FT-Raman Spectrometers

An FT-Raman spectrometer is often simply an FTIR spectrometer adapted to accommodate the laser source, filters to remove the laser radiation and a variety of infrared detectors. 

Figure 6 reproduces the Raman spectra in the region 800-1200 cm-1 reported by these authors for pure silicalite (sample 1) and for two TS-1 samples, 3 and 5, which contain 1.4 and 3.0 wt% Ti02. The spectra shown in Fig. 6a were recorded with a Fourier transfrom (FT) Raman spectrometer at an excitation wavelength of Aexc = 1064 nm (9398 cm-1), whereas those shown in Fig. 6b were excited with a UV-laser line at Aexc = 244 nm (40,984 cm-1). With each excitation wavelength, the pure silicalite gives rise to weak bands at 975 and 1085 cm -1 and a complex band centered near 800 cm-1. In the FT-Raman spectra of the dehydrated TS-1 samples (Fig. 6a), a band is clearly visible at 960 cm-1, the intensity of which increases with Ti02 content. 

IFS66-FRA-106 FT-Raman spectrometer equipped with a liquid-Nj cooled Ge-diode detector. Samples were in small glass capillary tubes at 23°C. The spectra were calculated by averaging -200 scans followed by apodization and fast-Fourier-transformation to obtain a resolution of -2 cm and a precision better than 1 cm . The spectra were not corrected for (small) infensity changes in detector response versus wavelength. 

Figure 2-9 Optical diagram of an FT-Raman spectrometer. The heart of the instrument is the interferometer head, consisting of the beam splitter and fixed and moving mirrors.

A FT-Raman spectrometer with excitation from a Nd YAG laser at 1.064 /im and an GalnAs detector was used to collect the Raman data. The range for the Raman spectra was 400-3,200 cm-1 at a resolution of 6 cm-1 and 200 mW power. 

Figure 3.5-13 Influence of sample and instrument parameters on the observed intensity of a Raman line under the conditions of NIR FT Raman spectrometers. The relative output voltage of the Ge detector (cooled with liquid nitrogen) is given for a sample with the absorption spectrum of water. The upper curves, r = 0, represent a liquid sample with a thickness of 1 and 2 cm, respectively. The lower traces, r = 10, 100, and 500, represent a coarse, medium and fine powder, respectively. Abscissa below absolute wavenumbers, above Raman shift.

Sec. 3.5.4 demonstrates that, by analyzing a sample of benzene in a 90° arrangement with an NIR FT Raman spectrometer, a laser power of 1 Watt produces a radiant power... 

Samples were characterized by using X-ray diffraction (XRD) on a Shimadzu XRD-6000 diriiactometer (CuKa radiation), physical adsorption of nitrogen on a Quantachrome NOVA 1000, Fourier transform iniiared (FTIR) spectroscopy on a Biorad spectrometer using the KBr method, Raman spectroscopy on a Bruker FRA 106/S FT-Raman spectrometer, and scanning electron microscopy (SEM) on a Joel JSM-5600LV. 

Interferometric Raman spectroscopy Interferometric Raman Spectroscopy is a measurement technique that utilizes time-domain or space-domain measurements of electromagnetic radiation or other type of radiation for collecting Raman spectra based on the coherence of a radiative source. An example is a Fourier transform (FT) Raman spectrometer. 

Figure 1. Raman spectra of Si02 nc-Si (gray line) and Al203 nc-Si (black line) measured by FT-Raman spectrometer.

The FT-Raman spectrometer used HP-532/1-50/1-4CH is a commercially available system from Kaiser Optical Systems. The system contains a green LASER with a wavelength of 532 nm and an output power of 35 mW. The power ouq>ut at the probe head is about 10 mW. The focused probe head has a sapphire window and can be used up to 280 °C and 60 bar pressure. The probe head is connected with a 100 pm optical fiber with a CCD detector for the Raman scattered light and an FT-spectrometer. The FT-spectrometer has a spectral coverage from 100 to 4400 cm and a spectral resolution of 5 cm [3]. 

Figure 6 shows a typical application of the on-line FT-Raman spectrometer in a pilot plant for a process optimization. During the running production, the amount of used catalyst is reduced step-wise. The operator gets a direct feedback of the reaction system. In this case the amount of catalyst can be reduced by about 20%. 

This last example shows that the on-line FT-Raman spectrometer is a powerful tool to optimize processes in both ways you can create a cost-effective process, which produces a high quality product. 

Figure 1.5. Spectra of rhodamine 6G obtained with a 514.5 nm laser and dispersive spectrometer (upper) or an FT-Raman spectrometer and 1064 nm laser (lower). Intensity scales differ greatly, with the upper spectrum being much more intense.

Figure 4.6. Spectra of 0.1 M Na2S04 dominated by detector noise. Spectrum A is 0.1 M Na2S04 in water, spectrum B is after subtraction of a spectrum of the cell containing only water. Spectra are from an FT-Raman spectrometer with a germanium detector.

Figure 5.2. Schematic of a nondispersive, FT-Raman spectrometer. A single detector monitors photons with all Raman shifts, after each has been modulated by a multiplexer such as an interferometer. Raman spectrum is obtained by Fourier transformation of the detector output (interferogram).

For example, FT-Raman spectrometers have relatively large input apertures and etendue, and can often collect light reasonably efficiently from an unfocused laser spot. It is possible to position the smaller laser mirror in Figure 6.4A on the sample side of LI, allowing the laser to be unfocused or at least less tightly focused. If the laser spot is 1 mm instead of 100 pm, the power density decreases by a factor of 100 (Table 6.1). This procedure generally causes loss of signal compared to the focused case, but by a factor much smaller than 100. 

A schematic drawing of an FT-Raman spectrometer is shown in Figure 9.3. The wavelength analyzer is a Michelson interferometer adapted from an FT-IR spectrometer. FTIR was developed to a high level of refinement before FT-Raman was introduced in 1986, and many components were transferred from FTIR to FT-Raman with minor modification. Many vendors offer FT-Raman attachments to otherwise conventional FTIR spectrometers, so that both techniques share the same interferometer. Dedicated FT-Raman spectrometers are also available but still share many components with FTIR systems. 

Figure 93. Schematic of FT-Raman spectrometer based on a Michelson interferometer . v is the moving mirror displacement, a and b describe two different paths of light split by the beamsplitter.

Figure 9.5. FT-Raman spectra of solid nylon obtained with 1064 nm excitation (245 mW) and a Bruker IFS/66 FT-Raman spectrometer with a germanium detector. Resolution was 4 cm in all cases, and the number of scans and associated total measurement time are shown. Blackman-Harris apodization, 2 x zero filling.

Figure 9.10. Black diagram of an FT-Raman spectrometer with 180° collection (Ml in place) or 90° collection (Ml absent). Optional lens permits tight or weak laser focus at sample.

Excellent interferometers were developed for FTIR and adapted for FT-Raman with minimal change. Since many FT-Raman spectrometers are configured as accessories to FTIR spectrometers, the interferometers are identical. An... 

Figure 9.11. Optical schematic of Bruker FT-Raman spectrometer based on an IPS 66 FTIR system.

For FT-Raman spectrometers, an equivalent one-point calibration is more reliable because interferometers are less prone to mechanical errors. Nearly all interferometer designs include a well-defined reference wavelength (often a He-Ne laser at 632.8 nm), which is used to control data acquisition. In addition, observed FT frequencies are calculated from a large number of individual measurements, so minor mechanical jitter and random timing errors are averaged out. Provided the laser and reference frequencies are known accurately, an observed FT-Raman frequency is quite accurate, and the one-point calibration is usually adequate. 

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