ARTICLE Total Internal Reflection Raman Spectroscopy

ARTICLE Total internal reflection Raman spectroscopy

Phillip R. Greene and Colin D. Bain Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA, UK. E-mail: [email protected]

Introduction to micrometres in thickness. In all cases, 1(a)] or through the sample itself [Figure Within the last two decades, Raman spec- our experiments were carried out on a 1(b)], if the sample is transparent. troscopy has become established as a modified commercial Raman spectrom- The principles of total internal reflection fast, non-invasive, non-destructive analyt- eter (Renishaw System 1000) equipped will be familiar to readers experienced ical tool. Development of commercial with a 2 W frequency-doubled Nd:YVO4 with attenuated total reflection Fourier Raman spectrometers has been spurred laser operating at 532 nm (Millenia 2, transform infrared (ATR-FTIR) spec- on by technological improvements in Spectra-Physics). troscopy. We will summarise here the lasers, optical filters and charge-coupled salient features of the theory of TIR and device (CCD) detectors. Modern Raman Total internal reflection use them to illustrate how a TIR geome- microspectrometers achieve high sensi- There are two basic requirements for TIR- try can confer surface-selectivity and tivity from liquids and solids with volumes Raman scattering from a thin film at an improved sensitivity on Raman spec- of only a few femtolitres. However, the interface. The first is that the interface is troscopy. sensitivity and spatial resolution of accessible to light from at least one side. commercial Raman systems is not yet The second is that the medium through Surface selectivity sufficiently good to allow routine charac- which the laser beam is delivered has a A beam of light incident on an interface terisation of ultrathin (<5 nm thickness) higher refractive index than the second is bent away from the surface normal as films at interfaces without the use of medium. Under these conditions, it is it passes from the optically more-dense enhancement mechanisms, such as elec- possible to arrange that the laser beam is medium (i.e. that having higher refractive tronic resonances or surface plasmons in totally reflected at the interface with only index) to the optically less-dense roughened metal surfaces achieved with an evanescent wave penetrating into the medium. The reflected portion of the surface-enhanced Raman scattering second medium. It is this evanescent beam is said to undergo internal reflec- (SERS). wave that generates the Raman spectrum tion. There exists a critical angle of inci-

Raman scattering by evanescent waves from the thin film at the interface. Figure dence, θc, above which all of the light is was first demonstrated by Ikeshoji et al. 1 shows two sample geometries that we reflected back into the optically dense in 1973.1 Although this early work used a have employed: scattered light can be medium and none of the incident energy bulk sample, it demonstrated the poten- collected either through the prism [Figure is transmitted into the optically rarer tial for obtaining unenhanced Raman spectra of ultrathin films at interfaces. Largely due to the explosive growth of SERS in the 1970s, the alternative approach of total internal reflection (TIR)- Raman spectroscopy received little atten- tion. With modern commercial instrumentation, however, TIR-Raman can be applied to a wide range of different physical and chemical systems, without some of the restrictions imposed by surface or resonance enhancement. This article provides an introduction to the principles and practice of TIR-Raman spectroscopy, and outlines recent devel- opments of the technique. We focus on examples from our own research group, Figure 1. Schematic diagrams showing configurations for TIR-Raman spectroscopy of an opaque where we have used TIR-Raman spec- (a) or a transparent (b) sample. For clarity, schematic (a) shows a cross-section through the inci- troscopy to study films from nanometres dent plane.

8 SPECTROSCOPYEUROPE www.spectroscopyeurope.com The Golden Gate™ HT At Up To 200°C, It’s For Those Who Take Their Applications To Extremes

Polymerisation Processes

Phase Change Studies (Solids To Liquids)

Degradation And Decomposition Studies

Polymorphism Studies Due To Temperature

Kinetic Studies Of Chemical Reactions

The Golden Gate™ HT has a real type IIa diamond brazed into a low thermal mass top plate, and is an absolute necessity to ensure the highest possible thermal conductivity in any ATR system. The diamond, set in close proximity to high power heaters, ensures that rapid and efficient heating is achieved. Right up to 200°C. (The heaters are low voltage and fitted with thermal fuses as standard for completely safe operation). With all this thermal power comes a high level of temperature control, to within +/- 1°C., and, like all Golden Gate™ systems, it is easy to clean and operate allowing for rapid sample turnaround. The Golden Gate™ HT is ideal for testing curing reactions such as polymerization, degradation, decomposition and for thermo- chemical studies. Extreme needs demand extreme solutions. THE GOLDEN GATE™ HT Nothing Touches It

