A Powerful Tool For Material Identification: Raman Spectroscopy - NAUN

INTERNATIONAL JOURNAL OF MATHEMATICAL MODELS AND METHODS IN APPLIED SCIENCESE hν(1)orE hcν,(2)where h is Planck’s constant, ν is its frequency, ν is thewavenumber, thus the reciprocal wavelength. Mentionedspecific energy E have to fulfill following quantum condition E E p Eq hν hcλ.(3)Ep and Eq are energies of different quantum states in which amolecule can exists, c is the velocity of light and λ is thewavelength. So the energy of a molecule will be changed by anamount E, if the molecule absorbs or emits the photon.Raman effect occurs when a researched material isirradiated by an intense monochromatic light. A major part oflight beam usually from near infrared, visible or nearultraviolet range is scattered without changes in frequency, noenergy is gained or lost, i. e. Rayleigh (elastic) scattering, apart is absorbed and a remained tiny fraction, important for theorigin of the spectra, is non-elastically scattered. Afterinteraction of the photon with the molecule, particularly withthe electron cloud and the bonds of the molecule, the photonevokes molecule excitation from the ground state to a virtualenergy state.When the molecule relaxes it emits a photon and it returnsto a different vibrational or rotational state. The energeticdifference, between the ground state and the final state, resultsin a shift in the emitted photon’s wavelength. Energy leveldiagram illustrates Fig. 4. The schematic principle of theRaman spectroscopy is shown in Fig. 5.If the molecule absorbs energy, i.e. the final state is moreenergetic than the initial state then the emitted photon of alower frequency generates a Stokes line. If the molecule losesenergy, the emitted photon of a higher frequency generates ananti-Stokes line. Wavenumber (Raman) shifts carry analyticalinformation on differences between the individual quantumlevels and play the key role in substance identification. TheseFig.4 Energy level diagram of Rayleigh and Raman scatteringIssue 7, Volume 5, 2011Raman shifts depend on the specific molecular geometry of thematerial and are independent on the incident photonwavelength, i.e. on the excitation wavelength of the laser [3].There can exist several characteristic shifts for certain materialwhich originates the Raman spectrum.Mostly only a more intensive (Stokes) part of the spectrumis measured. Both Stokes and anti-Stokes are approximatelysymmetrical towards the zero shift of the wavelength thatcorresponds with the incident laser line wavelength.Distribution of the lines in the spectrum informs about a sort ofthe bonds in the molecule. Every individual substance has itsown unique Raman spectrum characteristic only for therespective substance.Raman spectrum represents a dependence of intensity ofthe scattered light (in arbitrary units) on wavelength or on aRaman frequency shift (measured in cm-1). Intensity of theRaman scattering depends on several factors as the excitationwavelength of the used laser, used excitation power, changesin polarizability, the amount of Raman active moleculesilluminated by the laser beam and temperature. The intensityof a Raman band is theoretically described by George Placzek[4].In spite of the fact, that the particularity of Ramanspectroscopy is remarkable, the conversion efficiency ofRaman effect is rather poor, since only a scarcity (about 10-7)of the initial photons are non-elastically scattered. Hence thedetection of very low concentrated molecules is limited.In order to enhance sensitivity, to improve intensity, toreach better spatial resolution and other improvements numberof variation of Raman spectroscopy has been developed:Surface Enhanced RS, Resonance RS, Transmission RS,Spontaneous RS, Tip-Enhanced RS et al.Fig. 5 The sample is irradiated with laser, molecule vibrates,filter eliminates intense Rayleigh scattering, the gratingdisperses the light onto a detector to generate a spectrum,which gives the information about molecule bonding andprovides a chemical fingerprint utilizable for identification.1207

