Chemical mapping of surfaces by FT-IR microscopy

Chemical mapping of surfaces by FT-IR microscopy
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FT-IR Surface Mapping (Image Courtesy : http://www.eag.com/)

FT-IR microscopy proved to be a very versatile tool for compositional characterization of materials. Over the years it has proved to be an asset for chemical mapping of surfaces with the addition of the microscope. Such mapping includes identification of impurities, defects and compositional in- homogeneities in a wide range of applications:

Studies on cultural heritage materials and artifacts

The nondestructive character of FT-IR spectroscopy permits surface mapping and collection of large number of spectra over small time intervals thereby providing a distribution of chemically identified compounds. Inorganic pigments are being studied by extending the range of study from 8000 to \(50 cm^-^1\)(IR and Far- IR). This range provides useful information on mural paintings, damage by corrosion as well as efficacy of protective coatings.

Polymer Laminates

Multilayered polymer laminates often come with layers only a few microns thick so high resolution is required to reveal the chemical composition of such layers. The commercially available imaging systems come with resolutions down to 10 μm or lower.

Pharmaceutical tablets

Chemical imaging provides information on some critical quality parameters that is not readily available using conventional techniques. It is valuable in troubleshooting during manufacturing operations and scale-up of pharmaceutical processes. The combination of thermo micro-methods in combination with FT-IR microscopy gives valuable information on active ingredient – excipient interactions and compatibilities. Such information is of great value in development of new formulations.

The full potential of FTIR imaging applications could be exploited with advances in technology which are briefly highlighted below:

Detectors

Early day detection employed large focal plane array detectors which had limited applicability due to low sensitivity, reliability and limited spectral range.The situation improved remarkably with introduction of mercury cadmium telluride detectors with individual liquid nitrogen cooled elements. This made possible multiple scanning of complete spectrum range several hundred times per second. By moving the sample across the detector it became possible to scan images of variable sizes and aspect ratios for enabling scanning the desired surface area to maximum advantage.

Multiple variable imaging resolutions

Depending on the chemical differences, sample or article size the imaging system resolutions can be reduced to 10 micro-meter size. This saves valuable time when very high resolutions and sampling times are not necessary for routine mapping applications. Fast scanning identifies areas of in-homogeneity at low resolutions which can be followed by high resolution mapping which has proved to be useful in most situations.

Attenuated total reflectance(ATR)

Small surfaces of particles or even very thin polymer laminates have posed challenges for the analyst. Even precise sectioning often fails to maintain sample integrity and spatial diffraction poses additional problems.

Attenuated total reflectance comes to the rescue in such situations. The sample need not be cut into thin sections. Only mounting in an embedding resin and polishing is sufficient. Images tend to be sharper and spectra show fewer artefacts due to beam divergences. Resolutions below 10 μm are now possible with ATR accessory.

FTIR imaging today is meeting diverse challenges of material characterization and the power of software is accelerating analysis time to the benefit of the analysts.

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