FASTLINK / CIRCLE 005 FOR FURTHER INFORMATION THE SOLUTION IS SPECAC. WHAT’S THE PROBLEM? WWW.SPECAC.COM Specac Ltd., River House, 97 Cray Avenue, Orpington, Kent BR5 4HE UK +44 (0)1689 873134 Specac Inc., 410 Creekstone Ridge, Woodstock, GA 30188 USA Toll Free 800 447 2558 A part of Smiths Group plc ARTICLE

medium, i.e. there is total internal reflec- –1 tion. From Snell’s Law, θc = sin nti, where nti = nt / ni and nt and ni are the refractive indices of the transmitting and incident medium, respectively. Although there is no flux of energy across the interface, there is still an electromagnetic field in the second medium in order to satisfy the boundary conditions on the electric and magnetic fields at the interface.2 The transmitted wave propagates along the surface in the plane of incidence with its Figure 2. Variation of penetration depth of Figure 3. Graph showing the variation of electric field, E, decaying exponentially the squared electric field dp / 2, relative to Fresnel factors (| K |) with angle of incidence, with distance, z, normal to the interface the wavelength, λ, of the laser, as a function for beams of different polarisation, incident

[Equation (1)]. For this reason, it is known of the angle of incidence (θi), for TIR at the through a cubic-ZrO2 phase (n = 2.2) onto as an evanescent (“quickly fading”) wave. interface between a cubic-zirconia prism an interface with a polymer of refractive (n = 2.2) and a polymer (n =1.5). index 1.5. The first subscript of K gives the E =−exp()βz polarisation of the incident electric field, E 0 while the second subscript describes the 1 2 2π sin2 θ  beams with a shorter wavelength. Thus field at the interface. where β =− i 11 ()  2  the penetration depth for a TIR-Raman λ  nti  spectrum is small and constant at all β is the electric field amplitude decay wavenumbers in a given spectrum. Since in isotropic media. p-Polarised incident coefficient, θi is the angle of incidence, λ dp varies with θi, it is possible, in principle, light will, in general, give rise to fields in is the wavelength of incident light and E0 to obtain a depth profile of the surface of the x and z directions and s-polarised light is the electric field at z = 0. The penetra- a sample simply by changing the incident to a field in the y direction. tion depth, dp, of the electric field, into the angle. As Figure 2 shows, tight control over The dependence of the K-factors on optically rarer medium is given by the incident angle is required for depth incident angle (Figure 3) has two impor- dp =1/β. Except for angles very close to profiling by this method. tant aspects. First, Kpz and Ksy are

θc , dp is less than λ, and the electric field maximised when light is incident at the becomes negligible at a distance of only Improved sensitivity critical angle. The K-factors at θc are a few wavelengths into the second The relative proportion of incident energy several times greater than for normal inci- medium. Since incoherent Raman scat- transmitted and reflected from an inter- dence. Even after allowing for transmis- 2 tering is proportional to the intensity of face depends on the incident angle, the sion through the air–prism interface, Ey the incident light, and hence E2, the ratio of refractive indices across the inter- (which determines the strength of an s- Raman signal, drops off twice as fast as face, and the polarisation of the incident polarised Raman spectrum) in the TIR the electric field: >98% of the Raman light. The electric field in a thin film at an geometry is 1.6 times greater than in signal originates from a distance <2dp interface is related to the field in the inci- normal incidence through air. 2 from the interface. Figure 2 illustrates how dent medium by Fresnel factors, which The second feature is that Kpx becomes the penetration depth of the squared we will label K. For example, the K-factor zero at the critical angle. Consequently, a electric field depends on the angle of inci- for a conventional confocal Raman spec- p-polarised beam incident at θc produces dence. Note that dp→∞as θi →θc. trometer in which the laser beam is at an interfacial electric field that oscillates As in ATR-IR, the penetration depth is normal incidence in air onto a polymer purely in the z-direction. The special case minimised when the relative refractive with a refractive index of 1.5 would be 0.8. of TIR at θc is the only way to produce an index nti is smallest (i.e. when incident In Figure 3, we have plotted Fresnel electric field with purely z-oscillations at a media have high refractive indices). factors for the interface between a ZrO2 dielectric interface. TIR therefore enables

