ATR FT-IR imaging

Sampling volume with a depth of few micrometers from the surface of the ATR crystal 

ATR FT-IR spectroscopy has been studied for many years. It relies on the attenuation of internally reflected IR light at the interface between a material of high refractive index (ATR crystal) and one of lower refractive index that absorbs frequencies of IR light (the sample) when the critical angle is exceeded. When coupled with an FPA detector, the data allow ATR FT-IR images to be generated. 

The applicability of ATR FT-IR imaging ranges from micro-ATR imaging using a Ge microscope objective to the use of ATR accessories with focused or expanded optics without recourse to the microscope [3]. Thus, the range of areas in the samples that could be measured simultaneously using FPA detectors and a prismshaped ATR crystal is from 500 pm X 700 pm to 1.6 cm X 2.1 cm (see Section 9.2.3 for further discussion). The measured area with macro-ATR imaging depends on the specific optics used in these ATR accessories and also on the size of the pixels in the array detector. Most of the currently used detectors have a pixel size of 40 pm X 40 pm or 60 pm x 60 pm. 

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Figure 9.6 ATR FT-IR imaging results of paraffin oil for the v(CH/CH2/CH3) absorption bands at 3000-2800cm . (a) 2-D ...

Figure 9.9 ATR FT-IR images of the distribution of a release coat (PVSC) on the PP-backing of an adhesive tape. Sample 1 ( bad ) has a heterogeneous (a) and sample 2 ( good ) a homogeneous (d) distribution of the release coat. The absorbance gradient observed for the original PP-backing is demonstrated in panels (b), (c), (e) and (f). Reproduced with permission from Ref [38] 2006, Society for Applied Spectroscopy.

Figure 9.11 IR images of a poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) film measured by ATR FT-IR imaging (Ge ATR objective) with the pressure stages 1-3 (evaluation of the left wing of the v(C=0) band see Figures 9.12 and 9.13).

Figure 9.12 ATR FT-IR imaging spectra of the poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) film measured with pressure stage 3 of the Ge ATR objective comparison of two spectra of the red (amorphous) and blue (crystalline) regions.

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]. 

ATR-FT-IR Imaging for Pharmaceutical and Polymeric Materials From Micro to Macro Approaches... 

The applicability of ATR-FT-IR imaging ranges from micro ATR imaging using a microscope objective to the use of ATR accessories with focused or expanded optics, without need to use the microscope. The ATR crystal in a prism shape... 

I 70 ATR-FT-IR Imagingfor Pharmaceutical and Polymeric Materials Table 10.1 Summaty of the capabilities of different ATR-FT-IR imaging approaches. 

Figure 10.2 ATR-FT-IR images of a copper grid with a thick film of PDMS pressed behind and measured using (a) the ordinary and (b) the new imaging diamond ATR accessory (c) the white light image of the copper grid measured with a 5x objective.

As with ordinary ATR spectroscopy, ATR-FT-IR imaging results can be analyzed quantitatively, some recent examples including the study of tablet dissolution in water. In this case, the concentration profiles of hydroxylpropylmethylcellulose (HPMC) and niacinamide, at different stages of the dissolution process, were utilized to provide an understanding of the drug release mechanism [54]. Using this technique, it could be shown that the concentration profiles of different components could be obtained with the partial least squares (PLS) method. Here, with... 

Figure 10.5 ATR-FT-IR image of mineral oil measured with a ZnSe ATR accessory showing the integrated absorbance at 1480-1420cm. The histogram showing the number of detector pixels for different values of the integrated absorbance at the same range for the ATR-FT-IR measurement.

Figure 10.7 ATR-FT-IR images of the polymer matrix showing the distribution of (a) carbon fiber (b) distribution of the absorbance of the band at 1011 cmImage size is 50 rm X 50 rm.

Without compatibilizer Figure 10.8 ATR-FT-IR images showing the distribution of the absorbance band of (a) PE and (b) LDPE with and without compatibilizer. 

The in situ compaction study is only possible by using hard ATR crystals, due to the high compressive force used for tablet compaction. Consequently, diamond is the material of choice here, and use of the diamond ATR accessory to obtain ATR-FT-IR images of compacted tablets has been demonstrated previously [35]. ATR-FT-IR images of a model pharmaceutical formulation containing... 

Figu re 10.9 Micro ATR-FT-IR images of a tablet showing the distribution of caffeine, starch and HPMC. 

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