INTERNATIONAL JOURNAL OF MATHEMATICAL MODELS AND METHODS IN APPLIED SCIENCES IV. ADVANTAGES AND DISADVANTAGESOF RAMAN SPECTROSCOPYA. Advantages of the methodRaman spectroscopy has a number of indisputable advantageswhich appreciate scientist form variety spheres. These benefitsgo towards the growth of popularity of Raman spectroscopy inlaboratories worldwide. The method is: Non-destructive. Sample can be after Raman analysisconsequently treated by other procedures. Nondestructiveness also enables potential as in-vivodiagnostic tool in medicine providing information aboutboth the chemical and morphologic structure of tissue inquasi-real time. Non-contact. No contamination of a sample happens.This is convenience and safe. For example dangerous andtoxic samples or those with unpleasant smell, substanceswith unknown composition, properties, and history fromthe scene of an accident, samples, that are unstable to airor moisture can be measured through protective orcovering layers or packages from other materials as glassor polymers. Spectra of these packages can be latersubtracted using the software designed for manipulationwith data. Raman spectroscopy therefore appears as aneasy and strikingly useful technique analyzing andidentification of materials even in packing. No need for sample preparation prior the analysis isrequired - that is convenient and prompt. No physical orchemical conditioning of sample (diluting, pressing intopellets, etc.) is necessary. Highly sensitive. High spatial resolution is in the orderof micrometers. Currently the most commonly usedRaman spectrometers are combined with microscopes.Then only very small volume (about ones of µm indiameter) of a sample is needed for collecting Ramanspectra when using Raman microscopy. Thisinterconnection yields many benefits e. g. for forensicscience investigation of trace amounts of evidences. A rapid method. High quality Raman data are acquired invery short periods of time, often within seconds. E. g.chemical analyses generally take minutes or even hours.This kind of analysis definitely spares time. The rapidityof the method yields further potentialities inherent in ascope of almost immediate response and quasi real timeprocesses e. g. chemical changes kinetics monitoring. Applicable to a wide range of substances. Via Ramanspectroscopy is possible to measure liquids, transparentsolids and gases. Samples can be in a form of powder,crystal set, fiber, thin layer, gel, solution, etc. with noconcern with sample size, shape or thickness. Themethod is also appropriate for analyses of organic andinorganic compounds. Providing possibility of aqueous solutions explorationsince water is very weak Raman scatterer and generallyalmost does not interfere with Raman spectral analysis.This is a great advantage in comparison with infraredspectroscopy where the presence of water or moisture isunfavourable.Issue 7, Volume 5, 2011 Granting highly specific chemical “fingerprint”. Eachcompound gives rise to a unique Raman spectrum.The standard spectral range ideal for both organic andinorganic samples covers from 100 cm-1 to 3200 cm-1andcan be covered by a single spectra recording.The intensity of spectral features is directly proportionalto the particular species concentration.Remote analyses can be realized. Transmission of laserlight and Raman scattered signal can be done by fibreoptic probes over long distances – up to hundreds ofmeters far from the base of Raman analyzer.B. Drawbacks of Raman spectroscopyAs with all problems we solve and take into account relatedadvantages, we have to consider either the weak side limitations. In spite of many advantages of Ramanspectroscopy a well known competing process can appearalong with the Raman scattering: fluorescence. Fluorescencesignal is generated by a part of matter that undergoes anelectronic transition to an excited state what initiate emissionof light at a wavelength which is changed from the excitationlaser. A mechanism controlling both effects is similar anddetermines that if one of the phenomena occurs, the secondwill likely as well. Fluorescence initiation probability versusprobability of Raman scattering initiation is in the order of 1photon in 103-105 versus 1 photon in 106 – 109. Hence wealways have to consider fluorescence interference. Ramanspectra, for instance, of certain biological samples are oftenoverlaid by fluorescence when visible wavelengths of laser areused. There exists ways to avoid or at least minimize thisadverse effect A solution often consists in a selection ofsuitable laser wavelengths preferably with lower photonenergy. As the laser excitation wavelength gets shorter theRaman scattering intensity increases but fluorescence