Reflective cells

It is interesting to note that a similar specttum of the 0-0 band of the a-X system, leading to the same value of the absorption intensity, has been obtained using a Fourier transform spectrometer (see Section 3.3.3.2) but with an absorption path, using a multiple reflection cell, of 129 m and half the pressure of gas. 

The use of a resonance cavity results, as mentioned, in a sensitivity that is approximately one order of magnitude greater than that for a normal reflection cell. The consequence is, however, a sacrifice in time resolution, which is typically also of one order of magnitude. 

Figure 2.40 Schematic representation of the external reflectance cell design commonly employed in in situ IR experiments, if the working electrode is a semiconductor, then the semiconductor/ electrolyte interface can be studied under illumination with, for example, UV light by directing the beam perpendicular to the IR beam, as shown.

For sensing applications, high bending losses restrict the applicability as (flexible) light pipes. Practical applications are mostly restricted to gas cells, where the hollow waveguide acts as a compact multi-reflection cell to increase the sensitivity in comparison to single-pass cells. 

Sensitivity of cancerous cells to copper may reflect cell DNA content. Two closely related rat hepatoma cell lines differed in sensitivity to copper toxicity by a factor of four DNA content in each cell line decreased with increasing copper concentrations, but at different rates. Severity of toxicity was associated with increasing accumulations of copper in the cell nucleus and with decreasing DNA (Toussaint and Nederbragt 1993). 

Similar work was performed by Shaw et al.3 in 1999 when they used FT-Raman, equipped with a charge coupled device (CCD) detector (for rapid measurements) as an on-line monitor for the yeast biotransformation of glucose to ethanol. An ATR (attenuated total reflectance) cell was used to interface the instrument to the fermentation tank. An Nd YAG laser (1064 nm) was used to lower fluorescence interference and a holographic notch filter was employed to reduce Rayleigh scatter interference. Various chemometric approaches were explored and are explained in detail in their paper. The solution was pumped continuously through a bypass, used as a window in which measurements were taken. 

Well-designed reflectance cells have made this strong absorber of water useful in the process vat. Using long path-length gas cells also allows the analyst to monitor head-space gases in order to follow bioreactions. 

In reflectance cells, in contrast, the IR beam is directed through an IR-transmit-ting crystal which has a surface which is in intimate contact with the sample solution. The system is arranged such that one or more internal reflections of the IR beam occur at the interface of the crystal and sample. At each reflection, an evanescent wave is generated which penetrates a short distance into the sample medium. 

The rate for the Ir reaction was measured at temperatures up to 120 °C and CO pressures up to 6 bar in PhCl by IR using a cylindrical internal reflectance cell... 

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. 

When light is directed onto a sample it may either be transmitted or reflected. Hence, one can obtain the spectra by either transmission or reflection. Since some of the light is absorbed and the remainder is reflected, study of the diffuse reflected light can be used to measure the amount absorbed. However, the low efficiency of this diffuse reflectance process makes it extremely difficult to measure 120) and it was speculated that infrared diffuse reflection measurements would be futile 120). Initially, an integrating sphere was used to capture all of the reflected light121) but more recently improved diffuse reflectance cells have been designed which allow the measurement of diffuse reflectance spectra using FT-IR instrumentation 122). 

Fourier Transform Infrared Spectroscopy (FT-IR) measurements were made using a Nicolet Instruments 740 FT-IR spectrometer. A horizontal attenuated toted reflectance cell equipped with a 45° zinc-selenide crystal trough wets used. Spectra of neat solutions were obtained by co-addition of 256 scans at 4 cm- resolution. 

See reference (7) for optical arrangement and reference (13) for multiple reflection cell details. 

Pickering and Eckstrom evaporate catalyst metals, such as rhodium or nickel, onto the mirrors of a White multiple (20-40) reflection cell (68). A series of scans are taken before and after adsorption of the gases. The comparison of the two sets of curves is accomplished by a method described by King et al. (69), in which the detector output is fed to an ana-log-to-digital converter for recording on punched cards. The final graphs are plotted with an IBM accounting machine. This technique makes it possible to study absorption bands of the order of 0.03 to 0.05% of the radiation. 

ATR studies of the biocorrosion of submerged copper surfaces have been reported. The IRE of a cylindrical internal reflectance cell (CIRCLE) was coated with a thin copper layer via a vacuum deposition technique (105). The copper layer reduces the sampling depth of the radiation outward from the surface of the IRE. Therefore, the intensity of the water bending band will vary with copper layer thicknesses of 4.1 nm or less. The copper layers were shown to be stable to exposure to water alone, but the presence of acidic polysaccahrides in the water caused a reduction in the copper layer thicknesses (106.107). The adsorption of a model compound, Gum Arabic, onto the coated IRE was detected by increases in the C-O stretching band of the pyranose units near 1050 cm"1 (106). 

Fig. 3. Total internal reflection cell used for the interfacial fluorescence lifetime measurement (left) and the external reflection absorption spectrometry (right).

Subsequently, Melsheimer and Schlogl (1997) used various reflectance cells and tested pressed and unpressed powder samples, the spectra of which were characterized by different intensity distributions. From correlations of the integral intensity of individual components to the H2S conversion with time on stream it was deduced that S2O32 and S22 species "affect the steady state" when SO2 is used as an oxidant—and S2O32- species when O2 is the oxidant. 

Fig. 6. (a) The IR spectrum of gallane vapor (i) trapped in a solid nitrogen matrix at ca. 20 K and (ii) at a pressure of ca. 5 mm Hg and temperature near 270 K (contained in a cell fitted with Csl windows and having a path length of 10 cm), (b) Part of the IR spectrum of gallane vapor at a pressure of ca. 0.05 mm Hg contained in a multiple-reflection cell set to a path length of 6.5 m and maintained at ca. 290 K (reproduced with permission from Ref. 56 copyright 1991, American Chemical Society). 

Fig. 9. Portions of the IR spectrum of gallaborane vapor at a pressure of 0.15 mm Hg and maintained at ca. 290 K, showing the two v(B—Ht) fundamentals the vapor is contained in a multiple-reflection cell set to a path length of 3.9 m (37, 65).

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