Instruments evanescent wave

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

The configuration most often used in SPR instruments relies on the phenomenon of total internal reflectance and was developed by Kretchmann (Fig. 8).71,73 Total internal reflectance occurs when light traveling from a medium of higher refractive index toward a medium of lower refractive index reaches the interface and is reflected back completely into the higher refractive index medium. An important side effect of total internal reflection is the propagation of an evanescent wave across the interface into the medium of lower refractive index. 

Rosenberger, A. T. Rezac, J. P., Whispering gallery mode evanescent wave microsensor for trace gas detection, In Biomedical Instrumentation Based on Micro and Nanotechnology Mariella, R. P., Jr. Nicolau, D. V., Eds. Proc. SPIE 2001, 4265, 102 112... 

Fig. 7. Schematic diagram of the evanescent wave dynamic light-scattering instrumentation (Reproduced with permission from Fytas et al., Science 274 2041 Copyright (1996) American Association for the Advancement of Science)...

Sensing of evanescent waves with an optical tip has been proposed for use as an optical device to sense AFM forces by means of an optical microlever which is illuminated by a laser under conditions of total internal reflection and which is connected to an atomic force tip [77], Thus tunneling photons from the microlever to the optical tip at the evanescent light coupling may be used for the feedback loop. This instrument combines noncontact AFM and PSTM techniques. 

Sensitivity of the biosensor is a complex issue. Ultimately, the level of detection is determined by the binding affinity of the recognition element, the ratio of signal-to-noise from both the sample and the hardware, and the sensitivity of the instrument. The two predominate sources of noise in the system are (1) nonspecific retention of the fluorophore within the evanescent wave, and (2) stray excitation light. 

We have demonstrated that polycyclic aromatic hydrocarbons can be rapidly measured by using the DNA intercalation -fluorescence technique. The limits of detection obtained are sensitive enough to be used for measurements on environmental carcinogens Future studies will utilize evanescent wave technology with fiber optics and immobilized DNA to develop a portable real time instrument for in situ measurements of carcinogens.. 

The instrument used in this study was the TravellR (Smiths Detection), which employs a miniaturized Michelson interferometer and an integrated diamond ATR sample interface represented in Figure 8. An evanescent wave extends beyond the diamond surface and is partially attenuated by substances within 0.5-3.1 pm of the diamond surface in the range of 4000-650 wavenumbers. An embedded, on-board computer uses an automated search algorithm that compares infnued spectral features to digital spectral databases. 

Abstract This chapter describes recent breakthroughs in the instrumentation for far-ultraviolet (FUV) spectroscopy. The key technique is attenuated total reflection (ATR) that is frequently used in the infrared region. ATR technique decreases the absorbance of samples with strong absorptivity because of the penetration depth of the evanescent wave less than 100 nm. Therefore, ATR-FUV spectroscopy realizes the measurement of FUV spectra of samples in liquid and solid states. Some applications (in-line monitoring, characterization of polymers and time-resolved spectroscopy in sub-microsecond) are introduced in terms of instrumentation. This chapter explains not only the detail of the instruments but also the mathematical correction for ATR spectra to separate the absorption and refraction indices. 

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