Surface Raman sampling considerations

Recent developments in Raman equipment has led to a considerable increase in sensitivity. This has enabled the monitoring of reactions of organic monolayers on glassy carbon [4.292] and diamond surfaces and analysis of the structure of Lang-muir-Blodgett monolayers without any enhancement effects. Although this unenhanced surface-Raman spectroscopy is expected to be applicable to a variety of technically or scientifically important surfaces and interfaces, it nevertheless requires careful optimization of the apparatus, data treatment, and sample preparation. 

When investigating opaque or transparent samples, where the laser light can penetrate the surface and be scattered into deeper regions, Raman light from these deeper zones also contributes to the collected signal and is of particular relevance with non-homogeneous samples, e.g., multilayer systems or blends. The above equation is only valid, if the beam is focused on the sample surface. Different considerations apply to confocal Raman spectroscopy, which is a very useful technique to probe (depth profile) samples below their surface. This nondestructive method is appropriate for studies on thin layers, inclusions and impurities buried within a matrix, and will be discussed below. 

Considerable interest has been shown recently in the chemistry of species adsorbed on zeolite surfaces. For example, a Raman spectroscopic investigation of zeolites and the molecules adsorbed thereon has been undertaken. All the zeolites examined give weak Raman spectra in nearly all cases the samples gave rise to an excessive background, and in order to minimize the problem it was necessary to use high-purity materials. Although these spectra are not as informative as the i.r. framework frequencies, they still show differences between zeolite structures. The Raman spectra of adsorbed... 

Consideration needs to be given to the sample interface and the measurement technique used. Some techniques relate to the whole sample whereas others are very much surface measurements. For example, microwave spectroscopy and infrared transmission measurements provide values on the bulk sample whereas X-ray fluorescence and Raman spectroscopy are very much surface techniques, only penetrating the sample to a limited degree. 

FTIR and Raman spectroscopies can in principle provide infomiation about surface functionality on fibers and composites. In both techniques, however, the sampling depth is considerable, which means that if there is any chemical functionality in the interior of the fiber or composite, then the information is ambiguous. Nev-... 

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