD cameras. However, when coupled to a pulsed laser system, the ability to gate the detector can have two advantages: the gating of the detector can be co-ordinated with the laser pulse to restrict data collection to the time period where Raman-scattered photons are expected to reach the detector, while excluding the ambient light photons outside of this time period; and, accurate gating around the laser pulse also allows effective discrimination of the Raman signal from possibly longer-lived fluorescent or luminescent components in the target sample (as these signals will reach the detector after the Raman-scattered photons) [20,21].
3. Performance Raman spectra have been collected at different stand-off distances in the range 3–533 m (e.g., [9,12,18,22]). The limits of detection (LODs) for a particular sample will vary for each stand-off Raman instrument but will be especially affected by the excitation-laser wavelength and the stand-off distance. Carter et al. [15] used a stand-off Raman system operating at 27 m and utilizing a 532-nm excitation laser with a maximum power of 140 mJ per pulse and pulse width of 5 ns. They estimated that their LOD was of the order of 250 parts-permillion (ppm) for RDX [15]. Others state much higher LODs (e.g., the remote Raman system developed at Brookhaven National Laboratory in the USA, whose overall estimation is that their system is capable of detection, at levels sufficient for identification, from samples with a minimum of 850 g/m2 at a distance of 533 m and with 1 min of signal integration [12]). However, LODs for stand-off Raman analyses of the majority of samples are not currently known. 1238
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The sensitivity to environmental conditions (e.g., temperature, rain, snow or air pollution) has not been extensively studied. However, Pettersson et al. [16] have shown that it is possible to record stand-off Raman spectra in heavy rain and through snow with little appreciable difference to the spectra obtained [16]. Wu et al. found that measurements through light marine fog could reduce the Raman signal by two orders of magnitude [12]. This difference between rain/snow and fog stems from the fact that, during the timescale of a Raman experiment, snow and raindrops can be viewed as being large and stationary and the chances of them being in the path of the laser beam are small, so they do not greatly affect the passage of the laser light. When we consider fog, however, the particles are much smaller and more dispersed through the path of the laser beam. This results in greater diffraction and reflection of the laser beam by the fog particles, reducing the strength of the Raman signal reaching the collection optics. A field-portable system designed by Sharma et al. [13] has successfully measured benzene, in both clear-glass and brown-glass bottles, at a distance of 10 m. While a Raman spectrum of the sample in a clear-glass bottle only required a collection time of 1 s and the sample in a brown glass bottle required 5 s [13], this shows the potential of stand-off Raman spectroscopy for identifying substances in a range of different containers.
4. Applications Today, stand-off Raman detection is used for a wide range of studies where access to the sample can be difficult, dangerous or destructive. Such applications include: atmospheric and geological measurements (e.g., [21]), where direct access to the sample is often limited or prohibitively difficult; identification or monitoring of explosives or chemical spills (e.g., [9,15]), where direct access to the sample could be dangerous or toxic; and, studies of ancient artifacts or artworks (e.g., [23]), where more direct measurements may damage a priceless object. Table 1 summarizes the stand-off distances and instrumentation parameters for these different applications. 4.1. Geology and geophysics One of the most active groups in the field of stand-off Raman is that of Sharma in Hawaii, whose research focuses on geological and environmental measurements using both field-portable and laboratory-based stand-off Raman instruments. These instruments operate at a laser wavelength of 532 nm (a frequency-doubled Nd:YAG laser), with a power of 35 mJ per pulse, a width
Application Geology Examples include calcite, marble, feldspar, Chemical Detection CCl4, sodiumferrocyanide Acetonitrile and cyclohexane on topsoil Air pollutants SO2 and kerosene Explosives Examples include TNT, PETN, TATP, RDX, HMX, Semtex
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Art and Archaeology Objects of art in museums, paintings on chapel wall Other Fundamental studies of ATOF vs. spectrograph and CCD vs. iCCD
Stand-off distance (m)
Laser wavelength (nm)
10–65 10–50 10–120
532 (pulsed) 532 (pulsed) 532 (pulsed)
16.7 533 3–30
Repetition rate / Laser power at source
Optical geometry Coupling regime
Wavelength dispersion
Detector
Ref.
