Attenuated total reflectance length

Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), 24 111-114 Attenuation, 77 132-133 Attenuation length (AL), 24 87-89 Attrition, catalyst deactivation mechanism, 5 256t... 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

Due to the high absorbency in the MIR region and to comply with the Beer-Lambert law, the path-length of the cuvette must be about 10 pm. The development of the attenuated total reflectance (ATR) device allows the sampling problems encountered when collecting spectra from opaque and viscous samples to be overcome (Figure 20.1). 

Sabo et al. [35] developed an LC/FTIR interface for both normal and reversed phase chromatography using an attenuated total reflectance cell. The cylindrical internal reflectance flow cell contained in a ZnSe crystal, had a nominal volume of 24 pi and an equivalent transmission cell path length of 4-22 pi over the usable range. On-the-fly spectra of the components from a 100 pi sample of a solution containing 2% of acetophenone and ethyl benzoate and 1% of nitrobenzene gave clearly identifiable spectra. Nevertheless, relative to other LC/FTIR systems this was not a very sensitive device. 

Another approach, internal reflection spectroscopy (IRS) or attenuated total reflectance (ATR) spectroscopy, takes advantage of the total reflection observed when a light beam is directed to the back of the OTE at an angle greater than the critical angle. Since the reflection takes place at the electrode-solution interface, the light beam is attenuated only by molecules close to the electrode surface. The penetration depth is about 1000 A in a typical experiment. Because of this very small effective cell length, the electrode is most frequently used in a multiple reflection mode [Fig. 44(b)]. 

The main problem in IR measurements in situ is a strong IR absorption by the solvent water. Therefore the measurements in the classical transmission mode are practically excluded (the path length must not exceed 15 tan). Short path lengths are provided by the attenuated total reflection (ATR) mode. The probing light beam undergoes multiple total internal reflection in the ATR element (e.g. a cylindrical rod made of ZnSe, with two cone-sharpened ends) and each reflection... 

The use of infrared spectroscopy in the Earth and environmental sciences has been widespread for decades however, until development of the attenuated total reflectance (ATR) technique, the primary use was ex situ material characterization (Chen and Gardella, 1998 Tejedor-Tejedor et al., 1998 Degenhardt and McQuillan, 1999 Peak et al., 1999 Wijnja and Schulthess, 1999 Aral and Sparks, 2001 Kirwan et al., 2003). For the study of environmental systems, the strength of the ATR-Fourier transform infrared (FTIR) technique lies in its intrinsic surface sensitivity. Spectra are collected only from absorptions of an evanescent wave with a maximum penetration depth of several micrometers from the internal reflection element into the solution phase (Harrick, 1967). This short optical path length allows one to overcome any absorption due to an aqueous phase associated with the sample while maintaining a high sensitivity to species at the mineral-water interface (McQuillan, 2001). Therefore, ATR—FTIR represents a technique capable of performing in situ spectroscopic studies in real time. 

The Infrared Internal Reflectance Approach An interesting alternative method of minimizing the solution path-length is to employ internal reflectance or attenuated total reflectance (ATR) and this was the approach adopted by the authors of the first report on the acquisition of in situ infrared spectra Mark and Pons [39] employed the ATR approach, [or, as they termed it, frustrated multiple internal reflection (MIR)-see below], to... 

The mid-IR spectral range contains much denser and more selective information compared to the near-IR, where overtone and combination bands of the fundamental molecular vibrations occur. As the intensity of bands in the mid-IR is higher, optical path lengths are on the micrometer scale. Such path lengths can be achieved with a special technique that uses attenuated total reflection, and which renders optical materials compatible with aqueous biofluids. Recently, dry films of biosamples of nanoliter volumes have been successfully applied for reagent-free... 

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