Fluorescence Problem

Colored samples or impurities in polymer samples may absorb laser radiation and re-emit it as fluorescence. The intensity of fluorescence can be as much as 104 times higher than that of Raman scattered light. The fluorescence problem is the major drawback of using Raman spectroscopy. Thus, a Raman spectrum can be completely masked by fluorescence. Three main methods can be used to minimize fluorescence  

Irradiate the sample with high-power laser beams for a prolonged time to bleach out the impurity fluorescence  

Change the wavelength of laser excitation to a longer wavelength (near infrared). The chance of fluorescence is reduced because the excitation energy of the laser beam is lower and 

Use a pulsed laser source to discriminate against fluorescence because the lifetime of Raman scattering (10-12-10-1° s) is much shorter than that of fluorescence (10 7—10 9 s). Thus, an electron gate can be used to preferentially measure the Raman signals. 

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CAHRS and CSHRS) [145, 146 and 147]. These 6WM spectroscopies depend on (Im for HRS) and obey the tlnee-photon selection rules. Their signals are always to the blue of the incident beam(s), thus avoiding fluorescence problems. The selection ndes allow one to probe, with optical frequencies, the usual IR spectrum (one photon), not the conventional Raman active vibrations (two photon), but also new vibrations that are synnnetry forbidden in both IR and conventional Raman methods. 

Another method, called photobleaching, works on robust soHds but may cause photodecomposition in many materials. The simplest solution to the fluorescence problem is excitation in the near infrared (750 nm—1.06 pm), where the energy of the incident photons is lower than the electronic transitions of most organic materials, so fluorescence caimot occur. The Raman signal can then be observed more easily. The elimination of fluorescence background more than compensates for the reduction in scattering efficiency in the near infrared. Only in the case of transition-metal compounds, which can fluoresce in the near infrared, is excitation in the midvisible likely to produce superior results in practical samples (17). 

Raman sensors can be used for all three physical states. It possible to measure gases, though at the expense of sensitivity due to the lower sample density, liquids and solids of different forms and shapes. Basically any Raman active substance could be detected, also in aqueous solution, provided substance and the sample matrix permit Raman measurements (fluorescence problem, absorption,. ..) and the analyte concentrations are sufficiently high. 

When the conventional Raman effect was discovered by Sir C. N. Raman, the expectations were very high. It was believed that the technique would be a major tool for chemical analyses. Those expectations were not realized primarily because of fluorescence problems, where fluorescence completely masks the Raman spectrum (see discussion on fluorescence in Chap. 1). The advent of FT-Raman makes Raman spectroscopy more useful in chemical... 

Besides aiding in the fluorescence problems in Raman scattering by performing the measurements in the near-IR region, FT-Raman can be of aid in... 

Raman spectroscopy failed to live up to its original expectation when the technique was discovered. This was due to instrumental problems, high cost of the instrument, and the fluorescence problem. However, with improvement in instrumentation, the use of a near infrared laser with FT-Raman, the introduction of fiber optics, the number of applications (some of which were discussed in Chapter 3) has escalated. The applications are expanded in this chapter, which deals with materials applications involving structural chemistry, solid state, and surfaces. Additional applications are presented in Chapter 5 (analytical applications), Chapter 6 (biochemical and medical applications) and Chapter 7 (industrial applications). 

Fluorescence problems occurring with conventional Raman spectroscopy precluded the use of this technique in studying food and agricultural substances. However, with the advent of FT- Raman, renewed interest has arisen in these studies. 

Heretofore, Raman spectroscopy has not played a role in forensic science because of the fluorescent problems and the sample alignment, which is time-consuming. As a consequence, the technique was never seriously considered as a routine tool to study forensic materials. However, with the development of FT-Raman spectroscopy, the technique is now being reexamined. One such application in forensic science follows (15). 

Instrument Calibration Sampling Techniques Fluorescence Problems Raman Difference Spectroscopy Miniature Raman Spectrometers References General References... 

The transition from amorphous carbon-containing deposits to graphite-like species and finally to graphitic carbon typically proceeds via polyaromatic heterocycles (Guisnet and Magnoux, 2001), which are not easily detected by conventional Raman spectroscopy because of fluorescence problems (Chua and Stair, 2003 Li and Stair, 1996). The use of UV excitation provides a powerful means to circumvent fluorescence problems and tackle the identification of the carbonaceous deposits (Chua and Stair, 2003). This subject was discussed in detail by Stair (2007). Polyaromatic deposits were burned off very quickly upon restoration of oxidizing conditions (Boulova et al., 2001 Mul et al., 2003 Puurunen and Weckhuysen, 2002 Puurunen et al., 2001). 

The advantages of UV Raman speetroscopy in avoiding fluoreseenee from eoke deposits and eatalyst impurities and interference from luminescence at elevated temperatures were mentioned above. When it became apparent that the fluorescence problem was avoided, measurements were attempted of hydrocarbons adsorbed in catalytically active zeolites and during hydrocarbon conversions under catalytic reaction conditions. The spectra suggested that interference from sample damage caused by the ultraviolet laser was a serious problem. 

Near-infrared surface-enhanced Raman spectroscopy Some of the major irritants in Raman measurements are sample fluorescence and photochemistry. However, with the help of Fourier transform (FT) Raman instruments, near-infrared (near-IR) Raman spectroscopy has become an excellent technique for eliminating sample fluorescence and photochemistry in Raman measurements. As demonstrated recently, the range of near-IR Raman techniques can be extended to include near-IR SERS. Near-IR SERS reduces the magnitude of the fluorescence problem because near-IR excitation eliminates most sources of luminescence. Potential applications of near-IR SERS are in environmental monitoring and ultrasensitive detection of highly luminescent molecules [11]. 

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