Micro FT-IR imaging

For small samples or in order to achieve a higher spatial resolution, the microscope is used that is specially designed for FT-IR measurements. In this case, a motorized mirror in the microscope enables a change between single-point 

Type of measurement Magnification Numerical aperture Imaging area 

For transmission and reflection measurements, a 15x objective with a NA of 0.4 and an imaging area or field of view (FOV) of 260 pm x 260 pm is used (see Table 8.1). 

Micro ATR measurements in the reflection mode are performed with a special ATR objective with a Ge crystal. The tip has a diameter of 100 pm and is shaped slightly convex in order to ensure an optimal contact between the ATR crystal and the sample. It is a 20x objective with an NA of 0.6 and an FOV of 50 pm x 50 pm (see Table 8.1). 

If a larger FOV is required, the macrochamber IMAC is used instead of the microscope. It consists of an optical and a sample compartment and connections for 

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Table 9.1 The objectives applied for the micro FT-IR imaging measurements and their corresponding instmmental parameters.

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 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). 

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... 

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

In this chapter, we have presented some of the recent developments and applications of ATR-FT-IR imaging to pharmaceutical samples and polymeric materials. The use of both micro and macro modes in ATR-FT-IR imaging has been discussed. Imaging in the ATR mode may be very versatile, and offers imaging with different spatial resolution and different fields of view. It is important to remember that ATR imaging provides information only about that layer of the sample which is adjacent to the surface of the ATR crystal, and that the thickness of this layer may range from a fraction of a micrometer to several micrometers. However, the opportunity exists to obtain chemical images from layers of different thickness in... 

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. 

Overview of ATR FT-IR Imaging Approaches Micro (Ge), Macro (Diamond, Si), Expanded FOV (ZnSe), Variable Angle... 

Mixing of polymers is an important process in the polymer industry by combining the strength of different polymers through blending, new products with desirable physical properties can be produced [2]. FT-IR imaging with a micro-ATR objective has been used to study the effect of a compatibilizer on the mixing of two immiscible polymers, namely polystyrene (PS) and low-density polyethylene (LDPE). The compatibilizer used in this study is a triblock copolymer of polystyrene-f -poly(ethylene-butylene)-f)-polystyrene (SEES). The blends are prepared with a micro-extruder, which allows small amounts of the materials to be blended [2]. The two polymers are easily characterized by their specific absorption bands at 1492 and 1450 cm for PS and the band at 1466 cm for LDPE. 

The micro-ATR FT-IR imaging approach is readily applicable to study compacted tablets. The contact area of the ATR crystal is circular with a diameter in the region of 100 pm. This area is small enough to easily establish a homogeneous and intimate contact between the ATR crystal and the tablet. To prove this concept, a model tablet, composed of about 3 wt% caffeine in a matrix of starch HPMC of 4 6, was compacted and imaged using the micro-ATR method and the result is shown in Figure 9.10. 

Hanging-drop Crystallization Another common protein crystallization technique is the hanging-drop crystallization method, where a drop of protein in precipitation solution is suspended above a reservoir until crystallization is realized. This protocol was mimicked in a recent study in which the protein/precipitant solution was suspended from a Ge ATR crystal [99]. The resulting images obtained allow discrimination between protein adsorbate and salt crystal formation (Figure 9.21). In situ micro-ATR FT-IR imaging of the protein crystallization... 

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