Detectors Raman scattering

Figure C3.1.1. The basic elements of a time-resolved spectral measurement. A pump source perturbs tlie sample and initiates changes to be studied. Lasers, capacitive-discharge Joule heaters and rapid reagent mixers are some examples of pump sources. The probe and detector monitor spectroscopic changes associated with absorjDtion, fluorescence, Raman scattering or any otlier spectral approach tliat can distinguish the initial, intennediate and final...

Raman scattering is essentially undelayed with respect to the arrival of the incident light, in this technique the detector is activated only during each laser pulse and deactivated at all other times. This allows only Raman signals to be recorded but fluorescence signals and detector noise are gated out (Fig. 19). Improvement in Raman signal to fluorescence ratio has been achieved as illustrated in Fig. 20. The technique, however, at present seems to be restricted by several instrumental limitations [37). 

Time-gated detection offers the possibility to suppress background signals correlated with the excitation pulse. Direct and multiple scattered excitation light as well as Raman scattering reaches the detector at t 0, and can be effectively suppressed by opening the first gate a few hundred picoseconds after t = 0. 

Radioactive label, 330 Raman diffusion, 184 Raman scattering, 227 Ratio fluorimeter, 228 Rayleigh scattering, 226 Real mean, 385 Red-shift, 196 Reference electrode, 347 Reflectron, 298 Refractive index detector, 59 Relative response factor, 78 Relative standard deviation, 387 Reliability, 389 Resolving power, 282 Response factor, 77 Restrictor, 98 Retardation factor, 88 Retention factor, 14 Retention index, 41 Retention time, 7 Retention volume, 14 RP-18, 53 RSD, 387 Ruhemann, 112... 

The intrinsically low intensity of Raman scattering strongly influences both the sensitivity and penetration depth of SORS and its variants. Dominant noise components (photon shot noise or thermal/dark count [1]) can be minimised relative to signal by increasing absolute signal levels. In many Raman systems, collection optics, laser power and other relevant parameters are usually maximised for optimum performance of the system current detectors (CCD devices), for example, have detection efficiencies approaching 100%. Typically, acquisition time provides the only straightforward means available... 

Fourth, while fluorescence was not a problem in this system, some bioreactor components may be highly fluorescent. Until more experience is gained, this makes it difficult to make generalizations about the likelihood of success with new systems. Finally, water is a weak Raman scatterer and can be hard to track. While Ulber etal. considered the challenge in obtaining a spectrum of the aqueous cell culture broth to be a disadvantage, others consider it an advantage since these solutions overwhelm mid- or near-infrared detectors.33 34 36... 

Figure 3. Schematic of turbulent combustor geometry and optical data acquisition system for vibrational Raman-scattering temperature measurements using SAS intensity ratios. Also shown are sketches of the expected Raman contours viewed by each of the photomultiplier detectors, the temperature calibration curve, and several expected pdf s of temperature at different flame radial positions. The actual SAS temperature calibration curve was calculated theoretically to within a constant factor. This constant, which accounted for the optical and electronic system sensitivities, was determined experimentally by means of SAS measurements made on a premixed laminar flame of known temperature. Measurements of Ne concentration were made also with this apparatus, based on the integrated Stokes vibrational Q-branch intensities. These signals were related to gas densities by calibration against ambient air signals.

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