Raman spectroscopy detector

Instrumental Interface. Gc/fdr instmmentation has developed around two different types of interfacing. The most common is the on-the-fly or flow cell interface in which gc effluent is dkected into a gold-coated cell or light pipe where the sample is subjected to infrared radiation (see Infrared and raman spectroscopy). Infrared transparent windows, usually made of potassium bromide, are fastened to the ends of the flow cell and the radiation is then dkected to a detector having a very fast response-time. In this light pipe type of interface, infrared spectra are generated by ratioing reference scans obtained when only carrier gas is in the cell to sample scans when a gc peak appears. 

In pyrroelecttic devices, a charge is developed across the film in response to heating and such devices may serve as k-detectors (see Infrared TECHNOLOGY AND RAMAN SPECTROSCOPY). Pie2oelecttic appHcations are promising as sound detectors, because for these, a charge is developed across the film in response to pressure. A review is available (74). 

Infrared (in) spectrometers are gaining popularity as detectors for gas chromatographic systems, particularly because the Fourier transform iafrared (ftir) spectrometer allows spectra of the eluting stream to be gathered quickly. Gc/k data are valuable alone and as an adjunct to gc/ms experiments. Gc/k is a definitive tool for identification of isomers (see Infrared and raman spectroscopy). 

Because the Raman cross-section of molecules is usually low, intense light sources and low-noise detectors must be used, and high sensitivities - as required for surface analysis - are difficult to achieve. Different approaches, singly and in combination, enable the detection of Raman spectroscopy bands from surfaces. 

The first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

Sampling techniques for Raman spectroscopy are relatively general since the only requirement is that the monochromatic laser beam irradiate the sample of interest and the scattered radiation be focused upon the detector. 

Figure 2. Experimental set-up for Raman spectroscopy. The desired laser line is isolated from other plasma lines by a narrow bandpass filter or broadband prism monochromator, then focused onto a sample in a capillary tube. A collecting lens placed at a 90° angle to the incident beam focuses the scattered light onto the entrance slit of a monochromator with output to a photomultiplier tube (in the case of a scanning instrument) or a diode array detector.

Dramatic improvements in instrumentation (lasers, detectors, optics, computers, and so on) have during recent years raised the Raman spectroscopy technique to a level where it can be used for species specific quantitative chemical analysis. Although not as sensitive as, for example IR absorption, the Raman technique has the advantage that it can directly measure samples inside ampoules and other kinds of closed vials because of the transparency of many window materials. Furthermore, with the use of polarization techniques, one can derive molecular information that cannot be obtained from IR spectra. Good starting references dealing with Raman spectroscopy instruments and lasers are perhaps [34-38]. 

Finally, the introduction of new detectors, such as diode arrays and charge-coupled devices (CCDs), has been a boon for Raman spectroscopy. CCDs permit the accumulation of light in the manner of photographic film additionally, their noise level is lower than that of the photomultiplier tube. In addition, by combining CCDs or diode arrays with optical dispersive elements, entire spectra mav be collected in fractions of a second. 

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