Transmission infrared spectroscopy, sample preparation

Ballinger, T.H., Wong, J.C.S., and Yates, J.T., Jr. (1992) Transmission infrared spectroscopy of high area solid surfaces. A usefid method for sample preparation. Langmuir, 8, 1575-1578. 

In the limited space available this paper has attempted to give an overview of the ways that transmission infrared spectroscopy has been applied to the study of high surface area materials. Developments in improved sample preparation and the use of isotopic substitution have been discussed. The more quantitative aspect of work accomplished in the last decade has been emphasized by giving examples of adsorbtion isotherms on individual sites and the subsequent reactivity of the adsorbed molecules with these sites. 

Infrared spectroscopy/microscopy certainly is the primary method of choice when organic substances have to be identified. Sample preparation usually is simple for identification purposes, but will be an issue for imaging experiments, and spatial resolution may then well be only in the range of a few micrometers, depending on the used experimental approach (transmission, ATR). 

The basal spacing (d 001) (DRX-Kristalloflex-805 Siemens) and the surface area (Micromeritics ASAP 2400) was obtained on the solids calcined at different temperatures. X-Ray diffraction patterns have also been obtained after ethylenglycol saturation of selected samples. High resolution transmission electron microscopy (HREM) was performed (Jeol 100 CX Temscan) on ultrathin preparations (LKB Ultratome type 8802A). TPD (NH3) and infrared spectroscopy (pyridine) allowed to evaluate the acid properties of the solid calcined at 4(X) and 600°C. 

Recently, we reported that an Fe supported zeolite (FeHY-1) shows high activity for acidic reactions such as toluene disproportionation and resid hydrocracking in the presence of H2S [1,2]. Investigations using electron spin resonance (ESR), Fourier transform infrared spectroscopy (FT-IR), MiJssbauer and transmission electron microscopy (TEM) revealed that superfine ferric oxide cluster interacts with the zeolite framework in the super-cage of Y-type zeolites [3,4]. Furthermore, we reported change in physicochemical properties and catalytic activities for toluene disproportionation during the sample preparation period[5]. It was revealed that the activation of the catalyst was closely related with interaction between the iron cluster and the zeolite framework. In this work, we will report the effect of preparation conditions on the physicochemical properties and activity for toluene disproportionation in the presence of 82. ... 

Infrared spectroscopy is a relatively simple technique, nondestructive, and versatile enough to analyze solids, liquids, and gases with a minimum of sample preparation. Polymers can be mixed with potassium bromide and then pressed into pellets. Films can be prepared from melt or cast from solution and can be studied easily. In bulk samples or powders, or if a concentration profile is needed, the reflectance technique is probably more suitable than transmission. 

To uniquely identify the intrinsic feature of the material, one method of sample preparation is to pelletize the explosive powders or crystals [14], It is standard practice in far-infrared (THz) spectroscopy to press samples into pellet form to measure the THz transmission spectra. When the sample is a powder with a grain size comparable to the THz wavelength (about 300 microns), the powder strongly scatters the THz radiation. Another method of sample preparation is to mix the material (e.g., RDX) with an inert matrix or filler material to create a pellet. The filler is typically a material that is transparent in the THz such as polyethylene. This allows dilute concentrations of a highly absorbing agent to be measured. 

Because Raman spectmm stems from the bonds vibrations, it provides an intrinsic nano-probing and offers a bottom-up approach of nanostmctured materials that comes as a good complement to other techniques such as transmission electron microscopy. X-ray diffraction, and infrared, and Mossbauer spectroscopy. Since almost no sample preparation is needed, Raman technique [12, 13] is commonly used to investigate nanomaterials. This could provide the phase identification and, possibly, size estimation [14]. 

For some sample types, for example a coated substrate, it is not possible to collect an infrared transmission spectrum, whereas in some cases (e.g. when there are concerns over the effects of sample preparation) it may be more desirable to collect a reflected spectrum. The most popular reflection techniques nowadays are internal reflection spectroscopy (IRS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)... 

Transient infrared spectroscopy (TIRS) is a mid-infrared technique [82] that has been developed to obtain spectra of moving solids and viscous liquids. TIRS spectra are obtained from the generation of a thin, short-lived temperature differential that is introduced by means of either a hot or cold jet of gas. When a hot jet is used, an emission spectrum is obtained from the thin, heated surface layer. This technique is known as transient infrared emission spectroscopy (TIRES). When a cold jet is used, the blackbody-like thermal emission from the bulk of the sample is selectively absorbed as it passes through the thin, cooled surface layer. The result is a transmission spectrum convoluted with the observed thermal spectroscopy. This method is known as transient infrared transmission spectroscopy (TIRTS). TIRS is ideally suited for online analysis because it is a single-ended technique that requires no sample preparation. This technique has been applied to the lignin analysis of wood chips [83]. 

Reflectance spectroscopy is commonplace for samples that cannot be prepared for transmittance measurements. However, reflectance measurements must be carefully conducted as the reflected beam is not only indicative of the composition of the sample but is also affected by surface conditions at the sample plane. This makes the reflectance spectra, though indicative of material chemistry, difficult to interpret and generally less useful for quantitative analysis. Since the polarization of the beam is maintained for reflectance, especially specular reflectance methods, examination of orientation at polymer surfaces using reflection techniques is attractive [10]. Reflection-absorption modes involve the transmission of the infrared beam through the sample and subsequent reflection to pass through the sample again. Usually, sample preparation is difficult for such experiments and they... 

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... 

A number of experimental alternatives to traditional IR transmission spectroscopy are suitable for overcoming some of these complicating experimental factors. In the technique of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Hartauer et al. 1992 Neville et al. 1992) the sample is dispersed in a matrix of powdered alkali halide, a procedure which is less likely to lead to polymorphic transformations or loss of solvent than the more aggressive grinding necessary for mull preparation or pressure required to make a pellet (Roston et al. 1993). For these reasons, Threlfall (1995) suggests that DRIFTS should be the method of choice for the initial IR examination of polymorphs. He has also discussed the possible use of attenuated total reflection (ATR) methods in the examination of polymorphs and provided a comparison and discussion of the results obtained on sulphathiazole polymorphs from spectra run on KBr disks, Nujol mulls and ATR. 

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