CCD-Raman spectrometer

A CCD Raman spectrometer coupled with a 10-mW He-Ne laser has been used to eliminate fluorescence because the long-wavelength excitation by the He-Ne laser is not as likely to cause fluorescent transitions (71). Because of its directional property, coherent anti-Stokes Raman scattering (CARS) is also effective in avoiding fluorescence interference (see CARS in Section 3.9). 

Since the first edition of the book, the expansion of Raman spectroscopy as an analytical tool has continued. Thanks to advances in laser sources, detectors, and fiber optics, along with the capability to do imaging Raman spectroscopy, the continued versatility of FT-Raman, and dispersive based CCD Raman spectrometers, progress in Raman spectroscopy has flourished. The technique has moved out of the laboratory and into the workplace. In situ and remote measurements of chemical processes in the plant are becoming routine, even in hazardous environments. 

We define the sample shot noise limit, SNRy, as that occuring when op, and Or are negligible, and the noise is due solely to analyte and background shot noise. This is a common cause in dispersive/CCD Raman spectrometers and deserves treatment in some detail. 

These FT-Raman and CCD-Raman spectrometers revolutionized Raman spectroscopy such that, within the space of about five years, about ten different Raman spectrometers based on multiplex and multicharmel technologies had been introduced commercially [32, 33]. Several of the CCD-Raman spectrometers were either designed for, or could be readily modified for, microspectroscopy. Although FT-Raman microspectrometers have been reported (e.g.. Ref [34]), they have not proved very popular for three reasons ... 

In practice, therefore, CCD-Raman spectrometers have proved to be far more successful for Raman microspectroscopy than FT-Raman spectrometers, and most instruments are based on this concept. 

Raman microspectroscopy was not a completely new concept. In 1966, Delhaye and Migeon [35] showed that a laser beam could be hghtly focused at a sample, and that Raman-scattered light could be collected and transferred to a spectrometer, with minimal loss. Their calculahons showed that the increased irradiance more than compensated for the decrease in the size of the irradiated volume. The first Raman microscope was reported by Delhaye and Dhamelincourt in 1975 [36], and an instrument based on these principles (the MOLE) was introduced by Jobin Yvon at about the same time. However, the optical scheme used for imaging, which employed global illumination, was inefHcient and it was not until the advent of CCD-Raman spectrometers that the advantages of Raman microscopy became apparent. 

M Fryling, CJ Frank, RL McCreery. Intensity calibration and sensitivity comparisons for CCD/ Raman spectrometers. Appl Spectrosc 47 1965-1974, 1993. 

In the initial pilot-plant experiments, an immersion probe was used as the interface to the sampling stream [32]. The analyzer itself was a dispersive CCD-Raman spectrometer using a 532-nm diode laser as the excitation source fitted in nitrogen-purged National Electrical Manufacuters Associate (NEMA)-rated enclosure (Class 1, Div. II [33]. In Figs. 15 and 16, characteristic spectra of several of the individual components in the process steam are shown. 

Modern Raman systems are ideally suited for at- or near-line analysis. Fibre-optic probes, which can be interfaced to CCD-Raman spectrometers with greater ease than to FT-Raman instruments, have greatly expanded the utility of Raman spectroscopy by taking the measurement capability to the sample [374], It is also relatively simple to interface Raman spectrometers to other techniques, such as chromatography, light scattering, XRD, DSC, etc. but this is not yet an active area of research. Everall [375] has reported off-line LC-Raman (LC-Transform) interfacing. 

In principle, Raman microspectroscopy is attractive because the practical diffraction limit is on the order of the excitation wavelength, which is about 10-fold smaller for Raman spectroscopy with a visible laser than for mid-IR spectroscopy. It is therefore possible to focus visible laser light to much smaller spot sizes (400 nm in air and 240 nm with an oil immersion objective) than may be examined by mid-IR radiation. For various instrument-based reasons [4], charge-coupled device (CCD) Raman spectrometers have in practice proved to be far more successful for Raman microspectroscopy than ET-Raman spectrometers, and most instruments are based on this former concept. One further important instrumental advantage of the microscopes used for Raman microspectroscopy is their confocal design [5]. As the out-of-focus rays from an illuminated volume... 

