Reflective index detectors

Why do we need separation techniques As will be discussed in Sections 5 and 6, state-of-the-art mass analyzers and tandem mass spectrometry allow mass spectrometry to be a powerful tool for the analysis of complex mixtures. The coupling of classical separation techniques with mass spectrometry further improves the utility of these combined techniques for mixture analysis. Mass spectrometers are the most sensitive and structure-specific detectors for separation techniques that, in general, provide more detailed and reliable structural information on components of complex mixtures than other conventional detectors (such as flame ionization, UV, reflective index detectors, etc.). 

Figure 5.7 Scheaatic dlagraa of refractive index detectors eaploying the principle of refraction (deflection-refractoseter), A, and the reflection principle (Presnel-refractoaeter), B.

Fig. 2 shows the spectral dependence of the reflection index for the above structure. Using PS in light detectors decreases the reflectance of the structures promoting more efficient device operation. 

Figure 5.17. Schematic diagram of a refractive index detector employing the principle of reflection (Fres-nel-refractometer).

Fig. 3.4. Schematic illustration of a refractive index detector (LKB 2142-010). The LED light source (950 nm emission) is directed through the flow cell and reflected back by a concave mirror. The Ught is split into two beams at the edge of a prism and directed towards two photodiode sensors. Reproduced from LKB technical brochure with...

The size of the signal seen by the differential refractive index detector reflects the concentrations. The size of the signals seen by the photodetectors is greatest for the highest molecular weight at all angles (which ranged from 19.0° to 132.5°) [28]. 

In comparison to infrared detectors, it is much more difficult for silicon-based optical detectors to achieve high QE over a wide bandpass. The main challenge is the tremendous variation of absorption depth shown in Fig. 8. In addition, the index of refraction varies significantly for A = 0.32-1.1 m, as shown in Fig. 10, making it difficult to optimize anti-reflection coatings for broad bandpass. 

Deviation refractometers are the most commonly used. This version of the DRI measures the deflection in the location of a light beam on the surface of a photodiode by the difference in refractive index between the polymer solution and pure solvent. The Fresnel-type refractometers operate on the principle that the intensity of light reflected from a glass-liquid interface is dependent on the incident angle and the RI difference between the two phases. The deviation and Fresnel detectors typically have cell volumes of 5 to 10 pi, detection limits of about 5 x 10-6 refractive index units (RIU), and a range of 10 7 to 10 3 RIU.156 The deflection-type DRI is relatively insensitive to the buildup of contaminants on the sample cell and is therefore of special utility in laboratories that process large numbers of samples, such as industrial laboratories. 

Figure 1. Schematic of the optical fiber system. Excitation light is launched into the fiber. Due to the refractive index differences between the fiber core and cladding materials, the light is internally reflected and travels through the fiber with minimal loss (see inset). The emitted light is carried back from the fluorescent sensor located on the tip of the fiber to a CCD camera detector. Reprinted with permission from Science, 2000, 287, 451-452. Copyright 2000 AAAS.

Automatic instruments are usually reflection-type instruments that measure the deflection of a beam of light as it passes from one medium, into the sample and then is reflected back to a detector. The angle at which the light beam exits the medium is related to the refractive index of the sample. Automated instruments are calibrated with standard substances of precisely known refractive index prior to use. 

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