Raman near-infrared excitation

Stichtemath, A., R. Schweitzer-Stenner, W. Dreybrodt, R.S.W. Mak, X-Y. Li, L.D. Sparks, J.A. Shelnutt, C.J. Medforth, and K.M. Smith (1993). Macrocycle and substituent vibrational modes of nonplanar nickel(II) octaethyltetraphenylporphyrin from its resonance Raman, near-infrared-excited FT Raman, and FT-IR spectra and deuterium isotope shifts. J. Phys. Chem. 97, 3701. 

Kneipp K, Kneipp H and Seifert F 1994 Near-infrared excitation profile study of surface-enhanced hyper-Raman scattering and surface-enhanced Raman scattering by means of tunable mode-locked... 

Raman spectroscopy has been widely used to study the composition and molecular structure of polymers [100, 101, 102, 103, 104]. Assessment of conformation, tacticity, orientation, chain bonds and crystallinity bands are quite well established. However, some difficulties have been found when analysing Raman data since the band intensities depend upon several factors, such as laser power and sample and instrument alignment, which are not dependent on the sample chemical properties. Raman spectra may show a non-linear base line to fluorescence (or incandescence in near infrared excited Raman spectra). Fluorescence is a strong light emission, which interferes with or totally swaps the weak Raman signal. It is therefore necessary to remove the effects of these variables. Several methods and mathematical artefacts have been used in order to remove the effects of fluorescence on the spectra [105, 106, 107]. 

M in concentration. This is in the range required for single-molecule detection. These sensitivity levels have been obtained on colloidal clusters at near-infrared excitation. Figure 10.3 is a schematic representation of a single-molecule experiment performed in a gold or silver colloidal solution. The analyte is provided as a solution at concentrations smaller than 10-11 M, Table 10.1 lists the anti-Stokes/Stokes intensity ratios for crystal violet (CY) at 1174 cm-1 using 830-nm near-infrared radiation well away from the resonance absorption of CY with a power of 106 W/cm2 [34]. CV is attached to various colloidal clusters as indicated in the table. Raman cross sections of 10-16 cm2/molecule or an enhancement factor of 1014 can be inferred from the data. 

The Raman spectrum of gases can now also be recorded with Fourier-Transform Raman spectrometers with near infrared excitation (Dyer and Hendra, 1992). Fig. 4.3-19 shows a survey spectrum of air obtained in 4 hours of sampling time (Bruker, 1993). The region of the rotational spectrum is presented on an expanded scale in Fig. 4.3-20, it can be compared with Fig. 4.3-18. The intensities of the lines below about 80 cm are weakened by the Rayleigh line suppression filter and the resolution is limited to 1 cm", mainly by the laser used for excitation. 

Near-infrared excited FT-Raman spectroscopy has recently begun to show promise (Schrader, 1990), because the fluorescence is drastically reduced. It has the Jaquinot advantage over classical Raman spectroscopy, which affords a better signal-to-noise ratio. FT-Raman is an excellent technique to supplement FTIR difference spectroscopy in investigations of intramolecular protein reactions because Raman spectra have the... 

A typical example of the characterization of a polymorphic system by FT Raman spectroscopy has been given by Gu and Jiang (1995) while an application of the technique with near infrared excitation to the polymorphic cimetidine system has been described by Tudor et al. (1991). The FT Raman technique has been compared to infrared diffuse reflection spectroscopy in the study of the polymorphs of spironolactone (Neville et al. 1992), and the pseudopolymorphic transition of caffeine hydrate (i.e. loss of solvent) has been monitored using the technique (de Matas et al. 1996). 

Hildebrand P, Keller S, Hoffmann A, Vanhecke F, Schrader B (1993) Enhancement factor of surface-enhanced Raman scattering on silver and gold surfaces upon near-infrared excitation. Indication of an unusual strong contribution of the chemical effect. J Raman Spectrosc 24 791-796... 

In the same year, Kneipp et al. published single-molecule Raman spectra for crystal violet (CV) adsorbed at Ag colloids dispersed in aqueous solution. The authors used near-infrared excitation at 830 nm, which is off-resonant to the electronic excitations of both the crystal violet and the isolated silver particles. 

FT-Raman spectrometer with near infrared excitation... 

FT-Raman Spectrometers with Near-Infrared Excitation... 

Keller S, Lochte T, Dippel B, and Schrader B (1993) Quality control of food with near-infrared excited Raman spectroscopy. Fresenius Journal of Analytical Chemistry 346 863-867. 

