Spatial Resolution of Micro FT-IR Imaging Measurements

For FT-IR imaging, a high spatial resolution is very important in order to visualize the distribution of different substances of a heterogeneous sample as precisely as possible. The size of the imaging area of, for example, a Ge ATR objective is 50 X 50 (xm. By using a FPA detector with 64 x 64 detector elements, an area of 0.78 X 0.78 (xm can be allocated for each detector element. However, this does not mean that such a spatial resolution can be achieved, because this parameter is limited not only by the pixel size and magnification of the objective but also by the diffraction of the IR radiation (see Chapter 1, Section 1.2.4). 

Chan and Kazarian were able to achieve a spatial resolution of 3-4 xm with micro ATR FT-IR imaging measurements with a Ge ATR crystal [37]. A recent report by Lasch and Naumann described the lateral resolution of FT-IR imaging measurements in transmission, whereby numerous results from measurements of a resolution target USAF 1951 and tissue samples with different imaging systems from different manufacturers and with different objectives were compared [41]. 

The theoretical definition of the spatial resolution requires a distance of 2r (where r is the Rayleigh criterion of resolution) in order to differentiate two adjacent points completely, and is limited by the wavelength of the IR radiation and the NA of the objective [42, 43] (see Chapter 1, Section 1.2.4). 

The spatial resolution can be improved by evaluating absorption bands at high wavenumbers. Unfortunately, characteristic high-wavenumber absorption bands are not always available for the calculation of FT-IR images. However, by using a Ge ATR objective with a high NA, Sommer etal. and Chan and Kazarian were able to optimize the spatial resolution by a factor in the range of the refractive index of the Ge crystal (n = 4) [37, 44]. 

1 The Test Systems In order to demonstrate the spatial resolution achieved in practical measurements, two test sample systems were investigated by FT-IR imaging measurements in the ATR mode and in transmission. The sizes of the imaging areas were 50 X 50 pm for the micro ATR FT-IR measurements (20x objective) and 260x260pm for the micro FT-IR transmission measurements (15x objective). 

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