Raman sampling chamber

With a special optical system at the sample chamber, combined with an imagir system at the detector end, it is possible to construct two-dimensional images of the sample displayed in the emission of a selected Raman line. By imaging from their characteristic Raman lines, it is possible to map individual phases in the multiphase sample however, Raman images, unlike SEM and electron microprobe images, have not proved sufficiently useful to justify the substantial cost of imaging optical systems. 

Raman spectroscopy interfaces for UHV chambers are not fundamentally difficult to design, since Raman sampling is generally tolerant of the window required in the UHV chamber. As shown in Figure 6.7, the window need not be near the focal point of the incident laser, so window background may be avoided. Some ingenious designs for the UHV-Raman interface have evolved... 

Figure 11. Raman spectra of the formic-biological system (A) shown with the vibration peaks of formic and diamond anvils used in this study. The outlined boxed region is shown at higher resolution (B) to quantify the successive decrease in the peak intensity of the C-H stretch of formic acid at pressures of 68,142, and 324 MPa. The equivalent formate concentrations (C), corresponding to each peak height change, are based on comparisons with a known calibration curve. All experiments were performed at 25°C, with diamond anvil cells with gold-lined sample chambers. Pressures were estimated using Raman shifts in quartz used as an internal calibrant.

Most of the component parts used in Raman spectroscopy such as the monochromator and sample chamber have the same functioning principle as in infrared spectrometers. All these were described in detail in section 2.2.1. 

In order to implement Raman spectroscopy, a reference sample is firstly installed in the sample chamber in moveable cuvette. Sulfur is usually used as the reference sample because of its high Raman activity. The reference sample is moved in the sample chamber until an optimum position is found. The optimum position is marked by maximum scattered radiation recorded by the detector. After the optimum position has been found, the reference sample is replaced with actual, sample. It is important to note that the geometrical characteristics of the actual sample must be the same as those of the reference sample. Calibration with regard to wavelength of the light used for the analysis depends on the actual sample and Raman spectrometer used. Different methods can be found in handbooks for a given Raman spectrometer. 

FT-Raman 1047- 1339 Bulk Medium-slow Sample chamber Analytical, routine High cm" accuracy High concentration Qualitative Quantitative... 

Figure 22 A comparison of the Raman spectrum of balsa wood recorded with an unfiltered 6-around-1 probe and an FT-Raman spectrometers standard macro-sampling chamber. Conditions 8 cm spectral resolution, 300 mW, 1064-nm excitation, (a) Probe background (recorded with a mirror, 1000 scans) (b) balsa and probe background (1000 scans) (c) scaled subtraction of (a) from (b) and (d) macro-compartment spectrum (50 scans). (Adapted with permission from Ref. 179.)... 

Figure 2 Rotational Raman spectra of N2 and O2 in ambient air within the sample chamber recorded in the two modes. Laser power 1 W at 514.5 nm, single pass, (a) PMT mode t - 25 minutes, slits widths = W2 = W3 = 50 /xm height hi = 20 mm (b) CCD mode = 20 seconds, slits Wi - 50 /xm, hi = 20 mm, W2 = 2600 /xm binning n = I pixel, = 50 pixels. Dashed vertical lines indicate the borders between spectral windows. (From Ref. 53, with permission.)...

Another attachment that can be effected to a Raman spectrometer is a remote probe. This consists of a fibre optic cable that passes the laser beam to a sample outside the conventional sampling chamber. This can be of use in many different applications, the most obvious of which is where the sample is too large or complex to fit into the instrument. In vivo biological studies utilize a fibre optic probe for the investigation of human tissue. Industrial process monitoring uses a Raman spectroscopic probe for online quality control during manufacture. 

Because Raman spectroscopy requires one only to guide a laser beam to the sample and extract a scattered beam, the technique is easily adaptable to measurements as a function of temperature and pressure. High temperatures can be achieved by using a small furnace built into the sample compartment. Low temperatures, easily to 78 K (liquid nitrogen) and with some diflSculty to 4.2 K (liquid helium), can be achieved with various commercially available cryostats. Chambers suitable for Raman spectroscopy to pressures of a few hundred MPa can be constructed using sapphire windows for the laser and scattered beams. However, Raman spectroscopy is the characterizadon tool of choice in diamond-anvil high-pressure cells, which produce pressures well in excess of 100 GPa. ... 

Figure 13.3. Two sampling geometries for surface Raman spectroscopy. (A) is 180° backscat-tering, and (B) uses a non-normally incident laser and normal collection through a window in a UHV chamber.

The susceptibility of solid surfaces to contamination often results in a requirement for an ultrahigh vacuum (UHV) chamber for preparation and observation of particular samples. For many materials, including metals such as platinum and nickel, adsorption of hydrocarbons and chemisorption of oxygen are quite fast at atmospheric pressure, and the surface must be isolated in UHV to prevent rapid degradation. In addition, a sample in UHV may be subjected to surface analytical techniques such as X-ray photoelectron and Auger spectroscopy to verify or corroborate Raman results. As a result, much of the early and well-characterized surface Raman experiments were carried out in UHV chambers operating below 10 torr (12). 

After vapor deposition and cooling in a vacuum chamber, the SERS substrate is usually removed, exposed to the sample solution or gas, and then the Raman spectrum is acquired. Exposure to air and/or water vapor... 

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