DRIFTS accessories

The sampling station was equipped with an overhead DRIFTS accessory. The sample holder was used for the background spectra without KBr, and 256 coadded scans were taken for each sample from 4000 to 400 cm at a resolution of 16 cm L Single-beam spectra of the samples were obtained, and corrected against the background spectrum of the sample holder, to present the spectra in absorbance units. Spectra were collected in duplicate and used for multivariate analysis. 

Detailed experimental procedures for obtaining infrared spectra on humic and fulvic acids have been reported previously 9,22,25-26) and will be briefly described here. Infrared spectra were taken on the size-fractionated samples by using a Fourier transform infrared spectrometer (Mattson, Polaris) with a cooled Hg/Cd/Te detector. Dried humic and fulvic materials were studied by diffuse reflectance infrared spectroscopy (Spectra Tech DRIFT accessory) and reported in K-M units, as well as by transmission absorbance in a KBr pellet. Infrared absorption spectra were obtained directly on the aqueous size-fractioned concentrates with CIR (Spectra Tech CIRCLE accessory). Raman spectra were taken by using an argon ion laser (Spectra-Physics Model 2025-05), a triple-grating monochromator (Spex Triplemate Model 1877), and a photodiode array detector system (Princeton Applied Research Model 1420). All Raman and infrared spectra were taken at 2 cm resolution. 

Difiuse Reflectance Infiated Fourier Transformed (DRIFT) lectra woe recorded on a Nicolet Magna 550 specIronKter, equipped with a Spectratech DRIFT accessory. Samples were analyzed ex-situ, after partial converaon of the soot in the thermobalance and dilution with KBr (1 100 by wd ). A resolution of 8 cm and 256 scans were applied to obtain the spectra. 

Instrumentation. The interface within a suitably constructed electrochemical cell to be investigated is placed in the sample position of a standard DRIFT accessory for an infrared spectrometer for a typical design, see [328,329]. Examples reported so far deal with solid polymer electrolyte fuel cells where the surface of the anode layer exposed to a mixed gas atmosphere containing both water and methanol is separated from the environment via a Cap2 window [331, 332]. Various oxidized species and penetrating methanol were observed. 

As in the case of DT, diffuse reflectance (DR) can be measured in the mid-IR region with an integrating sphere [131, 141]. However, this technique is used mainly in reflectometry, where repeatable measurements with full hemispherical collection of radiation are needed [142]. Better performances, up to 12% efficiency, can be obtained using accessories with the so-called biconical optical configuration, which focuses the reflected radiation onto the IR detector. The SNR of such spectra is sufficient if such an accessory is used in conjunction with a FTIR spectrometer. The first DRIFT S accessory was described by Fuller and Griffiths in 1978 [143]. After that many forms of the DRIFTS accessories... 

Figure 4.25. Optical diagram of DRIFTS accessory (1) flat mirror (50 x 50 mm), (2) flat mirror (70 X 70 cm), (3) concave spherical reflector, and (4) sample cell D = diffuse reflection S = specular reflection. Dashed lines show optical path of diffuse reflection. Solid bold line shows optical path of specular reflection. Reprinted, by permission, from B. Li and R. D. Gonzalez, Appl. Spectrosc. 52, 1488-1491 (1998), p. 1489, Fig. 1. Copyright 1998 Society for Applied Spectroscopy.

Another mechanical solution to the problem of Fresnel specular reflectance is cutting off the specular component at the exit aperture of the accessory, as shown in Fig. 4.25 [154]. The waveguide that filters the KM component from the scattering radiation in the cup-on-the-saucer DRIFTS accessory (Fig. 4.19) may also be regarded as a mechanical device. 

Figure 4.30. (a) Classical and p) modified DRIFTS accessories for depth profiling of fibers. Reprinted, by permission, from F. Fondeur and B. S. Mifchell, Spectrochim. Acta A 56, 467 (2000), p. 469, Fig. 2. Copyright 2000 Elsevier Science B.V.

Murthy et al [317] described artifacts in the variable-temperature DRIFTS spectra, which are caused by changing sample height due to the thermal expansion as temperatme is changed. To overcome this problem. Venter and Vannice [318] suggested a modification of DRIFTS accessories. The reader can find other temperature-controlled DRIFTS cells in Refs. [316, 319-326]. 

Diffuse Reflectance Infra-Red Fourier Tranfform Spectroscopy. Spectra were acquired in situ using a DRIFT accessory (Collector) equipped with an environmental cell (Spectratech). DRIFT spectra were recorded in nitrogen up to about 500°C (to avoid absorption bands due to physisorbed or hydrogen-bridged water) on a Perkin Elmer 1600 FTIR spectrometer (64 scans, resolution 8 cm- ). 

Figure 5.33 Simplified DRIFTS. The sample is mixed with KBr and placed in a cup in a DRIFTS accessory. Light from the interferometer hits the surface at an angle, and the specularly reflected light is blocked while the diffusely reflected light is captured by a curved mirror and directed toward the detector. The sample spectrum is ratioed against KBr. 

FIGURE 4.41 An optical diagram of a diffuse reflectance (DRIFTS) accessory. 

In addition to obtaining the spectra of powdered samples, DRIFTS accessories can be used to take the spectra of large intractable solids using a technique known as abrasive sampling. In this technique, abrasive paper is used to collect particles from the sample of interest silicon carbide is conunonly used. A convenient way of obtaining abrasive sampling spectra is to pnrchase (from an FTIR accessory company) pieces of precut abrasive paper with an adhesive on the back, one of which is seen on the left in Figure 4.45. 

FIGURE 4.45 Left A silicon carbide disk with an adhesive back. Center A metal post that fits into a DRIFTS accessory. Right A silicon carbide disk adhered to the metal post. 

Bulk polymer samples can be studied by using DRIFT [45,46]. For the study of the surfaces of bulk samples, a DRIFT accessory should perform the following functions ... 

A DRIFT accessory has been developed to examine local areas of bulky samples [47]. 

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