Raman spectroscopy is a vibrational spectroscopic technique which can be a useful probe of protein structure, since both intensity and frequency of vibrational motions of the amino acid side chains or polypeptide backbone are sensitive to chemical changes and the microenvironment around the functional groups. Thus, it can monitor changes related to tertiary structure as well as secondary structure of proteins. An important advantage of this technique is its versatility in application to samples which may be in solution or solid, clear or turbid, in aqueous or organic solvent. Since the concentration of proteins typically found in food systems is high, the classical dispersive method based on visible laser Raman spectroscopy, as well as the newer technique known as Fourier-transform Raman spectroscopy which utilizes near-infrared excitation, are more suitable to study food proteins (Li-Chan et aL, 1994). In contrast the technique based on ultraviolet excitation, known as resonance Raman spectroscopy, is more commonly used to study dilute protein solutions. 

Although fluorescence can be a serious problem in Raman studies of some biological systems, the availability of FT-Raman instrumentation which use near-infrared excitation has been suggested to eliminate this problem for many cases (Colaianni et al, 1995). 

J Barbillat, E Da Silva, B Roussel. Demonstration of low-frequency performance and microanalysis capability of multi-channel Raman spectroscopy with near-infrared excitation. I. J Raman Spectrosc 22 383-391, 1991. 

C Dyer, BJE Smith. Application of continuous extended scanning techniques to the simultaneous detection of Raman-scattering and photoluminescence from calcium disilicates using visible and near-infrared excitation. J Raman Spectrosc 26 777-785, 1995. 

Besides the established dispersive techniques using monochromators and polychro-mators, also the Fourier transform (FT)-Raman technique with near-infrared excitation has... 

Raman and infrared spectra have also been compared for a series of 1,4-benzodiazepines, including diazepam (Vallium) and of closely related compounds [16,17]. The complementary nature of these two vibrational spectroscopic techniques was highlighted and the data provided spectral features that allowed identification of the drugs. The value of Fourier transform Raman spectroscopy using a near-infrared excitation source was also demonstrated for these heterocyclic molecules which have a tendency to fluoresce with visible radiation from conventional dispersive Raman spectrometers. 

Figure 14 shows the improvement in Raman spectra via near-infrared excitation due to the elimination of fluorescence of humic substances. The top spectrum was taken with 514.5 -nm excitation, and the Raman signal is completely obscured by fluorescence. However, the bottom spectrum that was taken with 1064-nm excitation with an FT-Raman system provided two clearly observable Raman bands. The peak in the 1300-cm range (referred to as the D band) proves to be extremely valuable because it indicates the amount of disorder of carbon networks. Because of the presence of this band and the absence of second-order Raman bands of humic substances (2000-3000-cm range), Yang and Wang [99] have concluded that the backbones of humic substances are structurally disordered carbons (i.e., graphitelike) for most cases. 

Figure 14 Improvement in the Raman spectrum of peat humic acid in neutralized form by use of near-infrared excitation. The top spectrum was obtained with 514.5-nm excitation, which results in a large fluorescence background that obscures the signal. However, as shown in the bottom spectrum, with 1064-nm excitation, and after subtraction of thermal emission background, spectral features of peat humic are clearly discernible. (Reprinted with permission from YH Yang, HA Chase. Applications of Raman and surface enhanced Raman scattering techniques to humic substances. Spectr Lett 31 821-848, 1998. Copyright 1998 Marcel Dekker, Inc.)...

The only way to overcome all of these problems is to use a nondestructive in situ analytical technique. Given the small quantities of colorants present in trace samples, ultaviolet (UV)-visible microspectrophotometry is one of the few analytical techniques that can be used in situ. Unfortunately, the electronic spectra data obtained provide only a very limited amount of molecular information to compare samples and help in the identification of a dye. The technique is very poor where discrimination of dyes in the mixture is required and this can be a key piece of information, for example, if a fiber is to be related to a garment. Thus, Raman spectroscopy using near-infrared excitation and SERRS are techniques with very considerable potential in forensic analysis. 

Since its inception Raman spectroscopy has been plagued by fluorescence, which can totally overwhelm the Raman effect. Because fluorescence is an electronic effect of lifetime different from that of the Raman effect, it can be gated out by a Kerr-effect filter. The times involved are femtoseconds hence, such gating has been described only recently by Matousek [5] and it is certainly very expensive at this time and not commercially available. Less expensive, but not exactly inexpensive, is using ultraviolet excitation. Several commercial ultraviolet (UV) Raman instruments are presently available. Ultraviolet excitation can reduce fluorescence of carbons significantly, although not entirely. Near-infrared excitation has been used for the same purpose. Again, it is not a perfect eliminator of... 

Meyerab, M. W, and Smith, E. A. [2011] Optimization of silver nanoparticles for surface-enhanced Raman spectroscopy of structurally diverse analytes using visible and near-infrared excitation, Ana/yst, 136, 3542-3549. 

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