20 Hz, 35 mJ/pulse 20 Hz, 35 mJ/pulse 20 Hz / -
Co-axial Co-axial & oblique Co-axial
Fiber optic Direct Direct
Spectrograph Spectrograph Spectrograph
iCCD iCCD iCCD
[12,17,20] [12,17,20,23] [26]
488 & 809 (CW) 266 (pulsed) 266 (pulsed)
- / 100 mW 30 Hz / 20 Hz, 7 mJ/pulse
Coaxial Coaxial
Transfer optics Transfer optics Transfer optics
Spectrograph Spectrograph Spectrograph
CCD iCCD iCCD
[8] [11] [21]
200
2 J, 2 pps ruby
-
-
-
-
Photo-multiplier
[3]
10 27 & 50 30 20 & 55
532 (pulsed) 532 (pulsed) 532 532 (pulsed)
Coaxial Coaxial
Fiber optic Fiber optic
Coaxial
Fiber optic
Spectrograph Spectrograph Spectrograph Spectrograph
iCCD iCCD iCCD iCCD
[12] [14] [16] [15]
-
514.5 (CW)
20 Hz, 35 mJ/pulse 10 Hz, 140 mJ/pulse 13 Hz, 20 mJ/pulse 10 Hz, 280 mJ/pulse (20 m) 10 Hz, 450 mJ/pulse (55 m) - / 500 mW
Fiber-bundle
Fiber-bundle
ATOF
Photo-multiplier
[18]
<1
785 (CW)
- / 300 mW
Probe head
Fiber optic
Spectrograph
CCD
[10,23,28,29]
15
532 (pulsed)
10 Hz, 47 mJ/pulse
Co-axial
Fiber optic
Spectrograph & ATOF
CCD and iCCD
[19]
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Table 1. Summary of instrumentation parameters for stand-off Raman systems discussed in the text
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of 8 ns and with a beam divergence less than 0.8 mrad [18,21]. The laboratory-based system can achieve clear, well-resolved Raman spectra of a range of minerals at a distance of 10 m with single laser-pulse excitation [24], as well as recording clear spectra of various minerals and chemicals at 50–100 m with a fast integration time of 1 s [25]. Both laboratory-based and field-portable stand-off Raman spectrometers have been used to identify a wide range of different mineral substances, including quartz, mica, calcite and marble [18,21,26]. The studies showed that these Raman systems can distinguish between closely-related minerals (e.g., dolomite, gypsum and fluorapatite), which are all Ca2+ based but contain differing anions [24]. They also showed that, when studying glasses of plagioclase composition, it was possible to distinguish between glasses formed during volcanic processes and those formed as a result of impact [18]. 4.2. Explosives detection Several studies have assessed the applicability of standoff Raman systems for remote identification of high explosives and associated compounds. They utilized both visible and UV excitation to characterize a range of explosives, including RDX, TATP, PETN, TNT and urea nitrate [15]. Additionally, explosive chemicals (e.g., ammonium nitrate and potassium perchlorate) have been detected at distances of 100 m with a 1-s integration time [25]. These explosives and related substances cover both nitrogen-based and peroxide-based explosives, showing the versatility of Raman spectroscopy for explosives detection. Carter et al. [15] developed a stand-off Raman system capable of detecting explosive compounds at 27 m and 50 m, with the 27 m set-up providing the best signalto-noise ratio, due to the overlap of laser-spot size and collection optics giving very efficient light collection [15]. This system utilizes a frequency-doubled Nd:YAG laser, producing 532-nm excitation, and a 10-ms gate width to allow good spectra collection in ambient-light conditions. They also studied the effects of laser power on their explosive samples and found that, for most of those studied, there appeared to be no significant laserinduced degradation of the sample. However, for TNT, spectroscopic and visual analysis indicated that laser powers greater than 3.4 · 105 W/cm2 can induce photo-degradation or thermal degradation of the sample, highlighting the importance of experimental parameters in measuring unknown explosive samples [15]. Gaft and Nagli [17] studied the pros and cons of using both green and UV excitation wavelengths for detecting explosives. They found that, for 532-nm excitation, Raman detection of explosives could be hampered by the presence of other materials in the background (e.g., metal surfaces, plastics or cloth that 1240
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the explosives may be contained in or deposited on) with such materials giving rise to luminescence [17]. When moving to 355-nm and 266-nm excitation using conventional laboratory-based systems – third and fourth harmonics of a Nd:YAG laser – the problem of luminescence from other materials could be compensated for by the increasing Raman cross-section of the explosive molecules at UV wavelengths [17]. For a number of explosive materials, with fundamental absorption bands in the UV, the move to UV-excitation wavelengths increases the likelihood of generating resonance or pre-resonance Raman signals that will provide an additional increase in signal intensity [27]. Gaft and Nagli found that, for the majority of explosives, the signal-to-noise ratio was much better when using 266 nm than when using 355 nm. However, RDX and its derivatives were the exception to this as a much clearer spectrum was recorded using 355 nm [17,27]. A further study [27] investigated the Raman crosssections of some explosives at different wavelengths in more detail. Finally, Sharma et al. also used a field-portable system to identify two explosives: b-HMX and TATB (distance 10 m, 10 s integration) with both low wavenumber (150–1800 cm 1) and high wavenumber (2990– 3225 cm 1) regions, giving rise to characteristic spectra that were well-defined fingerprints for the two explosives [13]. 