Diffuse Reflectance DRIFTS

FIG U RE 4.40 Left An agate mortar and pestle used to grind powders for infrared analysis. Right A filled DRIFTS sample cup. 

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

FIGURE 4.42 The DRIFTS spectrum of a ground aspirin tablet diluted in KBr. 

As you can see, the peaks in the Kubelka-Munk spectrum are smaller and have different relative intensities than those in the absorbance spectrum. To be clear, the only difference between these two spectra is how the y-axis units are plotted peak positions are the same. The equation derived by Kubelka and Munk relates the intensity of diffusely reflected light to concentration [8]. If you are going to perform quantitative DRIFTS you must measure the spectra in Kubelka-Munk units to obtain a calibration line, similar to how absorbance spectra must be used for quantitation in transmission experiments (see Chapter 5). However, if one is doing qualitative analysis, DRIFTS spectra plotted in absorbance or Kubelka-Munk units may be used. Since the DRIFTS experiment does not produce a true absorbance spectrum, it is best to call the y-axis units of a DRIFTS spectrum plotted in absorbance diffuse absorbance. All the DRIFTS spectra in this chapter are plotted in diffuse absorbance. 

FIGURE 4.43 A picture of two different micro-DRIFTS sample cups. 

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In solvent-elimination LC-FTIR, basically three types of substrates and corresponding IR modes can be discerned, namely, powder substrates for diffuse reflectance (DRIFT) detection, metallic mirrors for reflection-absorption (R-A) spectrometry, and IR-transparent windows for transmission measurements [500]. The most favourable solvent-elimination LC-FTIR results have been obtained with IR-transparent deposition substrates that allow straightforward transmission measurements. Analyte morphology and/or transformation should always be taken into consideration during the interpretation of spectra obtained by solvent-elimination LC-FTIR. Dependent on the type of substrate and/or size of the deposited spots, often special optics such as a (diffuse) reflectance unit, a beam condenser or an FITR microscope are used to scan the deposited substances (typical diameter of the FITR beam, 20 pm). 

The acquisition of solid-state FTIR spectra suitable for use in the characterization of polymorphic impurities is performed using either the Nujol mull technique, diffuse reflectance (DRIFT), or attenuated total reflectance (ATR). One should avoid the use of pelleting techniques to eliminate any spurious effects associated with compaction of the KBr pellet. The simplest approach is to prepare a mull of the sample in mineral oil, sandwich this between salt plates, and measure the spectrum using ordinary transmission techniques. The main drawback of the mull technique is that regions in the IR spectrum overlapping with carbon-hydrogen vibrational modes will be obliterated because of absorbance from the oil. 

In a similar vein, Wilson et al. [61] recorded the FTIR spectra of fruit jams of different types using a diffuse reflectance (DRIFT) samphng accessory. Despite distortions in the spectra, associated with the use of the DRIFT technique, the different jams were found to exhibit characteristic and reproducible patterns, indicating that jams of different fruit content can be differentiated on the basis of their FTIR spectra. The authors suggested the potential utihty of this technique for the detection of adulteration of jams, for instance, by the substitution of fruit content by cheaper vegetable material. 

One of the major sample-handling problems in FTIR analysis of carbonaceous materials is that many of them are effective blackbody absorbers and thus are too opaque for direct transmission analysis in the midinfrared spectral region. Addition of KBr intensifies the signal to obtain transmission infrared spectra. It is time consuming, and grinding conditions and moisture are known to affect the spectrum of the sample [238]. Alternative techniques such as specular reflectance, diffuse reflectance (DRIFT), photoacustic spectroscopy (FTIR-PAS), and total... 

Figure 12 Attachments for IR analysis of surfaces (a) attenuated total reflectance (ATR) (b) reflection-absorption (RAIR) (c) diffuse reflectance (DRIFT). (From Ref. 2.)... 

Infrared spectroscopy is a widely available technique and has been applied extensively in the study of microporous solids. Using Fourier Transform analysis, sensitive detectors and operating either in transmission or in diffuse reflectance (DRIFT) mode, powders can give spectra with high resolution and sensitivity. The method is most valuable when analysing the interaction of molecules with adsorption sites (acid or base) - this is described in Chapters 7 and 8. It does give some structural insights, however, for example on the environment of protons and on the presence of framework and non-framework cations. 

FTIR Fourier transform infrared spectroscopy transmission, diffuse reflection (DRIFTS), and attenuated total reflection (ATR) Identiflcation/structure of (adsorbed) species, adsorbate-adsorbent interaction... 

Spectra of insoluble solid phenolic prepolymers or crosslinked products can be recorded by diffuse reflectance (DRIFT) and attenual total reflection spectroscopy (ATR). Reflection spectroscopy techniques are qualitative methods for analysis of insoluble materials [228], which work by reflecting the infrared light off the surface of the material to be analyzed. Table 7 gives a summary of wavenumbers of absorbance bands in modified phenolic materials. 

Specular reflectance (Fresnel) Diffuse reflectance (DRIFTS) Reflection-absorption (RA) Grazing angle... 

One of the strengths of FTIR spectroscopy is its diverse range of sampling techniques. Examples include attenuated total reflectance (ATR), diffuse reflectance (DRIFT), photoacoustic (PA), grazing angle, microspectroscopy and more specialised techniques such as synchrotron-radiation-based FTIR (SR-FTIR) microspectroscopy 11-12). The following section outlines ATR and SR-FTIR microspectroscopy in more detail and then provides specific applications of their use for the analysis of C. neoformans, S. favosa and L crassa. 

Emission Spectroscopy Diffuse Reflectance DRIFT Infrared Microscopy Photoacoustic Spectroscopy PA Rheo-Photoacoustic Spectroscopy Ellipsometiy... 

Infrared spectroscopy is predominantly performed in the Fourier-transform mode and then commonly abbreviated as FTIR. The great advantage of FTIR spectroscopy is the great number of measurement options (and accessories), that allow spectra to be taken conveniently from just about any kind of sample. Polymeric powders can be characterized by pressing them into the conventional KBr pellets, but also, without any sample preparation, by diffuse reflectance (DRIFT). Very thin films of polymers can be measured in the conventional transmission mode, but any kind of film (thick or thin), as well as large polymeric objects, can be measured by ATR. ATR probes can also be used to characterize solutions... 

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