Materials such as cubic zirconia (ZrO2), prism and the same polymer. The more components of the Raman polaris- with a refractive index of 2.2, low fluo- subscripts of each K-factor refer to the ability tensor to be measured than does rescence and no substrate bands at polarisations of the incident beam (s- or conventional beam delivery along the wavelengths >700 cm–1 make for good p-polarised) and the polarisation of the surface normal. coupling prisms in TIR-Raman spec- field at the interface (x, y or z; the plane troscopy. of incidence is the xz plane, with z as the Practical considerations

Since dp varies with λ, TIR-Raman spec- surface normal). Light polarised parallel There is no surface sensitivity exactly at troscopy has some advantages over ATR- to the incident plane (p-polarised) is the critical angle, but with an incident

IR spectroscopy in that TIR-Raman independent of light polarised perpen- angle slightly larger than θc , we can experiments employ monochromatic laser dicular to the incident plane (s-polarised) achieve a good compromise between

10 SPECTROSCOPYEUROPE www.spectroscopyeurope.com Corner the Performance Advantage with JASCO FT-IR Introducing the New FT/IR-4000 and FT/IR-6000 Series Highly Stable Corner Cube Interferometer with AccuTrac™ DSP Technology Guarantees the Highest Level of Performance. • Robust High Sensitivity FT-IR Systems for a Wide Range of Routine and Critical Analysis Application • IQ Accessory Recognition System Programmable for any Commercially Available Sampler • Rapid Scan and Step Scan Options for Real Time Analysis of Reaction Kinetics and Time Resolved Imaging With over forty years of experience in infrared spectroscopy, JASCO introduces a new series of advanced FT-IR instrumentation and sampling accessories. The FT/IR-4000 and FT/IR-6000 Series set a whole new standard for performance, flexibility, and ease of operation. Each compact model offers unsurpassed reliability and the highest signal-to-noise ratio in the industry. Whatever your application or budget there's a perfect JASCO FT-IR solution...not only for today but for tomorrow.

Comparison Proven

FASTLINK / CIRCLE 006 FOR FURTHER INFORMATION JASCO EUROPE s.r.l. Via Confalonieri 25, 22060 Cremella (Co), Italy Tel: +39-039-956439 Fax: +39-039-958642 Internet: http://www.jasco-europe.com JASCO INTERNATIONAL CO., LTD., 4-21, Sennin-cho 2-chome, Hachioji, Tokyo193-0835, Japan Tel: +81-426-66-1322 Fax: +81-426-65-6512 Internet: http://www.jascoint.co.jp/english/index.html JASCO INCORPORATED, 8649 Commerce Drive, Easton, Maryland 21601-9903, U.S.ATel:+1-800-333-5272 Tel:+1-410-822-1220 Fax:+1-410-822-7526 Internet:http://www.jascoinc.com FT-IR • UV-Vis • Fluorescence • Chiroptical • Chromatography ARTICLE

depth sensitivity (Figure 2) and interfacial electric field (Figure 3). Typically the evanescent wave penetrates to a depth of <200 nm, imparting excellent depth resolution compared to confocal Raman spectroscopy, which usually achieves a depth resolution of ~2 µm at best. In cases where depth resolution is not important, operating at the critical angle gives optimum sensitivity. Several other >ÊÞÕÊ}iÌÊ>Ê factors can improve the sensitivity of TIR- Figure 4. Raman spectra of PEN (2.5 µm Ài`ÕVi`ÊÀ>ÌiÊ Raman techniques compared to confocal thick) on the surface of a PET film (200 µm Raman experiments. Where the required thick): (red) θi < θc; (blue) θi > θc (TIR). ÊÃÕLÃVÀ«ÌÊÌÊ depth discrimination is provided by TIR, Conditions: 532 nm, 0.2 W, 10 s red) 5 min -¶ the microscope can be used simply as (blue), Olympus ULWD 50× dry objective. efficient collection optics (not as a means to reduce the depth of focus). vÊÞÕÊ>ÀiÊ>ÊiLiÀÊvÊÌ i\ Consequently, the field of view of the and PET peaks are clear. The blue spec-