4.3. Chemical detection A number of chemicals (e.g., cyclohexane and acetonitrile) have been used as substances to test the performance of newly-developed stand-off Raman systems. However, several authors have extended the application of such systems to more complex chemical samples. Wu et al. [12] studied carbon tetrachloride in acetonitrile and carbon disulfide in cyclohexane as models of chemical spills, while Ray et al. [22] showed that it is also possible to detect cyclohexane when it is placed on topsoil. Angel et al. [9] designed their system to measure solids, semi-solids and gases in storage tanks. They stated that a typical mixed waste tank will contain both liquid and solid wastes and studied a mixture of 10% sodium ferrocyanide and 10% sodium nitrate in a sodium-chloride matrix as a model system for such mixed wastes. Their stand-off Raman system could detect both substances in the waste mixture simultaneously, as the Raman bands of NO3 and ferrocyanide do not overlap [9]. 4.4. Art and archaeology While the stand-off distances required for many of the other applications described above are generally on the scale of meters, for reasons of accessibility and safety, the stand-off distances required for measuring objects of art are often much smaller, but there still needs to be sep-
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aration between the spectrometer and the object to prevent damage or contamination of the artifact. Also, the use of pulsed lasers is not advisable, due to the potential damage to the delicate sample surface. Vandenabeele et al. [23] have studied the performance of a number of commercially-available and bespoke Raman instruments designed with measurements at short standoff distances in mind. They used these spectrometers to study titanium oxide and strontium yellow (SrCrO4) as models for paints on objects of art. These studies were also extended to paintings on canvas and the identification of organic elements (e.g., beeswax) in pottery vessels [23]. The portable bespoke Raman instrument described in ref [23], described in full in [10], has been used to study a wide range of different art and archeological objects. These objects ranged from Egyptian sarcophagi to paintings on paper, textile and wood. In all cases, even with problems arising from ambient light and fluorescence from varnish layers, Vandenabeele et al. could identify the composition of most pigments present on these artifacts [28]. Such mobile Raman instruments can also detect the degradation products of pigments, identifying areas where paint colour has changed as a result of aging (e.g., medieval wall paintings at the chapel of Ponthoz in Belgium [29]).
5. Stand-off Raman combined with other laserbased techniques Sharma and co-workers have proposed that additional developments could centre around combining stand-off Raman detection with other techniques [e.g., laser-induced breakdown spectroscopy (LIBS) and laser-induced native fluorescence (LINF)] [18]. SharmaÕs conventional stand-off Raman system can be modified using a flash lamp and a delay generator to illuminate the sample at the correct time, allowing Raman and LINF spectra with 10-ns resolution at a 9-m stand-off distance. The LINF spectra give complementary information to the Raman spectra, allowing measurements of rare-earth and transition-metal ions at ppb levels as well as sensitive detection of organic materials [18]. A prototype instrument has been developed, combining Raman with LIBS and using the frequency-doubled Nd:YAG wavelength of 532 nm [18,30]. Due to the differences in power-density requirements for LIBS and Raman spectroscopies, the two are collected sequentially, with the laser power for Raman-spectra collection reduced slightly, through reducing the laser power or defocusing the laser, in order to minimize the possibility of sample ablation [30]. Further studies have shown that, with a pulsed laser operating at dual wavelengths (532 nm and 1064 nm), it is possible to measure LIBS and Raman spectra of minerals (e.g., calcite and gypsum) [31].