In light of the fact that Fourier transform instrumentation was largely responsible for expanding Raman spectroscopy into the analytical laboratory, it is perhaps interesting to consider why Raman spectroscopy is so popular today but Fourier transform Raman does not play the dominant role. After a discussion of the poor sensitivity of NIR Raman spectrometry using a scanning monochromator with PMT detection in Section 18.1, it was stated To improve this situation, either a multichannel or multiplex measurement was needed and the multiplex measurement came first. Multichannel measurements came very shortly afterward, however, and instruments based on polychromators with silicon-based charge-coupled-device (CCD) array detectors have become more popular than FT-Raman spectrometers. In this section we compare the performance of FT- and CCD-Raman spectrometers. 

With such an obvious advantage of sensitivity to CCD-Raman spectrometers, we must ask why FT-Raman spectrometers have found their place in the marketplace. The answer is found in one word— fluorescence The shot noise introduced by fluorescence is so great that it offsets the sensitivity advantage of CCD-Raman spectrometers. Unlike the case for CCD-Raman spectrometry, fluorescence-free Raman spectra can usually be measured with a Fourier transform spectrometer in 5 or 10 minutes from almost any type of sample. 

Figure 18.10. Raman spectra of trinitrobenzaldehyde measured with (a) a FT-Raman spectrometer and (b) a CCD-Raman spectrometer equipped with a 782-nm diode laser. Spectra of hexanitrobiphenyl measured in the same way are shown as (c) and (d), respectively (Reproduced from [13], by permission of Springer-Verlag, Vienna copyright 1995.)...

Small, portable Raman systems that can be used in the clinic are very important. It is often difficult to obtain ethics committee permission to remove human specimens from the clinic. The system should be small and tough, and it must be enough sensitive to detect the weak Raman spectra of biological tissues. We recommend to check carefully the toughness of Raman spectrometer, CCD detector, and laser as well as performance, before purchasing. It is warm and humid in the clinic. The system should be air cooled and does not emit radio wave not to affect clinical instruments. 

These are now commercially available although the performance is still inferior to Si-based CCDs. Quite clearly, substantial additional development is needed in this area to achieve the ideal process Raman spectrometer. 

Figure 3-36 Raman spectrometer built around an AOTF. The sample is mounted on a microscope slide positioned 45 degrees relative to the incident laser. Raman scattering is collected and spectrally filtered with the AOTF. Holographic Raman filters are placed after the AOTF to Eliminate intense Rayleigh scatter befre the image is focused onto a liquid-nitrogen-colled CCD. (Reproduced with permission of Ref. 101.)...

Samples for spectroscopic measurements were prepared by incubation of the Ag-AAO substrates in identical aqueous solutions (3 ml) of 10 M CuTMPyP4 for 1.5 h. Raman spectra were taken using home-made Raman spectrometer equipped with a liquid-nitrogen cooled CCD detector. Spectra were excited with 441.6 nm radiation from He-Cd laser. 

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]. 

An important qualifier is required for the statement that etendue remains constant through the system. The usable etendue is determined by the minimum AS2 product for the system. This minimum could be determined by the laser spot size, the entrance slit, or the detector area and is often related to the limiting aperture applied to the definition of //. For many Raman applications, the etendue is determined by the spectrometer/detector combination (e g., the // of a spectrograph and the area of a CCD pixel). Increasing the Af2 product has provided much of the motivation for building Raman spectrometers with lower // and maximum etendue. 

At the time of this writing, the Raman spectrometer market is approximately split between dispersive (spectrograph/CCD) and nondispersive (FT-Raman) instruments. Both types have their pros and cons, which enter into a selection for a given application. Several generalizations are listed in Table 5.3. These... 

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