NÊ ÀÌÃ Ê >ÃÃÊ-«iVÌÀiÌÀÞÊ detection optics can be increased with- trum was obtained with θi > θc (TIR) and -ViÌÞ out compromising the depth resolution. only the PEN peaks are observed. In a A large area of the interface then confocal Raman spectrum of the same NÊ iÕÌÃV iÊiÃiÃV >vÌÊvØÀÊ contributes to the signal and produces laminate (not shown), the peak at >ÃÃiÃ«iÌÀiÌÀi spectra with higher signal-to-noise ratios. 1615 cm–1 from the PET substrate was NÊ-VjÌjÊÀ>X>ÃiÊìÊ Since larger laser spots can be employed, still visible. -«iVÌÀjÌÀiÊìÊ >ÃÃi higher laser powers can be used without exceeding the damage threshold of the Frictional contacts NÊ-ViÌDÊ V>ÊÌ>>> sample. Delivering the laser beam inde- Friction in solid–solid contacts can be Ì iÊÞÕÊV>tÊÕÃÌÊVÌ>VÌÊ pendently of the collection optics also reduced by ultrathin films of boundary allows the use of higher laser powers. lubricants, which are often no thicker than ÞÕÀÊÃViÌÞÊ>`ÃÌÀ>ÌÊ Only scattered light enters the microscope a monolayer. TIR-Raman spectroscopy vÀÊìÌ>Ã° and the scattering intensity is too low to can provide molecular-level detail about vÊÌ]ÊÃÕLÃVÀ«ÌÃÊÌÊ -Ê damage the objectives and beam split- monolayers at solid–solid interfaces, if we ÃÌ>ÀÌÊvÀÊÞÊ%ÎÇä]ÊvÀÊ ters. In combination, these factors can arrange that at least one of the solids is >ÊÃÕLÃVÀ«ÌÊÌÊÌ iÊÜiLÊ result in a signal-to-background improve- an optical component. ÕÀ>°Ê ment of two orders of magnitude in TIR- Figure 5(a) shows a spectrum of a Raman scattering compared to confocal Langmuir–Blodgett (LB) monolayer of 3 ÀÊÀiÊvÀ>Ì]ÊÛÃÌ Raman spectroscopy. zinc arachidate [Zn(C19 H39CO2)2] at the

ÜÜÜ°«ÕL°V°ÕÉiÃ° Ì interface between a CaF2 prism and an Applications MgF lens. The sensitivity of unenhanced ÀÊVÌ>VÌÊÕÃÊ>ÌÊÌ iÊ>``ÀiÃÃÊ 2 Polymer laminates TIR-Raman scattering is sufficient to LiÜ Polymer films a few hundred microme- provide monolayer spectra of excellent Ê*ÕLV>ÌÃ tres thick are commercially important in quality in only a few minutes. The use of ÈÊ >ÀÌÊ applications such as plastic bags and soft- one curved and one planar surface >ÀÌ drink bottles. Polymers are often lami- imparted a well-defined contact area with V iÃÌiÀ nated with thin coatings of other an average pressure of 90 MPa, and the 7iÃÌÊ-ÕÃÃiÝÊ*"£nÊä9]Ê1 polymers to modify the surface proper- lateral resolution of our experiments /\Ê³{{£Ó{În££ÎÎ{ ties and much effort is directed towards (~30 µm) is sufficient to examine only the characterisation of these surface films. the centre of the region of contact \Ê³{{£Ó{În££Ç££ Figure 4 shows a TIR-Raman spectrum between the two surfaces. Comparison \ÊÃÕLÃJ«ÕL°V°Õ from a 2.5 µm coating of PEN [poly(ethy- with a TIR-Raman spectrum from the 7\ÊÜÜÜ°«ÕL°V°Õ lene naphthalate)] on PET [poly(ethylene CaF2–air interface shows that the appli- terephthalate)] acquired by pressing the cation of pressure does not cause major

polymer against a cubic-ZrO2 prism [Figure disruption of the monolayer [Figure 5(a)]. 1(B)]. The red spectrum was obtained Since the monolayer peaks fall in regions

FASTLINK / CIRCLE 007 with θi < θc , in which case the bulk poly- of the spectrum where the substrates give FOR FURTHER INFORMATION mer film is sampled and both the PEN no signal, exclusion of bulk signals was