tends tobe less troublesome as the laser wavelength gets longer [5].Thereby spectrometers equipped by several lasers withdifferent excitation wavelengths are recommended toovercome the problem with fluorescence [6]. The interferingluminescence background can be also in some cases reducedby so called “bleaching”, i.e. prolonged sample illuminationwith the laser beam antecedent to concrete measurement [7].Even though Raman spectroscopy can be applied onmeasuring an extend spectrum of materials, metals and alloysare not Raman active.Other disadvantage of Raman spectroscopy is thesensitivity of the method. In spite of the fact, that theparticularity of Raman spectroscopy is remarkable, theconversion efficiency of Raman effect is rather poor since onlya scarcity (about 10-6) of initial photons is non-elasticallyscattered. In other words highly sensitive equipment (lasers,detectors, filters), what usually means also quite expensive, isdemanded.One more disability is eventual damage or laser-induceddegradation of a sensitive or light absorbing sample (especiallywhen examining unknown matter) through intensive laserradiation however experienced analysts can anticipate thiscomplication in the right preference of measurementprocedure.1208

INTERNATIONAL JOURNAL OF MATHEMATICAL MODELS AND METHODS IN APPLIED SCIENCESV. APPLICATIONSRaman spectroscopy has been reborn in recent years. As themore sophisticated spectroscopic and computer technologyaccess increased, Raman spectroscopy has become a tool fornot only purely research, but also for contrivable routineindustrial analyses. These days, Raman spectroscopy findsapplications, which number is still rising, in variety scientificand industrial disciplines and branches such as: Material sciences including semiconductor industry,nanotechnology, solid state physics, synthetic polymermaterials and others. Raman spectroscopy is one of thebest-known methods, it is of central importance for allthe specification and structural analysis of almost allkinds of materials (amorphous, partially crystallic,transparent, non-transparent samples, samples withdifferent surface textures). Nanotechnology. Raman spectroscopy is suitabletechnique for characterization of nanomaterials as nanocomposites, nano-sized crystals, polymers andsemiconductors developed in a form of films, wires ordots or modern and ancient ceramics. The methodprovides determination of nanocrystals, chirality,semidiameters in nanomaterials, also characterizingmicromechanical behavior or synthesis processes control(polymer curing, laser ablation, electrochemicaldeposition). It is a good tool for mapping and probingnanophases dispersed in matrix. Semiconductor industry. Raman spectroscopy allowssemiconductor impurities determination in siliconesubstrates and diamond-like carbon coatings (a pointmeasurement on silicone can be obtained in about 0.1second), identification of defects particles on the materialsurfaces. Such results notably affect device yields and theeconomics of the process line. Solid-state physics. Raman spectroscopy systems findapplication in material characterization, finding thecrystallographic orientation, etc. Synthetic polymer materials. Raman spectroscopyserves for real-time monitoring of polymerizationreactions for the purpose of controlling the processingtime, for plastic identification for recycling purposes,measuring the thickness of polymers such as protectiveand coating films, evaluation of polymer material underapplied strain or quality control of incoming/outgoingproducts. Chemistry. Raman spectroscopy provides a chemicalfingerprint for identification of a molecule, sincevibrational information is specific to the chemical bondsand symmetry of molecules. Except for identification themethod is used for characterization and analyses oforganic and inorganic substances, including carbonmaterials, solvents, films and for chemical processesmonitoring. Forensic sciences. Forensic scientists often deals withreally unknown samples that cannot be reproduced,require multiple analyses, but their amount is limited.Raman spectroscopy/microscopy is becoming a tool ofmajor importance in forensic science since it satisfiesIssue 7, Volume 5, 20111209most of forensic examination criteria, as are mentionedabove in advantages: it is a non-destructive, non-contactmethod without the necessity of sample preparation andpossibility