Transmission Sampling Overview

FIGURE 4.1 An illustration of transmission sampling, where the infrared beam passes through a thin film of sample before impinging on the detector. The letter L denotes the path-length, which is the thickness of sample encountered by the infrared beam. 

Works on many sample types Inexpensive tools Quality spectra History/tradition 

Opacity problem Trial and error may be involved May be time-consuming May be destructive of sample 

Another disadvantage of transmission techniques is that they can be destructive. Transmission techniques involve grinding, compressing, or dissolving the sample, all of which make it impossible to recover the sample intact. If there is not much sample, the sample is valuable, or if it needs to be preserved for some other purpose, this is a problem. 

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The history of EM (for an overview see table Bl.17,1) can be interpreted as the development of two concepts the electron beam either illuminates a large area of tire sample ( flood-beam illumination , as in the typical transmission electron microscope (TEM) imaging using a spread-out beam) or just one point, i.e. focused to the smallest spot possible, which is then scaimed across the sample (scaiming transmission electron microscopy (STEM) or scaiming electron microscopy (SEM)). In both situations the electron beam is considered as a matter wave interacting with the sample and microscopy simply studies the interaction of the scattered electrons. 

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. 

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