12 SPECTROSCOPYEUROPE www.spectroscopyeurope.com ELAN eliminates the element of surprise from ICP-MS.

High throughput, yes. High sensitivity, yes. We’ve been the inorganic analysis leader High blood pressure, no way. With an ever since AA, so it’s no wonder we’re the ELAN® 9000, DRC-e or DRC II, the excitement name you can trust for ICP-MS. If you’d of ICP-MS is in performance and results. You like to experience astonishing productivity won’t get any training or downtime surprises at the detection levels you need, talk to a because we promise (and deliver!) turnkey PerkinElmer ICP-MS specialist and be sure to methods to get you up and running quickly. ask for a free Guide to ICP-MS. You can also Plus, all ELAN systems provide maximum request a guide, or sign up for one of our Web uptime with minimal maintenance. seminars, at www.perkinelmer.com/ICPMS.

CIRCLE 008 FOR SALES CIRCLE 009 FOR LITERATURE

800-762-4000 (U.S. and Canada) (+1) 203-925-4602

face are shown in Figure 6. The small ular waxes – a multiphase system that is penetration depth (~220 nm) of the typically between 100 nm and 1 µm evanescent wave restricts the size of the thick and contains crystalline, solid amor- water peaks, facilitating subtraction to phous and liquid amorphous compo- yield clear spectra of an immersed phos- nents. Foliar wax layers have mostly been pholipid bilayer, even in the fingerprint studied in the past as extracted, reconsti- region. The stretching mode of the tuted wax, and by techniques such as carbon–carbon double bond (at chromatography which are necessarily 1650 cm–1) is readily detected, even invasive, destructive and in vitro. Little is though there is only one such bond for known about the structures and proper- every two lipid chains. The relative inten- ties of cuticular wax layers in vivo. sities of peaks in spectra with different With TIR-Raman scattering, it has been polarisations could be used to infer the possible to obtain spectra from the cutic- orientation of particular functional groups ular waxes on the surface of living barley in such artificial membranes. leaves (see Figure 7). The ratio of peak Figure 5. (a) A TIR-Raman spectrum of an Experiments with bilayers of soluble intensities for the antisymmetric –1 LB monolayer of zinc arachidate on a CaF2 surfactants adsorbed on silica have (2882 cm ) and symmetric methylene prism in air, overlaid with a spectrum of the shown that, for a concentration of 1 mM, modes (2851 cm–1) indicates that the film confined between a CaF2 prism and an surfactant molecules dissolved in the bulk surface waxes are crystalline. Our TIR-

MgF2 lens under ~90 MPa pressure; (b) solution within the evanescent wave typi- Raman studies of leaves also contributed polarisation-resolved TIR-Raman spectra of cally contribute only ~4% of the total evidence for trace amounts of carotenoids an LB monolayer of zinc arachidate on a surfactant signal in TIR-Raman spectra. At in the cuticular waxes. silica prism in contact with an LB monolayer higher concentrations the relative impor- Spectra of cuticular waxes provide a of deuterated zinc arachidate on a CaF2 lens. tance of the bulk surfactant signal particularly elegant demonstration of TIR- Conditions: (a) 0.9 W, 532 nm, s-polarised, increases, but it is relatively straightfor- Raman spectroscopy, since leaves present

15 min, θi = 49° (solid–air interface) or ward to account for the bulk contributions a number of practical obstacles. Leaf

θi = 78° (solid–solid interface), Olympus to spectra. surfaces are rough and may be easily ULWD 50× dry objective; (b) similar, except damaged by pressure; contact with a

