Evanescent wave absorbance spectroscopy

Optical detection for the system described here is based on evanescent wave absorbance spectroscopy (EWAS). This method takes advantage of the reflections of light... 

Figure . Top Standard absorption measurement Bottom evanescent wave absorbance spectroscopy.

Fiber-optic techniques offer increased sensitivity relative to conventional bulk optic approaches. For example, evanescent wave (EW) spectroscopy is significantly more sensitive than bulk attenuated total reflection spectroscopy. Fiber-optical EW spectroscopy is the only technique suitable for use with highly absorbing or scattering media. 

ATR is one of the most useful and versatile sampling modes in IR spectroscopy. When radiation is internally reflected at the interface between a high-refractive index ATR crystal (usually Ge, ZnSe, Si, or diamond) and the sample, an evanescent wave penetrates inside the sample to a depth that depends on the wavelength, the refractive indices, and the incidence angle. Because the penetration depth is typically less than 2 pm, ATR provides surface specific information, which can be seen as an advantage or not if surface orientation differs from that of the bulk. It also allows one to study thick samples without preparation and can be used to characterize highly absorbing bands that are saturated in transmission measurements. 

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

Quantitative determinations of the thicknesses of a multiple - layered sample (for example, two polymer layers in intimate contact) by ATR spectroscopy has been shown to be possible. The attenuation effect on the evanescent wave by the layer in contact with the IRE surface must be taken into account (112). Extension of this idea of a step-type concentration profile for an adsorbed surfactant layer on an IRE surface was made (113). and equations relating the Gibbs surface excess to the absorbance in the infrared spectrum of a sufficiently thin adsorbed surfactant layer were developed. The addition of a thin layer of a viscous hydrocarbon liquid to the IRE surface was investigated as a model of a liquid-liquid interface (114) for studies of metal extraction ( Ni+2, Cu+2) by a hydrophobic chelating agent. The extraction of the metals from an aqueous buffer into the hydrocarbon layer was monitored kinetically by the appearance of bands unique to the complex formed. 

Internal reflection spectroscopy (2), also known as attenuated total reflectance (ATR), is a versatile, nondestructive technique for obtaining the IR spectrum of the surface of a material or the spectrum of materials either too thick or too strongly absorbing to be analyzed by standard transmission spectroscopy. The technique goes back to Newton who, in studies of the total reflection light at the interface between two media of different retractive indices, discovered that an evanescent wave in the less dense medium extends beyond the reflecting interface. Infrared spectra can conveniently be obtained by measuring the interaction of the evanescent wave with the external less dense medium. 

Time resolved evanescent wave induced fluorescence spectroscopy is a powerful method for the investigation of dye molecules at interfaces. This technique has been used on studies on the popular photosensidzer aluminium phthalocyanine tetra/sulphonate absorbed at fused silica/methanol interfaces . 2nd harmonic detection of sinusoidally modulated two photon excited fluorescence can also be used to obtain luminescence spectra ". ... 

If sample material is in contact with the totally reflecting surface of the prism, an evanescent wave in the sample extends beyond the reflecting interface and the evanescent wave will be attenuated in infrared regions.The intensity of this wave decays exponentially with the distance from the surface of the ATR crystal. Due to the fact that the electromagnetic field passes only a few micrometers of the sample, this method is insensitive to sample thickness and therefore useful for analysis of strong absorbing or thick materials. Influencing factors for FT-ATR-IR-spectroscopy are as follows ... 

The penetration depth of the evanescent wave into the electrolyte for in situ ATR infrared experiments is generally taken as ca. X/10 on this basis, in the region of the O—H stretch of liquid water, s = 55 mol dm cm [42], an absorbance of <0.1 would be expected, and this is the essence of the application of ATR techniques in in situ infrared spectroscopy. 

Aqueous solutions have traditionally posed a problem for IR spectroscopy due to the fact that water is a strong absorber of IR radiation. This difficulty, for aqueous solutions and other strongly absorbing liquid and solid samples, can be overcome by using attenuated total reflection spectroscopy. In this technique, the phenomenon of total internal reflection is used in such a way that it is only the evanescent wave associated with total internal reflection that enters the sample. The evanescent wave penetrates the sample very short distances only, hence the advantage for strongly absorbing species. 

According to the Lambert—Beer s law the absorbance is linear with the absorption coefficient, concentration c and volume of the probed sample. Assuming that the absorption coefficient, concentration and cross-section do not change, it can be supposed that the absorbance of a sample is linear with thickness. This is actually true for most data recorded using transmission or similar sampling techniques. In the case of ATR spectra the exponential decay of the evanescent wave has to be taken into account if the band area and film thickness are to be correlated. For use in ATR spectroscopy, the Lambert—Beer s law can be empirically modified to ... 

When light traversing an optically dense medium approaches an interface with a more optically rare medium at an angle exceeding a critical value, Bent = sin (rerare/ dens), total internal reflection occurs and an evanescent wave of exponentially deca5ung intensity penetrates the rarer medium. This phenomenon is at the heart of certain spectroscopic methods used to probe biomolecules at interfaces (199). In total internal reflection fluorescence (TIRF) spectroscopy (200-202), the evanescent wave excites fluorescent probes attached to the biomolecules, and detection of the emission associated with their decay provides information on the density, composition, and conformation of adsorbed molecules. In fourier transform infrared attenuated total reflection (FTIR-ATIR) spectroscopy (203,204), the evanescent wave excites certain molecular vibrational degrees of freedom, and the detected loss in intensity due to these absorbances can provide quantitative data on density, composition, and conformation. 

The phenomenon of internal reflection was first reported in IR spectroscopy in 1959. It was observed that if certain conditions are met, IR radiation entering a prism made of a high refractive index IR transmitting material (ATR crystal) will be totally internally reflected (Figure 4.3). This internal reflectance creates an evanescent wave which extends beyond the surface of the crystal into the sample held in intimate contact with the crystal. In regions of the IR spectrum where the sample absorbs energy, the evanescent wave will be attenuated. 

While the SFA provided direct evidence as to the thickness of a polymer layer adhering to a surface, TIRF provides a measure of the surface area concentration, r. TIRF spectroscopy makes use of the total internal reflection of light at the interface between a solid adsorption substrate of relatively high refractive index and a polymer solution of lower refractive index. Ihe total internal reflection, however, generates a standing evanescent wave, a nonclassical penetration of the light into the lower refractive index phase. The evanescent wave may penetrate some 60 to 65 nm into the lower refractive index phase, be absorbed by it, fluoresce, and so on (72,73). 

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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