of application on a wide range of materials.In-situ measurements can be realized, meaning nocontamination of evidences during taking samples [4].Raman microscopy brings benefits of optical microscopy,what means that the sample can be surveyed undermicroscope, particles or locations isolated andconsequently their Raman spectra acquired.Method is used for forensic analyses and rapididentification of trace amounts of substances in evidentialmaterials as are paints, inks from documents, pigments,explosive particles, inflammables, drugs, illegal activeingredients, fibers, gunpowder residues, chemical andbiological agents, plastics and other various forensicevidences. An example of Raman spectra of cocaine isshown in Fig. 6. In most cases forensic examinationcomprise comparative analyses, hence the spectralibraries and databases are required. In case of havingcomparable samples it is easy to demonstrate the matchor identify counterfeit for instance of documents orhistorical artworks.Fig.6 A – Raman spectrum of cocaine in plasticcontainer, B – Raman spectrum of plastic bag, C – thedifference spectrum of cocaine obtained by subtraction ofspectra A-B. Security forces, fire brigades. Identification ofunknown or hazardous substances, by instance detectionof explosives or drugs. These units often work withportable spectrometers enabling identification right inaction. Appreciable benefit of method is potentiality ofmeasuring samples through containers. Pharmaceutical industry. The role that Ramanspectroscopy plays in pharmaceutical research,development and manufacturing is still rising. Ramananalysis is an efficient tool for control of quality andpurity of pharmaceuticals, active substances andexcipients (even through packing). Counterfeitspharmaceutical signify a health risk since they maycontain harmful impurities, wrong ingredients orincorrect amounts of active pharmaceuticals. Raman

INTERNATIONAL JOURNAL OF MATHEMATICAL MODELS AND METHODS IN APPLIED SCIENCESFig. 7 Raman spectrum of high-grade urothelialcarcinoma (UC; dashed) tissue, low-grade UC tissue(dotted), and normal tissue (solid) – the notable peak at1584 cm-1. [8]Fig. 8 Schema of Raman spectrometeroccurrence in nature. In living environment can be foundin several modifications. The most frequent form ismetallic chromium Cr(0), trivalent chromium Cr(III) andhexavalent chromium Cr(VI). Trivalent and hexavalentchromium compounds are produced in largequantities and are accessible to most of the population.However, Cr(VI) is a carcinogenic substance and maycause health risks.[9] There is a possibility of conversionof Cr(III) into Cr(VI). It is already proved that Ramanspectroscopy can distinguish Cr(III) from Cr(VI) [10](Fig. 9, Fig. 10). Obtained results showed thecomplication with fluorescence that masks the Ramanspectra of chromium compounds contained in naturalspectroscopy has been also successfully used foradulterated pharmaceuticals revealing. Medicine and biology. Applications benefiting Ramanspectroscopy involve e. g. DNA analyses, prognoses anddiagnoses of carcinomas (example displayed in Fig. 7),measuring blood and tissue oxygenation, study ofbiological systems. With regard to non-destructivenessand contactless the great potential of the method for invivo medical examination is studied. Food Product. Raman spectroscopy serves for detectingbacteria and contaminants in food product ormeasurements of fatty acid unsaturation in food oils andother ingredients. Geology and mineralogy – Raman spectroscopy servesfor identification of the principal mineral phases orclassification of rocks, etc. Art. The main intention of any analytical studies of anyancient artwork should be to get as much information aspossible by non-destructive methods. Raman/MicroRaman spectroscopy is suitable and effective for thispurpose. Raman examination of artworks and artefacts(the most often are paintings and pottery) reveals worthyinformation for conservators or those of general historicalinterest. Knowledge of artistic materials in connectionwith the time period or the location in particular regionand also about the artists’ modus operandi can be gained.Followed issues are solved at the present time usingRenishaw inVia Basis Raman microscope (schema in Fig. 8)with the 514 nm excitation Argon ion laser at Faculty ofApplied Informatics, Tomas Bata University in Zlin, CzechRepublic. The methodology for the detection of small hexavalentchromium concentrations with the use of Raman spectroscopyis developed. Chromium is an element of a commonIssue 7, Volume 5, 2011Fig. 9 From above: Raman spectrum masked by fluorescence,Raman spectrum of Cr(VI) and Cr(III)1210