θi = 80°, ~33 MPa pressure, 5 min (s- Leaves prism is imperfect, but the technique still polarised) or 10 min (p-polarised). The outermost layer of a leaf surface is has ample sensitivity. Notably, our spec- called the cuticle. The most important tra were recorded with a 532 nm (green) fraction of foliar cuticles comprises cutic- laser, yet the spectra contain minimal not a major concern in this system. To fluorescence from the underlying leaf maximise sensitivity, the angle of inci- constituents and pigments. It is possible dence was very close to the critical angle, to avoid fluorescence from plant material and scattered light was collected from a relatively large area at the interface (~500 µm2). Spectra recorded from the solid–solid contact under different polarisation conditions (sx, sy, px, py) are shown in Figure 5(b). The absence of peaks in the px and py spectra demonstrate that the hydro- carbon chains in the monolayer are oriented perpendicular to the interface. Figure 6. TIR-Raman spectra of a bilayer of egg lecithin (palmitoyl oleoyl phosphatidyl- Silica–water interface choline, POPC) at the silica–water interface, For our third example, we consider TIR- before (blue) and after (red) subtraction of Figure 7. TIR-Raman spectrum of cuticular Raman spectroscopy of lipids and surfac- the pure buffer spectrum. Note that residual waxes on the surface of a barley leaf, tants adsorbed at the silica–water noise levels after subtraction are larger in pressed against a cubic-ZrO2 prism. The interface. These systems demand sensi- regions with large background signals. prism adds two broad humps to the baseline tivity to one or two monolayers of surfac- Conditions: 532 nm, 1.8 W, incident beam s- (at 3040 cm–1 and 2700 cm–1), which have tant as well as suppression of the bulk polarised, collected light polarised in y-direc- been removed by subtraction. Conditions: signals from dissolved surfactants and tion, θi = 68°, Zeiss 40× water immersion 0.2 W, 10 min, 532 nm, s-polarised, laser water. Spectra from a single bilayer of egg objective, 350 s [CH stretching region, (a); spot size ~(20 × 30) mm, θi = 56°, lecithin adsorbed at the silica–water inter- 1200 s (fingerprint region, (b)]. Olympus ULWD 50× dry objective.

14 SPECTROSCOPYEUROPE www.spectroscopyeurope.com ARTICLE

with infrared probe beams, but not with- depth resolution arises from the evanes- Acknowledgements out significantly reducing the sensitivity cent wave and not from the confocal We would like to thank the following and depth resolution. We employed a parameter of a tightly focussed laser members of the Bain research group, high incident angle that limited the pene- beam, it is possible to use relatively large who supplied some of the data presented tration depth dp / 2 to 60 nm (refer to laser spots (>10 µm diameter), higher in this article: Chongsoo Lee (egg lecithin Figure 2), so the laser field was confined laser powers and wide entrance apertures spectra), Sarah Haydock and David to the wax layers. Leaves are susceptible on the spectrometer, leading to higher- Beattie (zinc arachidate spectra). to laser damage as well, but the barley quality spectra, shorter acquisition times Laminated polymer samples were kindly was not burned because it was only and less sample damage. provided by Neil Everall (ICI plc). The exposed to the evanescent wave, the We have shown that TIR-Raman scat- work was supported by Syngenta and the laser spot was relatively large (therefore tering can be used to study an assort- EPSRC. low in intensity), and the prism acted as ment of interfaces, simple and complex, an efficient heat sink. robust and delicate. All the TIR-Raman References experiments were based on a commer- 1. T. Ikeshoji, Y. Ono and T. Mizuno, Appl. Conclusions cial Raman microscope and a conven- Optics 12, 2236 (1973). Both in theory and in practice, TIR-Raman tional laser system. While commercial 2. M. Born, E. Wolf and A.B. Bhatia, spectroscopy offers several advantages accessories for TIR-Raman are not yet Principles of optics: electromagnetic over conventional Raman scattering available, there is no reason why TIR- theory of propagation, interference methods. The main benefits associated Raman scattering should not take its and diffraction of light. Cambridge with TIR-Raman techniques are excellent place as a standard technique in the tool- University Press (1999). depth resolution and improvements in box of the vibrational spectroscopist, just 3. D.A. Beattie, S. Haydock and C.D. the strength of the incident field due to as diamond ATR systems have done for Bain, Vib. Spectrosc. 24, 109 (2000). favourable electric field effects. Since infrared absorption spectroscopy.

Is it possible to be a KnowItAll in NMR, IR, MS, and Raman? · Access Over 860,000 Spectra · Build Libraries of Your Own Data · Award-Winning Software Tools

Predict Process Search Manage Report

Get Your Free "Make Me a KnowItAll" CD at KnowItAll® www.knowitall.com/se0804 (enter code: SE0804) Informatics System Call +44 (0) 20 8328 2555

FASTLINK / CIRCLE 010 FOR FURTHER INFORMATION www.spectroscopyeurope.com SPECTROSCOPYEUROPE 15