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Although the two techniques cover different spectral ranges, with LIBS a total of 650 nm and Raman (at 532 nm) 144 nm, they have compatible resolution requirements, with LIBS-emission lines resolved at 2500 (k/Dk) corresponding to a resolution of 7.5 cm 1 for Raman spectroscopy [30]. Finally, while the detection parameters are less significant for LIBS, due to the higher signal intensity than that of Raman spectroscopy, both techniques can benefit from the advantages of a gated detector reducing the effects of ambient-light contributions to the background signal [30]. 6. Future directions Stand-off Raman spectroscopy is still an emerging field and, for the most part, its future directions are likely to be application-driven. Advances that make the instrumentation smaller, and therefore more portable, as well as improvements in performance providing better signalto-noise ratios and LODs will not only improve the applicability of stand-off Raman to the topics described in this review but also broaden the range of fields of application. The rejection of ambient light, through choice of excitation wavelength or gating the detector, has been applied where the stand-off distances are on the scale of meters (e.g., explosives detection and geological measurements). However, discrimination of ambient light for measuring art and archaeology has not been developed extensively. Most stand-off Raman instruments that we described contain a single laser wavelength, which, whilst often making the overall design more portable, can lead to problems analyzing certain substances at particular excitation wavelengths {e.g., identifying azurite (Cu3(CO3)2(OH)2), a blue pigment used in Renaissance paintings, that does not scatter strongly when a 785-nm laser is used [23]}. In such cases, where samples may be colored, the ability to switch between different excitation wavelengths would be advantageous. Acknowledgements The research leading to these results received funding from the European Union-funded Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 218037. References [1] Committee on the Review of Existing and Potential STandoff Explosived Detection Techniques, Existing and potential standoff explosives detection techniques, 0-309-09130-6, National Research Council, Washington, DC, USA, 2004. [2] European Space Agency, 2009 (http://www.esa.int/SPECIALS/ ExoMars/SEMSZIAMS7F_0.html). [3] J. Cooney, Proc. Symp. Electromagnetic sensing of the Earth from Satellites, Brooklyn Polytechnic, Brooklyn, New York, USA, 1965.
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[20] J.C. Carter, J. Scaffidi, S. Burnett, B. Vasser, S.K. Sharma, S.M. Angel, Spectrochim. Acta, Part A 61 (2005) 2288. [21] S.K. Sharma, P.G. Lucey, M. Ghosh, H.W. Hubble, K.A. Horton, Spectrochim. Acta, Part A 59 (2003) 2391. [22] M.D. Ray, A.J. Sedlacek, M. Wu, Rev. Sci. Instrum. 71 (2000) 3485. [23] P. Vandenabeele, K. Castro, M. Hargreaves, L. Moens, J.M. Madariaga, H.G.M. Edwards, Anal. Chim. Acta 588 (2007) 108. [24] A.K. Misra, S.K. Sharma, P.G. Lucey, Appl. Spectrosc. 60 (2006) 223. [25] A.K. Misra, S.K. Sharma, P.G. Lucey, R.C.F. Lentz, C.H. Chio, Proc. SPIE 6681 (2007) 66810C. [26] S.K. Sharma, A.K. Misra, P.G. Lucey, S.M. Angel, C.P. McKay, Appl. Spectrosc. 60 (2006) 871. [27] L. Nagli, M. Gaft, Y. Fleger, M. Rosenbluh, Opt. Mater. (Amsterdam) 30 (2008) 1747. [28] P. Vandenabeele, J. Tate, L. Moens, Anal. Bioanal. Chem. 387 (2007) 813. [29] P. Vandenabeele, K. Lambert, S. Matthys, W. Schudel, A. Bergmans, L. Moens, Anal. Bioanal. Chem. 383 (2005) 707. [30] R.C. Wiens, S.K. Sharma, J. Thompson, A. Misra, P.G. Lucey, Spectrochim. Acta, Part A 61 (2005) 2324. [31] S.K. Sharma, A.K. Misra, P.G. Lucey, R.C. Wiens, S.M. Clegg, Spectrochim. Acta, Part A 68 (2007) 1036.