Attenuated total reflection cell design

Figure 2.39 (a) Schematic representation of the experimental arrangement for attenuated total reflection of infrared radiation in an electrochemical cell, (b) Schematic representation of the ATR cell design commonly employed in in situ 1R ATR experiments. SS = stainless steel cell body, usually coated with teflon P — Ge or Si prism WE = working electrode, evaporated or sputtered onto prism CE = platinum counter electrode RE = reference electrode T = teflon or viton O ring seals E = electrolyte. 

A special O-ring cell design is needed for in situ infrared (IR) vibrational characterization of an electrochemical interface. The absorption of one monolayer (i.e. <1015 cm 2 vibrators) can be measured if the silicon electrode is shaped as an attenuated total reflection (ATR) prism, which allows for working in a multiple-in-ternal-reflection geometry. A set-up as shown in Fig. 1.9 enhances the vibrational signal proportional to the number of reflections and restricts the equivalent path in the electrolyte to a value close to the product of the number of reflections by the penetration depth of the IR radiation in the electrolyte, which is typically a tenth of the wavelength. The best compromise in terms of sensitivity often leads to about ten reflections [Oz2]. 

A variety of cell designs for internal reflection or attenuated total reflection spectroscopy have been reported, demonstrating the flexibility of the technique. In the following, we describe only a few examples in some detail. 

By far the most common use of mid-infrared radiation for process analysis is in the non-dispersive infrared analysers that are discussed below. The widespread use of FTIR spectrometers in the mid-lR has yet to be fully realized in process analytical apphcations. The requirements for the optical components and the wavelength sta-bihty of the instraments available have, until recently, detracted from the use of this region of the spectrum in on-line process analysis. Optical fibers that provide such a benefit to the apphcations of NIR (see below) are not available for the mid-IR in robust forms or forms that are capable of transmitting over more than a few tens of metres. Improvements and developments to sample cells, particularly designs of attenuated total reflectance (ATR) cells, for use with mid-lR are being made and will influence the application of the technique. An impressive list of apphcations including both FTIR and the NDIR approaches has been compiled (2, 3]. 

A cell for characterising the diffusion of small molecules through thin polymer films using attenuated total reflectance (ATR) FTIR spectroscopy was described. The cell was designed to be used with precast (commercially extruded) polymer films, thus enabling the as-processed transport properties of the film to be studied. The cell was used to measure the diffusion of carbon dioxide, amyl acetate and limonene, and simultaneous diffusion of the individual components from a 50/50 mixture of amyl acetate and limonene through the thin polymer films (HDPE, LDPE and PS). Diffusion coefficients measured with the ATR technique compared favourably with values obtained from gravimetric measurements with the same penetrants and polymer samples. 20 refs. 

A few authors have utilized a grating spectrometer in conjunction with an electrochemical cell specially designed for attenuated total reflection (ATR). 

Figure 4.2 Schematic of basic probe designs transmission cell, immersion probe (retroreflecting and transmission), attenuated total internal reflection.

---