INTERNATIONAL JOURNAL OF MATHEMATICAL MODELS AND METHODS IN APPLIED SCIENCESFig. 10 Fibers of leather with linked hexavalent chromiumFig. 11 3D map of epoxy resin crosslinking processpolymers. This situation has been so far solved bybleaching – prolonged illumination of the sample beforeown measurement, but without success. On the otherhand, shortening the exposure time to tenths of secondsallowed recording of the Raman spectra. Raman spectroscopy has been applied on monitoring ofthe curing process of epoxy resins. Epoxy resins are oneof the most versatile polymers with a number of good toexcellent properties and owing to them are epoxy basedmaterials intensively applied in many technical areas andindustries. Due to a time series measurement can thekinetics of crosslinking be controlled and the changes inactive chemical groups and bonds recorded and analyzed.The progression of crosslinking process dependence ontemperature has been also studied. In Fig.11 is displayed3D map of crosslinking reaction at the temperature 25 Crecorded over 30 minutes with evident peakcorresponding to gelation time. Application of Multi-Walled Carbon Nanotube(MWCNT) for the purpose of conducting layers creationis investigated. Raman spectroscopy is very useful in thedetection of carbon form in fundamental research andalso in industrial usage. This technique is able todistinguish affect properties of electric conductivity ofdifferent forms of carbon (Fig.12.), because of evidentRaman spectral variances, and also map them. While, forexample, SEM indicates only presence of elementalcarbon in the sample [2]. Different concentrations ofMWCNT are used for shielding electromagnetic fields orelimination of static charge and others. Determination of inks within the scope of security usageis an upcoming problematic at present time dealing withthe possibility of revealing ink document falsifications.Ten different blue ball-pen inks with the known producerwere examined for the purpose of ink identification andaging and eventual document dating. Actual results showIssue 7, Volume 5, 20111211Fig. 12 Raman spectra of different modifications ofcarbonvery similar Raman spectra at the inks from the sameproducer e.g. KOH-I-NOOR, other are comparable witha few differences, which, however, should be sufficientfor the identification. Two samples were sheltered bystrong fluorescence and despite different setting ofmeasuring parameters, Raman spectra was impossible toobtain. This can be after all also a sort of hint foridentification following by other methods. Another application deals with the mapping of thedistribution of active substances in pharmaceuticals. InFig. 13 and Fig. 14 are for illustration shown 3D maps ofdistribution of acetylsalicylic acid and ascorbic acid in aAspirin.Other applications are prepared in present time.

INTERNATIONAL JOURNAL OF MATHEMATICAL MODELS AND METHODS IN APPLIED SCIENCESFig. 14 3D map of ascorbic acid in a Aspirin.Fig. 13 3D map of acetylsalicylic acid in Aspirin.VI.CONCLUSIONRaman spectroscopy is rapidly progressing method. Raman/Micro-Raman spectrometers are becoming essential andunnecessary equipments for researchers in variouslyspecialized laboratory such as forensic, pharmaceutical,laboratories, for development of nanomaterials, laboratoriesfocused on examination of artworks or for controllingproducts.Raman spectroscopic technique was studied as aninnovative method for obtaining information about a structureand properties of a wide range of materials, which can be usedin almost all technical and industrial branches. Measurementson a concrete device InVia Basis Raman Microscope wererealized. Possibilities of both a structure and properties ofselected materials were verified. Actual applications werehighlighted to demo

identified without problems. A demonstration of infrared and Raman spectrum of styrene-butadiene rubber can be seen in Fig. 2. Fig.2 Infrared and Raman spectrum of styrene-butadiene rubber From time to time a complication with other quantum process fluorescence, which occurs with a time delay, can appear and disrupt acquiring of Raman spectra. III.